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Re: [alfa] Low pressure AC hose insulation



In response to a question from Joe Elliott:
You inquire about insulating the suction line from the evaporator to the firewall; I will assume that you mean from the evaporator through the firewall to the suction (low pressure) port of the compressor. Your logic is almost correct but subtly flawed. As you say, insulating the suction line should ultimately reduce the temperature of refrigerant entering the condenser, but that's a good thing. Here's why. It does lower the temperature gradient between refrigerant and ambient, and thus (using "Ohm's law") reduce the heat flow out of the condenser (bad). But it also lowers the amount of heat that must be dissipated--the absolute temperature of the refrigerant times its specific heat (good). The good outweighs the bad, as can be seen from the limiting case: imagine refrigerant already cooled to ambient--it will provide no gradient for the condenser, but the output will be at ambient--as cold as you can hope for. As the incoming refrigerant temperature increases, the thermal efficiency of the condenser increases, and so does the net temperature drop (as you speculate). But the increased temperature drop is less than the increased incoming temperature. Thus the output temperature rises.

The above is simply to convince you that insulating the suction line isn't a priori a bad thing--it's done in home AC's for example. But an equally important point is that the situation is more complicated, involving superheat and subcooling. These relate not only to heat flow but to the change of state (heat of vaporization) that really gives AC it's oomph. They determine the extent to which the condenser and evaporator are fully utilized. One caveat along these lines: if insulating the suction line allows liquid refrigerant to make it through the evaporator and all of the way back to the compressor inlet, you can destroy the compressor.

Bob Wilkinson
72 Spider
That's pretty much the conclusion that I had come to--thanks for the explanation. I should have thought to analyze the limiting case!

I'm not too worried about liquid reaching the compressor. I think the systems are generally designed so that refrigerant leaving the evaporator is several degrees above the dew point--I can't imagine that the designers would be counting on the refrigerant being warmed as it passes through the engine compartment in a hose. Additionally, for the air conditioner to assist in demisting the windshield in the winter without destroying the compressor, there must be a significant difference between those temperatures when the weather is warmer, right?

But this raises another question that I'd never really thought about before: if the evaporator ices up (not uncommon in GTV-6's given their wimpy blower motor), will that prevent enough energy from entering the refrigerant that it might leave the evaporator as a liquid?

-Joe
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