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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale.
He holds a Bachelor of Business Administration and an associate degree in educational administration. He
has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status:
refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg
committee member for the past eight (chairman in 2011 and 2012). Currently he is the treasurer
(Johannesburg council) as well as president (national council).
SETTING REFRIGERATION
PRESSURE SWITCHES
By Grant Laidlaw
A pressure switch serves as a reliable operating and/or safety control
device and all HVAC&R technicians should be able to set one.
James asks: Hi, Grant. We really have difficulties when
working on refrigeration equipment that does not operate
with a thermostat. The underbar systems are a typical
example. Temperature is controlled by a pressure switch.
The older technicians just give out settings but cannot
describe how they come to these settings. This is somewhat
of a thumbsuck method. If you could describe how to
determine the settings and please, if possible, describe a
method for the practical setting of a pressure switch. Maybe
this is outdated?
H
i, James. Modern systems tend towards electronics for
control, but yes, the underbar systems and many smaller
walk-in cold rooms still operate on a pressure switch for
temperature control.
A good understanding of a refrigerant’s pressure
temperature relationship is essential in understanding how this
type of system is controlled.
These systems do not have defrost heaters or timers.
As we know, the formation of frost on an evaporator surface
is very undesirable. In the case of a pump-down system, heater
elements will be installed below the evaporator. This is, of
course, controlled by a defrost timer. In the situation where we
have an installation temperature of above 0°C, the air flowing
through the evaporator will remove frost build-up during the
compressor off cycle.
This type of installation uses a pressure switch for
temperature control and operates on what is called the
automatic defrosting cycle.
The automatic defrosting cycle operates as follows:
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RACA Journal I June 2019
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During the “ON” cycle the coil is usually at a temperature
below the freezing point of water (0°C). This will cause a
frost accumulation on the evaporator.
During the “OFF” cycle the evaporator warms up enough
so that the frost melts from the coil before the compressor
starts again.
To enable the frost to melt during the “OFF” cycle it is obvious
that the installation temperature must be above the melting
point of ice. As a general rule, the minimum installation
temperature of a system operating on the automatic defrosting
cycle is 1°C.
One of the aspects that we do need to understand is that
the suction pressure does not remain constant. The average
refrigerant pressure is the pressure of the refrigerant in
the evaporator taken halfway through the running cycle as
measured on a time basis. This should not be confused with the
halfway point between the cut-on and cut-off pressure.
Referring to the temperature/pressure chart, the average
refrigerant temperature is obtained.
A general rule, which may be followed for this type of
system, is that the average suction pressure is usually about
14kPa above the cutout point, excluding the pressure drop in
the coil and suction line.
The average suction pressure is therefore 14kPa above the
minimum suction pressure or the cut out pressure. This point
is called the minimum suction pressure. The average suction
pressure is the refrigerant pressure at the evaporator but
the pressure switch that controls the system is located at the
condensing unit. Due to the effect of gas friction losses in the
coils there is a pressure drop in the evaporator and suction line.
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