Professionals who care
WHY COMMISSIONING IS SUCH 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 .
In this final article of the series , I look at inverter speed limits and their relation to ensuring the correct pipe work installation .
You ’ ve heard the common expression , “ you never stop learning ” – even after 12 years of working within the industry , there is always something new to learn every day . Last year , I started a new job role and as any new employee , the first thing I underwent was training and induction . During my training , I was introduced to a computer software program that used inverter scroll compressors within their DX cooling units .
While playing with this computer program , I noticed that I was able to adjust the inverted speed of the compressor .
This variable within the program puzzled me , I mean , why on earth would you be able to adjust the maximum and minimum speed of the inverter compressor ?
After having an in-depth discussion with some of the other engineers , I learnt that setting the minimum inverter speed is important for the selection of the refrigerant pipe selections .
So how does one select the correct refrigerant pipe size for a DX system ? Well , we measure the equivalent length , determine number of bends , and then size piping accordingly at maximum load .
But let me ask , what happens when the maximum load does not exist ? How does the system adjust to the change in load ?
When the load drops off , the expansion valve closes , and the compressor reduces in speed . When the compressor speed reduces , the velocity of refrigerant within the pipe also reduces which causes oil return problems .
So how does one solve this problemz ? Constantly changing the refrigerant pipe sizes when the compressor reduces in speed is not a solution , so limiting the minimum compressor speed is the next step .
So how does this all work ? Well , we provisionally size the refrigerant piping for maximum load , but we also consider the minimum compressor speed where we can still accomplish acceptable refrigerant velocities within the selected pipe .
By doing this , we ensure that refrigerant velocities are maintained to acceptable limits and prevent the failure of the compressor .
So , it ’ s important to understand that the minimum speed of an inverter compressor is dependent on the equivalent refrigerant pipe length and expected heat load on the very first day of operations .
Before ending off I would like to mention that the above is not applicable to system that contain oil separators , but it is still good practice to confirm and check that the correct refrigerant piping has been selected and installed .
Wishing you a great month ahead .
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www . hvacronline . co . za RACA Journal I October 2021 41