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Originally Posted by jhurliman
I've been trying to figure this out all day. Are you saying a system with more cooling capacity (btu's) will be able to reach a lower temperature? It seemed to me the minimum temperature was based on the refrigerant and having a compressor+condenser+evap+capillary+etc to match it. Is one R-22 system able to reach lower temperatures than another? Because a 1500 btu/hour (439.6 watts) system would more than take care of a single PC as far as bringing it to ambient; it's the "overclocking" of phase change systems and running them cooler than they are intended to go where I have very little understanding.
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The temp of the evaporator depends on the boiling temperature of the refrigerant. This can be looked up on a chart, or with a free tool like Refrigeration Utilities. In theory you can get almost as low as you want by decreaseing the low side pressure (well eventually you have perfect vaccum, but within reason). In reality though you can't do this.
What happens is as you lower the low side pressure, it gets harder and harder to get any lower because the refrigerant flow through the system has to be at a certain level to match the CPU power load. So while you can keep makeing the cap tube longer and the compressor more powerful, it becomes less and less effective because you have to push a minimum refrigerant through at all times and this prevents the vaccumm from getting very low (typically people seem to aim for around 1 atm of absolute pressure w/ R22 in direct die systems).
Thats why people cascade. You can't make the high too high otherwise you'll break the compressor, and you can't go too low or your capacity drops off real fast. So you break the system down into discreet steps each with a reasonable pressure difference.
You're right about wattage rateing. Its pretty much meaningless, except to figure out the power of the motor. And generally I think something around 1/8 to 1/3 HP is the sweet spot in terms of performance/energy bill.
I'm still collecting parts though, so I could be wrong.