Professionals who care
WHY COMMISSIONING IS AN
IMPORTANT PART OF AN HVAC PROJECT
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 hard lesson to learn as an HVAC engineer .
few years ago , I designed a DX cooling unit which did not
A perform as expected . When this happened , the first thing that came to mind was that I had made a mistake and there were going to be serious financial penalties .
I got into a panic and started to doubt my own abilities . I took one deep breath and decided to backtrack my design and cross check all my calculations .
After one week of investigation , I confirmed that all my calculations were correct but what puzzled me was the reason behind the under-performance of this HVAC system .
I then decided to go to site and watched how the performance of the system was being measured and for some strange reason , the supply air temperature was higher than initially expected .
I then asked my colleague to tell me the return air temperature and what made me fall off my chair was that the return air temperature was as per my design .
So why on earth was this unit not giving me the required performance ? At that moment , I thought about control logic
and setpoints . So , I took the initial design selection sheet and started to cross-check if the cooling unit was set up as per the design setpoints .
The first deviation from the design was that the unit was set to control the compressor on return air temperature . The second deviation was that the superheat temperatures setpoints were lower than the design conditions .
Now initially this didn ’ t seem like a major problem but after further research , I discovered that both these deviations could greatly impact the performance of my design . Since the return air temperature was at the required setpoint , the control logic would tell the compressor to stay at its current state . The lower superheating setpoint meant that the expansion valve would open and request more refrigerant mass flow into the evaporator .
This in turn would cause the controller to instruct the compressor to speed up to meet the required refrigerant mass flow . Now in this case , the control logic must choose one instruction as a priority . Does it control the compressor according to the return air temperature setpoint or the superheating setpoint ?
For this particular unit the return air temperature takes preference so the compressor modulates at its current state even though the system calls for more refrigerant mass flow .
As time progressed , I watched the system try to constantly rebalance itself and eventually an alarm was heard which made everyone think that this cooling unit was rubbish and did not work .
Because the cooling unit was not meeting the required supply air temperature and alarms kept going off I learnt a very important lesson that day . This was that programming the unit to operate at the design setpoints is vital for correct operations of a cooling unit and success of a project .
My greatest takeaway from this lesson was the importance of communication between the design engineer and the field service technicians who commission the system .
I was the person who did not give the field service technician the setpoints and I felt responsible for the incorrect operations of the cooling unit .
We all learn from mistakes and grow . Moving forward , I now always schedule a project handover meeting when we move into the commissioning phase of a project .
So , is the commissioning of a system that important ? Well in my next article , I am going to show how setting the incorrect inverter speed limit can cause a compressor to fail .
Wishing you a successful month ahead and chat soon . RACA
www . hvacronline . co . za RACA Journal I August 2021 45