By Michael Young (Pr. Eng.)
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.