nibot | (no subject)

(no subject)

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.)

Flat | Top-Level Comments Only

Sure you can. It's what air conditioning does, only AC uses a fan over the radiator and Freon instead of water.

On a somewhat related thread, I thought of using the cold water line for the toilet in my bathroom to dehumidify the room. Basically, I'd just extend the water line using 30 feet of copper tubing with a drip pan underneath. I have more or less decided not to do it since the water would warm up quickly unless the toilet was flushed a lot and the system probably wouldn't remove much water.

The key difference I'm getting at is that supposedly a radiator radiates heat (eg, infrared?). I want to know whether it can absorb radiant heat. I have no doubt that it can act as a heat exchanger. In other words, if a radiator can heat via radiation (as opposed to convection/conduction), then can it cool in the same way?

What's the difference? Radiating heat and absorbing cold are the same idea just in different directions, right?

I just realized that you must be talking about an indoor steam-driven radiator, as opposed to the one in a car. They're both heat exchangers but the indoor type is more of a radiator in that it works primarily through radiation, not convection, as you said. I imagine you could get one to work as a cooling device but the problem would be surface area/convection. Without much surface area (car radiators have tons) or any forced air, you'd probably just be cooling the immediate area around the radiator.

The tinkerer's ugly but functional solution would of course be to replace the steam radiator with a car radiator or something similar.

I imagine the reason for freon instead of water is that you can get a much greater temperature differential between the room and freon than between the room and water. I think it's also the case that you can normally get water to a greater temperature differential above the room temperature than below it. If the room is about 70 degrees, then you can get the water almost 140 degrees warmer, and only about 35 degrees colder. You can get it about 170 degrees warmer than a 40 degree room, and only about 60 degrees colder than a 95 degree room, so Newton's law of heating/cooling would suggest that the conduction work will be about twice as effective at heating as at cooling.

And actually, you could probably do even better for heating, because ultrahot steam will still flow through a radiator, while ice won't.

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.

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.

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.

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).

To make a refrigerator, you need a compressible fluid; that's why water wouldn't work.

Close, but actually water can work - if you are working at extreme enough pressures. Actually, water is used as a refrigerant in one device I know of that uses a chemical dessicant to create the pressure differentials used in the refrigerant cycle.

You do need a compressible working "fluid," but the 'working fluid' is always a vapor when it is compressed (HVAC engineers have some of the most screwed up lingo you'll ever hear - the go metric folks don't even waste their breath on us!). Compressing a fluid is not part of the refrigerant cycle, although sometimes a poorly operating system will get fluid into the compressor, a condition knowns as 'slugging' (due to the sound and destructive effects).

Flat | Top-Level Comments Only