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Heating and Storing Asphalt at HMA Plants_ T-140 | Hvac | Natural Gas
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This technical paper is published by Heatec, Inc. of Chattanooga, Tennessee, a division of Astec Industries. It is hoped that the information contained in the paper will benefit the hot mix asphalt paving industry as a whole. Individual copies may be obtained free of charge by contacting the company. This paper reflects the considerable experience and knowledge of several individuals at Heatec. Chief contributors were as follows: Jim May Tom Wilkey Michael Swanson Jürgen Daub Gene Farrow John Clayton Dave Clum Mark Moon Bryan Eley Frank Eley The contributors have endeavored to provide factual information in an unbiased way. However, the statements and recommendations are strictly the opinions of the individuals and are not in any way intended as a warranty of the products or materials described.
© 1999 Heatec, Inc. All rights reserved.
2 3 3 4 6 9 10 11 12 12 12 13 13 15 15 17 18 19 Abstract Heat conservation Considerations Basic needs Increasing temperature Vs maintaining it Heating systems Direct-fired tanks Hot oil heaters Expansion tanks Electric heaters Heating fuels Heavy fuel preheaters Fuel heating values Monitoring fuel usage Heater thermal efficiency Impact of efficiency Efficiency factors Heatec heaters 20 20 21 22 24 25 25 28 28 29 32 34 35 36 37 38 39 39 40 Determining efficiency Case histories Burners Heat loss Proper insulation HMA plant heating costs Heat requirements Portability Equipment layout Piping Filters and valves Asphalt pumps Hot oil pumps Asphalt metering Calibration Heater controls Emissions Containment Horizontal Vs vertical tanks
Hot mix asphalt producers face numerous choices when buying heating and storage equipment for liquid asphalt. Making the right choices is not always easy because there are many things to consider. The first concern is usually the initial cost of the equipment—even when ample financial resources are available. Although production volume is usually a major factor in equipment costs, it’s by no means the only one. Other factors include such things as whether the equipment needs to be moved frequently, how much land area it will occupy, whether mixes will include polymers, allowable emissions, restrictions on ground water pollution, the types of fuels or energy readily available, and operating costs. This paper discusses all of these factors. We believe that contractors should have a good understanding of these subjects before purchasing equipment. That way
So. but also in many other countries of the world. it makes sense to take a close look at all issues that could in time have a positive or negative impact (Figure 2). a major concern of governments and citizens alike. Heat conservation Perhaps the most fundamental issue concerned with heating and storage of asphalt is the conservation of energy (Figure 1). While some issues—such as the thermal efficiency of asphalt heating—may seem . not only in the United States.Page 3 Figure 1 Waste not Want not Conservation of energy Is A Major Concern Figure 2 We need To look Ahead I F YOU OWN OR PLAN TO purchase HMA (hot mix asphalt) production equipment the information in this paper can help you understand many issues you face. In just a few years what may have initially appeared to be insignificant may either result in significant savings or significant waste. Fortunately. depending on how the issue was initially addressed. Any higher initial costs are quickly recovered and turned into savings. it is possible to conserve energy and save money at the same time when heating and storing asphalt. In recent years conservation of all natural resources—especially energy resources—ranks high. It will give you insight into important issues that are often overlooked or little understood when planning the heating and storage of asphalt at a HMA plant. It is Considerations less important than considerations for drying aggregate. These asphalts require heating and storage significantly There are many important considerations whether replacing old equipment or purchasing an entirely new system. all are important in the long run. Details are important! There are now a number of new issues that have come about recently from the use of PMACs (polymer-modified asphalt cements).
Asphalt is normally delivered to the HMA facility in a liquid state.Page 4 Figure 3 Asphalt Is Usually Delivered By Tanker Truck Figure 4 A Barrel Melter Is Used For Asphalt Shipped In Barrels different from virgin or neat asphalts. Some HMA facilities—mainly those in remote locations and on small islands— buy solidified asphalt in barrels. depending upon the arrangements with the supplier. Viscosity is a measure of a fluid’s resistance to flow and is related to its temperature. Heating. The viscosity of the asphalt must be low enough to allow it to be pumped from the delivery truck into the storage tanks. poise and centipoise. This requires special equipment known as barrel melters (Figure 4). they normally have a higher viscosity. Moreover. Thus. They must usually be maintained at higher temperatures and must be continuously agitated or mixed to keep the polymers from separating. please refer to Technical Paper T-133 entitled. Basic needs Heating and storing asphalt is a basic function of all HMA facilities. In some instances asphalt is transported aboard ships in . The actual temperature of the liquid asphalt at time of delivery may vary somewhat. which may affect piping and pumping needs. Viscosity is commonly expressed in units known as SSU (Saybolt Second Universal). For additional information on PMACs. Mixing and Storing of Modified Asphalt. This paper is available from Heatec. But most asphalt terminals deliver asphalt to HMA plants at a temperature suitable for making hot mix or slightly higher. the solidified material must be melted into a liquid before it can be pumped into the asphalt storage tanks. The higher the temperature the lower the viscosity or resistance to flow. usually by tanker truck (Figure 3).
the heating equipment must increase its temperature to meet that specified for use. However. heating equipment for some types of plants nearly always performs additional roles. When asphalt is used to make hot mix it must be within a specified temperature range. This . Moreover. some operators allow it to cool down. The containers usually have heating coils or provisions for a burner so the asphalt can be re-heated in the container after it reaches its destination. But when the asphalt is delivered to the plant at temperatures lower than required for making hot mix. Whenever stored asphalt is to be left unused for an extended period. This calls for more heating capacity. On relocatable and stationary plants heating is needed to maintain the temperature of the hot mix after it leaves the mixer. Again. which may affect the choice of equipment. This usually requires heating drag conveyors. this may affect the choice of equipment.Page 5 Figure 5 Shipping Containers May Be Used To Transport Asphalt insulated shipping containers (Figure 5). the role of the asphalt heating and storage equipment is to maintain that temperature until the material is used. traverse conveyors and the cones of hot mix storage silos. And sometimes it is necessary to heat hot mix additives and heavy fuel oil. as already noted. usually from about 300 to 325 degrees F. Accordingly. Some of these added roles may require greater heating capacity. higher temperatures are usually required for PMACs. when asphalt is delivered to the plant at a temperature suitable for use. Maintaining its temperature requires only enough heat to replace that lost during storage and pumping. in addition to replacing all heat lost during storage and pumping.
A typical heating and storage system for a large HMA plant that uses PMACs is shown in Figure 7.Page 6 HOT OIL PUMP UNLOADING PUMP METERING PUMPS DIRECT-FIRED ASPHALT TANK WITH SCAVENGER COILS Figure 6 JACKETED PIPING DRUM MIXER Typical Heating And Storage For A Small HMA Plant VERTICAL TANK WITH COILS HORIZONTAL TANK WITH COILS HOT OIL PUMP FUEL PREHEATER TRAVERSE CONVEYOR METERING PUMPS HOT OIL HEATER ADDITIVE TANK Figure 7 UNLOADING PUMP DRAG CONVEYOR JACKETED PIPING SILO CONE DRUM MIXER Typical Heating And Storage For A Large HMA Plant conserves heating energy and minimizes oxidation of the asphalt. it pays to buy your asphalt at the temperature at which it can be used and to use it before it loses much of its heat. The heating system should be capable of restoring the temperature within a short period of time. And it pays to . This practice is especially appropriate for PMACs that have been heated to higher temperatures than used for virgin asphalt. A typical heating and storage system for a small HMA plant is shown in Figure 6. Increasing temperature Vs maintaining it It costs less to maintain the temperature of asphalt than to increase its temperature. Accordingly.
127.000 100. Maintain Temperature Over 24 Hours do everything possible to keep it from losing heat.000 200. Heat Required To Raise Asphalt Temperatures Energy Needed Raise Temperature 25 Degrees F 4.000 600.000.800.000 1. Heat required to raise one pound of AC one degree = 0.000 10 Degree F Increase 40.500.000 3.000 lbs.000 gallons AC @ 8 pounds per gallon = 240.000 2.000 3.Page 7 Heat Required (Btu) Quantity (Gallons) 1.127. One gallon No.000 675. The amount of heat required to raise the temperature of various quantities of asphalt is shown in Figure 8 (and does not include any additional heat to make up for losses during heat up).000 20. 2 fuel produces 132.000.5 Btu.000 200. Heat required does not include extra heat to make up for heat lost while heating.500.000 1.127. Using well-insulated tanks and piping is the most cost-effective way of minimizing heat loss.000 1.500.000 1.000 Btu.000 1.400. Suppose your asphalt is delivered to your plant at a temperature that is 25 degrees F lower than needed to make hot mix.000.000 270.000 5 Degree F Increase 20. Specific heat (degrees F) = 0. Energy to Raise Temperature Vs.000 Heat required = weight x specific heat x temperature difference.000 600.5 Figure 9 Btu Gallons No.810 8.000.000 30.200.000 500.000 6.000 900. In both cases assume that the tank has 3 inches of insulation. Heat to raise temperature includes heat to make up for heat loss during the 24 hours.000 135.000 2. Consider the energy required to heat a 30.000 300. Weight = 8.000 500.000 pounds of AC one degr = 120.000 400.3 Maintain Temperature @ 300 Degrees F 1.000 1.000 1.000 400.000 2.000 800.000 25 Degree F Increase 100. It takes 4.810 31.000 35. raising the temperature requires more than three times the amount of fuel needed to maintain the temperature (Figure 9).200. a difference of 3.5 Btu per pound per degree F.000. But it only takes 1. Heat required to raise 240.000 600.000 Btu.750 10.127.000 700.000 gallon tank of asphalt to increase its temperature 25 degrees F (from 275 to 300 degrees F) over a period of 24 hours.810 Btu to maintain a temperature of 300 degrees F for 24 hours.000 300.000 Btu LHV.000 Figure 8 5. Tank has 3-inch insulation.0 pounds per gallon.000 15. Thus.100. 2 Fuel Temperature is raised and maintained for 24 hours.000 1.000 25.810 Btu to increase the temperature 25 degrees F and to make up for losses that will occur during a 24 hour period. .000 405. 30. Then compare that with the energy required to maintain a temperature of 300 degrees F over the same period.000 15 Degree F Increase 60.500.
