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Old 11-26-15, 09:38 PM   #118
jeff5may
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Ok, with your smallish compressor of under a ton, your loop will most likely be able to digest whatever you throw at it until something breaks. For the most mass flow, a suction pressure near where it is now (90ish psig) is about as high as it should get. This correlates to around 60 degF SST, and you were getting 65 degree suction line temperature. This 5 degrees of superheat is dangerously low for a cap-tube metered rig! I don't know what your ground temperature is, but unless it's really deep, it's not going to be above 65 degrees all year long.

On the condenser side, a 230 psig discharge pressure translates to 120 degF saturation temperature. I imagine the compressor discharge temperature was higher than that. Since you are moving a lot of mass through the compressor at a high compression ratio, the compressor is working as hard as it can. You are probably at that 50% loaded condition, where the entering gas is highly superheated. It travels a while losing superheat until it reaches the saturation temperature. At that point, it begins to condense at constant (120 degrees) temperature. Half way through, it is all liquid and heat transfer tapers off as the liquid temperature drops. Liquid travels way much slower than gas, so the liquid sits around doing nothing for a long time in the end of the condenser. The more charge goes in, the more useless work the compressor does and the more liquid backs up in the condenser.

In the air-source units I have put together, the compressor was never working against this much head pressure unless it was cooling on the hottest days of the year. The high head pressure lays waste to your energy efficiency. It is much more efficient to move more air (or water)flow through your hx at a lower saturation temperature. The air (or water) doesn't "feel hot" coming out, but you are moving more of it, and more of the effective surface area of your hx will be in a saturated state with less pressure. For the same btu transfer, your compressor runs cooler and uses less power.

When you step up to compressors with larger capacities, they have enough muscle to push your system out of its comfort zone. If nothing is in place to sense and stop bad things from happening, they will. More energy causes bigger problems faster.
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