Professionals who care
By Michael Young , Pr . Eng .
MICHAEL YOUNG
Michael Young is a trainer , coach and mechanical engineer in the HVAC industry . He graduated from the University of the Witwatersrand in the field of Mechanical Engineering ( B . Sc Mech Eng ) in 2008 and qualified as a Professional Engineer ( Pr . Eng ) in 2013 . Michael is passionate about promoting knowledge and helping other young engineer grow within the industry through his training workshops and coaching sessions . Michael can be contacted on michael @ myhvaccoach . com or 073 171 2311 for any questions or HVAC training needs .
THE EASIEST WAY TO KILL
THE PERFORMANCE OF A DX COOLING SYSTEM
Part 4 : Understanding the construction of the system
Today we will introduce a concept that often gets overlooked when selecting the refrigerant pipe sizes for a DX system . There are many different types of DX system constructions out there and the thing that I would like to focus on today comprises units that contain fixed speed compressors with a single refrigerant circuit .
In this type of construction , you may have one fixed speed compressor as a standalone compressor or two fixed speed compressors that have been connected in tandem that form a single refrigerant circuit .
In this type of arrangement , it is important to size the refrigerant pipe sizes on the lowest cooling capacity that can be delivered by the circuit . So , when there is only one compressor , you size the refrigerant pipes on its capacity . If you have a tandem compressor arrangement , you will size the piping on the capacity of only one compressor .
Now this may sound incorrect but let ’ s think about what is happening . In a tandem type of arrangement , there will be an
instant in time where only one compressor will be operational depending on the current load of the room .
When only one compressor is operational in a tandem type of configuration , the refrigerant pipe sizes must be selected to ensure the velocity within the pipe are within acceptable limits to ensure adequate oil return .
When the load rises , the second compressor may be activated . Now this poses a problem as when both compressors are operational , the pressure loss through the refrigerant line may be too high , considering the line has been sized for the operation of one compressor .
So this is where the dilemma lies for a tandem fixed speed compressor type of arrangement with no oil separator . Do you size the refrigerant line for the operation of a single compressor , or do you size the refrigerant line for the operation of two compressors in tandem ?
Some may argue that you should size for two compressors in operation as this is the required design load . So , let ’ s see what is the best- and worst-case scenario . If we size for one compressor in operation , we will always ensure that we get adequate oil return to the compressor . When the second compressor starts , the pressure loss is excessive , and we will lose some cooling capacity .
If we size for two compressors in operation , the velocity may be low and oil will not return to compressor when part loads exist . So , what ’ s the correct way forward ? Do we lose some cooling capacity at full load operation or do we risk compressor failure by having inadequate oil return ?
Feel free to email me on michael @ myengineeringcoach . co . za and join us next month to find out the answer .
Wishing you a successful month ahead and chat soon . RACA
" It is important to size the refrigerant pipe sizes on the lowest cooling capacity that can be delivered by the circuit "
www . refrigerationandaircon . co . za RACA Journal I January 2023 47