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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 over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
WELCOME TO THE SOLUTIONS PAGE
By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry . I will endeavour to enlighten . We are going back to basics as I have questions coming in that indicate that the basic understanding and skills necessary to work in our industry are simply not in place .
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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 if there is no thermostat ? Thank you .
Hi Khulani , this situation is found in older walk-in fridges and underbar fridges , although I have seen some new underbar fridges working on this principle . These systems are out in the field and do not have electronic temperature controllers . In these instances , a low-pressure ( LP ) pressure switch does not only operate as a safety device , but also as a control device .
This means that in these systems , the LP switch in reality , controls the temperature of the installation and is , as a consequence , critical to the commissioning of the unit .
Khulani , in the last issue I described the procedure for setting the LP switch in practice . However , the question remains , how does one determine the settings needed to obtain the desired installation temperature ?
Given that what we want to achieve is to have the system run and cycle correctly to achieve a specific temperature , we need to determine the cut-in and cut-out pressure settings for the setting of the pressure switch .
The following represents a tried-and-tested method of calculation and the setting of the LP switch in practice . When compared to what I call the ‘ thumb suck method ’, the time saving is considerable . With all guesswork removed , the accuracy and improved quality of workmanship is well worth the effort . In addition , the understanding of the system that this method affords the technician is invaluable .
This type of system is used for refrigeration as opposed to freezing and is typically used for the storage of cold meats , cheeses , beverages or anything that does not require freezing . This type of system does not pump down , nor have defrost elements and associated controls , but runs on what is called the automatic defrost cycle .
RACA Journal I October 2023
I have used R134A as an example but using the correct pressure temperature chart enables this method to be used for any refrigerant .
On a system having an installation temperature of 5 ° C , the automatic defrost cycle operates as follows :
• 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 ( the evaporator fan remains running with the air temperature blowing through the coil at 5 ° C ) 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 ( in my example 5 ° C ). As a general rule , the minimum installation temperature of a system operating on the automatic defrosting cycle is 1 ° C .
Installation temperature control by means of a LP switch : The ‘ pressure-temperature ’ relationship of a refrigerant and the following formulae are applied in determining the LP switch settings in order to control the average temperature of an installation . Formulae : Box temperature / installation temperature ( BT ) Evaporator temperature ( ET ) Temperature difference ( TD ) always given in ‘ K ’.
BT TD ET
= ET + TD = BT - ET = BT - TD
Assume that we have a cold room controlled by a pressure switch and fitted with a forced draft evaporator connected to a condensing
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