653. It will take about 1.834. billion (1.000 gallon tank of AC at a temperature of 300 deg F before shutdown. But the same tank with no insulation takes 126.463. Assumes a 30.463.3 Tank With 6-inch Insulation 289 Degrees F 1. An automatic timer can be used to start up the heating system to restore asphalt temperature in time for plant re-start.650 1197. Assumes a 1.418.091 Btu to restore the temperature and likewise takes about 2.000 gallon tank of asphalt at 300 degrees F for a month.9 Figure 11 Shutdown is for 60 hours. Energy Used To Restore Heat After Shutdown Now. But it takes over one U. It takes only 18.Page 8 Energy Needed Btu Gallons No.248) Btu to maintain the non-insulated one.296 Btu to maintain the tank with 3-inch insulation.2 million Btu heater using 75 percent of its output to reheat the tank. Heat to restore temperature includes heat to make up for heat loss during restoration. consider the energy needed to maintain a 30. 2 Fuel Tank With No Insulation 1.798 320 175 30. 2 Fuel To Restore Tank With No Insulation 252 Degrees F 126. Compare the energy needed to maintain a noninsulated tank for that length of time with a tank that has 3-inch insulation and one that has 6-inch insulation (Figure 10). A 30.367.475.367. Non-insulated Tanks Temperature & Energy Bulk Temperature After 60-Hours Btu To Restore After Shutdown Gallons No.034.382 Figure 10 9.000 gallon tank of asphalt with 6-inches of insulation requires only 1.834.283 Btu to restore the temperature to 300 degrees F after a 60-hour shut- down.5 hours. Having well-insulated tanks and piping also makes it feasible to shut-off heating equipment over week ends or for other extended periods of non-operation.248 Tank With 3-inch Insulation 33. depending upon the size of the heater and how much of its heat is needed for other equipment. Figure 11 shows the amount of heat required to restore the temperature.7 hours.650 Btu to restore the temperature and will take about 140 hours.S.000 gallon tank.475.283 12.296 Tank With 6-inch Insulation 18.1 Tank With 3-inch Insulation 281 Degrees F 2.653.418. It takes 33. .382 Btu for the tank with 6-inch insulation.091 23. These are significant differences. Energy Used By Insulated Vs.034. The same tank with 3 inches of insulation takes 2. Maintain AC temperature at 300 deg F for 30 days.
Asphalt in contact with tank inner surfaces solidifies. asphalt in the uninsulated tank solidifies sooner than in the insulated ones. Heating systems Two basic types of heating systems are commonly used at HMA plants. One system employs what is known as a direct-fired tank (Figure 12). Both use heating fuels.Page 9 Figure 12 Plant With Direct-Fired Tank Figure 13 Large Stationary Plant With Hot Oil Heater And Tanks You may have noted that the table shows the bulk or average temperatures of the three tanks range from 252 to 289 after being off for 60 hours. An electric heating system (which does not use heating fuels) is sometimes used. . But an interesting phenomenon occurs as the asphalt cools. You may have expected a greater temperature drop for the uninsulated tank. The other system employs a hot oil heater and tanks with heating coils (Figure 13). Nevertheless. it takes considerably more heat to restore the temperature of both the uninsulated one and the one with 3 inches of insulation because they will lose more heat while being re-heated than the one with 6 inches of insulation. This solidified layer retards heat loss to a major extent. creating a highly effective insulation. Moreover.
The hot oil is piped to the asphalt storage tank where it heats the tank coils. Moreover. Another type. Moreover. It can employ scavenger coils the same way as the direct-fired tank. And the coils heat the asphalt or contents of the tank. The other type of heating widely used is an indirect system. Scavenger coils are very limited in their ability to heat other components. This limits the amount of heat available for heating other equipment. Oil at this temperature is suitable for heating jacketed asphalt lines. Its exhausts on the same end as the burner. By simply using additional piping circuits the hot oil can be used to heat additional tanks and virtually any plant component. Oil is pumped through the coils. These are plants that usually need only one asphalt storage tank. If anything in addition to the asphalt needs to be heated. The maximum scavenged temperature of the thermal oil will be range from about 250 to 300 degrees F. emulsion tanks and heated fuel tanks. enabling them to scavenge heat from the asphalt. metering systems. It employs a hot oil heater and tanks equipped with heating coils The heater heats oil (thermal fluid) as it is pumped through the heater. The direct-fired tank is well-suited for small portable asphalt plants using a surge bin . drag conveyors. The coils are totally independent from the burner and fire tube. all as separate units (Figure 15). which in turn heats the asphalt that surrounds it.Page 10 RETURN TUBES FIRE TUBE COMBUSTION CHAMBER BURNER JACKETED EXHAUST STACK HOT OIL SCAVENGER COILS LEVELS INDICATORS Figure 14 ASPHALT FUEL BURNER MAIN FIRE TUBE TWIN RETURN TUBES OVERFLOW PIPE INSULATED BULKHEAD Cross-section Of Direct-Fired Tank With Scavenger Coils The direct-fired tank is an asphalt storage tank that has a burner mounted on one end of the tank. or about 25 to 50 degrees below the asphalt temperature. the temperature of the oil flowing through it is limited by the temperature of the asphalt. Direct-fired tanks A heating system using a direct-fired tank has the advantage of lower initial cost than most other systems. Larger plants usually need more than one asphalt tank. However. The only way a direct-fired tank can heat other components is by use of scavenger coils or by use of a booster heater. It is important to match the capacity of the heater with the heat load. and fuel tanks. known as a two-pass fire tube. Direct-fired tanks are not well-suited for larger plants and for plants that use PMACs. especially if it has a two-pass fire tube. The system using electric heat is very similar to the one using the direct-fired tank. The burner heats the fire tube. and for small batch plants. Some systems have a trailer-mounted coil tank with the hot oil heater mounted on the gooseneck of the trailer. They are positioned above the fire tube so they are immersed in the asphalt. runs the full length of the tank and doubles back toward the burner. runs the full length of the tank and exhausts on the end away from the burner. it has relatively few parts and requires very little maintenance. large plants usually need to heat a number of plant components such as drag conveyors. asphalt silos. the tank can be equipped with scavenger coils. One type of fire tube. Systems with hot oil heaters often have a single hot oil heater and one or more coil tanks. The thermal efficiency of the tank is also very high. The burner fires directly into a fire tube or heating chamber located inside the tank (Figure 14). known as a single-pass fire tube. Because the hot asphalt is the source of heat for the scavenger coil. And they don’t usually require heat for other plant components. The tank has an electric heating coil instead of the burner and fire tube. carrying the scavenged heat to other plant components.
Layers of coke have insulating properties similar to fiberglass insulation. The film temperature is the temperature of the thermal boundary layer in the asphalt or material surrounding the fire tube. Accordingly. some plants use scavenged heat to pre-heat the cone of a surge bin prior to its use. Another disadvantage of a direct-fired tank is the need to always retain enough asphalt in the tank to cover the fire tube and scavenger coils. it’s possible to shut off any component not in use. An asphalt temperature recovery rate of 1 to 3 degrees F per hour is typical. each controlled independently. The outside surfaces of a fire tube are significantly hotter than the outside surfaces of coils heated by hot oil. . The hotter surfaces cause the asphalt in contact with them to have a higher film temperature. this type of system may cost more than a direct-fired system for some plants. the direct-fired tank is better suited to maintain temperature rather than to raise temperature. Moreover. It is likely to cost more for a small portable plant that needs only one asphalt storage tank and little extra heating. This is not a problem with virgin asphalt because of its higher film temperature rating. The system may have several hot oil heating circuits. if the direct-fired tank is unable to provide adequate heating for other purposes it becomes necessary to add a booster heater. Even so. This requires retaining anywhere from 12 to 20 percent of the tank’s total volume. Even a small hot oil heater is capable of heating several storage tanks along with several plant components. Allowing the level to drop below the fire tube and scavenger coils exposes their surfaces to air and will cause the film of asphalt on their surfaces to oxidize or coke. the temperatures of each component can be varied independently. Direct-fired tanks are not well suited for heating PMACs because of unfavorable film temperatures and mixing limitations. Another disadvantage of a direct-fired tank is a low rate of heat transfer because the fire tube has a limited amount of heating surface. Moreover. the material breaks down. (Please refer to Technical Paper T-133. And because it is a more sophisticated system than a direct-fired tank it has more parts that are subject to maintenance.) Hot oil heaters Systems with hot oil heaters are the most versatile systems. hot oil heaters rival direct-fired tanks in thermal efficiency. a build-up of coke on heating coils significantly reduces thermal efficiency or the ability of the heating surfaces to heat the asphalt. polymers have lower film temperature ratings that would be exceeded when heated by a fire tube. its maintenance is relatively low—not a matter of major concern.125-INCH COIL HEATER LUG ALUMINUM SKIN (OPTIONAL) TEMPERATURE CONTROLLER 1/4-INCH STEEL TANK WALL LEVEL INDICATOR ACCESS MANWAY LADDER LIQUID ASPHALT HEATING COILS TEMPERATURE TIE-DOWN LUG 10-INCH CONTROL VALVE STEEL CHANNEL SADDLE OVERFLOW PIPE EXPANSION JOINT Cross-section Of Coil Tank With Hot Oil Heater oil at this temperature is not suitable for maintaining the heat of an asphalt storage silo because the oil temperature is lower than the temperature of the hot mix in the silo. At this point it may be more feasible to use a system with a hot oil heater in lieu of the direct-fired tank. However. Thus. When the film temperature exceeds the maximum recommended film temperature for the material.Page 11 Figure 15 HELICAL LIFTING 0. depending on the volume stored in the tank. Moreover. Thus. However. oil at a temperature lower than that of the hot mix would actually steal heat from the hot mix. any size of plant and for any type of asphalt material including PMACs. They can be used for virtually any type of plant. More heat is possible from a hot oil heater than a direct-fired tank due to burner limitations of a direct-fired tank. Accordingly. However.
plus additional concerns. That’s because one or more power plants in a network that supplies your electrical energy may use fossil fuels and may cause pollution of their own. the burner in a hot oil heater can raise the oil temperature higher than scavenger oil systems that are heated by the asphalt and limited by its temperature. The tank is usually incorporated on the heater or on the tank. The asphalt heating system often uses the same fuel as the aggregate dryer. Two key concerns are availability of the fuel and cost per Btu. And it may prove to be the most expensive means of heating. Some may favor using electric heating out of concern for conserving natural resources and reducing environmental pollution. Using electrical energy doesn’t always achieve that goal. its possible to get an electric hot oil heater. Synthetic fluids such as Dowtherm or Therminol can be heated as high as 750 degrees F. The temperature of the oil in a hot oil heater is limited only by the properties of the oil. The tank should be filled with nitrogen to minimize oxidation of the hot oil. Because hot oil heaters do not use fire tubes they can use larger burners. However. Expansion tanks Systems with hot oil heating circuits must have an expansion tank to allow for thermal expansion of the oil. A hot oil heater system is always the one of choice where there is little or no difference in initial cost. Increased demand may dramatically increase electricity rates (Figure 16). But electric heating systems are problematic. Fuels that produce high levels of emissions are not well-suited for asphalt heating. fuel choice is usually based on dryer usage.Page 12 Figure 16 Electrically Heated Tank Direct-fired tanks employ fire tubes. For example. Heating with electricity significantly increases the power demand for a HMA plant. Thus. Moreover. Its temperature needs to be at least 25 degrees F higher than the mix in the silos. But therein lies a myth. However. using electricity for heating may merely displace one’s concerns. This applies to systems using hot oil heaters as well as systems using direct-fired tanks with scavenger coils. Thus. A hot oil heater can heat the oil to a temperature high enough to maintain the temperature of mix in storage silos. electric heating can be cost effective in a limited number of areas where electric power is available at extremely low rates. And its possible to get an electrically heated asphalt storage tank. conventional hydrocarbon oil such as Exxon HT43 can be safely heated to 600 degrees F. the amount of emissions it produces when burned can be an overriding factor. which limit the size of the burners that can be used. The size of the tank varies according to the amount of heating oil in the system. The desired temperature can be preset on the temperature controller. And because the dryer uses significantly more fuel than the heating system. its versatility may outweigh the additional cost. Asphalt heating systems using directfired tanks and hot oil heaters work with a variety of fuels. such as its flash point. which will automatically maintain the set temperature. Electric heaters Heating fuels It is possible to get electrically heated counterparts of fuel-fired systems. And even when it costs more. even though those same fuels may be satisfactory for a dryer. An electrically heated tank has virtually all the same limitations as a direct-fired tank. That’s because there are some important differences . This significantly extends the life of the oil.
