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[nibot]

This is sort of a physics question: Could you pump cold water through radiators to cool your house? The obvious flaw is something like "cold doesn't radiate," but, then, don't we have the general principle that a good antenna for transmission is usually a good receiving antenna too, and hence the cold "radiator" should absorb thermal radiation from other objects? (In addition to cooling by convection.)

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[eviladmin.livejournal.com]

Universities and other large institutions use a chilled water system which basically runs water through something like a car radiator and has a fan blow through the radiator to chill the air. During the winter cold water is exchanged for hot. This allows for an efficient central heater/chiller and cheap one moving part (fan) local units. A thermostat controls the fan and therefore the temperature.

[(Anonymous)]

I always thought that the reason for freon was that it had a convenient boiling point - on the 'hot' side it's a gas, and on the 'cold' side it's a liquid (for human-friendly definitions of hot and cold). Hence, you can effectively 'pump' the latent heat of vapourisation around - typically a fairly large number. Freon being non-flammable is, of course, a rather nice bonus. I suspect that this is the reason it is used instead of the variety of alcohols which have a similar boiling point.

[surpheon.livejournal.com]

Yup, its about boiling and condensation. Freon is neat because it boils and condenses at about the temperatures we want when compressed to sane levels. And it is non-toxic, non-reactive and non-flammable (butane is also used as a refrigerant at times, heck water can be used as a refrigerant).

A change in state (liquid to vapor) absorbs or releases a huge amount of energy. Remember shivering when you get out of the shower? Water on your skin is going to vapor and sucking heat out of you. Here's the neat bit, the temperature at which it occurs can be altered by changing the pressure of the working fluid. So, you use a compressor to put freon vapor under pressure until its phase change point, in this case the point at which it condenses into a fluid and releases a buncha btus and turns into a liquid (boiling in reverse), is about 120F. Allow the freon to condense on a metal tube which gets hot, and blow outside air over the tube to keep it cool enough to keep condensing freon. Heat is rejected to the 'cool,' up to 110F outside air, being blown over the tube.

Now, for the cool part (pun intended). When you compressed one side to make the freon change phase at a high temperature, you sucked another chamber to a very low pressure. So low, that if you squirted a little bit of the now liquid freon into it, it would boil (sucking up btus) at a temperature of about 40F. Air conditioners use a throttling valve to squirt liquid refrigerant onto a surface on the very low pressure side, where it immediately boils off - sucking heat out of the surface and the air flowing over the other side. This creates a vapor, which you squish through the compressor (its a vapor, so you can use a simple turbine fan type compressor - bad things can an do happen if any fluid refrigerant hits the compressor) until its pressure is so high it condenses at 120F on the hot side, rejecting heat and the cycle continues.

So, if you are bored enough to still be reading, freon works because its boiling/condensing temperature can be manipulated at reasonable pressures between the temperatures we want - 40F or so to cool the inside air, and 120F+ to reject heat outside on hot days. Freon also has nice properties regarding the number of btu's it sucks up and spits out in the phase change process.

[surpheon.livejournal.com]

Yup, and the water that runs through the fan coils is then cooled by big ass 'chillers' that cool water using a standard vapor compression cycle. As a side note, water is a miracle liquid for cooling. It's heat capacity is amazing, and its phase change is a fundamental part of every large system in the form of cooling towers. The chillers evaporate water directly to the air to dump heat at a rate of almost 1000 Btus/lb of water evaporated. 1000 Btus is enough to bring a quart and a half of water to 210F (note that going from 210F to a vapor at 213F would take over 6000 additional btus - phase changes take energy!).

The T-stat usually controls the valve on the coil rather than the fan because small variable speed fans are pricy, but thats a minor quibble. And cheaper variable speed fans are in the pipeline (which is cool, because reducing air flow rather than liquid flow through the coil would save more energy - pumping air is more turbulent and inefficient).