Glycol Effect on Chilled Water Coils
Glycol Effect on Chilled Water Coils
(OP)
What am I missing here? I know glycol has a derating effect on heat transfer, but the numbers I am seeing here don't make sense. I know that based on density and specific heat changes, a solution of 30% propylene glycol at 40F should have less than a 10% derate in heat transfer capability. Correct?
I ran a chilled water coil selection using 40 degree water. I then kept the airflow and inlet temperature the same, and I kept the entering fluid temp and GPM the same. All I did was switch from water to 30% propylene glycol, but the total heat transfer was significantly less (34% less). See attached selections.
What am I missing to explain this difference over what the density and specific heat changes would suggest? Thanks for your help
I ran a chilled water coil selection using 40 degree water. I then kept the airflow and inlet temperature the same, and I kept the entering fluid temp and GPM the same. All I did was switch from water to 30% propylene glycol, but the total heat transfer was significantly less (34% less). See attached selections.
What am I missing to explain this difference over what the density and specific heat changes would suggest? Thanks for your help





RE: Glycol Effect on Chilled Water Coils
By keeping the glycol flow the same as the water flow you've had a disproportionate impact on the LMTD which is impacting on your heat transfer. Sorry I can't comment further without looking up the program and the acronyms used for the inlet/outlet fluid temperatures.
As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"
RE: Glycol Effect on Chilled Water Coils
Water is a great heat transfer medium, low viscosity, high specific heat, high thermal conductivity. It's hard to beat. If only it wasn't susceptible to freezing and corrosion.
I have some reference material which gives you an indication of how the inside heat transfer coefficient changes with fluid properties but it's at work, will have to look it up tomorrow.
RE: Glycol Effect on Chilled Water Coils
Effect of a change in viscosity on the tube side inside heat transfer coefficient is (u2/u1)^0.47, turbulent flow.
For thermal conductivity, it's (k1/k2)^0.67
For specific heat ratio, it's (Cp1/Cp2)^0.33
For mass flow rate, it's (G1/G2)^08 (you have a constant volumetric flow rate which is not a constant mass flow rate).
I didn't dig into the inside heat transfer film coefficient equation to confirm the above order but I suspect that is why viscosity is u2/u1 which the others are the original value divided by the new value.
This is only 1 part of the overall heat transfer coefficient. You have the air side resistance and any fouling coefficients that could be built into this program, these tend to off-set the effect of a change in the inside heat transfer coefficient. Given the change you are seeing, the air side isn't as controlling as much as I thought it might be.
RE: Glycol Effect on Chilled Water Coils
But if I am understanding correctly, every piece of heat transfer equipment may no longer deliver the heat it did before. Even if flow is increased by the "correction factor" listed in the glycol literature. Correct?
RE: Glycol Effect on Chilled Water Coils
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RE: Glycol Effect on Chilled Water Coils
I would be delighted for an article that actually discusses all of this. All I can find are either generic ones that just list "correction factors" or ones that list tables of physical properties without explaining how they affect the heat transfer.
RE: Glycol Effect on Chilled Water Coils
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RE: Glycol Effect on Chilled Water Coils
Just a reminder about adding glycol to existing systems that don’t have it now. There are 3 main detriments to adding glycol to a system:
1. Glycol has less heat capacity than water.
2. Glycol is more difficult to pump than water.
3. Glycol has less thermal conductivity than water.
Most of the glycol literature does a good job of explaining the first 2 detriments; however, the literature almost always neglects to mention the third. Having less heat capacity means that it takes more glycol to deliver the same amount of heat as straight water. So it takes more of a heavier fluid to convey the same heat energy as water. It takes more pump head to move the glycol around, and you get less heat out of it. The glycol literature does a good job of reporting this and lists “multipliers” or “correction factors” to account for the difference. Most of the literature boils it down to saying that a certain percent increase in flow will make all of the problems go away. They also point out that a pump change may be required due to the increased head, but otherwise, they lead you to believe that more GPM is the answer. THIS IS ONLY PART OF THE STORY!
It must be understood that this only accounts for the heat capacity of the glycol and NOT its ability to TRANSFER that heat. Glycol also has less thermal conductivity, so it will not give up the heat as easily as water. Almost all of the literature neglects this part, which can come back to bite you. If you are installing a new system with glycol, all of the equipment, heat exchangers, coils, etc. will be selected with glycol, so there should be no issues. If, on the other hand, you are putting glycol in an existing water system and re-using coils, you may now have grossly undersized heat transfer devices.
The magnitude of the problem changes with fluid temperature, and it is much more prominent for cooling applications. For heating with a glycol temperature of 180 degrees, the heat transfer from a coil will be on the order of 10% less than it otherwise would be with straight water. You may be able to get away without changing the coil. A 40 degree cooling coil with glycol, on the other hand, may only deliver on the order of 50% of the heat transfer that it did with straight water.
So bottom line, if you are adding glycol to a chilled water system, be extremely careful that your equipment and coils are going to function correctly. Simply increasing the GPM won’t cut it!
RE: Glycol Effect on Chilled Water Coils
At 200*F Ethylene glycl has 4x the dynamic viscosity of water.
At 40*F it is 30x.
je suis charlie
RE: Glycol Effect on Chilled Water Coils
RE: Glycol Effect on Chilled Water Coils