Support
This pressure drop in an average installation is
Cut In = BT +1˚C
= 236.5 kPa
5 CUT- IN
about 21kPa.
Cut In = 4˚C
On R132a pressure chart
To calculate the suction pressure in the
COP = Cut out Pressure
4˚C = 236,5 kPa
CIP
=
Cut
in
Pressure
evaporator from a reading taken at the
SP = Suction Pressure (97.9 kPa Cut out)
PD = Pressure Drop (21 kPa)
condensing unit, we must consider the pressure
EP = Evaporator Pressure (118.9 kPa)
ASP = Average Suction Pressure (132.9 kPa)
drop in the coil and suction line.
MSP = Minimum Suction Pressure (14 kPa)
The cut-in point of the pressure switch is
1
determined by the installation temperature when
AVERAGE SUCTION PRESSURE
= 132,9 kPa Average Refr. Temp. -
automatic defrosting is required. Water freezes
(Compressor runnning, ½ way into
Box temp = TD
cycle)
and ice melts at approximately 0°C, but in practice
a slightly higher temperature is necessary to
ensure fairly rapid defrosting of the evaporator. If
SP = EP - PD
= 21 kPa
4 PRESSURE DROP IN COIL
= 118.9 kPa - 21 kPa
the required installation temperature is 3°C, the
= 97.9 kPa Cut out point
evaporator can safely be allowed to reach about
3 MINIMUM SUCTION
4°C during the defrost cycle.
=14 kPa
EP = ASP - MSP
PRESSURE
= 132.9 kPa - 14 kPa
Therefore to determine the “range” (cut in
= 118.9 kPa at evaporator
2 CUT OUT POINT
= 97,9 kPa
setting) for this type of system one simply adds
1°C to the required installation temperature.
System operating pressures.
Converting this temperature to pressure will
give you the correct setting for the pressure
is properly adjusted for the best food preserving conditions.
switch. This does not dramatically change the product stored
The method used to check the TD of any installation is to first
temperatures, which remain fairly constant.
determine the fixture or box temperature (BT). This is very easily
done by using a pocket thermometer to check the temperature
CALCULATION OF LOW-PRESSURE SWITCH
of some product that has been in the fixture for at least 10
SETTINGS IN PRACTICE
hours. In this way a peak temperature reading (either high or
Assuming that we have a cold room fitted with a forced air
low) will be avoided.
evaporator connected to a condensing unit, the desired cold
The next step is to determine the average refrigerant
room temperature (BT) is 3°C, and automatic defrosting is
temperature
(ET). For example, consider a kitchen refrigerator
required. The plant has been designed to give a temperature
using R134a on which the switch is set to cut off at 90kPa and to
difference (TD) of 9K. This gives an ET of -6°C, converted to
cut in at 234kPa to hold a cabinet temperature of 3°C.
pressure equals 132.9kPa.
As discussed before, a general
method
which may be followed in
= ET - minimum suction pressure rule - pressure drop in coil
determining the average refrigerant
Cut-out pressure
= 132.9kPa - 14kPa - 21kPa
temperature is that the average suction
= 97.9kPa cut-out pressure
pressure is usually about 35kPa above
= BT + 1°C (defrost cycle)
the point at which the switch cuts out.
= 3°C + 1°C
Cut-in pressure
The temperature corresponding to this
= 4°C (convert to kPa)
pressure may then be assumed to be
= 236.5 kPa cut-in pressure
the average refrigerant temperature.
Therefore, the average refrigerant
James, this gives you the answer to the first part of your
pressure for the above application is:
question.
90kPa + 35kPa = 125kPa, and the average refrigerant
It is also important to understand that the Temperature
temperature (ET) will be -7°C.
Difference (TD) of an evaporator has an impact on the humidity
Now to determine the TD on this application we merely
within the installation, which in turn affects the products stored.
subtract the -7°C from the 3°C fixture temperature and we find
It is imperative that the proper TD be employed. The TDs
the TD to be:
required for forced air circulation are closer than those for
TD
=
BT - ET
gravity air circulation, but the same general rules apply to both.
=
3°C - (-7°C)
The technician should be familiar with these TD figures, as he
=
10K
will check many installations to determine whether or not a job
www.hvacronline.co.za
RACA Journal I June 2019
55