Layout of purge air regulation for reducing air consumption
Application note
–
Energy management function at adsorption dryer PDAD
–
1.10 Page 13 of 17
2.5
Use of a pressure sensor for purge regulation
If a pressure sensor is used, it’s best to use a programmable variant. In the final analysis, this type of pressure
sensor is a pressure transmitter and a process regulator in a single component. Although a programmable
pressure sensor is a bit more expensive than a simple mechanical pressure switch, it can be much more
favourably configured. Hysteresis can typically be reduced or eliminated, which permits immediate reaction to
changes in system pressure which determine air flow.
If a pressure sensor is used, the dryer is wired to the contacts of a switching relay which are closed in the
absence of current. When the pressure sensor is activated (figure 3, page 10: e.g. at an overpressure of 6.55 bar
or 95
psi), the purge function is shut down via the relay contact until the pressure sensor’s output drops out
again at the selected pressure value (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.
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 pressure sensor is critical with
regard to constant dew point reduction.
The following drawing 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 6:
Use of a pressure sensor for purge regulation (this setup is only an example and the configuration can be changed depending
on the application)
Dryer
regulator
Pin 2
Pin 1 (reads out
+5 V DC signal)
Signal output
Return signal
* Current-free when closed
–
open to deactivate the purge function
DC supply power
Electronic
pressure switch
Relay
S
ig
n
a
l
o
ut
COM
