View Full Version : Low back pressure compressor (LBP) in high back pressure system (HBP)

03-03-2016, 09:47 AM

I'm a newbie here, but was following this forum for quite some time. I'm an electronics engineer who somehow have an inexplicable attraction to refrigeration and HVAC in general.

So the question:
I'm planning to design a small power r134a heat pump (~250w) with capillary flow control. Evaporating temperatures (forced air flow) would stay around +10C (50F) and condensing temperatures around 50C (122F). By my understanding, in this situation the HBP compressor with 3,4 cub. cm should be used, BUT - If I used the LBP compressor with the 6 cub. cm displacement, but increase the superheat (by prolonging the cap. tube) to keep the current consumption of the compressor not to go over the designed nominal, would it be OK? Would the compressor lifetime expectancy be somehow altered? Because to my knowledge LBP compressor in HBP designed system would be overloaded and HBP compressor in LBP system would not get enough refrigerant flow to cool down the motor.

The question why is pretty simple - LBP compressors are significantly cheaper. I understand that compressors quality vary immensely. But by general experience - if the "economy-class" compressor is rated for lets say 250w cooling power - is it real to expect it's lifetime to be at least 5 years while keeping it at nominal or just a bit over nominal power?

By the way, I might still have a misunderstanding about the condensing and evaporating temps - am I correct that these are the mediums around the coils temperatures (lets say forced air)?

Thank you for the answers.

04-03-2016, 02:33 AM
I have this feeling you are pushing the proverbial up hill with a pointed stick.
134a in an inefficient refrigerant on a good day and is pretty much banned and taken out of production, capillary is OK at high temps., but LBP compressors not, they rely on cold /cool vapour over the motor windings to keep itself cool, high suction, no saturation compressor will fail.
Next good idea please. \

04-03-2016, 07:14 AM
High suction superheat = high discharge superheat = compressor winding overheating!

04-03-2016, 06:21 PM
Shortly, it is no good idea to use a LBP compressor with +10C evap. temperature.