1. Field
This invention relates to the hot mix asphalt plant used to manufacture asphaltic mixes to pave highways.
2. Description of the Related Art
The components of the state of the art hot mix asphalt plant combine process methods to heat and dry aggregates, add coarse and fine aggregate dust particles and mix them with asphalt. The raw material virgin aggregates release dust particles as they are heated and dried that must be collected using mechanical separation for the coarse dust particles along with fine dust collection using fabric filters or baghouses to eliminate air borne particulate from entering the atmosphere. Asphalt plants use rotary dryers to tumble the aggregates in the presence of heat to remove the moisture and heat them to the process temperature.
Drying aggregates with the rotary dryer involves a physical separation process designed for the purpose to remove the liquid phase or moisture using thermal energy. The liquid in this case is water that is liberated by the process of vaporization leaving a solid or aggregates with a trace of residual moisture. The dryer is a direct heat dryer as heat transfer for drying is accomplished by direct contact between the wet solid aggregate with hot gases from a combustion burner. This invention describes the dryer as a rotating cylindrical shell, slightly inclined to the horizontal with the heating medium flowing in a countercurrent direction to the flow of aggregates through the dryer. The dryer is equipped with flights or lifters on the interior for lifting and moving the wet aggregates through the hot gas stream as they pass through the cylinder.
As the gases pass through the dryer the aggregates become hot and dry thus liberating aggregate dusts that cling to the aggregates. Some aggregates liberate dust as they are tumbled through the rotary dryer. The dryer operates under a suction pressure as the dust particles are pulled through the dryer and into the coarse particle mechanical dust separation devices and finally into the fabric filters for dust collection. Those dust particles must pass from the rotary dryer to the dust collection devices generally using ductwork or large round pipes or rectangular ducts to move the moisture, products of combustion and dust to the dust collection devices. This industrial size external ductwork usually requires the use of a large lifting crane to assemble the ductwork from the rotary dryer to the dust collection devices and filter house needed to collect the hot dry aggregate particles. Moving the asphalt plant from one location to another becomes cumbersome as lifting equipment is needed to assemble and disassemble the plant rotary dryer from the dust collection equipment.
Making asphaltic mixes requires a recipe using a combination of sand and varying sizes of aggregates. Multiple bins are needed to contain the sand and aggregates. The bins are filled using construction bucket loading tractors from the site stored aggregates piles.
The aggregate bins are each fitted with an underlying variable speed belt feeder that proportions the needed flow of raw material to combine the hot mix recipe.
The raw aggregates are generally conveyed with a belt collection conveyor to a large vibrating screen to insure debris or oversize materials are eliminated from the flow of aggregates to insure a smooth road surface. A vibrating screen device of the size needed on the asphalt plant generally must be conveyed as a separate unit and mounted to a large wheeled trailer assembly in order for it to be conveyed for the portable plant. Bins of this size and capacity normally require a large individual wheeled trailer assembly as they are moved as a separate component of the portable asphalt plant. The movement of aggregates from the aggregate bins and oversize vibrating screen requires the use of a long portable conveyor belt assembly to move the aggregates to the rotary dryer for processing. These portable conveyors are fitted with truck axles and wheels as they are moved along with the portable asphalt plant.
The dust particle size separation and collection equipment needed on the portable plant is generally conveyed as a separate component with truck axles and wheels that also must be positioned on the plant site to mount the large connecting duct pieces between the rotary dryer and dust separation equipment. The large portable rotary dryers as well are conveyed as a separate component with truck axles. Ductwork connection and feed conveying plant components must be connected to the rotary dryer once it is in place at the portable plant site.
Once the hot mix asphalt is produced it must be conveyed away from the rotary dryer, stored and weighed and then transferred to a dump truck to be carried to the road paving site. Each of these devices must be made portable and carried along with the other plant components as a separate piece of equipment with truck axles as trailer assemblies.
Each time a portable hot mix asphalt plant is moved, each site must be graded, leveled and compacted to insure the proper soil surface conditions exist to allow the equipment to be assembled and parked for alignment and safety. As described, the portable hot mix asphalt plant can easily contain ten individual plant components. Transporting and preparing the site represents time taken away from actual plant production; the cost to prepare the site, grading for each piece of equipment and the cost of additional site preparation materials such as soil and aggregates to prepare and level the site.