Refrigeration Savings in Cold Weather

[billjg]

Is there any way to configure a chiller system to get free refrigeration in the winter? I need to cool water from 50F to 40F. In theory, in cold weather (<30F) I can; A. run the water through an air-cooled exchanger or B. pipe an evaporative chiller to a condenser coil exposed to cold air located above the chiller. In the latter case the system would act passively like a heat pipe, and use existing equipment.

Does anybody do this?

Thanks,

billjg

 

[djack77494]

billjg,
I wouldn't be concerned with designing for energy savings in cold weather. This would be like designing your home heating system for summer operation. In warmer weather, the heater will run at ever decreasing frequency unless/until it is no longer needed at all. Then it will just shut off. Your cooling (A/C) system does the same 6 months later. If you have a constant refrigeration load, you will have a constant expense. It will be lower in cold weather and higher in hot weather, but you will always need to circulate your refrigerant through the system to "move" your heat around. (Maybe a freezer in a house in Greenland would have been a better example.) {
Doug

 

[dcasto]

Think about this: In the winter you run your water chiller or refrigerator. The heat rejected from the condensor heats up your house, so, your furnance does not run. Presto-chango-free heat or free cooling of the water, your pick.

 

[NickE]

I do know that the convienience stores (specifically CITGO in Downtown Houghtion) in the upper peninsula have a method for cooling the refridgerators using outside air, in the winter....

The one year when the temp dropped way, way down (<-20F) the controls system must have stuck, every thing in the one fridge froze.....

 

[billjg]

Thank you for all of your responses.

NickE, they probably use a heat pipe which is a tube closed at both ends partially filled with freon. One end is exposed to heat and the other exposed to cold. The cold end must be physically higher. Freon evaporates from the hot end, absorbing heat, and condenses on the cold end, rejecting heat. The condensed freon returns by gravity to the hot end. This will continue at some rate until there is temperature equilibrium. Sunfrost used to offer a feature like this. Check it out:

http://www.sunfrost.com/passive_refrig.html

If I might be allowed to think out loud, the "at some rate" part is the catch. The rate is determined by the hot and cold side surface area, heat transfer coefficient and overall temperature difference. For a small size like a freezer, a practically sized unit can do the job, but for 100 tons of industrial refrigeration, a heat tube would be enomormous! You can get and assist by putting fins our your cold end, and a fan, which would be an aircooled exchanger. Add a compressor to increase the condensation temperature and you have a refrigeration system.

We now use a closed circuit Baltimore Air Cooler tower to condense ammonia (our refrigerant). In the winter, the water temperature never goes below 70F, that is, the cooling is evaporative even in winter.

If we susbstituted the tower with aircooled exchanger, which uses convective cooling, then we would get the maximum benefit from cold weather, but would probably still need the "assist" from the compressors (although they would not work as hard). However, aircooled exchangers are more $$$ per unit of heat transferred. My guess is that is all comes down to economics. Someone has certainly done this anaylsis, but in light of renewed conservation and green efforts, it may be worth a second look. Hope that wasn't too pedantic!

billjg

 

[jistre]

Hmm. It seems to me that when you'd opt for the free chilling the environment provides, you get benefit from free refrigeration in exchange for increased costs of control systems.

You could run the water through an external air-liquid exchanger to prechill the water before it goes through the refrigeration unit, but you'd have to then introduce some sort of control unit that would take inputs from sensors and determine how much water needs to be bypassed through the exchanger to prevent freezing the water, overshooting your target temp, etc. It would also most likely have to have some way to purge the bypass system and shut it down when it gets so cold that even at 100% bypass, the water would freeze in the system. I suspect that the capital costs of the additional controls and equipment required wouldn't be offset by the savings in refrigeration unless you're talking about a LOT of refrigeration. Then, economies of scale may work in your favor.

 

[TXiceman]

It is really pretty simple in a water chiller. I presume you are running the unit in the summer to cool as normal. Using water side controls and 3-way valves, divert the flow through an air cooler coil. If the water chiller is an air cooled unit, you can mount a water coil above the unit condenser and cool it using the condenser fans with the compressor shut off. This is often referred to "free cooling" of an economizer.

One company to contact that can build the units is RAE Corp in Pryor,OK (

http://www.rae-corp.com).

Ken

Ken
KE5DFR