Why Chilled Water Supply Temp = 7C?

[murphymok]

Hi All,

A lot of people are using the rule of thumb for chiller design, Chilled Water Supply = 7C, and Chilled Water Return = 12C. Why is that the case? I heard this can be explained using the psychrometric chart. Please advise.

Thanks in advance.

MM.

 

[carlosgw]

I'd like someone to expain (using the chart)it too. It was/is a good compromise between cold enough to be effective for cooling and not so cold as to be inefficient and freeze the chiller. Only the first part of this would be shown on a psych chart.
Without checking, I am assuming 7C and 12C corrispond to 45F and 55F.

 

[Turtle77]

A very good site for information on Air Psychrometrics and mini tutorials in understanding psychrometric charts and getting the set points you want is available at the Engineering Toolbox website. See the following link:

http://www.engineeringtoolbox.com/air-psychrometrics-properties-8.html

Click on

http://www.engineeringtoolbox.com/psychrometry-8_516qframed.html

for an adobe interactive tutorial which covers using psychrometric charts. (It takes a few seconds to load though)

They've got a lot of good information on other engineering stuff too!

Best of luck!

 

[quark]

I don't think it is hard and fast. However, many references can be drawn with 7[sup]0[/sup]C chilled water temperature. For instance, normal industrial requirement for space conditions is 22+/-2[sup]0[/sup]C and 55+/-5% RH. For the worst case scenario, with 22[sup]0[/sup]C and 50%RH, the dew point is about 50F. If you allow a bypass factor of 5% and considering losses in the chilled water system 45F chilled water is good.

Further, that is about the safe temperature a chilled water system can maintain. Generally, you shouldn't go below 5[sup]0[/sup]C and you are safe with an antifreeze thermostat setting at 6[sup]0[/sup]C.

Incase of trouble, you can reduce the temperature to one more deg.C(say, during high latent load conditions).

etc.etc. I hope somebody else will come up with some more points.

Regards,

 

[lilliput1]

In US standard rating of chillers is 45°F (7.22°C) supply and 55°F (12.78°C) return. With 45°F chilled water supply you can get 53-55°F cooling coil db and close to saturation which is ideal for typical office applications. For pharmaceutical applications w/ 100% outdoor air & 78°F wb outdoor design, it is not uncommon to use 42°F supply instead. It is desireable to limot coils to maximum 8 or 10 row and 14 fins per inch. It would be difficult to clean coils with more rows & finning.
Studies have shown optimum temperature rise is 14, 15°F instead of 10 for economy of pumping & pipe sizing.

 

[sethuME]

generally the supply air temperature of the AHU will be +5 deg. C of the supply chilled water temperature.in pshychrometry chart refer what should be the supply air temperature to maintain the inside condition,with its RH.the supply air has to maintain the room temperature say 24 deg. C then the supply air can be even at 18 or 16 deg.But the RH cannot be maintain.

 

[digitrex]

I would like to take this opportunity about chiller temperature control.

In my office chilled water system, the Return CHW temperature is being used for control of the chillers. I find it strange. Do you think we should change to regulate the Supply water temperature instead of the Return?

 

[ChrisConley]

In a nutshell digitrex, if the return water is coming back cold, then the system isn't using the chilled water. This means that the chiller can send out warmer supply water.

If the return water is coming back warm, then the system needs more cooling and the chiller will send out colder supply water.

So supply and return water temperatures need to be monitored by the chiller.