www . refrigerationandaircon . co . za RACA Journal I October 2023 35
Support unit . The desired cold room temperature ( BT ) is 3 ° C , and automatic defrosting is required . The plant has been designed to give a temperature difference ( TD ) of 9K , that is the difference in temperature between the air in the cold room and the refrigerant boiling off in the evaporator ( ET ) is 9K . The system runs on R134a .
Therefore : ( ET ) = ( BT ) room temperature ( 3 ° C ) - TD ( 9K ) = - 6 ° C Refrigerant temperature in evaporator ( ET ).
Referring to the R134a pressure temperature chart the pressure equivalent to a temperature of – 6 ° C is 135kPa .
AVERAGE SUCTION PRESSURE The suction pressure , however , 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-in and cut-out pressure . Referring to the temperature / pressure chart , the average refrigerant temperature is obtained . By subtracting the average refrigerant temperature from the installation temperature , the operating TD of the installation is obtained . The point at which the pressure switch must cut out , is below that of the average suction pressure .
A general rule which may be followed is that the average suction pressure is usually about 14 kPa above the cutout point , excluding the pressure drop in the coil and suction line . This point is called the minimum suction pressure which in this case is then 135 kPa - 14 kPa = 121 kPa .
This is the refrigerant pressure at the cooling unit whereas the pressure switch 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 . This pressure drop in an average installation is about 21 kPa , made up of approximately 14 kPa through the suction line and an additional 7 kPa through to the mid-point of the evaporator . To calculate the suction pressure at the condensing unit , we must subtract the pressure drop in the coil and suction line ( 21kPa ).
The minimum suction pressure at the condensing unit for a 3 ° C installation temperature = 121 kPa - 21 kPa = 100 kPa . This figure is then the cut-out point of the pressure ( 100kPa )
The cut-in point of the pressure switch is determined by the room temperature when automatic defrosting is required . Water freezes and ice melts at approximately 0 ° C , but in practice a slightly higher temperature is necessary to ensure fairly rapid defrosting of the evaporator . As the required room temperature is 3 ° C , the evaporator can safely be allowed to reach about 4 ° C during the defrost cycle . Again , referring to the pressure temperature chart , we find that the equivalent pressure of R134A for 4 ° C = 229 kPa . If the cut-in point of the pressure switch is set at approximately 229 kPa , automatic defrosting will result .
TO SUMMARISE Assuming that we have a cold room fitted with a forced air evaporator connected to a condensing unit , the desired cold room temperature ( BT ) is 3 ° C and automatic defrosting is required . The plant has been designed to give a temperature difference ( TD ) of 9K . This gives an ET of – 6 ° C converted to pressure equals 135 kPa .
Cut-out pressure = ET - General rule - Suction loss = 135 kPa - 14 kPa - 21 kPa = 100 kPa cut-out pressure
Cut-in pressure = BT + 1 ° C ( defrost cycle ) = 3 ° C + 1 ° C = 4 ° C ( converted to kPa ) = 229 kPa cut-in pressure
Setting the pressure switch to these values will achieve the desired installation of a correctly designed system .
If the evaporator TD is unknown , you can determine the value as follows :
The first step in checking the TD of any installation is to determine the fixture or box temperature ( BT ). This is very easily done by using a thermometer to check the temperature of some article which has been in the fixture for at least 10 hours . In this way a peak temperature reading ( either high or low ) will be avoided .
The next step is to determine the average refrigerant temperature ( ET ). For example , consider a kitchen reachin refrigerator using R134A on which the switch is set to cut-out at 90 kPa and to cut in at 234 kPa to hold a cabinet temperature of 3 ° C .
As discussed before , a general method which may be followed in determining the average refrigerant temperature is that the average suction pressure is usually about 35 kPa , ( 14 kPa general rule + 21 kPa suction loss ), above the point at which the switch cuts out . The temperature corresponding to this pressure may then be assumed to be the average refrigerant temperature . Therefore , the average refrigerant pressure for the above application is :
90 kPa + 35 kPa = 125 kPa . Therefore , the average refrigerant temperature ( ET ) will be 7 ° C .
Now to determine the TD on this application we merely subtract the 7 ° C from the 3 ° C fixture temperature and we find the TD to be : TD = BT - ET = 3 ° C - ( 7 ° C ) = 10K
Another method typically used on a new installation where no product is present or when for some reason the average box temperature cannot be determined is : After allowing
www . refrigerationandaircon . co . za RACA Journal I October 2023 35