Other fuels. But the fuel must be preheated and the burners are subject to reliability and maintenance problems. Natural gas produces very low emissions. requiring the heater to have a special lining to facilitate clean-out. 5 or No. Or. The special provisions are not usually cost-effective. 2 fuel oil are all highly appropriate fuels for both aggregate dryers and asphalt heaters. including propane. It may be used to pre-heat the fuel flowing directly to the burner when a single pass through the heater raises its temperature adequately for atomization. you must know how many Btu the fuel produces per gallon. The pre-heating lowers the viscosity of the oil so it can be atomized by the burner. Preheaters are typically used to heat No. 6 fuel oils for aggregate dryers. per pound.000 Btu). Natural gas is usually purchased by the cubic foot (CF) or by 100 cubic feet (CCF). LP gas. No. they are problematic even for large asphalt heaters. Most LP gases produce low emissions. The preheater usually consists of a shell-and-tube heat exchanger with the capacity to increase the temperature of the fuel 100 degrees F at a rate of 1. The fuel oil is heated as it passes through the shell surrounding the tubes. 1 and 2 fuel oils generally produce fairly low emissions. but some LPs may produce a higher level of NOX than others. A preheater is usually connected in the fuel supply line between the fuel tank and dryer. which is how many Btu are produced by a given amount of the fuel. The fuel may be recirculated to the fuel tank if Fuel heating values To determine the cost of fuel per Btu you must know its heating value. No.Page 13 Figure 17 Heavy fuel preheater in the ability of aggregate dryers to use such fuels and the ability of asphalt heaters to use those fuels. 1 fuel oil and No. Moreover. Special burners are available. Most liquid fuels. heavy oil and waste oils are difficult to burn and produce high levels of emissions. state and local ordinances governing allowable emissions may affect the choice between these fuels. Natural gas. Heavy fuel preheaters A heavy fuel preheater is used to preheat heavy fuel oil so it can be burned by a fuel oil burner. (Figure 17). these fuels leave heavy residues in the heater. these fuels are not suited for small (3 million Btu or less) asphalt heaters. Furthermore.) Consequently. The first problem is getting a burner that will burn these fuels. Federal. Thus. additional heating is needed. per cubic . The tubes are heated by thermal fluid (hot oil) from a hot oil heater as it circulates through the tubes.000 gph. such as No. but is sometimes purchased by the therm (100. it can be mounted inside the fuel tank. (Heavy oil is also known as No. 6 fuel oil or Bunker C. are purchased by the gallon. 4 fuel oil.
their representatives may not know that there is a difference. the difference between the two values is not widely known or understood. The heating values of various fuels are shown in Figure 18. the values used here are for fuels commonly used in the US. LHV is the net or useable amount of heat (Btu) produced by a fuel.698 gal 1. the values used here are for fuels commonly used in the US. Thus.500 Btu/CCF Figure 18 3. Electrical energy is based on a resistive load. as applicable. 2 fuel oil has a LHV of approximately 132. However. HHV is the gross amount of heat (Btu) produced by a fuel. Electrical energy is based on a resistive load. Unfortunately. The difference between HHV and LHV is due to hydrogen in the fuel. you should assume that Btu values shown on fuel bills are gross values unless specifically stated otherwise. or per therm.154 cu ft Net or LHV 0. 5 Fuel Oil Propane (LPG) Natural Gas Electricity No.843 per gal 0. of Therms (100. which is normally lost as water vapor or moisture. It has a HHV of approximately 140.000 Btu per gallon.105 gal 0.758 gal 0. it is usually more appropriate to use LHV.000 Btu/gal 90. No. Working With Therms foot.667 gal 1. a difference of 7700 Btu. Both values are sometimes used in calculating the amount of fuel required for heating asphalt and drying aggregate.009 per cu ft Amount Required For One Therm (100.433 per gal 0.714 gal 0.000 Btu/gal 143. HHV and LHV do not apply to electricity.000 Btu) Gross or HHV 0.497 cu ft 29.962 CCF 96. 2 Fuel Oil No.105 CCF 110.400 per gal 1. 5 Fuel Oil Propane (LPG) Natural Gas Electricity Gross or HHV 140.034 per Kwh CCF stands for 100 cu ft.Page 14 Type of Energy No.250 Btu/gal 84.040 per CCF 0. However.186 gal 1.345 Btu/gal 905 Btu/Cu Ft 90.500 per gal 0.010 per cu ft Net or LHV 1. And when calculating fuel costs you need to take the difference into account. The actual heating values of various fuels vary from one region to another. compare the heating values of the most commonly used fuels. It has a HHV .300 Btu per gallon.040 Btu/Cu Ft 104. Moreover.905 per CCF 0.000 Btu/CCF Net or LHV 132.905 per gal 1. HHV and LHV do not apply to electricity.320 per gal 1.500 Btu/gal 1. 2 Fuel Oil No. However.000 Btu/gal 150. To get an idea of the difference. Two types of heating values are used to indicate the amount of heat a fuel can produce: LHV (low heating value) and HHV (high heating value).000 Btu) Gross or HHV 1. The actual heating values of various fuels vary from one region to another. One cubic foot of natural gas has a LHV of approximately 905 Btu.300 Kwh Figure 19 0. Most suppliers of fuel oils and gas cite gross values for their fuels. High and Low Heating Values Type of Energy No. but do state whether they are net or gross.413 Btu/Kwh CCF stands for 100 cu ft.
20 $0.01 $0.02 $1. 2 fuel oil at LHV to produce one therm.69 $0.00 $1. 2 fuel oil at $1.14 $0.76 $0.60 $0.27 $0.03 $1.154 cubic feet (0.00 per gallon. which is approximately 1. gas or electric meter solely for the asphalt heating system separates those costs.55 $0. the values used here are for fuels commonly used in the US.30 $0.03 Each column of cost comparisons relates the costs of various types of energy to each other based on heating values.02 $0.00 per gallon is equivalent to a cost of $1.01 Cost Comparisons Based On Heating Values $0.000 143.62 $0.02 $0. Although the thermal efficiency of a direct-fired tank is very similar to that of a hot oil heater.345 90.01 $0.000 Btu per cubic foot.10 $1. 5 FUEL OIL PROPANE (LPG) NATURAL GAS ELECTRICITY Heating Value (Net or LHV) Btu/gal Btu/gal Btu/gal Btu/CCF (see note) Btu/Therm Btu/Kwh 132.02 $0.64 $0.26 $0. 5 fuel oil unless it is less than $1. A therm is 100. if No.30 $0.15 $0.09 for No.09 $0. which takes approximately 96.30 $0.00 per gallon it doesn’t pay to choose No. the costs of heaters with different thermal efficiencies as discussed in subsequent portions of this paper. For example.91 $0.48 $0. Suppliers of natural gas may bill their customers either by the hundred cubic feet (CCF) or by the therm.50 $0.80 $0. 2 FUEL OIL NO. 5 fuel oil for the same Btu.38 $0. it wouldn’t pay to use electricity unless it is less than $0.76 $0.03 per Kwh when No.21 $0.413 Billing Units Per Gallon Per Gallon Per Gallon Per CCF Per Therm Per Kwh $0.51 $0.77 $0. The actual heating values of various fuels vary somewhat from one region to another. 2 fuel oil is $1.20 $1. However.33 $0. Fuel costs are a significant part of the overall operating costs of a HMA facility.58 $0.87 $0.758 gallons of No. It is especially helpful when comparing Thermal efficiency affects the amount of fuel an asphalt heater uses.250 84. Monitoring fuel usage Heater thermal efficiency It is smart to separate costs for drying aggregate from the costs for heating your asphalt.23 $0.040 Btu per cubic foot. this discussion applies specifically to hot oil heaters.43 $0.75 $0. natural gas is normally billed at its gross heating value.54 $0. the cost of No. It is important to note that natural gas suppliers normally calculate a therm based on the HHV of natural gas.09. Equivalent Energy Costs Figure 20 of approximately 1.03 $1. It takes approximately 0.500 100.45 $0. The thermal efficiency of a hot oil heater relates the amount of heat (Btu) that the .19 $0.34 $0.13 $0.65 $0. Thus.32 $0.000 Btu (Figure 19). 2 fuel oil is $1.82 $0.90 $0.38 $0.83 $0.70 $0.Page 15 Type of Energy NO. Likewise.68 $0.01 $0.40 $0. Installing a fuel.22 $0.961 CCF) per therm. It takes approximately 100 cubic feet (1 CCF) of natural gas at LHV to produce one therm.70 $0. CCF stands for 100 cubic feet. This can serve as a guide to selecting a fuel from those that have acceptable levels of emissions.41 $0.61 $0. The net heating value of one cubic foot of natural gas is 905 Btu. However. a difference of 135 Btu. Knowing the true costs for asphalt heating makes it possible to accurately determine the cost benefits of upgrading heating equipment. Figure 20 compares the cost of fuel per Btu.45 $0.000 3.98 $0.53 $0.19 $0.
