Tube-in-tube refrigeration dryers, Figure 3, operate by cooling a mass of aluminum granules or bronze ribbon that in turn cools the compressed air. As the tube-to-tube refrigeration dryer cycles, a thermometer in the granule mass senses its temperature. As the temperature rises, a switch turns on the refrigeration unit. When the temperature drops to a cut-off point, refrigeration stops. These dryers are designed to produce dew points of 35° or 50° F.

Water-chiller refrigeration dryers, Figure 4, use a mass of water for cooling. An extra heat exchanger is needed to maintain chilled water flow through the condenser, as well as a water pump. Dew points can be between 40° and 50° F. Water-chiller dryers cycle as they operate.

Direct-expansion refrigeration dryers, Figure 5, use a refrigerant-to-air cooling process to produce pressure dew points of 35° F below standard operating conditions. (100° F temperature at compressor inlet, 100 psig, 100° F ambient - from the NFPA standard). No recovery period is necessary, so direct-expansion refrigeration dryers run continuously. The cost difference between cycling and continuous operation is difficult to calculate. The difference in electrical power consumption between cycling and non-cycling refrigerated dryers is outweighed by the value of continuous operation of the plant's pneumatic equipment.

Membrane-type dryers are gas-separation devices. They consist of miniature membrane tubes made of plastic materials compounded to allow water vapor to pass through when there is a vapor pressure differential. They work as your lungs do, venting water vapor each time you exhale.

Typically this membrane material is formed into bundles of thousands of individual fibers from one end of the dryer to the other. Water vapor escapes through the walls of the fiber to a sweep chamber from where it is continually vented to atmosphere as a gas. A fraction of the dried air is routed through the sweep chamber to continuously purge and exhaust moisture vapor.

Industrial-grade membranes can be used for years to dry air continuously. They respond spontaneously to any change in inlet conditions. They perform at temperatures between 40° and 150° F (ambient or inlet), and handle pressures from about 60 to 300 psig. They will deliver a consistent outlet dew-point reduction anywhere between these extremes. The inlet flow rate and pressure determine the outlet dew point suppression. In other words, membrane air dryers deliver a consistent level of drying protection that follows the rise or fall of the inlet dew point temperature, and can easily be sized to follow the ISA recommended 20° F pressure dew point suppression below ambient. Outlet pressure dew points can also be selected as low as 50° F. Flow capacities are relatively low, but modules can be installed in parallel for higher flows.

Prefilters mounted immediately upstream from the membrane dryer keep out liquids and solids to allow an almost unlimited service life. Because water vapor passes right through the membrane material, it does not accumulate there, so membranes do not become saturated and never need to be regenerated. Membranes have no moving parts to wear out. They are non-electric and suitable for most hazardous locations. They require no RF shielding or protection. They use no refrigerant gas or potentially dusty desiccants.

They make no noise. And, they can be mounted in any orientation. Their low-mass components are inherently vibration-resistant. Because they are static, inert devices, they never need service or adjustment and don't require monitoring devices. Made of plastic and aluminum, they do not rust or corrode and don't need painting. They have almost no pressurized volume, so most pressure code restrictions do not apply.

Note: membrane gas separators will remove other gases too. Some membrane-type compressed air dryers can reduce outlet oxygen concentrations (or not permeate oxygen at all). Consult the manufacturer to determine if membrane can be used for breathing air.