www . refrigerationandaircon . co . za RACA Journal I June 2024 41
Professionals who care
By Michael Young , Pr . Eng .
MICHAEL YOUNG
Michael Young is a trainer , coach and a pre-sales 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 .
HOW TO COOL THE DATA CENTRE IN THE 21ST CENTURY ( PART 6 )
Part load operation within a data centre : things to consider when using a DX system .
The design of a data centre begins with the specification of the operating air conditions as well as the required cooling load . In many instances , these operating conditions are based when the data centre is at 100 % cooling load .
While the design is often based upon 100 % load , it is seldom that a data centre will operate at this requirement once the system has been installed and started . On the surface , part load operations may not seem to be a problem as the use of inverter compressors and fans modulate in speed to meet the required load .
The above statement holds true up to a certain point but should loads be too low , the system will start to experience operational errors . So , what is meant by this statement ?
Let ’ s imagine we have a data centre that is operating on a return air condition of 35 ° C and a supply air temperature of 24 ° C . The compressor and the fan will modulate to constantly meet these setpoints .
The compressor and supply air fan will only be able to modulate down to the prescribed minimum setting as allowed by the onboard cooling units controller . Should the heat load within the data centre be lower than the minimum speed limit , the unit may begin to display certain alarms or , in some instances , shut down .
Now there are many alarms that may occur but let ’ s discuss one common alarm which is ‘ low superheat ’ and why it occurs when there are low loads . So , within this type of system , the expansion valve , compressor and supply air fan are all able to modulate .
In the case where low loads exists , the fan and compressor decrease in speed up to the minimum point as mentioned previously . In addition to this , the unit tries to maintain a specified superheat value within the evaporator coil as well .
At these low loads , airflow and refrigerant mass flow are at their minimum values . Since the load within the data centre is very low , the refrigerant within the evaporator is unable to reach the super heat setpoint as well .
In this instance , the expansion value continues to close until it reaches the minimum setpoint . As time passes , the system continues to monitor these superheat values and if they do not increase to the desired setpoint , the ‘ low superheat alarm is triggered ’.
This error often goes away on its own and does not cause the unit to shut down , but what is this alarm telling us ? If all components are operating at the minimum operating condition and should the unit continue to operate , there is a chance that the liquid refrigerant within the evaporator will not completely change phase into a vapour .
Should liquid refrigerant leave the evaporator and enter the compressor , there is a good chance the compressor heads will become damaged as liquid refrigerant is an incompressible fluid .
To prevent this from occurring , the unit is forced to switch the compressor off until the load begins to increase again . As you can imagine , constantly switching components on and off is not ideal as the energy consumption of the unit will increase .
So , what can be changed to avoid this from happening at part load operations . Find out next month …
Wishing you a successful month ahead . RACA
www . refrigerationandaircon . co . za RACA Journal I June 2024 41