By Michael Young, Pr. Eng.

Part 4: Understanding the construction of the system

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

Today we will introduce a concept that often gets overlooked when selecting the refrigerant pipe sizes for a DX system. There are many different types of DX system constructions out there and the thing that I would like to focus on today comprises units that contain fixed speed compressors with a single refrigerant circuit.

In this type of construction, you may have one fixed speed compressor as a standalone compressor or two fixed speed compressors that have been connected in tandem that form a single refrigerant circuit.

In this type of arrangement, it is important to size the refrigerant pipe sizes on the lowest cooling capacity that can be delivered by the circuit. So, when there is only one compressor, you size the refrigerant pipes on its capacity. If you have a tandem compressor arrangement, you will size the piping on the capacity of only one compressor.

Now this may sound incorrect but let’s think about what is happening. In a tandem type of arrangement, there will be an instant in time where only one compressor will be operational depending on the current load of the room.

When only one compressor is operational in a tandem type of configuration, the refrigerant pipe sizes must be selected to ensure the velocity within the pipe are within acceptable limits to ensure adequate oil return.

When the load rises, the second compressor may be activated. Now this poses a problem as when both compressors are operational, the pressure loss through the refrigerant line may be too high, considering the line has been sized for the operation of one compressor.

So this is where the dilemma lies for a tandem fixed speed compressor type of arrangement with no oil separator. Do you size the refrigerant line for the operation of a single compressor, or do you size the refrigerant line for the operation of two compressors in tandem?

Some may argue that you should size for two compressors in operation as this is the required design load. So, let’s see what is the best- and worst-case scenario. 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 is excessive, and we will lose some cooling capacity.

If we size for two compressors in operation, the velocity may be low and oil will not return to compressor when part loads exist. So, what’s the correct way forward? Do we lose some cooling capacity at full load operation or do we risk compressor failure by having inadequate oil return?

Feel free to email me on and join us next month to find out the answer.

Wishing you a successful month ahead and chat soon.

About Michael Young

Michael Young, Pr. Eng.

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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