Adding glycol to a chilled water system

[Takket3]

I have a 100% water chilled water plant that is going to get new chillers and be filled now with a 40% propylene glycol solution.

I need to check the existing cooling coils on the AHUs to make sure they are adequate to meeting the cooling load with the reduced capacity.

I know from the manufacturer's specific heat data that our Btu/hr = 500 * GPM * dT formula changes to 456 * GPM * dT

But, what about thermal conductivity? Do I need to factor that in? The thermal conductivity of water is 0.336 (BTU/hr*ft^2)(degF/ft). The glycol solution is only .215. Won't this further degrade coil performance?

 

[EdStainless]

If flow velocities are high then TC is not a major factor.
The 10% loss in heat capacity is the big one.
Will you be running colder? What about changes in flow (density and viscosity changes will impact pump performance)?

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

Why introduce even PG? Are you intending to operate at 0C or lower ambients? Why PG is relatively safe from a toxicity viewpoint, it's still a gummy kind of liquid, and the handling thereof is not trivial. I believe there's also some corrosion susceptibility increase with any glycol.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[lilliput1]

PG will affect capacity of coils, chillers and pumps. Also it expands more so additional expansion tanks may be required. To offset loss in cooling capacity you would need to lower chilled water supply temperature but this will reduce chiller capacity even more. Instead consider running chilled water pumps with chiller off during winter below 35 degrees F and allowing preheat coils to heat the downstream cooling coils or provide electric or steam heat tracing on outdoor insulated chilled water piping to prevent freezing.

 

[willard3]

Glycol/water will also change the viscocity so pump head will change.

 

[Takket3]

Thanks for the replies.

Basically we are putting it in because the Owner wants it. Their property management group is spread thin and they don't want to be bothered with draining and filling the system in the winter to prevent freezing. So they are willing to accept all the trade offs.

I told them we will have to revisit and maybe replace the pumps so we are aware of the head increase.

Sounds like the thermal conductivity isn't THAT great a deal which is mostly what I was worried about.

Thanks!

 

[IRstuff]

How far below freezing can the coolant get? One reason for the hassle of using EG is that at -30C and below, PG is extremely sluggish, while EG is still behaving as a liquid.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[EdStainless]

We used alcohol in our chiller system. Depends on the system and the reason.
The correct additive package of corrosion inhibitors is also a vital part of this process.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[cranky108]

I know of at least one case where the seals were not made to hold glycol.

 

[amorrison]

Agree with cranky108.
Make sure the pumps have mechanical seals - packing allows capillary action causing "driping of fluid.

 

[georgeverghese]

PPEG solution would have poorer heat transfer properties in comparison to plain water, also pump flow may be reduced due to higher imposed backpressure. As suggested, check pump seals for compatibility with PPEG solutions also.
Physical props for PPEG solutions required for cooling coil heat transfer and pressure drop calcs are available in text books and maybe on the net also.

 

[dbill74]

How much of the system is outside besides the chillers? For all the issues mentioned above, another possible option would be to heat trace piping exposed to ambient.

 

[BronYrAur]

I'm just seeing this post but absolutely you need to consider the thermal conductivity. The glycol literature is great at explaining the heat capacity difference. Usually it is explained away by just recommending something like a 10% increase in flow. Most of the literature also covers the increased pumping head as well, but almost none of them (that I have seen) talk about the thermal conductivity. I once ran a bunch of coil selections and did a comparison. I found as much as a 50% reduction in heat transfer, depending on the conditions. I don't have that at my fingertips to elaborate further.

In heating applications, the thermal conductivity was almost a non-issue, but with cooling it was huge!

The best thing you can do is have the coil manufacturer run both conditions and make a comparison.

 

[lilliput1]

The AHU cooling coil performance will be degraded. To compensate have coil performance calculation done to still match required duty by combination of reducing the chilled glycol supply temperature and increasing flow. Note the chiller capacity would be reduced. You would have to add additional capacity (chiller and cooing tower) if spare is not available. Your chilled water pumps would have to be replaced. You need to add chilled water pumps & condenser water pumps if additional chiller & cooling tower is required.