derating cooling coils for chilled water with glycol

[pgmmce]

I am inquiring about a derating factor to be applied to the performance (sensible & latent)of a chilled water coil with 30% propylene glycol. The chiller mfg. suggests a 5% derating for the chiller performance. Is this derating the same for the coil/air handler. If not, why not?

 

[willard3]

Read here:

http://www.dow.com/heattrans/prod/glycol/

 

[mechanicaldude]

You can find the specific heat and density of the glycol solution from data in ASHRAE Fundamentals. With these two numbers you get a "K" factor for the solution. The formula for water is Q = K * GPM * delta T where Q is heat transfer in BTU's, delta T is your change in water temp. (typically about 12 degrees for chilled water) and K = 500 for plain water.
You can find the new K value from the data in ASHRAE, but it is dependent on the % of the solution and the water temperature. If I assume that the water temp. is about 40 degrees F and the delta T is 12 degrees F, your new "K" value for your situation is 471. 471/500 = .942 or about 6% less.

In short, the derating of your coil will depend on the percent of glycol and the temperature of the water, but you are looking at about a 5 - 6 percent derating. So what the manufacturer told you for the chiller should also applies to the coil. FYI the colder the water temp the more the derating factor there is for the glycol.

 

[pgmmce]

Thank you,
I agree with you but when I was sizing cooling coils with a mfgs. rep., he said the output he was getting from his program derated the output by 30%. It did not make sense to me that the derating factors would be different from chiller to air handler/cooling coil.Ever heard of that?

 

[mechanicaldude]

I'm not sure. I've used different manufacturers software to size coils before (specifically Carrier's) and I know that these programs had trouble accounting for the glycol accurately. This was confirmed to me by the sales reps who use the software so I'm not sure I trust all the results when it comes to glycol and these software selection programs.

Now I can't tell you that these problems haven't been fixed or that the program you were using had that same issue, but that is my experience with it.

If you still aren't sure call a few sales rep from Carrier, Trane, York and just ask them as a general question how there cooling coils would be derated by adding 30% prop. glycol.

 

[pgmmce]

The issue to me is how the heat transfer can be different in the chiller versus the cooling coil.
It shouldn't be, but maybe I am missing something.

 

[mechanicaldude]

No you're not missing anything. The derating should be the same for the chiller and coil.