TM 5-3895-374-10
air than is stoichiometrically required. By doing this, the number of oxygen molecules available for the carbon and
hydrogen molecules has been increased. For this reason the stoichiometric air requirement is frequently called the
theoretical air requirement, and the additional air is known as excess air, generally stated as a percentage of
stoichiometric 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 air
wastes fuel. This happens because the excess air will be at the same temperature as the other components of the exhaust
gas, and as part of the exhaust gas, it is exhausted from the stack. The rise in temperature of the excess gas consumes
energy (BTU’s). Exhausting hot excess air that was not needed to promote complete combustion is like throwing fuel into
the air.
1-15. Drum Mixer
The drum mixer is the component which completes the process of producing the hot asphalt mix. This rotating drum
accepts an aggregate mix, dries the aggregate, and mixes the liquid asphalt with the aggregate to produce the asphalt
mix. This process is accomplished on a continuous basis so that the mix will be consistent in content and temperature. A
steady flow of aggregate, hot gas and hot liquid asphalt are al that is required. This simple statement of the process
however does not properly indicate the complexity of operation of the Asphalt Mixing Plant. The plant has two surge points
to allow the accumulation of materials in order to make the continuous operation of the plant much simpler. A surge bin is
located at the drum mixer discharge point to collect the finished product (asphalt mix) and allow continuous operation of the
drum mixer. Bins are used to hold the aggregate so as to ensure a steady stream of aggregate. These four bins also
allow the production of different aggregate mixes. Each of the four bins are individually controlled and the blend can be
changed by controlling the speed of the feeder on each binThis allows a different amount of each aggregate to be fed to
the feed conveyor, therefore producing a desired blend by controlling the proportion of each type of aggregate.
IV. PRINCIPLES OF OPERATION
1-16. Setting the Mix
The asphalt hotmix design is provided to the operator of the Asphalt Mixing Plant by the engineer or supervisor of the
construction project. The raw materials required, aggregates and asphalt cement, have been provided and are ready for
use. The mix design is input into the blending controller if it is not already in memory. Aggregate moisture content must be
provided daily and input into the controller. The density of
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