Evap Temp Effect on Compressor Capacity

I really appreciate the detailed data that some manufacturers have available to the public on the internet. For example, Bristol Compressors has detailed performance specifications for all of their compressors online for anyone to see. Not only do they have the specs for standard rating conditions, but you can also enter in your own. This allows you to see the effect that changing system conditions has on the compressor capacity, energy use, and EER. For example you can see what happens if the evaporator temperature drops from 45°F to 40°F. At first, that does not seem like a very big deal. But in the case of a Bristol H81J223ABC, that change drops the capacity from 21,800 Btuh to 18,800 Btuh and drops the EER from 9.8 to 8.9. Now ask yourself this question: “How easy would it be to have an evaporator temperature 5°F too cold because of a dirty air filter, dirty coil, restrictive duct, slight undercharge, partially clogged filter drier, incorrect piston (orifice), incorrectly adjusted TEV – and I am sure you can think of a few more.  In fact there are so many ways to be just a little off it seems likely to occur. Unfortunately, the effect on system performance is not little. The capacity drops by 14% (18,800/21,800 = 0.86 or 86%) and the EER drops by 9% (8.9 / 9.8 = 0.91 or 91%).  For a more dramatic drop in capacity, look at what happens to a commercial refrigeration compressor when the evaporator temperature drops from -10°F to -20°F. The Bristol L61J113ABC produces 7000 Btuh at its -10° rating point. At -20°F this drops to 3800 Btuh. At only 10°F colder, the compressor capacity drops to 54% of its rating! That extra 10°F colder costs dearly in electricity. With some creativity and exploration you can use comparisons of different operating points to illustrate many issues in HVACR systems.

You can find this interactive compressor playground at www.bristolcompressors.com Engineering Support > Compressor Search or just CLICK HERE  Enter whatever details you want to start the search. Once on the page for a particular compressor, click “Specific Point” towards the top right of the screen to enter your own operating conditions. Have fun!