Layout of purge air regulation for reducing air consumption
Application note
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Energy management function at adsorption dryer PDAD
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1.10 Page 11 of 17
2.4
Use of a simple pressure switch for purge regulation
Many compressors are furnished with pressure switches which include additional contacts, by means of which the
dryer’s purge function can be shut down when the compressor is in the discharged state. Unfortunately, many of
these switches have considerable dead zones or a large hysteresis (difference or reset point between the charge
(on) and discharge (off) settings, which in most cases cannot be changed). The amount of air which can flow
through the dryer while the compressor is being discharged must be taken into consideration, because it may be
considerable (in particular if the hysteresis is large or the receiver is very moist.
A separate pressure switch is often a better alternative, because the user can adjust it independently of the
compressor’s pres
sure switch. The following applies when selecting a separate pressure switch: the smaller the
hysteresis the better. If a separate pressure switch is used, the dryer is wired to the contacts at the pressure switch
which are closed in the absence of current. When the switch is activated (figure 3: e.g. at an overpressure of
6.55 bar or 95 psi), the purge function is shut down until the pressure switch is once again released (in this
example at an overpressure of 6.2 bar or 90 psi). The point at which the switch is released can be adjusted in
accordance with the system.
If it’s determined that the dew point drops to an unacceptable level, the point at which the switch is released must
be increased. Please note that many pressure switches have a fixed hysteresis value, in which case the activation
and deactivation points change in correspondence to each other.
It’s very important to understand that the purge function can
only
be shut down at times when little or no air is
required. If this function is shut down at any other time, the drying agent cartridges may be permanently damaged
thus resulting in the loss of drying performance, and possibly a mechanical defect as well. The cartridges may
never be entirely saturated!
Fundamentally, the solution with a simple pressure switch is critical with regard to
constant dew point reduction.
Figure 4 on page 11 shows how the above mentioned components might be connected for regulation of a dryer’s
purge function. Please note that specific details regarding the components are manufacturer-independent
–
the
manufacturer’s specifications must be complied with in any case.
Figure 3:
Adjustment of the simple pressure switch or pressure sensor
Purge function off (switch activated)
Purge function off (switch activated)
Purge function on (switch deactivated)
Purge function on
Adjust depending on system
(pressure switch deactivation point)
S
ys
tem
p
res
su
re (
p
si
g
)
