www . refrigerationandaircon . co . za RACA Journal I August 2024 35
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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 . We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place .
Andre asks : Hi Grant , I ran into confusing information with regard to expansion valve feeler bulb location . I understand that the 12 o ’ clock position is not always best . We have just lost a compressor due to the expansion valve feeler bulb incorrectly installed . How does one select a valve for a system ? Could you help on this as well as expansion valves hunting , Thanks
Hi Andre , what we are dealing with here forms part of refrigerant controls - thermostatic expansion valves are still common in industry with electronic versions freely available for some time now . Assuming that we all understand what is , the effect of , the importance of , measuring and setting of superheat we can look at the thermostatic expansion valve .
Let us begin with how to select the correct thermostatic expansion valve . In most cases your supplier will be of assistance but in the field , you may have a situation where labels are missing , and information is not readily available .
You will need the following information :
• Type of refrigerant
• Maximum evaporator load
• Evaporating temperature
• Maximum and minimum condensing pressure
• Maximum and minimum liquid temperature
• If pressure limiting is required
• If an external equaliser is required
PROCEDURE :
1 . Determine pressure difference across the valve as follows . Subtract evaporating pressure from highest condenser pressure Subtract evaporating pressure from lowest condenser pressure
Use the average of the two to determine pressure difference between evaporator and condenser
From this value deduct the other possible pressure differences , for example :
• Liquid line and accessories such as solenoid valve , filter drier , shut-off valves , heat exchanger etc . ( use 100 kPa if exact loss is not known )
• Heat exchanger friction losses , if applicable
• Pressure loss ( gain ) due to vertical lift ( drop ):
• Deduct 12 kPa per meter that the TEV is above the condenser
• add 12 kPa per meter that the TEV is below the condenser
• Pressure drop across the distributor :
• Deduct 100 kPa if a distributor is used ( in the case of an externally equalised TEV )
2 . Determine the liquid temperature leaving the condenser ( condensing temperature minus sub cooling ). Reduce this temperature further if a long liquid line is used to determine the liquid temperature entering the valve .
3 . Determine the correction factor for the temperature of the liquid entering the valve .
4 . Calculate the corrected valve capacity required by dividing the maximum evaporator load by the liquid correction factors . 5 . Select the valve from the appropriate capacity table for the evaporator temperature and the pressure drop available ( corrected capacity vs . pressure drop at the correct evaporating temperature ).
6 . The valve capacity should equal or slightly exceed ( max . 5 %) the design rating of the system . A valve with a capacity rating up to 10 % below the full load condition may be selected if the system is to operate at reduced loads for long periods of time , and if a slightly higher than normal superheat can be tolerated at full load conditions .
www . refrigerationandaircon . co . za RACA Journal I August 2024 35