By Michael Young, Pr. Eng.

Part 2: Understanding is the key to success.

Click here if you missed Part 1

Did you ever have a high school subject that you absolutely hated? I remember I had one such subject where I would drag my feet to class. The minute the lesson began, I would look at my watch and count down the mintutes until the bell would ring and the lesson would be over.

Since I disliked this subject so much, it was no surprise that I failed most of my class tests and exams. I still dream of this subject till this day and everytime I wake up after the dream, I think, “wow, that was a terrible nightmare.”

Looking back now in my late 30s, I have come to realize why I hated that subject so much. I guess I hated this subject because I did not understand why I had to do it, and the application of it.

It was only when I started to look at how this subject is applied in real life did I start to understand and appreciate the subject. I started to change my viewpoint to constantly understand the “why”. Why is this specific principle so important and why is it relevant to everything that I do?

It was at this point that I discovered that refrigerant pipe design falls under this category. Yes, it is true that computer software can speed up this process, but incorrect understanding of the topic can have some negative consequences in the operation of a HVAC system.

So when it comes to refrigerant pipe design, what are the most important principles that need to be understood? The first one is oil return and the second one is pressure drop.

So why is oil return so important? During the operation of a DX system, the compressor requires oil to lubricate various parts. However, when the refrigerant is in the gaseous phase, the oil is no longer misible in the refrigerant and gets discharged into the refrigerant line. (This is applicable to standard units that do not contain any oil seperators).

This oil needs to eventually make its way back to the comrpessor to prevent failure of this component. The determining factor that indicates bad oil return is the velocity of the refrigerant in the pipe.

If the pipe is too large, the velocity decreases and if the pipe is too small, the velocity increases. So what are the consequences of high or low velocities in the refrigerant piping?

If the velocity is too high, the refrigerant experiences excessive pressure drops which can decrease the cooling capacity of the unit. If the velocity is too low, the oil does not return to the compressor.

So the goal is to be able to find that ‘sweet spot’ where the velocity is high enough to ensure adequate oil return but the velocity is also not so excessive that the cooling performance of the cooling unit is reduced.

Unfortunately, we have run out of time to discuss the impact of pressure drop but join us next month to see how excessive pressure drops can reduce cooling performance.

Wishing you a successful month ahead and chat soon.

About Michael Young

Michael Young, Pr. Eng.

Image supplied

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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