33
Heatless Desiccant Dryers
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Instruction Manual
Section 7:
Energy Management
Using a Standard Pressure Switch for Purge Control
Many compressors come equipped with pressure switches
with extra contacts that can be utilized to switch off the
dryer’s purge when the compressor is in an unloaded state.
Unfortunately, many of these switches have significantly large
dead bands (difference between the load (on) and unload (off)
settings that in most cases cannot be altered). The amount of
air that can pass downstream, through the dryer, while the
compressor is unloaded must be taken into consideration as
it can be significant, especially if there is a large dead band
and/or large wet receiver.
Often, a stand-alone pressure switch is a better option
as it allows the user to adjust it independently from the
compressor’s pressure switch. When selecting a stand-alone
pressure switch, the smaller the dead band the better.
When using a stand-alone pressure switch, the dryer is wired
into the normally closed contacts on the pressure switch.
When the switch is actuated (at 100 psig in the example), the
purge shuts off until the pressure switch deactuates (at 95
psig in example). The point at which the switch deactuates,
may be adjusted to suit the system.
If it is determined that the dew point drops to an
unacceptable level, the deactuation point must be increased.
Please note that with most standard pressure switches, the
dead band is fixed such that the activation and deactuation
points move together.
It is extremely important to understand that the purge may
only be shut off during periods of low or no demand. This
feature, if used otherwise, can result in permanent damage
to the desiccant beds resulting in loss of drying capacity
and possibly mechanical failure. The cartridges must not be
allowed to become fully saturated at any time.
The following schematic offers an overview of the devices
mentioned as they might be connected for use in controlling
a dryer’s purge function. Please note that specific details
differ between device manufacturers, so in all cases the
manufacturer’s specifications should be adhered to.
Figure 7.2: Using a standard pressure switch for Purge Control.
(Puts out+5 VDC Signal)
PRESSURE SWITCH
(Field Installed)
N.C.
N.O.
DRYER
CONTROLLER
PIN 1
Signal Out
Signal Back
PIN 2
*
*
N
ormally
C
losed, open to turn purge off
