Professionals who care
By Michael Young , Pr . Eng .
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 .
THE EASIEST WAY TO KILL
THE PERFORMANCE OF A DX COOLING SYSTEM
Part 3 : Is Pressure drop too much of a good thing ?
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 . RACA
" Anything consumed in excessive amounts can be harmful and the same goes for excessive pressure drops ."
www . refrigerationandaircon . co . za RACA Journal I November 2022 37