By Michael Young (Pr.Eng)
The hard lesson to learn as an HVAC engineer.
A few years ago, I designed a DX cooling unit which did not 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.