Page 16 Eth = [ (Hinput – Hstack) /Hinput ] x 100 Where: Eth is percentage thermal efficiency (NET) Hinput is Heat input Btu/hour (NET) Hstack Stack loss Btu/hour MOISTURE STACK LOSS HOT OIL HEATER Figure 21 WALL LOSS GROSS HEAT INPUT (HHV) NET (LHV) AVAILABLE HEAT OUTPUT TO HELICAL COIL Heater Thermal Efficiency HEATER EFFICIENCY 50 PERCENT 60 PERCENT 70 PERCENT 80 PERCENT 85 PERCENT COST PER HOUR 1.00 = 132. the higher the efficiency. The thermal efficiency of a direct-fired tank is very similar to a hot oil heater.000 Btu per gallon 0.000 Btu per gallon 0.50 1. there is an easier method of determining a reasonably accurate indication of efficiency for hot oil heaters used in the asphalt industry. Calculating Heater Fuel Costs Per Hour burner produces to the amount of heat actually transferred to the thermal fluid flowing through its coil (Figure 21).00 = 132.00 = 132.000. a heater that is 85 percent efficient uses 85 percent of the heat produced to heat the fluid and wastes 15 percent.85 $15.000. No. 2 fuel oil LHV (low heating value) = 132.000.00 per gallon. Heaters that are 15 or 20 years old are apt to have efficiencies much lower than those available now.00 = 132. One with an optional combustion air preheater will save even more.63 $10. Consequently.000. This can be done by measuring the stack temperature and using a bar chart that shows efficiencies for various stack temperatures (Figure 28). It relates the amount of heat the burner .000 Btu per hour 1 X X $1.47 $8.70 1.000.000 Btu per hour 1 X X $1.000 Btu per gallon 0. No.000 Btu per hour 1 X X $1. The lower the temperature of the exhaust gas. The formula for calculating the net (LHV) thermal efficiency of a heater is shown in Figure 21.82 $9.15 $12. 2 fuel oil cost = $1. A more efficient one could save a lot of money on fuel costs and pay for itself in a very short time. All heat that does not go into the thermal fluid is wasted.000 Btu per gallon.60 1. However. And so is any heat that is lost by air leakage or as the result of poor insulation of the heater shell.000 Btu per hour 1 X X $1. Thus.91 Figure 22 Heating load = 1. Temperature of the exhaust gas is a good indication of efficiency where most wasted heat goes out the stack.000 Btu per gallon 0. all heat that goes out the exhaust stack is wasted heat.80 1.00 = 132.000 Btu per hour 1 X X $1.000 Btu per hour.000.000 Btu per gallon 0.
00 per gallon. many times more than the dryer runs during those same years. Figure 22 shows how to calculate the operating cost per hour for a heater that operates on No. Long Term Fuel Costs For Various Efficiencies produces to the amount of heat actually transferred to the asphalt surrounding the fire tube. 2 diesel oil (132.036.861 1.364 $1. That’s because many asphalt plants don’t meter fuel for the heater separately from the dryer.157.861 $1.61 Total Gallons 2.197. a heater that is 85 percent efficient saves $838.197.000 Btu per gallon). Its easy to calculate the operating cost of a heater when you know its thermal efficiency and input rating.025 Heat demand = 1 million Btu.696. the heater is often ignored. 7 days a week. The heater at a typical asphalt plant runs 24 hours a day.727 $1.684 over one 70 percent efficient.036.Page 17 Figure 23 Heater Efficiency (%) 50 60 70 80 85 88 Gallons per hour 15. Figure 23 shows a comparison of fuel costs for heaters with efficiencies of 50 to 88 percent.63 10.272.400 hours during its life-span of about 20 years. Compare the differences in fuel costs over the life of the heaters. Both are usually lumped .15 12. Consequently. Operation = 24 hours per day. And because the dryer burns fuel at a much higher rate than the heater.546 $1.364 1. But it’s easy to overlook the amount of fuel a heater uses. The amount of fuel a heater burns over its life and its cost depends on the heater’s efficiency.454. 40 weeks a year. 20 years (heater life).47 8.503 over one 50 Impact of efficiency A small difference in efficiency makes a big difference in how much fuel a heater burns over its life.157.970 1.727 1. It may easily run a total of 134.272. Fuel cost = $1. most operators probably don’t know how much fuel the heater alone uses. The efficiency of a direct-fired tank is about 2 percent higher than that of a hot oil heater because of operating temperatures.696. about 40 weeks a year. 2 fuel oil.546 1.970 $1. Even more astounding.82 9.025 Total cost $2. A heater with 85 percent efficiency saves a staggering $256.454.91 8. 7 days a week. Fuel = No. together. That’s a lot of running hours.
Higher turbulence causes better heat transfer and lower film temperatures. Moreover. the burner operates at a lower firing rate. heat transfer surface area. Helical coil heaters are inherently efficient for a number of reasons. reduce efficiency and are common in other types of heaters. positioning of the helical coil. Thus. And boosting efficiency from 85 percent to 88 percent with an air pre-heater saves an additional $40. Critical design factors include the flame pattern. But not all helical coil heaters have the same efficiency. effectiveness of the insulation and how well the unit is sealed. Increases up to 4 percent can be expected. This reduces the amount of heat that the burner has to produce.Page 18 Figure 24 Combustion Air Pre-heater On Helical Coil Heater Figure 25 Thermal Fluid Pre-heater On Helical Coil Heater percent efficient. Subtle design differences make significant differences in efficiency. far exceeding its extra cost. thermal fluid turbulence. The choice is between two types of heat exchangers: a combustionair preheater or an oil pre-heater.836. which can . The spiral path of a helical coil imparts much higher turbulence to oil flowing through it than the straight sections of serpentine coils. The combustion-air preheater is an airto-air heat exchanger (Figure 24). the oil always fully contacts its surfaces thereby eliminating stagnant zones. It uses the heat of the exhaust gases to preheat combustion air supplied to the burner to temperatures of 150–200 degrees F. combustion gas velocity. A secondary heat exchanger can be installed in the exhaust stack of a helical Efficiency factors coil heater to recover heat from exhaust gases and use that heat to increase efficiency.
Smaller temperature differences necessitate larger economizers.Page 19 Figure 26 Heatec Helical Coil Heater The thermal fluid pre-heater is an airto-oil heat exchanger and is commonly known as an economizer (Figure 25). This reduces the heat load on the burner allowing it to operate at a lower firing rate. With the economizer it’s the difference between the exhaust gases and the temperature of the return oil. It increases the backpressure on the burner exhaust and that can make burner adjustments more sensitive and temperamental. Hence. Moreover. The combustion-air preheater usually has a somewhat higher temperature difference. depending on capacity and fuel used. However. . The economizer does not increase NOX emissions and does not usually affect burner adjustments. But it is likely to cost more than an air pre-heater. With the combustion air preheater it’s the difference between the exhaust gases and the ambient air temperature. about 300 degrees F. Heatec heaters Our hot oil heaters are designed around a helical coil. despite being less efficient. our heaters have always been among the most efficient heaters available. Helical coil heaters are widely recognized for long life and low maintenance. The actual effectiveness of both types of secondary heat exchangers depends on temperature differences. As you would expect. The size of the economizer is governed mostly by the difference in temperature between the exhaust gases and return oil. Consequently. It uses the heat of the exhaust gases to preheat the oil as it returns to the helical coil in the heater. Actually. especially those in electronics and insulation. in addition to their high efficiency. The thermal efficiency rating for the current generation of these heaters without pre-heaters (Figure 26) is 85 percent. the efficiency varies slightly. larger economizers cost more than smaller ones. Apart from those issues the combustion-air preheater is usually more cost effective. there are a couple of drawbacks for the air-preheater. it usually increases efficiency a little more than the economizer if sized properly. we have continued to refine our helical coil heaters to take advantage of new technologies. about 525 degrees F. The greater the difference the greater the increase in efficiency. the same as the very first heaters the company built in 1977. And an air-preheater increases NOX emissions to some extent.
072. Our heaters with an optional combustion air pre-heater have a heat exchanger built into the stack. It has a steel cylindrical shell that houses a helical coil. Case histories We calculated the potential savings a HMA plant in Arizona could expect by replacing their old heater. Figure 27 shows the construction of our helical coil heater. heating its outer surfaces by convection. We will be happy to check the efficiency of your heater and tell you if savings are possible with a more efficient heater. Very little energy is lost through the shell. Heatec checks the efficiencies of heaters as a free service to HMA plant owners. It then travels through a duct that carries it to the combustion chamber where it augments heat from the burner. Hot combustion gases turn outwards at the end of the coil. The total cost of the new heater installed was $42. The hot gases pass through tubes of the heat exchanger and exit the heater.Page 20 BLOWER HELICAL COIL HEATER WITH COMBUSTION AIR PREHEATER COMBUSTION AIR PREHEATER HELICAL COIL Figure 27 CONVECTIVE TRANSFER RADIANT TRANSFER BURNER Cross section of Heatec Helical Coil Heater Equipping them with an optional heat exchanger boosts efficiency to 88 percent or more. A blower forces fresh air into the chamber surrounding the hot tubes where it is heated. By measuring the temperature of the gases exiting the exhaust stack . The hot fluid exits the heater and flows through piping to asphalt tanks and other plant components. So if you would like for Heatec to do this for you please give us a call. The new heater paid for itself in only 10 months. Radiant energy accounts for about 70 percent of the heat transferred to the coil. and looking it up on the chart shown in Figure 28 you can learn the efficiency. Determining efficiency The exhaust gas temperature of a heater gives a reasonably good indication of its efficiency. The tubes are sealed so exhaust gases do not enter into the chamber surrounding them. Thermal fluid is pumped through the hot helical coil and is heated by the process of conduction. The gases exit through the exhaust stack in the heater shell near the burner after traveling back the full length of the coil. It had an efficiency of about 52 percent. They double back into the annular space between the insulation and the exterior side of the coil. We estimated that a new Heatec heater would save them an average of $3882 per month. MO. The inner surface of the shell is covered with ceramic blanket insulation that has low thermal conductivity and low heat storage. Estimated savings are $5852 a month. Heatec offers the two types of secondary heat exchangers discussed above. On another occasion we replaced an old Hopkins heater (6 million Btu/hr) at an asphalt terminal in Kansas City. The burner fires through the center axis of the coil. Steel pads inside the shell support the coil leaving an annular space between coil and shell. Convection accounts for about 30 percent of the heat transferred to the coil. Its flame produces radiant energy that heats the inner side of the coil without impinging on its surfaces. They increase efficiency by recovering some of the exhaust gas heat that would otherwise be lost. The savings paid for the new heater in 11 months. The savings now increase the owner’s profits and will do so for the remainder of the heater’s 20-year life.
low. The high-low burner is a compromise between a non-modulating burner and a fully modulating burner. The non-modulating burner is wellsuited for direct-fired tanks because they lose heat very slowly. high-low burners are less popular for asphalt heating. Frequent cycling reduces its efficiency because of temperature overshooting. And it is less suited for use with hot oil heaters than a fully modulating burner. allowing the burner to remain off for long periods of time. high-low. Hot oil heaters are subject to frequent cycling when the plant heat load varies a lot. Heater Efficiency Figure 28 Stack Temperature (degrees F) Net Heater Efficiency (%) 70 75 80 85 90 95 Stack Temperature Vs. The non-modulating burner is not well-suited for hot oil heaters that are subject to frequent on-off cycling. Heater Efficiency Figure 29 High-Low Burner Burners Burners on equipment used for asphalt heating can be either non-modulating. A high-low burner (Figure 29) has three operating states: off. A nonmodulating burner has two operating states with no intermediate settings. Heat loads tend to vary a lot on plants with several asphalt tanks and a number of other plant components that are heated. Consequently.Page 21 1130 1100 1070 1039 1003 972 945 910 875 840 805 770 746 708 665 625 595 558 525 495 465 435 390 348 0 65 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 Stack Temperature vs. . A fully modulating burner is one that can be fired at variable rates ranging from off to high with numerous intermediate rates. and high. It is well-suited for applications where it does not need to frequently cycle on and off. It is either on or off. reliable and cost less than the other types. It doesn’t offer any real advantages for direct-fired tanks. or fully modulating. The non-modulating burner has the advantages of being simple.
