Question regarding pressures

[mburns]

If i look at an pressure-enthalpy chart i can see that the pressure in the evaporator (low side) is constant and you absorb heat (kj) and the pressure in the condensor (high side) stays constant as well when you reject heat.

So for example just throwing a couple of situations out there. For example

There is a leakage in the return air duct of an air con causes an excessively high suction pressure? Is it because you are absorbing to much heat causing a too high superheat which in turn increases the pressure?

Or for example in a condensor shouldn't the pressure be lower at the outlet because at the inlet it's a vapour and at the outlet it's a liquid so shouldn't the high side pressure be lower at the outlet?

Thanks guys still learning!

[Gary]

Start here:

http://www.refrigeration-engineer.com/forums/showthread.php?t=19701

[Tayters]

There is a leakage in the return air duct of an air con causes an excessively high suction pressure? Is it because you are absorbing to much heat causing a too high superheat which in turn increases the pressure.

Almost. If the duct leak causes a higher suction pressure this will be because the evaporator has more heat to deal with - i.e. a higher load. If the expansion device can't fill the evaporator with refrigerant quick enough then a high superheat will result. So it's the higher load causing the higher pressure. High superheat as a result (in your example) of not coping as effectivley as before with the higher load.


Or for example in a condensor shouldn't the pressure be lower at the outlet because at the inlet it's a vapour and at the outlet it's a liquid so shouldn't the high side pressure be lower at the outlet?

Treating this as an ideal system with no pressure drops, then high side pressure will be the same from compressor outlet to the expansion device. This pressure when converted to saturation temp gives temperature at which the refrigerant changes state. In the condenser it arrives as a gas as it's temperature is above it's saturation temperature for that pressure. As it cools down the pressure stays the same, refrigerant cools to below the saturation point as it condenses to a liquid.
Pressure shouldn't be lower, temperature should, to get the change of state.

Inreality there will be pressure drops but this will be due to (hopefully!) unavoidable system constaints such as pipe bends, frictional losses, risers etc.

Hope that helps,

Andy.