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Using a heat exchanger to increase latent heat extraction

DCcontrarian | Posted in Mechanicals on

This is a follow-up to a discussion from an article that was recently posted about dehumidifiers — https://www.greenbuildingadvisor.com/article/dehumidifying-with-portable-unit-vs-ducted-system?  *– and a discussion in the comments. I wanted to break that discussion into a new thread because it’s really a separate topic, and posts in the Q&A seem to get more and better comments.

I feel greatly indebted to Charlie Sullivan, who turned me on to the idea of using a heat exchanger in combination with a regular air conditioner to make a dehumidifier with very high latent heat removal. If you look at posts #8 through #11 in the comments, I expound on the idea in great detail, and I came to the conclusion that doing so would use about 75% less electricity for the same level of dehumidification as a standalone dehumidifier, or even a conventional whole-house dehumidifier.

The idea is that the output of an air conditioner is run through a heat exchanger, where air entering the air conditioner is cooled and air leaving is warmed. If the heat exchanger were 100% efficient the air would leave at exactly the same temperature as it entered at, there would be no sensible cooling. When the air was cooled, moisture in it would condense out, so there would be the same level of latent cooling as a similar-sized air conditioner would develop. 

This is such  dramatic improvement over conventional dehumidifiers that I’m wondering if there is a catch that I’m not seeing. 

Thoughts?

*(Ironically the article was a “Q&A Spotlight,” where questions here get expanded into full articles. So we’ve gone full circle!)

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Replies

  1. FrankD | | #1

    DC, there are a couple extra characters at the end of your link. Here it is for anyone trying to find the discussion:
    https://www.greenbuildingadvisor.com/article/dehumidifying-with-portable-unit-vs-ducted-system

    1. Expert Member
      DCcontrarian | | #4

      Thanks, not sure why it's doing that. I think I've fixed it now.

  2. Expert Member
    Akos | | #2

    Interesting.

    With a water coil, this is not too bad of a DIY add on. Get a 2nd coil put it after the cooling coil.

    Set the water flow rate and air flow rate through the cooling coil so the water outlet temperature gets to pretty close to room temperature. Run this water into the 2nd added in coil as a reheat. If you want cooling and dehumidification, bypass the water around this 2nd coil.

    A more universal setup would be to use a counterflow HRV core. One side gets the air flowing to the AC coil, the other side, the air out of the AC coil. If you want cooling, bypass the HRV core.

    1. Expert Member
      DCcontrarian | | #3

      The reheat is basically what I lay out in post #9, "Scenario two," in the original article. It is more efficient than a stand-alone dehumidifier, but not nearly as good as using a heat exchanger.

      Using a counterflow HRV core is exactly what I have in mind, all of the sensible heat you took out of the air gets returned and reused, so it's super-efficient.

      In theory it could be used on any air conditioner, so it's more of a configuration option to an existing system than a new piece of equipment. The one thing I'm not sure about is that your system has to be able to modulate pretty low. In the scenarios I lay out removing the sensible load and just having the latent load reduces the total load to about 40%. So the compressor has to be capable of running at 40%. The less humid it is, the lower that has to be.

      This is a reason that I think this approach is particularly well-suited to hydronic cooling, because with a buffer tank a chiller is essentially infinitely capable of modulation. The other reason is that people with hydronic systems often want to do 100% sensible cooling -- cooled floors or panels -- and this is a way of complementing that with 100% latent cooling with no energy penalty.

      1. Expert Member
        Akos | | #5

        The HRV route would not be too bad to try. You would need a pretty big core though, part to increase heat transfer with the small delta T but part to reduce friction losses.

        Big resi HRVs are around 200CFM and those need almost 0.3"WG pressure for rated flow on each side. So that would be 0.6" total, doesn't leave much for house ducting. If you can run at 100CFM it will probably work but definitely not using that with any modulating AC. It would work with a small hydro coil.

        If they would still make those dual core HRVs, you could re-plumb it with a small hydro coil and get fresh air and dehumidification in one box.

        1. Expert Member
          DCcontrarian | | #6

          Thanks, that's the kind of detail I'm looking for.

          The HRV's I'm looking at have a built-in fan, like this one:
          https://www.supplyhouse.com/Honeywell-Home-VNT5200H1000-TrueFRESH-Heat-Recovery-Ventilator-200-CFM

          So they shouldn't affect house ducting. What's more of an issue is just finding a HRV that has enough capacity to go on even a small ducted system.

          One issue I see right away is that unit draws 180 Watts for 200 CFM. In my scenarios, I was calculating 183 Watts for 100 CFM. So having the HRV powered ups the power consumption by about 50%.

          1. Expert Member
            Akos | | #7

            Not sure that is what you want to start with. That doesn't look like ECM blowers, so pretty high power use. It is also a cross flow core, so efficiency is not that great.

            I think taking say two of these in parallel:
            https://gasexperts.ca/product/lifebreath-hrv-hexagon-heat-core-68-221r/

            And using the blower of the HVAC to push through both sides might work. Might be able to get 150CFM or so through it at reasonable power.

            Still far cry of 400-600 cfm needed for a ducted AC setup.

  3. Expert Member
    DCcontrarian | | #8

    Akos, here's my mental image:

    There's an air handler with a coil in it which can provide either heat or cooling. When you're cooling, at the flip of a switch the air path changes, and instead of just going through the coil air comes in at the return, goes to the inlet of the heat exchanger, to the intake of the air handler, back to the other side of the heat exchanger and from there to be distributed. This is done with motorized dampers in the ductwork. So now you've got three distinct modes: heat, cool, and dehumidify.

    I think -- but I'm not sure -- that it would be simplest to have a booster fan for dehumidify mode, to make up for the additional resistance from the circuitous route. The alternative would be to have one fan that is big enough for dehumidify but can be turned down for heating or cooling.

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