Figure 32 shows an infrared photo of a well-insulated vertical asphalt storage tank and depicts temperatures on the skin and on the concrete at its base. which shows temperatures at similar points on a poorly insulated vertical tank. This conserves fuel. The cause of most heat loss in asphalt heating systems is too little insulation on some components and no insulation on others. when components are purchased. Heat losses can be minimized by proper use of insulation. Heat loss getting any benefit from it. It is used extensively on hot oil heaters. especially larger ones that heat several asphalt tanks and other plant components.Page 22 Figure 30 Fully Modulating Burner Figure 31 Insulation On Coiled Tank The fully modulating burner is the most efficient of the three types of burners (Figure 30). Thus. Because of the need to avoid wasted cost and to conserve our resources every effort should be made to avoid heat loss. It can be fired at a rate that closely matches the heat demand. Heat loss is the difference between the amount of heat energy produced and that used. This is especially true at a HMA facility because of the large amount of heat energy needed to produce hot mix. And when the system is installed all asphalt piping and hot oil piping should be properly insulated. a very cost effective solution (Figure 31). it is wasted energy. proper insulation should be specified. and can eliminate constant on-off recycling. Thus. It is heat produced without . Compare this photo with Figure 33. which is costly. Heat loss occurs when heat from materials and components in an asphalt system escapes into the atmosphere. reduces temperature overshooting.
Hot oil temperature = 350 degrees F.Page 23 Figure 32 Infrared Photo Of A Well-Insulated Tank Figure 33 Infrared Photo Of A Poorly-Insulated Tank JACKETED ASPHALT PIPING Asphalt Pipe Nominal Size Hot Oil Jacket Nominal Size 4 inches 6 inches 8 inches Loss Per Linear Foot Btu Per Hour Uninsulated Jacket 1598 2349 3057 Insulated Jacket 86 122 148 Loss Per Flange Btu Per Hour Uninsulated 1890 2600 3240 Insulated 120 134 178 Figure 34 3 inches 4 inches 5 inches HOT OIL PIPING Pipe Diameter 1-1/2 inches 2 inches 2-1/2 inches 3 inches Loss Per Linear Foot Btu Per Hour Uninsulated 676 846 1024 1243 Insulated 47 54 55 72 Loss Per Flange Btu Per Hour Uninsulated 1205 1660 2155 2485 Insulated 97 115 125 130 Revised 11-24-03 Asphalt temperature = 300 degrees F. Pipe insulation = 1-1/2 inches (Figure 35). Piping Heat Losses .
05) Minimum Recommended Insulation For Pipes Btu Per Hour Capacity (Gallons) 10. the thickest insulation is appropriate even in warm climates.992 50.118 21.000 Horizontal Tank No Insulation 633.000 25.050 1.621 1. Note the heat loss from various pipe sizes and temperatures as shown in Figure 34. the following components should be insulated to minimize heat loss: • All asphalt storage tanks • Hot oil heaters • All asphalt piping • All hot oil piping • Metering pumps • Unloading pumps While the need to insulate storage tanks and heaters is obvious to most people.786.054 46.217 26. Figure 37 compares fuel tanks.221.000 15. the appropriate thickness and using good workmanship when installing it. Proper insulation Not all insulation can be considered as proper insulation. in locales with low temperatures the losses are greater than in areas of high temperature. In cold climates it is very important to use the thickest insulation available for the application.347 18.889 25. Asphalt temperature = 300 degrees F.117 40. the greater the rate of heat loss.000 20. Generally. The Thermal Insulation Handbook by Turner and Malloy recommends insulation thickness for pipes as shown in Figure 35.660 27.179 33.006.850 791.933 58.Page 24 Insulation Thickness For Various Operating Temperatures Pipe Diameter 1 inch 1-1/4 inches 1-1/2 inches 2 inches 2-1/2 inches 3 inches 3-1/2 inches 4 inches 5 inches 6 inches 8 inches 150 Degrees F 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1-1/2 inches 1-1/2 inches 200 Degrees F 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1-1/2 inches 1-1/2 inches 300 Degrees F 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1 inch 1-1/2 inches 1-1/2 inches 400 Degrees F 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 1-1/2 inches 2 inches 2 inches 500 Degrees F 1-1/2 inches 1-1/2 inches 2 inches 2 inches 2 inches 2 inches 2 inches 2 inches 2-1/2 inches 2-1/2 inches 2-1/2 inches 600 Degrees F 2 inches 2 inches 2 inches 2-1/2 inches 2-1/2 inches 2-1/2 inches 2-1/2 inches 2-1/2 inches 2-1/2 inches 3 inches 3 inches Figure 35 Thickness according to Turner/Malloy (emmisivity 0.755 31.813 Figure 36 *Btu values are for new Heatec tanks and do not include heat for valves or connections.437. Asphalt Tanks—Maintaining Temperature The greater the temperature differences between the ambient air and these materials and components.018 1. Figure 36 compares asphalt tanks.536 Horizontal Tank 3-inch Insulation* 21.760 14.411 Horizontal Tank 6-inch Insulation* 11. You can see just how important insulation is by comparing the heat required to maintain the temperatures of various sizes of insulated and non- insulated tanks. although not . many overlook the need to insulate piping. But it is one of those details that shouldn’t be overlooked.000 40. Old tanks may require double the heat or more. Proper insulation involves using the appropriate type of insulation.562.000 30.753 1.886 1. the thicker the insulation the better. Thus. Moreover. But regardless of the locale.000 35.
Old tanks may require double the heat or more. R-value per inch is the reciprocal of Kvalue or 1/k. 2) fiberglass blanket.000 25. (This assumes that you must raise its temperature in order to use it. Using good workmanship means installing the insulation carefully. The estimate can provide important information for selecting new equipment. Figure 39 shows the daily costs for heating asphalt and plant components at a typical relocatable HMA plant. And it should not be compressed any more than absolutely necessary.273 11. Refractory could also be regarded as a type of insulation. 3) mineral wool blanket. We assumed that the plant was operating for 8 hours during the 24 hour period. Type of fuel = No.158 Figure 37 10. The insulation will usually always pay for itself over a period of time.330 9.570 3-inch Insulation* 5. fuel and plant components at a typical stationary HMA plant. and hot oil piping) along with the Btu required to maintain them at a specific temperature. and 5) calcium silicate.Page 25 Capacity (Gallons) Btu Per Hour No Insulation 170. Your estimate should include every heated component (including asphalt piping. Increasing R-values denote greater resistance to flow.548 387. or as the resistance for the thickness stated.) Also HMA plant heating costs Figure 38 shows the daily costs for heating asphalt.215 13. Heat demand is the key to choosing the appropriate capacity of the heating equipment and can enable you to closely estimate fuel costs. And six inches of material compressed to three inches is no better than three inches uncompressed.952 213.000 20. Be sure to include an appropriate amount of extra heat demand for raising the temperature of asphalt that you buy at a temperature lower than required for the mix. it has almost no effect on heat loss. The insulation should cover virtually all hot surfaces exposed to ambient air. The term most commonly used in asphalt heating to denote the efficiency of insulating materials is thermal conductivity. An estimate of the heat demand should be made when purchasing new asphalt heating equipment or upgrading old equipment.000 30. Fuel temperature = 150 degrees F Fuel Tanks—Maintaining Temperature as critical.504 271. A measure of the ability of insulation to resist or impede heat flow is denoted by its R-value. Doubling the thickness of the material doubles the R-value. However.” and is usually called the k-factor. It is typically marked on the wrapper or container of insulating materials. Note that daily usage shown in figures 38. Both R-value per inch and R-value per stated thickness are used in actual practice when heat insulating materials are being specified. Five types of insulation are commonly used in asphalt heating equipment: 1) ceramic fiber blanket. Figure 40 shows the daily costs for heating asphalt and plant components at a typical portable HMA plant. This term is very common in the building industry. It is used in combustion chambers to protect metal parts from the burner flame and to shape the flame.000 15. Thermal conductivity is a measure of the ability of a material to conduct heat flow. Compressing two inches of material to one inch cuts its insulating value to that of one inch of the material. Thermal conductivity. It is designated by the letter “k.941 7. is expressed in Btu per hour per square foot per degree F per inch of thickness of the material. It should be installed so that it is not subject to being wetted by rain storms. Heat requirements The amount of heat (Btu/hr) needed for heating asphalt and components at an asphalt plant is known as heat demand. 39 and 40 is not equal to 24 times the hourly usage. Thus. . It may take a little longer in warm climates. R-value can be expressed either as resistance per inch of thickness. the higher the R-value the better the insulation. 6 fuel oil. This is because the heat load on the hot oil system drops while the plant is operating.526 329.000 *Btu values are for new Heatec tanks and do not include heat for valves or connections. 4) molded foamglass. k. R-values are very easy to use for comparing the insulating properties of various materials.
