Piston

johnjohn89

Why charge wet bulb and dry bulb when system use piston metering device ? And why txv doesn’t charge by wet bulb and dry bulb ?

ChrisJ

As far as I know with a fixed orifice device like a piston you charge by superheat.

With a txv you charge by subcooling because the txv controls superheat, so you can't really charge by it.   Personally I watch superheat as well with a txv and adjust the txv if possible to get both superheat and subcooling where they need to be .

Subcooling is really just showing you how much reserve liquid refrigerant you have backed up in the condenser.  

pedmec

because with fixed metering device a greater portion of the refrigerant is used to remove the latent heat plus the sensible heat. so on a humid day your superheat will increase due to the higher latent heat load on the evaporator coil.
with a txv it will just open up to maintain the proper superheat due to the increased load.

johnjohn89

ChrisJ said:

As far as I know with a fixed orifice device like a piston you charge by superheat.

With a txv you charge by subcooling because the txv controls superheat, so you can't really charge by it.   Personally I watch superheat as well with a txv and adjust the txv if possible to get both superheat and subcooling where they need to be .

Subcooling is really just showing you how much reserve liquid refrigerant you have backed up in the condenser.  

Ture thanks

johnjohn89

pedmec said:

because with fixed metering device a greater portion of the refrigerant is used to remove the latent heat plus the sensible heat. so on a humid day your superheat will increase due to the higher latent heat load on the evaporator coil. with a txv it will just open up to maintain the proper superheat due to the increased load.

Great answer thank u so much