The refrigerant dryer uses a non cycling operation with a hot gas bypass control on the refrigerant flow to provide a constant dew point of approximately 35F at the tank pressure. The refrigeration circuit is hermetically sealed to prevent loss of refrigerant and lubricant and to protect against dirt.
The heat exchanger reduces the temperature of the compressed air passing through it. A separator/filter condenses both water and oil from the air and ejects the condensate through a drain. A temperature-sensing element controls the operation of the refrigeration system to maintain the temperature in the exchanger.
With a dew point of 35F and an average compressor tank pressure of 80 psi, air is dried to a dew point of 12F at 20 psi. Under severe winter conditions and where piping and devices are exposed to outside temperatures, the 12F dew point may not be low enough.
Desiccant Drying
A desiccant is a chemical that removes moisture from air. A desiccant dryer is installed between the compressor and the PRV. Dew points below -100F are possible with a desiccant dryer. The desiccant requires about one-third of the process air to regenerate itself, or it may be heated. To regenerate, desiccant dryers may require a larger compressor to produce the needed airflow to supply the control system and the dryer.
It may be necessary to install a desiccant dryer after the refrigerant dryer in applications where the 12F dew point at 20 psi mainline pressure does not prevent condensation in air lines (e.g., a roof-top unit exposed to severe winters).
The desiccant dryer most applicable to control systems uses the adsorbent principle of operation in which porous materials attract water vapor. The water vapor is condensed and held as a liquid in the pores of the material. The drying action continues until the desiccant is saturated. The desiccant is regenerated by removing the moisture from the pores of the desiccant material. The most common adsorbent desiccant material is silica gel, which adsorbs over 40 percent of its own weight in water and is totally inert. Another type of adsorbent desiccant is the molecular sieve.
A desiccant is regenerated either by heating the desiccant material and removing the resulting water vapor from the desiccant chamber or by flushing the desiccant chamber with air at a lower vapor pressure for heatless regeneration. To provide a continuous supply of dry air, a desiccant dryer has two desiccant chambers (Fig. 14). While one chamber is being regenerated, the other supplies dry air to the system. The cycling is accomplished by two solenoid valves and an electric timer. During one cycle, air passes from the compressor into the left desiccant chamber (A). The air is dried, passes through the check valve (B), and flows out to the PRV in the control system.