Why are air dryers necessary? All air compressors take in atmospheric and compress it eight or ten times to a pressure between 110 and 125 psig. All the moisture vapor and heat in the atmospheric air is also compressed and concentrated, so air at the compressor outlet is hot and wet. The temperature of the air at the compressor outlet can be as high as 350ºF. The air also can be saturated with water vapor. As this air cools in the receiver and plant piping, the water vapor in it condenses into water droplets.
A 25-hp compressor running at 75% capacity pumps 18 gallons of water into a plant air system on a day with average ambient humidity. An aftercooler condenses and removes about 11 gallons of this liquid, but that still leaves 7 gallons to collect in low spots, retard valve movement, damage production parts, and cause problems in general.
Types of air dryers
Figure 7-4 shows a cutaway view of (and the symbol for) a deliquescent air dryer. Wet air enters the dryer (which is a pressure vessel), passes up through a bed of hygroscopic chemicals, and flows on to the outlet. The chemicals (often a form of sodium) absorb moisture from the air as it passes through the bed. As they dry the air, the chemicals break down into a slurry of water and chemical drops that falls to the bottom of the tank. A manual or automatic drain keeps the water mixture from rising too high and mixing with inlet air flow.
A typical deliquescent dryer removes moisture to a dew point of about 40ºF. Air that has passed through this dryer must be cooled below 40ºF before any more moisture will condense. This dew point is satisfactory for most plants -- even during winter cold. However, the hotter the air passing through the chemicals, the less the amount of moisture the chemicals will collect. Thus it is important to keep the incoming air at or below 100ºF. This usually requires an upstream aftercooler to lower the temperature of the air being delivered from the compressor. Air at higher temperatures at the inlet results in higher dew points at the outlet.
Deliquescent dryers are the least expensive of the three types mentioned in this section, but they might cause problems in some installations. There is always a chance of the chemicals or their vapors being picked up by the air stream and sent into the pneumatic system. These chemicals are corrosive and can damage internal parts. Also chemicals must be replenished on a regular basis. This means shutting down the compressor or bypassing the dryer when chemicals get low. Finally, slurry must be removed.
Figure 7-5 shows a cutaway view of a refrigeration-type dryer. This unit condenses water vapor by cooling compressed air to lower its dew point. There are no chemicals to replace or break down, so the system can run as long as required. A refrigeration dryer is more complex and more expensive to purchase than chemical dryers, but has a lower operating cost. The main operating cost is electricity. Maintenance cost is minimal and life expectancy is high, so this type dryer can be cost effective over the long haul.
To keep operating cost down these systems usually have an upstream water-cooled aftercooler to cool compressor air and take out the bulk of the moisture. Normally a maximum incoming temperature of 100ºF is recommended. When a higher inbound temperature is present, oversize the refrigerated dryer to handle the extra energy removal. (Of course, this adds extra cost to the purchase of the unit, as well as increasing operating cost over the life of the system.)
Wet air enters the unit through an internal air-to-air heat exchanger. This unit is piped to pre-cool the hot inbound air and re-warm the cold dry air before it exits the dryer to enter the plant piping. This arrangement saves energy by removing some of the heat in the incoming air. It also keeps the plant piping from condensing water vapor from ambient air on its cold exterior surfaces and dripping water on the production floor.
The pre-cooled air then passes through a Freon-to-air heat exchanger that reduces its temperature to approximately 35ºF. This procedure condenses more water vapor to achieve a 35ºF dew. If plant temperature stays above 35ºF (as it does in most plants), there will be no more condensation inside the air piping. The condensed water drains from the dryer through a water separator.
The main potential problem with refrigerated dryers is they will freeze up if temperature is set too low or if the amount of air passing through them is low and intermittent.
Another approach to refrigerated drying is drying the air before it enters the compressor. One company offers chillers that take atmospheric air down to –40ºF. and feed it to the compressor. After it is compressed to 100 psi, the air has a pressure dew point of approximately 35ºF. Because the compressor takes in air that is denser and at such a low temperature, the heat of compression is negligible. Also, most of the airborne contaminants are removed during the cooling, condensing, and freezing process. These inlet air dryers use dual refrigeration units. While one is drying input air, the wasted heat from the drying unit defrosts the other.
Figure 7-6 pictures a twin-tank desiccant dryer that uses a hygroscopic material (such as silica gel or activated alumina) that collects water vapor but is not broken down by it. This type dryer is called an adsorber because it collects water vapor but once the moisture is removed by heat or other methods, the chemical is ready to work again. This desiccant dryer may achieve dew points of –40ºF or lower, so the air can be used in most outdoor circuits without fear of freeze ups.
Electric heaters and purge air from the opposite tank handle the drying process. Other methods are steam heat, dried purge air only, and desiccant replacement.
As wet compressed air enters the control valve, it is channeled to one of the desiccant tanks. (The control valves are usually set up to shift automatically, triggered by a signal from a device that monitors the output air’s dew point.) Wet air is forced through the desiccant material to take out water vapor, then sent on to the plant. Some of the dried air is diverted through an orifice to the spent tank, where it is heated and then passed through the wet desiccant in the other tank. Water again vaporizes and exhausts to atmosphere. This drying process continues at the rate necessary to maintain the required dew point.
Desiccant dryers are the most expensive type to operate and maintain. In particular, they are subject to failure if the incoming air contains carry-over compressor oil. The oil can coat the desiccant and make it incapable of collecting moisture. The main expense of operating these dryers is the energy used to dry out the idle tank of desiccant.