No.3 0.3 0. 2 fuel cost = $1.5 105. Natural gas cost = $0.2 3.9 1.69 per CCF.3 0. 36 Inch x 14 ft 100 feet of asphalt piping.5 0. Heavy fuel oil is used only for drum mixer. 24 inch x 50 ft 70 feet of asphalt piping.2 0.5 6.58 Figure 38 Asphalt temperature = 300 degrees F.620 966.533 243.640 55.984 40.940 14.06 CCF Natural Gas 13.0 7.567. Highlighted cells Include additional heat load for first hour cold start.1 0.000 1. Heavy fuel oil is used only for drum mixer. Heater efficiency = 85%.054 336.000 gallon vertical asphalt tank for PMAC Heavy fuel preheater Double Barrel drum mixer 9 ft x 46 ft Drag conveyor. Natural gas cost = $0. Heating Costs For A Typical Stationary Plant Hourly Usage Btu Two 30.750 238.020 1.1 0.500 176.52 Btu 2.2 0.8 4.014. Heating Costs For A Typical Relocatable Plant Hourly Usage Gallons No.3 0.500 238.250 2. 2-1/2 inch dia Total Quantity Cost 93.918 Figure 39 Asphalt temperature = 300 degrees F.240 880.6 0.439 9.081.5 0. Heavy fuel oil is used only for drum mixer.260 35.2 0.780 Figure 40 Asphalt temperature = 300 degrees F.389.1 1. No.48 CCF Natural Gas 1.391.1 6.2 11. Usage includes heating for unloading pump and metering. Tank insulation = 3 inches.1 8.97 2.7 228. Thermal oil temperature = 350 degrees F.2 31.709 43.5 4.750 9. Fuel temperature = 150 degrees F.500 9.2 0.00 per gallon.3 3.3 2.520 68.400 2.45 CCF Natural Gas 1.1 3.4 3. Pipe insulation = 1-1/2 inches. 4-inch dia jacket 230 feet of hot oil piping.3 12.1 16.685. Natural gas cost = $0.920 274.2 3.1 4. 2 fuel cost = $1. 2-inch dia Total Quantity Cost 42.1 3.812.1 22.7 0.23 Btu One 30. Pipe insulation = 1-1/2 inches.200 19. 2 Fuel 0.69 per CCF.3 25. Tank insulation = 3 inches.3 0. 2 Fuel 0.500 7.3 12.4 0.649 19.1 14.706. 36 inch x 33 ft 100 feet of asphalt piping.4 157. Tank insulation = 3 inches.2 9.8 0. Thermal oil temperature = 350 degrees F.234.248 Gallons No.940 14.5 0.5 0.920 726. Fuel temperature = 150 degrees F.600 114.360 10. Usage includes heating for unloading pump and metering.6 12. Thermal oil temperature = 350 degrees F. 36 Inch x 95 ft Two 200-ton storage silos Traverse conveyor.00 per gallon.Page 26 Hourly Usage Btu Two 30.4 0.5 2.1 0. Highlighted cells Include additional heat load for first hour cold start.6 59.58 CCF Natural Gas 29. Usage includes heating for unloading pump and metering.1 2.1 0.0 CCF Natural Gas 0. Highlighted cells Include additional heat load for first hour cold start.250 99.7 0.000 gallon horizontal coil tanks for asphalt Double Barrel drum mixer 8ft x 39 ft Drag conveyor.088.6 18.1 1.2 59.3 1.560 358.000 487.00 per gallon. 2 fuel cost = $1.5 1.375 1.6 0.3 3. Fuel temperature = 150 degrees F. Heating Costs For A Typical Portable Plant .4 11.305 8.080 6.984 38.640 254.575 51.1 25.0 Daily Usage (24 hours) Gallons No.69 per CCF.526 Gallons No. Heater efficiency = 85%.0 2.000 gallon Heli-tank for asphalt Double Barrel drum mixer 7 ft x 35 ft Drag conveyor on surge bin.6 25.4 0.620 961. 4-inch dia jacket 230 feet of hot oil piping.1 4.6 2.570 487. Heater efficiency = 85%. 2-1/2 inch dia Total Quantity Cost 93. No. 2 Fuel 20 9 72 10 15 21 4 2 3 157 156.58 CCF Natural Gas 29.0 0. 2 Fuel 20.1 0. 2 Fuel 9.7 86.4 6.1 0.255.1 6.9 0.000 1. Pipe insulation = 1-1/2 inches.080 17.255.8 0.1 0. 4-inch dia jacket 150 feet of hot oil piping.9 59.905 Daily Usage (24 hours) Gallons No.6 0. 2 Fuel 0.5 6.560 358.46 Btu Daily Usage (24 hours) Gallons No.0 1.2 2.1 0.3 36.6 2. 42 inch x 100 ft Three 300-ton storage silos Traverse conveyor.920 Btu 1.000 gallon horizontal coil tanks for virgin asphalt One 25.
000 lb Batch Plant Weigh Bucket & Pugmill.230 120.000 lb Batch Plant Weigh Bucket & Pugmill. Thus.000 Run 35.884 111. 8-inch cone insulation. 16.2 milion Btu/hour heater for the relocatable plant described in Figure 39.000 442.000 351. The total hourly usage can be used as a starting point for estimating heat demand for such plants. So.640 43.720 9.618 Figure 41 Double Barrel Drum Mixer 9 x 46 ft (hot oil jacket) Metering Pkg. Moreover.929 349. In some cases the factor could be up to 10 times. 500 tph.752 131. Drum Mix Plant. (per foot) Heavy fuel preheater 1000 gal/hour Silos have heated cones and gates.000 24. 24 Inch width (per foot) Drag conveyor.200 38.911 34. .735 21.000 250.000 lb Batch Plant Weigh Bucket & Pugmill. 39 and 40 show hourly and daily Btu usage for typical stationary. Add a safety margin of about 20 percent to the estimated demand to allow for unknowns and contingencies. 6.593 Run 17.Page 27 Btu per Hour Equipment Double Barrel Drum Mixer 7 x 35 ft (hot oil jacket) Double Barrel Drum Mixer 8 x 39 ft (hot oil jacket) Pre-Startup 213.590 33. 250-Ton Asphalt Storage Silo.000 lb Batch Plant Weigh Bucket & Pugmill. But remember that the totals in those figures is only a sum of the heat required for the components listed. 100–300 tph Metering Pkg. 8. 100-Ton Asphalt Storage Silo. We recommend a 1.000 168.000 214.085 90. Heat Demands For Other Components allow for asphalt tanks and storage silos you plan to add in the foreseeable future. 12. In both cases this is the heat loss that the hot oil system must replace.962 93.216 5. Figures 38. Heat demands are shown for pre-startup and run.171 117. 42 Inch width. 36 Inch width.000 275. the heat demand for your plant should start with the hourly usage for all of its specific components.719 30.270 38. relocatable and portable plants. To that you should add extra heat for startup plus all of the factors mentioned earlier. 6-inch sidewall insulation. Figure 41 shows heat demands for key components of drum mix plants and batch plants.698 3. 400–600 tph Weigh Bucket & Pugmill.000 300.750 390 585 683 336.184 101. It would be virtually impossible to accurately calculate all those factors because of unknowns. 300 tph. 300-Ton Traverse conveyor. We recommend increasing the total hourly usage by a factor of about 4 times. 14.725 50. Drum Mix Plant. it is appropriate to make a judgement based on experience.377 4. the total hourly usage does not include the extra heat needed during the first hour of a cold start.465 3. (per foot) Drag conveyor. 24 inches x 14 ft-8 inches Traverse conveyor. 400 tph.520 19. before heat from hot aggregate supplements heat of the hot oil system.709 Figure 42 Asphalt Storage Silo. 36 inches x 16 ft-8 inches Drag conveyor.000 200. 10. 4. 150-Ton Pre-Startup 100.736 4. we recommend a heater with an output of 2 million Btu/hour for the stationary plant described in Figure 38.363 25.000 150. Figure 42 shows the heat demands for various other components of a plant.000 lb Batch Plant Heat Demands For Key Components Btu per Hour Equipment Asphalt Storage Silo.000 135.091 215. And we recommend a 1million Btu/hour heater for the portable plant described in Figure 40.156 142.635 25. So.000 lb Batch Plant Weigh Bucket & Pugmill.000 lb Batch Plant Weigh Bucket & Pugmill. 200-Ton Asphalt Storage Silo.
but can be transported by conventional tractortrailers (Figure 44).Page 28 Figure 43 Heatec Heli-Tank Combines Coil Tank And Helical Coil Heater Pre-startup means that the plant is not running HMA. it can reduce operating costs long after the installation has been completed. If the demand is significantly underestimated you will get a heater that is too small to handle the plant temperature requirements. It may constantly cycle on and off. Cranes or lift trucks are required to load and unload these units. Heatec constantly discovers layouts that are not costeffective. Even so. components should be positioned so they are easy to access . Correctly estimating the demand will enable you to pick a heater of the optimum size. Consequently. Portability Most asphalt heating equipment is available as units that are either trailermounted. As an option they can include adjustable steel foundations that can be lowered to the ground to support the units at the job site. thereby reducing the load on the hot oil system. Moreover. which shortens its life. They can be transported by conventional tractortrailers. The bottoms of the skids are Equipment layout Asphalt heating and storage equipment should be laid out or arranged in the most cost effective manner for piping. Skid mounted tanks and heaters do not have wheels and suspension. Moreover. skid-mounted or framemounted. Over-firing is when the burner operates at a higher rate than the design capacity of the heater for extended periods in an attempt to maintain needed temperatures. heaters that are too small will cause over-firing.500 pounds per square foot. Run means that the plant is running HMA. Fully-modulating burners can normally fire at a rate low enough to reduce frequent on-off cycling. Over-firing decreases efficiency and increases the film temperature of the thermal fluid. the frames are designed to be supported by concrete pads or poured foundations. a load normally supported by well-compacted soil. except for the wiring and piping between the units. The bottom surfaces of the skids have large bearing surfaces so as to keep ground-loading to no more than 2. In a good layout. Thus. piping runs are as short as possible and pipes don’t double back unnecessarily. Good layouts can significantly decrease material and labor for installation of both asphalt and hot oil piping. Its run cycle may be too short for maximum efficiency. heat from the HMA replaces much of the heat lost. Moreover. A properly sized heater runs about 75 percent of the time while the plant is operating. the hot oil system replaces virtually all of the heat lost. However. Frame-mounted units are similar to the skid-mounted units. designed to support the units and to rest on either soil or paved surfaces at the job site. They have a fifth-wheel towing pin so they can be pulled by the same types of tractors used to pull conventional trailers. aside from the extra cost of the heater. Trailer-mounted tanks have wheels and suspensions to make them portable. All units are pre-wired and pre-piped. The reverse will happen when the heater is too large. moderate over-sizing is not a problem for hot oil heaters with fully-modulating burners. Heatec combines a coil tank and helical coil heater on the same trailer chassis to create a portable system known as a Heli-tank (Figure 43). hot oil heaters should not be grossly oversized. Cranes are not required when relocating these units. The goal of carefully estimating the heat demand is to get a heater that is neither too small nor too large. However. an undesirable operating condition.
This is especially important on plants that will have two or more tanks and a heater skid. The following parties must work together as a team to achieve good layouts: • The plant owner or operator. Two key factors govern good layout for piping. Moreover. And when the plant is idle the hot oil keeps the asphalt from solidifying. These factors affect setup and operating costs. the standard layouts reduce manufacturing costs and speed up delivery schedules. Heatec has developed standard layouts that greatly facilitate manufacturing the tanks and provide efficient piping. Good layouts can be achieved only by close coordination among all parties involved. Thus. has to be replaced. . plant owners and equipment salesmen would do well to closely consider a standard layout unless there are special circumstances that necessitate a custom arrangement. Piping In addition to equipment layout there are several important factors to consider when choosing hot oil and asphalt piping. • Those who manufacture the heating equipment and piping. Hot oil is pumped through the jackets to maintain the temperature of the liquid asphalt flowing through the lines when making HMA. such as a coil.Page 29 Figure 44 Skid-Mounted Tanks And Heaters Are Transported By Trailer for routine maintenance and in the event a major component. The second factor is the location of the inlets and outlets on the tanks and heater skid. • The HMA equipment salesman. The first factor is the heights and positions of the tanks and heater in relation to each other. All asphalt lines should have outer heating jackets. • Engineers who design the plant layout. • Engineers who design the asphalt heating equipment .
they are stronger than flexible lines and less likely to rupture when stressed—an important safety precaution for lines carrying hot asphalt. When traveling to another site. Ball joints are highly recommended in asphalt piping for portable asphalt plants. The jumpers provide fast heating on start-up and minimize unheated spots. the lines stay attached and are simply rotated into transport position. Some use only 1/2-inch jumpers. They make piping installation faster and easier (Figure 46). Small HMA plants with only one asphalt storage tank usually only need one . which slow down the heating and make it difficult to obtain the desired temperature. They allow up to 2 feet of misalignment between tank and mixer. The diameter of the jumpers is 1-1/2 inches. Moreover. As a result the units don’t have to be positioned with as much accuracy as would be required without them. much larger than those provided by some manufacturers. A minimum of three ball joints are required. The insulation is covered with a protective outer shell made of aluminum.Page 30 Figure 45 Flexible Jumpers Minimize Unheated Spots Figure 46 Ball Joint Piping Reduces Need For Precise Alignment Heatec jackets for portable plants incorporate flexible jumpers at the joints (Figure 45). Heatec offers insulation with 1-1/2 inch pre-formed fiberglass material. All hot oil lines and asphalt lines— including those that are jacketed—should be insulated to minimize heat loss.
