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Temperature Indicating Device

Aggregate temperature is measured by either a thermometer or a thermocouple attached to an indicating pyrometer. Pyrometers react much faster to changes in temperatures and are usually preferred.  An aggregate heat measuring device should be installed in the dryer discharge in full view of the burner operator.  This device is one of the most important plant control accessories and should be a reliable and accurate instrument.  Overheating of the aggregate can damage the asphalt during mixing, under heating makes the aggregate difficult to coat with the asphalt and difficult to place.

The Dust Collector

The dust collector (Figure 3-4) is generally operated adjacent to and in conjunction with the dryer, and is necessary for efficient plant operation. The collector eliminates or abates the dust nuisance that might result from exhaust air from the dryer.  Modern dust-collection systems are highly efficient. Provisions are usually made in the dust collecting system to return the collected dust back to the hot aggregate as it emerges from the dryer and picked up by the hot elevator.  If the collected dust is unsuitable for use in the asphalt concrete mixture, it may be removed from the collector and wasted. The dust collector fan(s) furnishes the draft that draws the flame and hot gases through the dryer.  Dust particles from the dryer and other parts of the plant are also carried in the current of draft air, which enters the dust collector at the upper periphery and goes into vertical motion.  The heavier dust particles in the air stream are separated by centrifugal force into the collector shell and fall to the bottom.  The finer dust may remain in suspension and be carried out the exhaust stack with the air. A wet wash system may be added to the dust collecting system to reduce the amount of fine dust being carried out the exhaust stack with the air.  There are several types of wet systems, but they all usually consist of a short tower, with or without baffles.  Exhaust from the dust collector enters the tower at the bottom and passes upward through a series of water sprays that remove the dust.  Use of a wet wash system usually will increase fan requirements by 10 to 15 percent because of the pressure loss in the tower.
 

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