By Michael Young, Pr. Eng.

Part 5: Site and technology can influence refrigerant pipe sizes.

Click here if you missed Part 1, Part 2, Part 3, Part 4

In our last publication we posed the question, “If we have two fixed speed tandem compressors in operation, do we size the refrigerant pipe sizes for both compressors in operation or do we size the refrigerant pipe sizes for one compressor in operation?”

If we size for one compressor in operation, we will always ensure that we get adequate oil return to the compressor. When the second compressor starts, the pressure loss could be excessive, and we will lose some cooling capacity. So what is the correct way forward?

Let’s look at the various circumstances on site. What is the possibility of the unit actually operating at the design load and how much vertical rise is there from evaporator to condenser? If there is a high possibility that there are not many fluctuations from the design load and the units are installed back to back, then sizing the piping for the design load with two compressors in operation may be the most ideal scenario.

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However, should you have a high amount of fluctuating loads and should you have a long pipe run with a high vertical rise, then I would recommend that you size the refrigerant piping for one compressor in operation and then determine the associated pressure loss.

Yes, you will have loss of cooling capacity in this configuration but it is better to lose say 1% to 4% cooling capacity than constantly have compressor failure on site. The loss of cooling capacity can also be offset by selecting a slightly larger size unit or cross checking how much contingency has been allowed in the initial heat and cooling load estimate.

Now remember, everything above is applicable to a DX system with tandem fixed speed compressors with no oil separator and high vertical rises.

But surely technology has improved and changed? What advances have been made since the introduction of variable speed compressors, oil separators and control logic.

Most advanced air conditioning systems today come with oil separators so oil return is no longer a problem. Some manufacturers have also added additional contingency with the integration of an oil return function.

So how does this oil return cycle function work? Well it varies from supplier to supplier but one way is where the cooling unit controller monitors the current compressor speed. Should the compressor speed be lower than a specific speed for a specific amount of time, it will initiate oil return mode at certain intervals.

So is the oil return function the solution to all our problems? What are the advantages and possible disadvantages of this function? Can we just forget about refrigerant pipe sizing if we use a system with oil return function?

I guess you will have to find out next month but feel free to email me on if you can’t wait until then.

Wishing you a successful month ahead and chat soon.

About Michael Young

Michael Young, Pr. Eng.

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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 or 073 171 2311 for any questions or HVAC training needs.

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