What is the difference between refrigerant-type compressed air dryers and regenerative desiccant-type compressed air dryers? What are the advantages and disadvantages of one over the other?
Karamvir Singh for Zondits, January 4, 2016
Answer: Let’s start by answering why you even need to dry the compressed air. The atmospheric air contains water vapor that begins to condense into liquid (water) in the compressed air system when the air cools past the saturation point. Note that the temperature at which the condensation begins is called the dew point. Whatever type of compressed air dryer you ultimately select, the objective is always the same: to prevent moisture from condensing in your compressed air lines where it can foul your air-operated equipment and processes.
The refrigerant-type dryer is the most popular, as the dew point obtained is acceptable in many general industrial plant air applications. The compressed air is cooled in an air-to-refrigerant heat exchanger to about 35°F, at which point the condensed moisture is separated and drained off. The air is then reheated in an air-to-air heat exchanger by means of the incoming air, which also is pre-cooled before entering the air-to-refrigerant heat exchanger. This means that the compressed air leaving the dryer has a pressure dew point of 35°F to 40°F. A lower dew point is not feasible in this type of dryer as the condensate would freeze at 32°F or lower. In applications requiring a much lower dew point, regenerative compressed air dryers are typically used. A typical regenerative desiccant dryer has a pressure dew point rating of -40°F, but dew points down to -100°F can be obtained. The regenerative dryers use a desiccant, which adsorbs the water vapor in the air stream. The moisture adheres to the desiccant, collecting in the thousands of small pores within each desiccant bead. The composition of the desiccant is not changed, and the moisture can be driven off in a regeneration process by applying dry purge air by the application of heat or a combination of both. Regenerative desiccant dryers are often specified to protect instrumentation and control systems, laboratory equipment, or moisture-sensitive process materials.
The advantages of utilizing refrigerant type air dryers include a low capital cost and a relatively low operating and maintenance cost. Additionally, the refrigerant type dryers are not damaged by oil in the air stream. The major disadvantage of refrigerant type dryers is the limited dew point capability. On the other hand, the primary advantage of using regenerative desiccant-type dryers is that a very low dew point can be achieved without potential freeze-up. The cost of operation is moderate considering the dew points achieved. The biggest drawback of regenerative desiccant type dryers is the high capital cost as well as the high maintenance cost. A periodic replacement of the desiccant bed (typically 3 to 5 years) is required. Note that in regenerative desiccant dryers the oil aerosols can coat the desiccant material, rendering it useless if adequate pre-filtering is not maintained.
The choice of the compressed air dryer system ultimately depends on the facility requirements and the end use.
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