Refrigerated Compressed Air Dryers
By far the most popular type of dryer, refrigerated compressed air dryers are competitively priced, economical to operate, provide a stable pressure dew point and do not require much maintenance. Most provide a pressure dew point of 35°F, but some less expensive models with smaller refrigeration systems are rated for the higher dewpoint of 50°F. There are two types of refrigerated air dryers: Cycling and Noncycling. The thing that both have in common is that they use a refrigeration system to cool the compressed air to a temperature as close to freezing as possible in order to condense out as much water as possible.
They incorporate a mechanical separator to remove the condensed water before sending the compressed air out to the plant. Most refrigerated dryers also contain a heat exchanger called a precooler/reheater which uses the cold air leaving the dryer to pre-cool the hot, wet air entering the dryer. This reduces the load on the refrigeration compressor and prevents the piping from “sweating” in humid areas. Refrigerated air dryers require electricity to operate and must be kept in areas that are maintained above freezing.
Non-Cycling Refrigerated Air Dryers
Non-cycling air dryers have refrigeration compressors that start as soon as you turn the dryer on, and don’t stop until you turn the dryer off. They are less expensive to buy than cycling dryers and are a good choice for applications where they will be constantly at full capacity. However, during periods of low compressed air usage they can experience problems.
Since the refrigeration compressor is running constantly there is cooling taking place constantly. If there isn’t enough heat being provided by incoming compressed air temperature in the dryer could fall below freezing,which will cause water condensed from the compressed air to freeze. At best this will mean an internal blockage with ice that shuts off your compressed air supply. At worst you get the blockage plus a split tube or heat exchanger caused by the expansion of water when it freezes.
To prevent this from happening noncycling dryers are equipped with a device called a hot gas bypass valve, which sends the heat that the refrigeration compressor would normally discharge to atmosphere back to the cold side of the system to keep it from freezing. As long as this valve operates properly you won’t have a problem. If it fails, the dryer will freeze.
Cycling Refrigerated Air Dryers
Cycling refrigerated dryers have refrigeration systems that run when cooling is needed and shut off when it is not. This concept is very similar to the refrigerator at home. If you open the door you will eventually hear the refrigeration system start. After you close the door the refrigeration system will continue to run for a while and shut off. If you open the door less, the refrigeration runs less. The refrigeration also runs less in winter than summer because cooler ambient temperatures cause less cooling loss through the walls of the refrigerator. What all this adds up to is Big Energy Savings!
The most popular and reliable cycling dryer is the Thermal Mass type. The refrigeration system cools a large, heavily insulated tank full of propylene glycol antifreeze and water to a temperature close to freezing. This cold fluid is used to cool the compressed air to condense the water out of it. When the cold fluid warms up a thermostat turns on the refrigeration compressor and cools the fluid down again. The benefit of a thermal mass cycling dryer is that the refrigeration system only runs as much as it has to. If you use less compressed air, or the ambient temperature is cooler, you will use less electricity and put less running time on the refrigeration compressor and condenser fan motor(s). The electrical savings from cycling dryers can add up to a lot of money often pay back on hte cost of the unit is very quick. Better yet, many power companies offer rebates for energy efficient equipment which helps to offset the purchase price.
Variable Speed Drive Refrigerated Air Dryers
Variable speed dryers are an alternative to thermal mass cycling dryers in obtaining efficient part-load operation. A temperature probe placed in the evaporator sends a signal to a variable speed drive which controls the refrigeration compressor, providing more or less cooling as needed. These dryers are best applied on installations that consistantly stay between 100% and 50% of dryer capacity. Below 50% capacity these dryers revert to a hot gas bypass system to prevent freeze-ups.