Designing a direct-expansion immersion coil to generate 1deg C water.

[Au Yeong W. S.]

Hi, everybody,

I've a problem in getting my hands on a evaporator to be used to generate icy water at 1 deg C from the market with return water at 5 deg C. The WTD is too narrow for falling film (pillow-plate) evaporator makers.
As the cooling capacity is not large, I don't wish to use a 2-stage system (glycol to water with gasketed PHE) and there are no small gasketed PHE in the first place.
The only solution is to design and build a finned evaporator dx-coil and immerse it in water. This is where I will certainly need some good advice and help from you guys.

I intend to select the evaporating temp at -10deg C but what values (rule-of-thumb?) should I use as the coefficient of over-all heat transmission for water to refrigerant in a 1/2-inch copper external-finned coil?
Fin/inch: 10 ? The water in the tank will be mildly agitated by the water pump, and perhaps with baffles as well.

Au Yeong

[Tesla]

Hi Au Yeong W. S.
I used to repair/rebuild coca cola post mix machines which used a bare 1/2" coil with an ice bank thermostat for temp control similar to what you have asked, If you have problems with control you could speed up the agitator for tighter control. No fins are needed and they may cause problems, the ice bank was about 1/2" or less and the evap temp you mentioned sounds about right.

[Brian_UK]

Or maybe try a co-axial evaporator.

http://www.wieland-thermalsolutions.de/commonmedia/content/media/en/prospekte_2/gbrohre/prospekte/da_r_wkewkc_08.pdf


Maybe you feel rich?? :) http://www.aquasystemsinc.com/frontinsidecover.html

[monkey spanners]

A small air blower is a good way to agitate the water and ensure even melting. A grid of pipes with small holes in fixed in the bottom of the tank will do this, you could feed your return water into this same grid if it is all sized well enough and suitable one way valves fitted, and the draw cooled water from the top from a similar grid of pipes.

If you are making one, don't forget the cooling coil once covered in ice will want to float so will need to be securly mounted on a frame which in turn is solidly fitted to the tank its in.

[michaelm]

B”H
In some application the tank was agitated using injectors. In your case the injectors could be plastic. Agitation is very important to achieve a good heat transfer.

[mad fridgie]

For low temp water agitation is used both for heat transfer and to give you cold water, "h" i hear you say at 4C water is most dense, so if you do not agitate and draw from the bottom of the tank, you may have ice but you 4C water