Newbie Question - Help!

[manuelcm]

Hi

This is kind of a newbie question.

Say you have a vapor compression cycle, like the one used in automotive air conditioning.

The compressor is at variable speed, whitin it´s working limits, and heat is flowing from hot to cold, thus changing the refrigerant's phase inside the evaporator.

Once the room being conditioned, is at a temperature that can't go any lower (because of the compressor, an heat exchangers' capacity), what happens if the compressor keeps working?

There is no more heat exchange......so, what happens with the refrigeran? it doesn't boil inside the evaporator anymore? Does the compressor take it in as liquid? is it harmful for the system?

Any help would be appreciated.

Thanks

[Andy P]

Dear Manuel,

If the compressor keeps working and the unit has reached the design temperature then the temperature will fall below design - seems reasonable. This will result in two things: suction pressure lower than design and air temperature lower than design. You say "there is no more heat exchange" but unless you have pulled down to -273C there will be heat to exchange, but at a lower temperature than you would like. So the temperature keeps getting lower, and so does the suction temperature. Then something probably will go wrong - the evap frosts up because it is not intended to run so cold, or the system has an LP cutout, or the compressor goes beyond operating limits and trips on high discharge temperature, or compressor oil flow is interrupted and the plant trips etc etc. If you manage not to hit any of these then eventually the pressure ratio will be so high that the compressor stops pumping (but keeps consuming power). This will also cause a high temperature trip - or maybe a motor burnout.

cheers
Andy P

[cooltruck]

if the compressor keeps running,and the Lp switch does not cut out you will start to draw liquid refrigerant to the compressor.which will remove oil from compressor and eventually seize compressor