In pneumatic systems, compressors draw in ambient air that always contains some amount of water vapor. The process of compressing air to around 100 psi raises the air’s temperature, but also dew point and water-holding capacity. Any subsequent cooling downstream will likely cause some water vapor to condense. Liquid water in pneumatic systems creates problems such as corrosion and bacteria growth, which, in turn, leads to sluggish controls or component breakdowns. Thus, air dryers are often needed to mitigate water issues.
In a typical system, air flows from the compressor to an aftercooler that lowers the temperature, causes condensation and removes the majority of water. Sometimes coalescing filters are installed to remove additional water. But if a circuit requires further treatment, air dryers are a must. They remove most or all of the water in compressed air before it reaches critical components or processes. Here’s a look at the major types.
Refrigerated air dryers remove water by cooling the compressed-air temperature and causing condensation. An internal moisture separator collects the liquid water and sends it to a drain. Refrigerated dryers typically generate air with pressure dew points between 35 and 40° F. They tend to be used in general plant operations. They may not be suitable for more-critical processes that demand extremely dry air, and they aren’t designed for circuits that see sub-freezing temperatures.
Refrigerated dryers are considered fairly economical to purchase and operate. They fall into two categories, cycling and non-cycling. As the names imply, one type runs intermittently and the other runs continuously. Users should consider cycling dryers, which only power up to meet demand—and thus reduce electricity consumption and energy costs.
Desiccant air dryers work on a different principle—they adsorb moisture from the air stream and onto a desiccant material in a reversible process. They produce low dew points, so they are a good choice in subfreezing conditions or when processes require extremely dry air. Two types are heatless and heated.
Heatless desiccant dryers house a desiccant material in two adjacent tanks, called the drying tower and regenerating tower. Moisture-laden compressed air flows into the drying tower, where it passes over and binds to the porous desiccant. Extremely dry air, with pressure dew points of –40 to –100° F, exits the dryer.
This adsorption process also generates heat, typically raising the air temperature by as much as 20° F. To remove water from the desiccant, about 15% of the dry air from the first tower is ported to the second, regenerating tower. There, the dry compressed air expands to atmospheric pressure as it travels through the desiccant and pulls water off the desiccant, aided by the higher air temperature due to the heat of adsorption. Moist air is then discharged from the system. The dryers cycle between drying and regenerating operations at regular intervals, so one tower always dries the incoming air.
Heated desiccant dryers, as the name implies, have a heater in the circuit. Much like the heatless version, desiccant in one tower removes moisture from the flowing air. Resulting pressure dew points again can range from –40 to –100° F. A second tower regenerates the spent desiccant. Valves divert approximately 8% of the air exiting the drying tower and pass it through a heater. This hot, dry air then passes over the desiccant in the regenerating tower, freeing the previously captured moisture. Moist air is then discharged, usually through a muffler, to the outside.
When weighing the virtues of heatless versus heated desiccant air dryers, keep in mind economics. The cost of generating compressed air can be sizeable, and heated dryers use about 50% less compressed air for regeneration. On the other hand, heaters can require a lot of electricity. Thus, users should review the specifics of an application to determine which type costs less to operate.
A related type of desiccant dryer is the heat-of-compression dryer. These are special versions of desiccant dryers that reuse heat generated by oil-free air compressors and are highly energy efficient. In HOC dryers, hot (often above 300° F) high-pressure air from a compressor first travels to the regeneration tower and frees moisture from the desiccant. Flow then exits and passes through an aftercooler, reducing air temperature to around 100°F and removing some water. Cool air then runs through the drying tower and exits with dew points of approximately –40° F.
HOC dryers coupled with oil-free compressors generate high-quality compressed air free of water and oil, and are suitable for food, beverage and pharmaceutical-manufacturing applications. HOC dryers cost more up front, compared to refrigerated and conventional desiccant dryers. However, because they’re powered by waste heat from the compressor, operational costs of HOC dryers are minimal.
Membrane air dryers rely on the selective permeability of specially engineered membrane materials, and on pressure differences inside the dryer. Small water molecules in the air can pass through microscopic pores in the membrane; larger nitrogen and oxygen molecules cannot.
A dryer consists of a cylindrical vessel filled with a bundle of hollow, membrane tubes. In action, untreated compressed air enters the dryer and flows through the tubes. Also, the volume outside the tubes but inside the vessel itself is at atmospheric pressure, creating a pressure differential across the tube walls. Inlet air passes through the tubes and the pressure differential lets only water molecules pass through the membrane, and dry air exits the unit.
A percentage of dry “purge” air (usually 10 to 20%) recirculates outside the tubes and carries the water vapor away. Depending on the design, pressure dew points can range from around 40° to as low as –40° F.
Membrane air dryers are compact, lightweight, require no power and have no moving parts, so they cost little to operate and do not require routine maintenance. These units are often recommended for point-of-use applications, near electrical or explosive hazards, and in remote locations.
With any type of dryer, experts recommend mounting standard and coalescing filters upstream. That keeps particulates, oil and liquid water from entering a dryer, maintains high efficiency, and helps ensure long life.
Another recommendation is don’t over-specify a dryer. Operating every part of a compressed air system at the lowest possible dew point is rarely necessary, is expensive and almost always wasteful. On the other hand, don’t skimp on drying and specify a dew point that‘s too high. Damage from water in a system is equally expensive. Experts say users should only supply the degree of dryness needed for each application, especially when it varies by process or machine. And, like any system, consider up-front and operating costs, flow rates and performance capabilities when specifying a dryer.