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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.
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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.
Dew Point: 35 °F
CFM Range: 10 - 20,000
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.
Dew Point: 35 °F
CFM Range: 10 - 19,200
High temperature refrigerated air dryers are designed to operate with compressors that don’t have aftercoolers. The refrigeration system in a standard dryer is sized for an incoming compressed air temperature of 100°F or below.
The compressed air temperature coming from a compressor without an aftercooler can run from 150°F to 200°F, and would overload and damage a standard dryer. High temperature dryers are designed for these temperatures and can deliver a 38° or 50°F pressure dew point (depending on make and model).
Dew Point: 38 - 50°F
CFM Range: 20 - 130
High pressure refrigerated air dryers are designed to provide dry air to operations such as PET bottle blowing machines, which require pressures around 700 PSI. They are also commonly made with corrosion resistant materials (stainless steel heat exchangers) to resist the high concentrations of aggressive contaminants present in compressed air at elevated pressures.
In addition to the heavy heat exchangers, these dryers will incorporate special drains to operate at high pressures. If a standard 680 psi model is not suitable for a specific application, a dryer with heat exchangers suitable for higher pressures can be built to order.
Pressure Rating:
up to 725 PSI
Dew Point: 35°F
CFM Range: 13 - 4300
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.
Dew Point: 35°F
CFM Range: 800 - 3000