Chilled Water Temperature & Pressure Relationship 1

[micksouth]

Hello All
Please could you offer some expert advice on how to calculate the temperature required to reach 9.5 bar, on a system which normally runs at 5.2 bar at 6 degrees centigrade.
If it makes any difference the chilled water contains approx 25% glycol anti freeze.
Thanks very much for your assistance.
Mick.

 

[25362]

I assume it all depends on the size of the expansion tank vis a vis the system's volume. The following link may be of help:

http://www.vemcoinc.com/pdf/Technical Library/Expansion Tank Application.pdf

 

[willard3]

Is that what you were asking about, water expansion?

It's certainly not clear from your question.

 

[Compositepro]

In a property designed system, the temperature does not affect the pressure .

 

[Sam654]

The only way pressure can increase due to temperature in a closed loop is if there is no expansion tank (or expansion tank is "solid") and there are no leaks. If you have some leaks, pressure would then be determined by your makeup source. The only thing I would add to Compositepro's post is "properly designed, operated and maintained" system....

 

[willard3]

Pressure does increase with temperature even with an expansion tank....just not very much and it is ignored in a system with an expansion tank.

 

[micksouth]

Thanks for the responses.
Basically, our CHW runs at 6c most of the day at around 5.2 bar.
When the chillers shut down over the weekend the CHW temp can rise as high as 30c on a hot day, which results in pressure rise and water being lost out of the Pressure Release Valve (set to 9.5 bar). What I need to calculate is what max temp to auto start the chillers in order to keep the pressure below 9.5 bar.
When the chillers restart on Monday morning, the pressurisation unit has to make up the lost water and sometimes trips out on excessive running. The pressure vessel has been checked and is ok but maybe the pressure increase is too much for it to cope with.

So to be clear: What temperature does the water (which normally runs at 6c at 5.2 bar) need to reach befor it exceeds 9.5 bar?

All answers welcome. Thankyou very much.

 

[IRstuff]

It wouldn't just be the chiller, per se, would it? Wouldn't you also need a recirculation pump? Otherwise, the warm water in the pipes are expanded and applying pressure to the reservoir.

TTFN
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[IRstuff]

and return from the recirculation. That's what my house has, a closed loop that recirculates hot water through the heater through the main line in the roof, and back to the heater. It's on a timer to turn on before we get up in the morning so that we don't stand there dumping gallons of cold water that was sitting in the lines in the roof overnight.

TTFN
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[DRWeig]

Again, micksouth, that answer (temperature to reach 9.5 bar) depends entirely on your system configuration.

1) Measure it. Log pressure and temperature over a hot weekend.

2) Fix your system. This shouldn't be happening at all. Do you have too much water and no air in your expansion tank? Is your expansion tank way too small?

3) If you decide that you must run the chillers to stop this problem, don't use temperature as an analog for what you want to measure (pressure). Put a pressure switch in there or a pressure transmitter.



Best to you,

Goober Dave

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[MTES93]

I agree w/ DRweig, address the apparent expansion tank issue rather than cycling chillers on when not needed.

The VEMCO link 25362 posted is a good refresher... Glad to see that a Montana rep firm link made the post. Nice!