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Technical Paper T-140

Heating and Storing Asphalt at HMA Plants

PREFACEThis 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 Jrgen 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.

Revised 11-24-03

Contents2 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

Page 2

Abstract

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 equipmenteven when ample financial resources are available. Although production volume is usually a major factor in equipment costs, its 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

the contractor can buy with confidence, without fear of costly surprises later. It is easy to focus on the initial cost of equipment and overlook how much it will cost to operate over its life. The life expectancy of the better equipment sold today is 20 years or more. So, operating costs can add up to a lot of money over 20 years and may greatly exceed the initial cost of the equipment. Consequently, where there is a choice of equipment with different operating efficiencies, the possible savings in operating costs may significantly outweigh its higher initial cost. Perhaps the most cost effective way of obtaining higher operating efficiencies is by the appropriate use of insulation. Insulation reduces heat loss significantly, especially on asphalt storage tanks and asphalt piping operating at temperatures of 325 degrees F. Thus, insulation reduces the amount of energy required

to replace heat losses, thereby reducing energy costs. So, when there is a choice of insulation thickness, it usually pays for contractors to obtain equipment with the thickest insulation offered. Contractors should also pay close attention to the thermal efficiencies of heaters. The better heaters offered today have thermal efficiencies of 80 percent and higher. If a plant has an old heater with an efficiency less than 80 percent it may pay to replace it with one of higher efficiency. This is especially true if the plant operates throughout most of the paving season each year. The more a contractor knows about these many subjects, the more apt he is to get the right equipment and to be satisfied that he made the right decisions. Likewise, he is less likely to be duped into buying equipment that does not fully meet his needs.

Please note that calculations for heat losses and heating requirements can vary widely depending upon the assumptions for weather conditions. Calculations in this document are based on an ambient temperature of 70 degrees F and a wind of 10 mph. We consider these conditions to be appropriate for general use. However, it may more appropriate to use a different ambient temperature and wind for a specific area or region. In any case, the reader should understand that the data presented in this document may not be the most appropriate for a specific location even though it is suitable for comparisons within the document.

AMBIENT TEMPERATURE AND WIND

Page 3

Figure 1

Waste not Want notConservation 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. 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.

a major concern of governments and citizens alike, not only in the United States, but also in many other countries of the world. Fortunately, it is possible to conserve energy and save money at the same time when heating and storing asphalt. Any higher initial costs are quickly recovered and turned into savings.

Heat conservation

Perhaps the most fundamental issue concerned with heating and storage of asphalt is the conservation of energy (Figure 1). In recent years conservation of all natural resourcesespecially energy resourcesranks high. It is

Considerations

less important than considerations for drying aggregate, all are important in the long run. In just a few years what may have initially appeared to be insignificant may either result in significant savings or significant waste, depending on how the issue was initially addressed. So, it makes sense to take a close look at all issues that could in time have a positive or negative impact (Figure 2). 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). These asphalts require heating and storage significantly

There are many important considerations whether replacing old equipment or purchasing an entirely new system. While some issuessuch as the thermal efficiency of asphalt heatingmay seem

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. They must usually be maintained at higher temperatures and must be continuously agitated or mixed to keep the polymers from separating. Moreover, they normally have a higher viscosity, which may affect piping and pumping needs. For additional information on PMACs, please refer to Technical Paper T-133 entitled, Heating, Mixing and Storing of Modified Asphalt. This paper is available from Heatec.

Basic needs

Heating and storing asphalt is a basic function of all HMA facilities. Asphalt is normally delivered to the HMA facility in a liquid state, usually by tanker truck (Figure 3). The viscosity of the asphalt must be low enough to allow it to be pumped from the delivery truck into the storage tanks. Viscosity is a measure of a fluids resistance to flow and is related to its temperature. The higher the temperature the lower the viscosity or resistance to flow. Viscosity is commonly expressed in units known as SSU (Saybolt Second Universal), poise and centipoise.

The actual temperature of the liquid asphalt at time of delivery may vary somewhat, depending upon the arrangements with the supplier. But most asphalt terminals deliver asphalt to HMA plants at a temperature suitable for making hot mix or slightly higher. Some HMA facilitiesmainly those in remote locations and on small islands buy solidified asphalt in barrels. Thus, the solidified material must be melted into a liquid before it can be pumped into the asphalt storage tanks. This requires special equipment known as barrel melters (Figure 4). In some instances asphalt is transported aboard ships in

Page 5

Figure 5

Shipping Containers May Be Used To Transport Asphalt

insulated shipping containers (Figure 5). 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. When asphalt is used to make hot mix it must be within a specified temperature range, usually from about 300 to 325 degrees F. However, as already noted, higher temperatures are usually required for PMACs. Accordingly, when asphalt is delivered to the plant at a temperature suitable for use, the role of the asphalt heating and storage equipment is to maintain that temperature until

the material is used. Maintaining its temperature requires only enough heat to replace that lost during storage and pumping. But when the asphalt is delivered to the plant at temperatures lower than required for making hot mix, the heating equipment must increase its temperature to meet that specified for use, in addition to replacing all heat lost during storage and pumping. This calls for more heating capacity, which may affect the choice of equipment. Moreover, heating equipment for some types of plants nearly always performs

additional roles. On relocatabl

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