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Hello all and thank you in advance for any help!

I have seen two general types of liquid-to-air heat exchanger test setups: an "open loop" (top) and "closed loop" design:

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I personally have used the open loop setup for my research and I haven't had any problems. However, I've seen others in my industry using the closed loop setup.

As far as I can tell, this setup is much more expensive due to the need for a cooling coil. While it does let you test with air inlet temperatures below ambient, and it does give more precise control over RH, I don't see why these features are necessary. Why would you want to test with air inlet temperatures below ambient? It's not like the absolute temperature of the air makes a big difference to HX performance - rather, it's the temperature difference between the HX and the air that matters. If I need a bigger temperature difference, I just turn up my water temperature. Thoughts?

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  • $\begingroup$ The cooling coil can be used to dump heat into a reservoir. $\endgroup$
    – Solar Mike
    Commented Oct 1 at 15:34
  • $\begingroup$ I don't understand. Why would you want to do that? Could it improve test accuracy or would it just be some energy efficiency thing? $\endgroup$
    – Emily Conn
    Commented Oct 1 at 15:56
  • $\begingroup$ because the medium is expensive and you want to re-use it. Steam locomotives used water once, typical rankine steam cycles re-use the water over and over. No one would just cool air for the fun of it. $\endgroup$
    – Tiger Guy
    Commented Oct 1 at 16:35
  • $\begingroup$ This about test rigs in a lab so energy/water efficiency isn't really a concern typically $\endgroup$
    – Emily Conn
    Commented Oct 1 at 19:30

2 Answers 2

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Just comparing the two systems you have shown, I can give you one example where you might prefer the closed loop version:

Lets say you're making a datasheet for a heat exchanger. It might say the following: "all figures specified at 30% RH, 20degC ambient".

With the open loop system you have shown you cannot control RH and temperature simultaneously. You could not create that exact run condition for your test.

You could compensate your data. But that adds more steps where there may mistakes or a reduction in accuracy.

Or you could change the datasheet to specify whatever conditions you actually tested it in. That's a no-go if you have multiple products and you want them all to be spec'd in standardized operating conditions.

So TLDR; If you want tight control of the temp and rh, and you want your conditions to be independent of the lab ambient environment, closed loop makes a lot of sense.

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After speaking with others in my industry, I determined that the following was the most significant reason:

Evaporator coils need to be tested in condensing air-side conditions because condensation on the coil impacts pressure drop and heat transfer performance. Thus, the test rig must be capable of creating condensing conditions.

If you just want to get the heat transfer performance of a condenser, there's no need to use a closed loop system.

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