By Michael Young, Pr. Eng.
Part 3: Is Pressure drop too much of a good thing?
Click here if you missed Part 1 or Part 2
Were you ever told that eating too much ice cream was bad for you? Yup, ice cream tastes great but it’s the loaded sugar that is harmful to your body if consumed in excessive amounts.
But does it hurt to treat yourself to a delicious cup of vanilla ice cream with chocolate sauce once a month? What’s the point to life if you can’t enjoy that and reward yourself from time to time?
Well just like ice cream, anything consumed in excessive amounts can be harmful and the same goes for excessive pressure drops when selecting the refrigerant pipe sizes for a DX system.
So why is pressure drop so important and why should we even consider it in a DX system?
To understand this, we need to understand the effects pressure plays on a refrigerant gas.
Now from high school we learnt that pressure is related to temperature. Think about pumping up a soccer ball where you insert the pump and start pumping air into the ball. After pumping up the ball for 30 seconds, feel the sides of the pump and the ball, what do you notice?
You may notice that the sides of the pump and the ball are warm. So, what you have just witnessed is the direct relationship between pressure and temperature. As we push more air into the ball, the pressure within the ball increases and so does the air temperature within the ball.
This very same concept holds true for all refrigerants in their gas phase. When we increase the pressure of a refrigerant gas, we increase its temperature and the same holds true for the opposite – whereby decreasing the pressure decreases its temperature.
So, how is all this related to the pipe sizing of the various refrigerant lines?
Let’s talk about the gas lines. If the sizing of the piping is too large, the velocity of the refrigerant gas decreases and this can pose a problem for oil returning back to the compressor, especially under low-load conditions. It’s important to note that this statement is valid for systems that do not contain oil separators.
If the size of the piping is too small, the velocity of the refrigerant within the pipe increases but the refrigerant vapour experiences a high pressure drop. A pressure drop in the suction line reduces a system’s capacity because it forces the compressor to operate at a lower suction pressure to maintain a desired evaporating temperature in the coil.
So, the goal is to find the optimal point whereby adequate velocities are maintained and pressure drops are within reasonable values. After conducting research, a guideline is to size the gas lines to have a pressure drop no greater than the equivalent of about a 1 K change in saturation temperature.
Now I know this may sound like a lot of information to take in but I’m here to support you. So, if you have any questions, feel free to email me on michael@myengineeringcoach.co.za.
Wishing you a successful month ahead and chat soon.
About 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.