Operation of the system is much more flexible because each circuit can be operated independently.Page 31 HOT OIL HEATER VERTICAL TANK WITH COILS HORIZONTAL TANK WITH COILS CIRCULATING PUMP MANIFOLD FUEL PREHEATER METERING PUMPS Figure 47 SIDE PUMPS (TWO) ADDITIVE TANK TRAVERSE CONVEYOR JACKETED PIPING UNLOADING PUMP DRAG CONVEYOR DRUM MIXER SILO CONE Using A Hot Oil Heater With A Manifold Facilitates Extra Circuits hot oil circuit with one hot oil pump. It also allows adding or removing tanks or other heat users to the hot oil system without affecting the flow of oil through the coil of the heater. The single pump circulates the hot oil through heated components and through the heater. One pump. when some units are not in use. which can significantly extend the life of both the coil and the thermal fluid. Each of the other pumps. Heatec helical coil heaters for single circuits are designated by the prefix HCS. There are other important advantages to having a pump used solely for circulating the hot oil. HC stands for helical coil and S stands for single circuit. its operating pressure and the size of the motor that drives it. Using extra circuits reduces the size of each pump. Plants with HMA storage silos and two or more asphalt storage tanks can usually benefit from using two or more hot oil circuits. only circulates oil through a single circuit. It ensures that there is always an optimum flow of oil through the helical coil of the heater regardless of the heat demand. known as the circulating or main pump. the hot oil to them can be shut off. serves solely to recirculate the oil through the heater. A manifold is used on the hot oil heater to split off the extra circuits. Thus. known as side or loop pumps. . This ensures that the coil always operates at favorable wall temperatures. each with its own pump (Figure 47).
. maintenance requirements. This is especially true if all of the piping is to be fabricated at the job site by an independent contractor. Strainers have a filter basket. Another should be used ahead of the unloading pump to protect it. either of perforated metal or wire mesh.Page 32 Figure 48 Y-Strainer Is Commonly Used. liquid asphalt and heavy fuel. A strainer is a type of filter used to remove debris or foreign material from hot oil. But Requires Shut Down For Maintenance Figure 49 Duplex Strainer For Heavy Fuel Eliminates Need To Shut Down Filters and valves HMA plant owners need to be informed about filters and valves to ensure that they get the ones most appropriate for their asphalt heating and storage system. A duplex strainer is commonly used where a heavy fuel preheater is installed (Figure 49). One can be serviced while the other operates. In order to replace or clean the basket of a Y-strainer used in a hot oil line it is necessary to shut down the heater and to shut off the line to the filter. protection from damage. how often asphalt samples are taken. etc. its temperature. One strainer should be used in the hot oil line to protect hot oil pump(s). It has two independent strainers and a valve that allows switching from one to the other. A Y-strainer is the type most commonly used (Figure 48). And another should be used ahead of the metering system to protect it. Where a heavy fuel preheater is used. A common mesh opening is 3/64-inch for hot oil and 9/64 for asphalt. a filter should be used ahead of its pump. Some of the factors that may affect their choice are as follows: the type of material being controlled.
Sock filters are very effective and are highly recommended for use with hot oil heaters to protect the hot oil pump (Figure 50). . about the same height above the bottom. they are usually leak-free and provide free-flow. They are virtually clog-free because of the way the valve is designed and mounted in the tank. The extra protection is worthwhile. The valves are usually manually operated and feature a screw stem operated by a hand crank. They often clog and require use of a torch to heat them and their connections to get the material to flow.) Moreover. It is made of a fine fabric mesh which collects dirt and debris from the hot oil flowing through it. even though such valves are not designed for sampling. Gate valves and other types of valves are sometimes used for taking asphalt samples. The filter is long and shaped somewhat like a sock. In a vertical asphalt tank it should be installed in the side. A sampling valve is a special type of valve that enables an operator to easily take samples of liquid from a tank (Figure 51).Page 33 Figure 50 Sock Filter Protects Hot Oil Pump Figure 51 Sampling Valve Facilitates Frequent Asphalt Sampling eliminating the need to shut down the system. It is appropriate to use a sock filter in addition to a strainer in the hot oil line. about 24 inches above the bottom. One should be installed in each asphalt storage tank. especially when asphalt samples must be taken frequently. In a horizontal asphalt tank the valve should be installed in one end. But valves specifically designed for sampling virtually eliminate the hazards and difficulty associated with valves not designed for that purpose. (They do not need a hot oil jacket. They are especially recommended for new installations and when replacing an old heater with a new one.
moves to close passage through the valve as it is actuated. When used for asphalt. They can be used in lines for hot oil. They are used in lines for hot oil. A variety of configurations and plug shapes are available.Page 34 Figure 52 Typical 3-Inch Asphalt Unloading Pump Plug valves are commonly used in piping of hot oil heaters and asphalt storage tanks. can be used to operate the valve through its 90 degree stroke. One is used to unload asphalt from delivery trucks. Globe valves can be used in piping for hot oil heaters. asphalt. They are not recommended for piping carrying liquid asphalt. can be used to operate the valve through its 90 degree stroke. Plug valves use a slotted cylindrical plug that rotates 90 degrees to control the flow. fuel oil. such as a pneumatic cylinder. Some check valves use a spring-loaded ball instead of a hinged gate. The valve has a hinged gate that is controlled by a modulating motor. Most gate valves are manually operated. Pneumatic or hydraulic actuating cylinders cannot be used to operate a globe valve. A butterfly valve cannot be used to totally shut off the fuel supply to the burner. the valve should have a hot oil jacket. The valve controls the firing rate or modulation of the burner in response to the temperature sensors and controls. Two asphalt pumps are usually required at all HMA plants. Gate valves are sometimes used in piping of hot oil heaters and asphalt storage tanks. The other is used to supply asphalt from the . hot oil and fuel. or gas to shut off flow. asphalt. An actuator. They are not recommended for gas. Three-way versions are sometimes used to switch flow from one circuit to another. They are used in lines for hot oil. The valve has a globe-shaped plug that moves to restrict the flow as its spindle is actuated. the valve should have a hot oil jacket. Butterfly valves are used in the fuel supply lines of modulating burners that operate on gas. However. A hinged gate in the valve automatically unseats and opens when pressure is applied to one side of it. similar to a guillotine. All use a slotted ball that rotates 90 degrees to control the flow. There are several issues concerning pumps. requiring multiple turns of a wheel on a threaded shaft. Others are parabolic-shaped. Pneumatic or hydraulic actuating cylinders cannot be used to operate a gate valve. An actuator. Two-way versions of the valve can be used to either shut off or regulate flow. It is known as an unloading pump (Figure 52). Most globe valves are manually operated. Plug valves can also be used in hydraulic and pneumatic circuits. asphalt storage tanks and hot mix storage silos. They can be used in lines for hot oil. asphalt. Three-way versions can be used to switch flow from one circuit to another. fuel oil or gas to regulate flow. Two-way versions of the valve are often used to either shut off or regulate flow. fuel oil. They are used to pump asphalt. Check valves are commonly used in fuel oil and hot oil piping of heaters. Some plugs are conical-shaped. gate valves are not recommended for use in lines carrying liquid that normally contains foreign matter or debris that can accumulate in their grooved seats and prevent complete shut off. A gate. requiring multiple turns of a wheel on a threaded shaft. such as a pneumatic cylinder. A check valve allows liquid or gas to flow only in one direction. or fuel oil. Ball valves are often used in piping of hot oil heaters and asphalt storage tanks. Asphalt pumps Asphalt heating and storage systems at HMA plants require a variety of pumps. or gas. The gate automatically seats and remains closed when pressure is applied to its opposite side. When used for asphalt.
the packing boxes should be heated. An asphalt unloading pump is used to transfer liquid asphalt from the tank of a delivery truck to a storage tank at a HMA facility. For years leaking pumps were believed to be unavoidable and were accepted as a routine maintenance problem. Plants with the asphalt metering system known as “a pump pushing a pump” has another pump. Full-jackets are strongly recommended.Page 35 Figure 53 Metering Package Using “A Pump Pushing A Pump” storage tank to the drum mixer or batch plant. A problem can occur with a supply pump when its inlet is higher than the outlet of the storage tank. Thus. which can severely damage the pump if allowed to continue. In any case.7 minutes per truck load. Pumps should not be subjected to a vacuum in excess of 15” Hg (inches of mercury). the packing will be short-lived and leaking will be a constant problem. But unloading time may not be a concern if you do not operate your own delivery trucks. On portable plants the pump is usually mounted on the gooseneck of a portable storage tank. Hot oil pumps A potential problem at virtually every asphalt plant is oil leaks from the hot oil pump. On relocatable plants the pump is sometimes mounted on the same skid as the hot oil heater. which is used passively for metering (Figure 53). On stationary plants the pump is usually independently mounted at a location that will conveniently serve two or more storage tanks. Asphalt pumps should be carefully selected to avoid maintenance problems and down time.000 tons of hot mix a year using 5 percent liquid AC can save about $4400 by switching from a 3-inch pump to a 5-inch pump. It is known as a supply pump. And if the delivery truck has a 3inch unloading line there is no advantage to having a 5-inch pump. 320 truck loads and a trucking cost of $50. The positive-displacement pump is the industry standard for pumping asphalt. The time it takes to unload tanker trucks delivering liquid asphalt cement to a HMA facility depends upon the size of the unloading pump and connecting lines.000 gallons of liquid asphalt cement. The high operating temperatures (about 400 degrees F) takes its toll on pumps.88 per truck load. Otherwise. Unloading pumps used usually range in size from 3 to 5 inches. Modified asphalt needs larger pumps and lines than used for virgin asphalts because it has a higher viscosity. A facility that runs 150. a 5-inch pumping system saves 16. Only those jacketed for hot oil heating should be used. A 3-inch pump unloads 200 gpm and takes 30 minutes to unload a tanker with 6. Supply pumps range in size from 2 to 5 inches. But a 5-inch pump unloads 450 gpm and takes only 13. Lifting the liquid creates a negative pressure or a vacuum at the inlet of the pump. regardless of the type of seal or packing used. and are usually picked according to the rating of the drum mixer or batch plant. The solution is to either place the pump at a lower level or place the storage tank at a higher level. This is based on a savings of $13. . The size used most frequently is a 2-1/2 inch pump.00 per hour. Excessive vacuum will cause cavitation.3 minutes to unload the same tanker. This is a condition that sometimes occurs with underground storage tanks. This makes the pump lift the liquid to its inlet before it can be pumped. providing 200 to 450 gpm respectively. Gear pumps often leak soon after the pump is put into service.
