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Q & A: water cooling devices

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Most recent answer: 10/09/2013
Q:
Hello, I recently purchased 2 water cooled machines... they run constantly and use around 1 Lts of water per minute. I need to chill the water rapidly as I can not use tap water to cool down the machine (Too much water wasted) and there is no point buying and industrial chiller as it is to big for only 2 machines. I bought an aquarium chiller (http://www.allpondsolutions.co.uk/marine-aquarium-chiller-l500.html) but it takes to long to cool down water, it takes around 15 minutes to lower 1 deegre for a mass of water of 200lts. How can I lower the water tempeture faster??? (The water that comes out of the machines is around 50C, it need to be cooled down to 30C)
- Eduardo (age 28)
Caracas Venezuela
A:

This problem becomes easier if one converts all the various specs to some standard units. You need to cool about 1000 gm of water by about 20 K every 60 sec. (I assume your 1 Lts number was for the combined machines, not for each one.) Since the heat capacity of water is ~4.2 J/K-gm, you need about 1600 W of cooling power. (twice that if that's just for one machine) The device you bought is giving you ~200,000*4.2/(15*60) W ≈ 920 W. It sounds like one solution would be just to buy a second device and run it in parallel. 

That device, by the way, isn't especially efficient. A good home air-conditioner can give more than 4W of cooling per 1W power used. It looks from the specs as if your device draws about 320 W of power, so 920 W cooling is mediocre. (However, I may be confused since I'm assuming that the 1.4 A current draw of the device is for British 230V supplies, not for standard 120 V Venezuelan supplies.) So, especially if you need 3200 W cooling rather than 1600 W, you might do best to buy just the compressor unit from a small split-style home air-conditioner, and use its cooling coils in a tank of flowing water.

There's another possible approach. Simply replacing all the water would use too much. However, cooling water by 20 K requires evaporating only about 4% of it, since the latent heat of evaporation is large. Sending your hot water through a sprayer to speed the evaporation might get most of the cooling you need and might still not require an unreasonable amount of water. The feasibility depends a lot on the price of water, compared to electrical power,  and on the typical relative humidity. 

Mike W.


(published on 10/09/2013)

Follow-Up #1: space-saving cooling

Q:
Mark, I really appreciate your help on this matter. The last idea of having the Hot Water spray out into the tank sounds like a way it can help, but the problem is that this is all built in a really small room. The machines I am talking about are actually Soft Ice Cream Machines. Now, what we are doing is having 2 ways to cool down the machines, one, with the water tank connected to the chiller,(two separate pumps, one to the chiller the other to the machines) the outlet of the machines comes out on top of the water tank, the outlet of the chiller comes at the bottom and in front, out to the machines again, when all fails and water gets to hot we conect the machines directly to the outside water supply. Now, the idea of purchasing another Chiller might not be an option, again, space is an issue. So my next question is this... even if the chiller fails in a busy day that a bunch of people hopefully want to eat ice cream (We have not oppened yet) I can use the water tap directly, but, I cant use it all day, problem is not cost, it is the amount of clean water wasted. Electricity on the other hand I can work with. So my next question is this: is there anything (other than ice, which we have already taken that option as our option C), that can cool water faster? If I poor refrigerant into the water, will that absorb the heat faster in which case it might also help the chiller work faster? We can leave the chiller working all night long and by the time the store opens water might be 20C lower, but I need to have it come down even lower so that it can last as long as possible. Having the lid of the tank opened on top might also help with evaporation, we have installed a small 12BTU Split air conditioning to also help out. Again, thanks for the help.
- Eduardo (age 28)
Caracas
A:

I think that ice is about as good as you're likely to do: large latent heat per volume, inexpensive, very low toxicity! So freezing as much as possible overnight and using it to augment the cooling during the day could help a lot. The latent heat of melting ice is equivalent to about 80° C of cooling water.

Is that split air-conditioner really only 12 BTU/hour? That's under 4W, which sounds tiny. Could it be 12,000 BTU/hr? That would be plenty for your ice-cream needs, with some cooling power left for the room. It would require a little extra plumbing to divert some of the cooling to the ice-cream system, but not too elaborate.

Mike W.

p.s I'm posting this without Lee's checking, since he's on a trip and the information seems timely.


(published on 10/09/2013)

Follow-up on this answer.