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Old 11-26-15, 04:20 PM   #116
jeff5may
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Ok, this is a really good question. The pt chart describes the saturation temperatures at a certain pressure for the refrigerant being used. I say temperatureS because for a blend, the refrigerant will exhibit "glide". The temperature where the blend begins to bubble or boil is different than the dewpoint where it completely vaporizes. While the refrigerant is changing phase, it will stay at the saturation temperature as closely as physics will allow. If there is not enough heat flow to complete the phase change, a mixture of gas and liquid will come out the other side of the hx at or near the saturation(pt chart) temperature.

After all the refrigerant has changed phase, it loses its ability to gain or shed heat without changing temperature. In an evaporator, heat is absorbed only when the refrigerant is colder than its surroundings. Higher dT means more heat transfer. When the liquid all evaporates, as it finds equilibrium with its surroundings the heat transfer tapers off real quick.

This is how we measure heat exchanger loading and effectiveness. If we feed, say 75 psi into your evaporator, it may only be half saturated. Let's say you have a 25 degree dT at this condition. The refrigerant boils off quickly due to the high heat flow in the first half of its journey. Once it changes phase, it has the whole second half to gain superheat and leaves the evaporator not cold. We say the evaporator is only half loaded and the leaving refrigerant is highly superheated. Not too good with a TXV, maybe not too bad with a cap tube.

Ok, lets say something changes and the entering liquid pressure rises to 90 psi. This drops your dT to 10 degF, so the refrigerant travels 90% the way through the hx before it all boils off. Once it leaves saturation, it only has the remaining 10% to be superheated. It comes out colder than in the first example. We say the evaporator is 90% loaded and the leaving refrigerant has x amount of superheat above SST (saturated suction temperature). This amount of loading usually only happens in TXV metered systems with a massive heat source.

Whether the first or second set of conditions will transfer more heat depends on lots of specific details with the rest of the system. Since evaporators can freeze or flood, we usually monitor them and superheat.

Condensers work much the same way: higher head pressure -> higher condensation temp -> higher dT -> more heat flow and subcooling.

Last edited by jeff5may; 11-26-15 at 04:59 PM..
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