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unver
03-03-2010, 11:07 PM
Dear Colleagues;
I am not much experienced in refrigeration, so my following question may seem little naive.

I know that the pressure difference between an evaporator and a condenser is maintained by the compressors volumetric compression rate and the size of the TEV orifice (at least thats what I think so far). So, for a given compressor and TEV, theorically we have a fixed evap. pressure, am I right? (considering that everything else is kept constant)

Question is that, is there a way to adjust evap. pressure (and therefore its temperature) so that we could adjust the temparature of the cooled medium? For example, I like to obtain cooled water at a precisely defined temperature. And this temperature may be any thing between 5 to 20C. Cooling load and condenser temperature are assumed to be constant .

I know that water temperature can be adjusted by on-off control of the compressor, but I think that this may not be a precise way of temperature control.

I like to thank all of the suggestions or comments ahead of time. Looking forward hearing from you.

Brian_UK
04-03-2010, 12:17 AM
You could utilise an electronic expansion valve to control the evaporator temperature but you must also consider maintaining the cooling of the compressor so some modifications to the basic circuit may be required.

Valve details here...
http://www.sporlanonline.com/100-20_092008.pdf

norseman
04-03-2010, 08:52 AM
It would be a task to find the correct eva. temperature. If you want to hold water at 5c you have to let the eva. temperature be lower so it could end up icing up the eva. or otherwise wait a day to cool down the water and still have to use an eva. temperature lower than the bath temperature you need.The system should have a good cirkulator to move the water over the eva. to get some decent temperature all over. If not the lower part would be cold and the upper much warmer.
We trade some waterbaths which use a very exact driven heater and the control by heat is for sure the best way. Those systems can be adjusted inside 0,001c and is controlling inside 0,005c. Your idea is not to reach such a level? Anyway, a controlhead with a pump and heater would give you a pretty good temperature and a good circulation could make a 5c without any freezing up if water is the medium.

Norseman

gregd1401
04-03-2010, 09:07 AM
Use of an evaporator pressure regulator to maintain suction pressure above freezing point and compressor thermostat control would give desired control effect.

mbc
04-03-2010, 09:47 AM
Use evaporator pressure regulator( Back pressure regulator valve )to maintain suction pressure
and to adjust it in (different temperature - delta T) it is better you put one gage on suction line before this valve

unver
05-03-2010, 07:00 PM
I like to thank All the colleagues that reply my question.

anna.savvy
07-03-2010, 10:20 AM
Thanks to univer...
you also several doubts of mine too.. thanks to all above..

anna.savvy
07-03-2010, 10:21 AM
sorry thats UNVER....*:D:D

Peter_1
08-03-2010, 08:08 AM
Unver , we have some systems running at +/-0.2°C in a closed and relative stable circuit.
Let's say you need 10°C supply, we cool an isolated drum at 5°C with a DT of +/- 2°C.
A 3-way watervalve (STAEFFA Control) in the return is bypassing some preheated process water to the supply water of 5°C, sensed by a PT100 in the supply

Or you can use an pressure evaporator regulator and regulate very precise the evaporating temperature, they even exist electrically controlled (Danfoss) http://www.danfoss.com/BusinessAreas/RefrigerationAndAirConditioning/Products/Detail/RA/Pressure-and-Temperature-Regulators/Pressure-and-Temperature-Regulators/EVMCV-pilot-valvesvalve-body-for-PMPMC/CVQCVMQ-constant-pressure-pilot-valve-electronically-operated/027B1139/060425b0-17b3-4d61-b7f4-8381fc53c647/c032f383-671c-46e7-b5e1-4e5f63b74b82.html

unver
08-03-2010, 10:46 AM
Dear Peter;
Thanks for the mixing valve suggestion. But we were trying to find an alternative solution to that. That was where my original question was originated from.

But let me ask then, is there an electronically controlled mixing valve in small sizes (about 10cmx10cmx10cm). Motor controlled mixing valves i have seen are usually very big for our 1kW cooling system.

Peter_1
08-03-2010, 06:39 PM
1 kW is new for us, do it with a controlled hot gas injection, very precise. Keep suction temperatures no too high so that Sh stays within margins