Support
Suppose in checking the TD on a particular installation it is
found that it is too great. The complaint in this case would very
well be one or more of the following:
1. Long running time, resulting in high operating costs.
2. Products drying out.
3. Meat discolouration.
4. Generally unsatisfactory refrigeration.
When too small a TD is encountered, the usual result is
excessive humidity in the fixture or the system efficiency may
be low.
In practice, after setting a low-pressure switch the operation
and fixture temperature must be observed for several cycles.
Particular attention must be paid to the following points:
• The settings must not cause the condensing unit to short
cycle.
• In an installation operating on an automatic defrosting
cycle, the evaporator must be completely defrosted before
the switch will cut in.
A good understanding of a
refrigerant’s pressure/temperature
relationship is essential in
understanding how this type of
system is controlled.
Example:
Assume the low pressure (LP) cut-in pressure needed is 185kPa
and the cut out pressure is 60kPa.
Step: 1
Set screw “R2”
Take cut-in pressure, assume it is 160kPa.
Turn 1/2 turn clockwise = ±180kPa.
Turn 1/8 turn clockwise = ±183kPa.
Turn 1/16 turn clockwise = 185kPa
Step: 2
Set screw “D”
Take cut out pressure, assume it is 90kPa.
Turn 1/2 turn anti-clockwise = ±68kPa.
Turn 1/4 turn anti-clockwise = ±62kPa.
Turn 1/16 turn anti-clockwise = 60kPa
Step: 3
Set screw “R1” (H.P.)
James, I hope that this helps you with pressure switch setting.
A pressure switch serves as a reliable operating and or safety
control device and all HVAC&R technicians should be able to
set one. As to your comment about pressure switches being
outdated, no, I think they are going to be around for a while!
Here is a new condensing
unit operating on R290
(small flammable label on
compressor) and that uses
pressure switches. Not
outdated yet!
PRACTICAL SETTING OF A PRESSURE SWITCH
One can set a pressure switch using nitrogen or set on the
system. When setting on the system manually pump down the
system to see at what pressure the compressor cuts in and
out at.
It is important to note that on the low side the range must
be set first.
You must use a manifold gauge set.
1. Set “R2” (Low pressure cut-in value)
2. Then set “D” (Low pressure cut-out value)
3. Then set “R1” (High pressure safely cut-out)
Thank you for all your questions. Send your problems
(and sometimes your creative solutions) to acra@
netactive.co.za with “Solutions Page” in the subject line.
You may include pictures.
References:
ACRA RACA
QUOTE OF THE MONTH
“It doesn’t matter how slowly you go, as long as you don’t stop.”
Confucius..
www.hvacronline.co.za
RACA Journal I June 2019
57