Condensing Drying; High-Pressure Drying; Refrigerant Drying; Desiccant Drying - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CONDENSING DRYING

The two methods of condensing drying are high-pressure
drying and refrigerant drying.

High-Pressure Drying

High-pressure drying may be used when main air piping is
kept away from outside walls and chilling equipment. During
compression and cooling to ambient temperatures, air gives up
moisture which then collects in the bottom of the storage tank.
The higher the tank pressure, the greater the amount of moisture
that condenses. Maintaining a high pressure removes the
maximum amount of moisture. The compressor should have a
higher operating pressure than is required for air supply
purposes only. However, higher air pressure requires more
energy to run the compressor. The tank must include a manual
drain valve or an automatic trap to continually drain off
accumulated moisture. With tank pressures of 480 to 620 kPa,
a dew point of approximately 21 C at 140 kPa can be obtained.

Refrigerant Drying

Lowering air temperature reduces the ability of air to hold
water. The refrigerated dryer (Fig. 14) is the most common
means of obtaining dry, compressed air and is available in
several capacities. It provides the greatest system reliability and
requires minimal maintenance.
HOT GAS
BYPASS
CONTROL
HEAT
EXCHANGER
REFRIGERANT
LINES
AIR IN
AIR OUT
REFRIGERATION
UNIT
REFRIGERANT DRYER
Fig. 14. Typical Refrigerant Dryer Airflow Diagram.
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 2 C at the tank pressure.
The refrigeration circuit is hermetically sealed to prevent loss
of refrigerant and lubricant and to protect against dirt.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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 refrig-
eration system to maintain the temperature in the exchanger.
pressure of 550 kPa, air is dried to a dew point of –11 C at
139 kPa. Under severe winter conditions and where piping
and devices are exposed to outside temperatures, the –11 C
dew point may not be low enough.

DESICCANT DRYING

desiccant dryer is installed between the compressor and the
PRV. Dew points below –120 C 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.
refrigerant dryer in applications where the –11 C dew point
at 138 kPa mainline pressure does not prevent condensation in
air lines (e.g., a roof-top unit exposed to severe winters).
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.
material and removing the resulting water vapor from the
desiccant chamber or by flushing the desiccant chamber with
CONDENSOR
air at a lower vapor pressure for heatless regeneration. To
provide a continuous supply of dry air, a desiccant dryer has
C1888
two desiccant chambers (Fig. 15). 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.
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PNEUMATIC CONTROL FUNDAMENTALS
The heat exchanger reduces the temperature of the compressed
With a dew point of 2 C and an average compressor tank
A desiccant is a chemical that removes moisture from air. A
It may be necessary to install a desiccant dryer after the
The desiccant dryer most applicable to control systems uses
A desiccant is regenerated either by heating the desiccant