www . refrigerationandaircon . co . za RACA Journal I July 2023 45
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 engineers grow within the industry through his training workshops and coaching sessions .
THE EASIEST WAY TO KILL
THE PERFORMANCE OF A DX COOLING SYSTEM
Part 6 : Oil return function in a cooling unit .
Let ’ s talk about control logic when it comes to oil return management . As mentioned in our last publication , units with oil management functionality increase the compressor speed when the compressor operates below a set speed for a certain amount of time .
When this function is engaged , the compressor increases in speed and the expansion valve will now begin to open . Now in certain instances , a low-pressure alarm may be triggered and this is where the problem with the oil return function may begin to occur .
So why is an alarm triggered when the compressor increases in speed ? Surely it is normal for a compressor to increase in speed ?
Well , the problem comes when there is low load within the space . So , in normal operations , low load will cause the
compressor speed to modulate to match the load within the space .
However , when the compressor speed increases and when there is low load , the refrigerant within the evaporator may not fully boil off and change phase into a gas . This results in a decrease in evaporating temperature and can trigger an alarm to prevent liquid floodback to the compressor .
If that was not enough of a problem , when this function is activated , the controller also looks at the compressor envelope . So when oil management is engaged , the controller must ensure that the compressor also maintains acceptable compressor operating condensing and evaporating conditions .
Now a cooling unit controller can only work on one task at a time and it can only control all the internal according to the task at hand .
So when oil management function is engaged , it initially increases compressor speed to the desired setpoints . However , while the unit operates at these setpoints , the controller also now monitors the low pressure as well as the compressor envelope limits .
Should the compressor envelope operation be on the limit , the control logic changes and the compressor , expansion valve and condenser fan speed now change to ensure compressor envelope operations are within acceptable limits .
Once the operating compressor envelope is within acceptable limits , the control logic once again changes to the monitoring of low pressure and operations oil return operation parameters .
So as you can imagine , there is a lot going on in the background of the control logic of a unit and when an alarm occurs , there are a lot of tests and series of events that need to occur for a specific alarm to be triggered .
That brings us to the end of this series . If you would like to learn more about refrigerant pipe design , feel free to email me on michael @ myengineeringcoach . co . za . RACA
" In normal operations , low load will cause the compressor speed to modulate to match the load within the space . "
www . refrigerationandaircon . co . za RACA Journal I July 2023 45