RACA Journal September 2023 | Page 43

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Khulani asks : Mr Grant , how do we set a fridge that only works on a low-pressure switch ? How do we set the switch and temperature ? Thanks .

Hi Khulani , yes this is a problem area for many people . Given that the low pressure ( LP ) switch in some installations controls the temperature as a thermostat would , it is critical to the commissioning of , for example , an under-bar fridge that the LP switch be set correctly . Many of these units , especially older units in the field do not have electronic temperature controllers or even thermostats . In these instances , an LP pressure switch does not only operate as a safety device but also as a control device .

Khulani , the following is a practical method to set a LP pressure switch :

METHOD Draw lay-out of switch set screws and write down . LP switch R1 q D q

R2 q to calculate the pressures needed to achieve your required temperatures . In the next issue I shall cover this topic in detail .

Thanks to everybody for the overwhelming response . I receive on average over 50 questions a month and cannot publish all of them . But keep them coming , as I may answer you directly . I look forward to hearing from you . RACA

Grant Laidlaw

Thank you for all for your interesting 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 .

“ Many of these units , especially older units in the field do not have electronic temperature controllers or even thermostats .”

1 . Set ‘ R 2 ’

( low pressure cut-in value ) 2 . Then set ‘ D ’ ( low pressure cut-out value ) 3 . Then set ‘ R 1 ’

( high pressure safely cut-out )

For example : Assume the LP cut-in pressure needed is 185 kPa and the cut-out pressure is 60 kPa .

Step : 1 Set screw ‘ R 2 ’

( range ) a . Run the installation and observe cut-in pressure , assume it is 160 kPa . ( Example ) b . Turn 1 / 2 turn clockwise = ± 180 kPa c . Turn 1 / 8 turn clockwise = ± 183 kPa d . Turn 1 / 16 turn clockwise = 185 kPa Step : 2 Set screw ‘ D ’ ( differential ) a . Run the installation and observe cut-out pressure , assume it is 90 kPa . ( Example ) b . Turn 1 / 2 turn anticlockwise = ± 68 kPa c . Turn 1 / 4 turn anticlockwise = ± 62 kPa d . Turn 1 / 16 turn anticlockwise = 60 kPa

Note : Always set the range setting first and then the differential setting .

And there you have it , an accurate , time saving and relatively simple method to set the LP switch .

Khulani , I assumed that the pressures needed were 185kPa cut-in and 65kPa cut-out . This is just an example ; you have