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Pump Myth #3: I AM Adding Glycol to My Existing Hot or Chilled Water System. No Big Deal

Well, it may or may not be a big deal. There are a number of issues to consider. We will not consider the issue of fluid compatibility with materials. That will likely not be an issue but best to check the compatibility of things like valve elastomers. The things you do want to worry about are heat transfer and pump performance.

This blog is mainly about pump performance, but let’s touch on heat transfer, too. But first, let’s go back a step and talk about the properties of glycol solutions that matter. As opposed to water, glycol solutions have different viscosities (resistance to flow), specific gravities, specific heats and thermal conductivities than water. The effects on performance are cumulative for the various properties (they add up). They depend on the concentration (30%-40%-50%?) and they depend on the temperature. As this is but a humble blog, and not a full-blown presentation, let’s avoid the murky details and go to the bottom line…

Heat transfer: I think I could make a safe statement that the heat transfer will be less effective after adding glycol. How much will vary on %, temperatures and approach temperatures. But you will likely lose capacity and the only way to accurately determine how much is to contact the equipment manufacturer of your coils, chillers, boilers, fan coils, etc. For sure, you will have to pump MORE glycol to get the same performance. Note that depending upon concentration, you WILL need about 5-10% more flow to achieve the same BTUH capacity at the design delta T of the system.

Pump performance: Again, this is but a blog but here is are a couple things to remember. 1) Higher percentage, more effect, 2) Temperature affects pump performance—cold temperatures (40 degrees F, for example), have much more effect than 190 degrees F). 3) Propylene glycol has greater effect than ethylene and 4) Glycol affects small pumps (<100 GPM) more than large pumps.

This is all covered in more detail in our course HYD-120 but here is an example: Let’s say that you want to go from water to a 40% propylene glycol solution in a system making 40-deg F chilled water. First of all, the friction resistance of the system would increase by about 40%. So right away, you are pretty far behind the eight ball! Now, realize that in addition to affecting the system, these parameters affect the performance of the pump itself. Per the factors presented in HYD-120, your existing pump will be about 10% short of its original capacity The head will be about 10% short…uh-oh…this temperature and concentration will result in about 40% more pressure drop than water at the design flow rate and almost 50% more to hit the new flow rate. On top of all that, your power required will be about 5% higher for this viscous glycol than water.

Net effect, very approximately, you need:
1.1 (more flow required) X 1.1 (pump flow derate correction X 1.05 pump power increase X 1.4 friction loss correction increase) = About 1.78 times as much power to make it all happen!

So, is the moral of the story to figure more to double the power and select bigger pumps for all glycol solutions? Nope. For a 500 GPM system utilizing 30%, 190-deg F ethylene glycol, the corrections would be basically insignificant!
So, the moral of the story is 1) dig into it, do the math and do not assume, 2) be very wary of glycol type and percentage, pump size and especially cold glycol! Sorry—it simply isn’t all that simple!

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