There are four main types of compressed air dryers:
- Refrigerated air dryers
- Chemical air dryers
- Desiccant air dryers
- Membrane air dryers
Let’s take a closer look at how each of these air dryers works and how to determine which is the best for your application.
REFRIGERATED AIR DRYERS
Refrigerated air dryers work by cooling the air to low temperatures and condensing the water vapour. When optimally designed, refrigerated air dryers can produce air with dew points to approximately 2°C. However, a refrigerated air dryer leaves some water vapour in the air and should not be used in water-sensitive applications.
CHEMICAL AIR DRYERS
Chemical air dryers pass the compressed air over beds of chemicals, usually calcium chloride and lithium chloride. These chemicals attract water vapour, and once saturated, they are discarded. The lowest dew point that can be achieved with a chemical air dryer is 15°C.
DESICCANT AIR DRYERS
Desiccant air dryers pass the compressed air through desiccant material that absorbs water vapour. When the desiccant is nearly saturated, the airflow is switched to the second unit of desiccant material while the first is regenerated. Either timers or dew point monitoring equipment can be used to control the regeneration phase. Desiccant air dryers can consistently deliver air at low dew points of -40°C to -73°C and are good choices when the compressed air is subject to freezing conditions.
Heated desiccant air dryers use heat to remove water vapour from the desiccant material that is not in use at that point in the cycle. Therefore, they need large amounts of steam or electricity to operate.
Heatless desiccant air dryers use the dry air generated by the dryer to remove water vapour from the desiccant material. These air dryers are ideal for delivering instrument quality air for critical applications.
MEMBRANE AIR DRIERS
Membrane air dryers use specially formulated membrane microtubes that are selectively permeable to water vapour. These microtubes are an excellent medium for producing dry air from standard compressed air. As the compressed air travels along the length of the membrane, water vapour spreads through the membrane to produce dry compressed air at the outlet. A small portion of the dry air is directed along the outside of the membrane surface to draw the moisture-laden air away from the membrane. Membrane air dryers are designed with no moving parts and no requirement for electrical supply and can reduce the dew point of the compressed air to as low as -40°C. Because membrane air dryers require minimal maintenance and are explosion-proof, they are ideal for remote areas, explosive environments, sub-freezing environments, and applications that need a consistent dew point.
SPECIFYING THE SUITABLE COMPRESSED AIR DRYER
To specify the suitable air dryer for a compressed air installation, you’ll need to keep the following in mind:
Do not over-specify. Drying the entire compressed air supply in a factory to dew points less than -40°C is wasteful. Instead, subdivide the compressed air supply by application, ensuring each point of use is provided dry air for the downstream application served.
Do not under specify. Damage from wet air can result in expensive maintenance, extended downtime, and lost product.
An air-drying system that only contains an aftercooler and a coalescing filter could create problems with condensation downstream from the aftercooler when the air still saturated with vapour are likely to condense when the ambient temperature is lower than the compressed air temperature.
Maximise the drying effect of pressure reduction.
Specify membrane air dryers for those parts of the system that require dew points of 2°C to 11°C and flow rates up to 17 NM3/min. Membrane air dryers can also be used for applications that need instrument quality air, air exposed to freezing temperatures, and water-sensitive applications that require specific flow rates.
For applications that need dew points lower than -40°C, a desiccant air dryer is ideal.
CONTACT COMPRESSOR VALVES & ACCESSORIES FOR DETAILS
For more information about our range of air dryers, contact a Compressor Valves & Accessories team representative.