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I think that's backwards
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Posted by Robert Kaplan (more from Robert Kaplan) on Wed, 3 Jul 2002 14:07:26 Share Post by Email
In Reply to: BAck'erds 'n' fore-erds, Quasi, Wed, 3 Jul 2002 12:38:25
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Let's say you have an evaporator pressure of 15psi. That means you have an evaporator temperature in the vicinity of 15F. If it stays at that temperature, the evaporator is going to freeze up. Which means that the temperature of the evaporator needs be increased. Since temperature and pressure are directly related, this means the gas pressue needs to rise.

I think we agree up to this point.

But....you say "when the bulb gets too cool, it means gas flow needs to be reduced." The idea being that restricting flow increases pressure.

This is where we disagree. Because, restricting flow increases pressure before the restriction, but reduces pressure on the other side of the restriction.

I think you are forgetting about the compressor on the other end of the low side hose. The compressor does 2 things. First, it pushes gas into the high side of the system. But, it also sucks gas from the low side. Like a vacuum pump.

Which means that when the valve closes (ie reduces refrigerant flow), the compressor is still sucking whatever is in the evaporator. If there's not much gas in the evaporator (due to the closed valve), the compressor pulls the pressure down fast since its sucking function is like a vacuum. Accordingly, closing the valve and reducing refrigerant flow, reduces the pressure in the evaporator, thereby reducing its temperature.

Consequently, to increase the temperature of the evaporator, you have to open the valve and increase gas flow to give the compressor more to suck on. The more the compressor is sucking, the less of a vacuum it can produce. And the higher the pressure in the evaporator.

So why is my valve doing the opposite?

posted by 64.91.16...
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