However.1 percent). Heatec has switched from gear pumps to centrifugal pumps as a solution to solving this problem (Figure 54). . Consequently. pressure. The system is commonly known as “a pump pushing a pump. It compensates for temperature variations in the material. The federal government has put strict limits on the amount of oil allowed to spill on the ground. Mass flow meters have a high degree of accuracy (plus or minus 0. which gradually increases over a period of time. One pump is active and the other is passive. there is only a small leakage. The standard continuous metering system used by Heatec has two pumps (Figure 53). The use of a centrifugal pump brings on special design considerations. if the system is not designed to limit these variations. thereby damaging the pump. an output register. So. inadequate flow at its inlet can cause the pump to cavitate. Heatec does not use the Brodie meter unless specifically requested by the customer. Leaking oil must be contained so it cannot contaminate the soil and ground water. Batch plants use a weigh pot to weigh liquid asphalt cement in the batch tower before the asphalt cement is dropped into a pugmill for mixing. Still another type of continuous metering system used for drum mix plants is the Brodie meter. and cost effective. The system can be calibrated to meet the most stringent accuracy requirements of plus or minus one-tenth of one percent (0. seals can be replaced at a convenient time before maximum leakage occurs. The system incorporates a temperature measuring system that compensates for changes in the volume of asphalt cement due to temperature variations. viscosity and temperature of the asphalt. This is an electronic measuring system. Oil leaks also need to be eliminated because they are a fire hazard. The passive one is forced to rotate by the liquid asphalt being pumped through it and functions solely as a flow meter. durable. It runs much quieter. Asphalt metering Two methods are used to add the specified amount of asphalt cement to aggregate for making hot mix asphalt. as required by the state of Illinois and other states. The system must also be designed to avoid excessive starting loads and overloads. The active one is motor-driven and functions as a conventional supply pump. The asphalt flowing through the passive pump causes its drive shaft to turn at a speed proportional to the flow rate.Page 36 Figure 54 Centrifugal Pump Minimizes Hot Oil Leaks But now environmental concerns have given more importance to solving the problem. An encoder on its shaft generates an electrical signal representing its revolutions. It is essentially a batch metering system. Moreover. the pump may not achieve its rated performance. Another continuous metering system sometimes used at drum mix plants is known as a mass flow meter. density. This signal is used by the computer in the control house to display the amount of asphalt cement used and its rate of flow. And when seals wear to the extent that they leak. So. It is compensated for temperature variations. they are significantly more expensive than other systems used for asphalt metering and are hard to justify solely on the basis of their increased accuracy. preventing oil leaks is an important goal. Our field trials revealed that seals in certain centrifugal pumps can go for years without leaking. The centrifugal pump Heatec uses has other advantages. It is air-cooled and runs cooler. The pump is more sensitive to pressure variations at its inlet. The Brodie meter is more expensive than the “pump pushing a pump” system and is less durable. The pot is mounted on load cells. Drum-mix plants use a continuous flow asphalt metering system.15 percent) and virtually no moving parts.” The system is very accurate and has proved to be very reliable. Thus. and a signal generator that produces pulses in proportion to the rate of flow. It uses a rotor assembly. The system has an electronic sensor and transmitter that measure flow rate.
a vertical (Figure 55) and a horizontal (Figure 56). Efficiency becomes significant when calibration must be done frequently. Heatec calibration tanks provide an efficient way to calibrate the metering system. a common practice in the industry. The tanks are available in two styles.Page 37 Figure 55 Vertical Calibration Tank Figure 56 Horizontal Calibration Tank Calibration Calibration tanks are recommended for calibrating the liquid asphalt metering system of a drum mix asphalt plant. The tank has built-in load . They save considerable time and money. The choice between the two styles is mostly a matter of personal preference. But now some states now require it every week. It wasn’t so bad when you only had to calibrate every month or two. Using a calibration tank is far more efficient than using a distributor truck.
4. You can run three cycles with the calibration tank in less time than it takes for one cycle with a distributor truck. The control panels should always be UL approved and may be either NEMA 4 or NEMA 12. Pump one or two thousand gallons of asphalt into the truck as indicated by the metering system. Drive it across a truck scale. economical burner management system is used for the burners on direct-fired tanks. Adjust the metering system to agree with the weight shown on the readout. 2. then pump the AC back into the AC tank. A control panel mounted on the heater houses key electronic components (Figure 57). They are factory calibrated and furnished with a calibration certificate. You may save two or three hours every time you calibrate. Repeat as many times as necessary. And again. 2. the savings in time and money is apt to be more than worthwhile. ignition. Reconnect the truck to the asphalt tank and pump the asphalt back into the tank. Just compare using a truck with using calibration tanks. setting off the chain reaction that caused the shutdown. This type of system works very well on direct-fired tanks because the burner does not usually cycle frequently. Note the weight shown on the readout of the calibration tank. You can normally tell from the lights which limit switch tripped first. Drive the truck back across the weigh scale and record its weight. In any case. Connect the truck to your asphalt tank. It has a sensor that shuts off the fuel if it does not detect the presence of a flame. The system provides a higher degree of accuracy than using a truck scale.Page 38 Figure 57 Heater Control Panel With Readouts And Status Lights cells and a digital readout. record its weight. Here’s what you have to do when using a truck to calibrate your metering system: 1. and flame monitoring protection. Repeat the whole process again. Traditionally. Switch the valves. 4. Adjust your metering system readouts to agree with the calculated weight. If a problem occurs that causes the burner to shut down. Disconnect the lines. when the thermostat calls for heat and closes the valve when the thermostat is satisfied. It uses a microprocessor to manage the burner controls and provide proper burner sequencing. 6. 3. a very simple. Here’s all you have to do using a Heatec calibration tank: 1. On hot oil heaters the burner management system is usually more sophisticated. Readouts and signal lights on the face of the panel indicate the current burner status and its operating history. there is no need to connect and disconnect asphalt lines each time the system is calibrated. They Heater controls A burner management system is required for automatically-ignited fuel burners. It opens the fuel valve and ignites the fuel . Subtract the truck’s empty weight from its loaded weight. The load cells are highly accurate. And don’t forget about improved accuracy. 5. and then drive it to your plant. Get an empty asphalt distributor truck. Maybe more. 7. But NEMA 4 panels give added protection and are preferred. Open the valves to the calibration tank and pump about 1000 gallons of AC into the calibration tank as indicated by the asphalt metering system. the lights identify all switches affected by the shutdown. Once the calibration tank is installed. 3. NEMA 12 panels are dust tight.
. Condensing the vapors turns them into a liquid state wherein they return to the Containment refers to a secondary enclosure for tanks that contain liquids.Page 39 Figure 58 Blue Smoke Condenser Minimizes Air Pollution protect against windblown dust and rain. Emissions liquid asphalt cement instead of escaping into the atmosphere through the tank vent or exhaust stack. It condenses gas vapors. Containment A blue smoke condenser is a heat exchanger sometimes used as a vent in tanks of heated asphalt cement to minimize air pollution (Figure 58). commonly known as blue smoke. usually fuel or asphalt cement. However. It functions to keep the liquid from escaping in the event of a leak or rupture in the primary tank. This greatly minimizes the release of pollutants into the atmosphere. gasses exiting the tank are cooled as they flow through the tubes. Thus. produced by heating light ends (volatile organic compounds) often present in the liquid asphalt cement. splashing water and hose-directed water. The cooling causes vapors of the VOCs to condense and to drain back into the tank. The main concern is for the environment. a containment can also minimize the impact that an accidental spill can have on operation of the HMA facility. Condensers used on asphalt storage tanks usually have a number of tubes with external fins. The tubes are cooled by ambient air circulating through the fins.
Thus. Consequently. On a stationary or relocatable tank the containment enclosure usually encloses only the bottom portion of the tank. Another advantage is that the surface area of asphalt cement exposed to atmosphere inside a vertical tank is smaller than that inside a horizontal tank for the same amount of liquid. there is less oxidation of asphalt cement in a vertical tank. However. the liquid in vertical tanks .Page 40 Figure 59 Concrete Wall Containment Encloses Several Tanks Figure 60 Tank With Built-In Containment Shell Containment enclosures are often no more than concrete walls built around a group of tanks (Figure 59). Horizontal tanks are installed so their long axes are horizontal and they occupy a ground area equal to their diameter times their length. when mixers are installed to keep polymers in suspension. Vertical tanks are installed so that their long axes are upwards and they occupy a ground area only the size of their diameter (Figure 61). Moreover. you can fit four vertical tanks in the space of one horizontal tank (Figure 62). Heatec offers containment enclosures for individual tanks fabricated from steel (Figure 60). They are easily equipped with wheels and suspension for highway travel. Horizontal Vs vertical tanks Heated asphalt storage tanks are cylindrical and may be either horizontal or vertical. On portable tanks the containment enclosure consists of a second tank that fully encloses the main tank—a double-walled tank.
a unit that combines a coil tank with a heater. as already noted.000 GALLON VERTICAL TANKS Figure 62 ONE 35. even though they can be portable. direct-fired tanks are not recommended for polymer-modified asphalts. such as a Heli-Tank. the best-suited tanks for both portability and polymer asphalts are horizontal tanks that have coils heated by hot oil. However. And where only a single tank is needed. there are other considerations when polymers are used and the tanks must also be portable.000 GALLON HORIZONTAL TANK Ground Area Of Vertical Tanks Vs. So. Horizontal tanks are best-suited for portability. Horizontal Tanks has better flow patterns than in horizontal tanks. is an excellent choice. .Page 41 Figure 61 Vertical Tanks Have Small Footprints FOUR 30. But.
Page 42 (Figure 61). (Please refer to Heatec Technical Paper T-133. Although the mixing action in a horizontal tank is not as good as in a vertical tank. current experience indicates that the mixing is quite satisfactory.) . SPRAY BAR PUMP Figure 63 HEATING COILS OMITTED FOR CLARITY INLET STAGNANT ZONE Horizontal Tank With Re-circulation System For PMAC MIXERS IN IDEAL LOCATIONS MANWAY Figure 64 SMALLER STAGNANT ZONE Horizontal Tank With Mixers For PMAC Heli-Tanks and other horizontal tanks heated with hot oil can incorporate either a spray bar re-circulation system (Figure 63) or mixers (Figure 64) to keep the polymer from separating from the asphalt.
Page 43 NOTES .
Page 44 NOTES .
COM . CHATTANOOGA.. TN 37410 • PHONES 423-821-5200 • 1-800-235-5200 WWW.HEATEC an Astec company 5200 WILSON RD.COM • E-MAIL: HEATEC@HEATEC.HEATEC.
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