TM 5-3895-374-10air than is stoichiometrically required. By doing this, the number of oxygen molecules available for the carbon andhydrogen molecules has been increased. For this reason the stoichiometric air requirement is frequently called thetheoretical air requirement, and the additional air is known as excess air, generally stated as a percentage ofstoichiometric air to fuel ratio is 1377 cubic feet per gallon, the amount of air needed for combustion is 1721 cubic feet.While excess air is necessary to ensure that all the fuel burns, it is possible to overdo a good thing. Too much excess airwastes fuel. This happens because the excess air will be at the same temperature as the other components of the exhaustgas, and as part of the exhaust gas, it is exhausted from the stack. The rise in temperature of the excess gas consumesenergy (BTU’s). Exhausting hot excess air that was not needed to promote complete combustion is like throwing fuel intothe air.1-15. Drum MixerThe drum mixer is the component which completes the process of producing the hot asphalt mix. This rotating drumaccepts an aggregate mix, dries the aggregate, and mixes the liquid asphalt with the aggregate to produce the asphaltmix. This process is accomplished on a continuous basis so that the mix will be consistent in content and temperature. Asteady flow of aggregate, hot gas and hot liquid asphalt are al that is required. This simple statement of the processhowever does not properly indicate the complexity of operation of the Asphalt Mixing Plant. The plant has two surge pointsto allow the accumulation of materials in order to make the continuous operation of the plant much simpler. A surge bin islocated at the drum mixer discharge point to collect the finished product (asphalt mix) and allow continuous operation of thedrum mixer. Bins are used to hold the aggregate so as to ensure a steady stream of aggregate. These four bins alsoallow the production of different aggregate mixes. Each of the four bins are individually controlled and the blend can bechanged by controlling the speed of the feeder on each binThis allows a different amount of each aggregate to be fed tothe feed conveyor, therefore producing a desired blend by controlling the proportion of each type of aggregate.IV. PRINCIPLES OF OPERATION1-16. Setting the MixThe asphalt hotmix design is provided to the operator of the Asphalt Mixing Plant by the engineer or supervisor of theconstruction project. The raw materials required, aggregates and asphalt cement, have been provided and are ready foruse. The mix design is input into the blending controller if it is not already in memory. Aggregate moisture content must beprovided daily and input into the controller. The density ofpage 1-23
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