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Thread: Hot gas reheat

  1. #1
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    Hot gas reheat

    Suppose you have a conventional rooftop packaged unit with DX cooling and hot gas reheat. Some, or all, of the hot gas is taken and routed into the reheat coil. After hot gas leaves the coil, it's a mixture of gas and liquid. In the systems I typically deal with (such as Trane Voyager) this gas is then routed into the liquid line before the filter dryer.

    Under part load condition refrigerant coming out of the reheat coil may not be all liquid. Thus feeding it into the evaporator will reduce its capacity.

    An alternate way of routing refrigerant out of the reheat coil is to route it back into hot gas line going back to the condenser.

    My question is which is a better way of doing this from a energy effeciency and operational standpoint?

  2. #2
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    Quote Originally Posted by dd98724 View Post
    An alternate way of routing refrigerant out of the reheat coil is to route it back into hot gas line going back to the condenser.
    You'd need a refrigerant pump to produce the DP needed to force flow back into the condenser and would lose efficiency from the power needed to run it.

    The older voyagers modified by Mosco (Morganizer) were much better units than the current crop of Voyager/Precedent units churned out by Clarkesville. IMHO, that is.
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  3. #3
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    I'm not familiar with Morganizer's mod. What did they do to the Voyagers?

  4. #4
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    http://www.morganizer.com/

    They set the standard for hot gas reheat dehumidification. When Trane saw how successful they were, they jumped on the bandwagon with their own unit.
    Low Pressure Forever!

    If you know heavy metal, you can work anywhere-Dave Andreson

    Anchors Aweigh my boys, Anchors Aweigh!
    Farewell to foreign shores, We sail at break of day. Through our last night on shore, Drink to the foam. Until we meet once more. Here's wishing you a happy voyage home!

  5. #5
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    Recently PM'd a number of sites for corn storage. Yes, I'm roundup ready, if you must know The hot gas reheat was tied back into the condenser outlet at a check valve which then flowed into the receiver.
    It's a sight to behold, especially when you think a check is leaking past and you're trying to manage the sight glass!

    But meanwhile...
    efficiency wise, I'd have to take an educated guess. But I leave that to those who have undertaken painful hours dissecting the theses of engineers and consultants.


    Meanwhile, do the specs call for the outer limits on the specific refrigerant used? (E.G. using 22 for a space spec'd for 50*DB and 55%RH. )
    It's great to be alive and pumping oxygen!

  6. #6
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    I would have to say that a rooftop unit with hot gas reheat is not conventional. This sounds like a makeup air unit to me. I startup new Aaon units all the time with dehum./hot gas reheat and have never paid attention to where the outlet of the hot gas coil enters the condenser section. I will look on a couple that I am starting up tomorrow and report back to ya. I would think that with air leaving a cold evap. coil and then passing through the reheat coil, condensing hot gas would be no problem unless there is a starved evap with high superheat.
    It might get loud!

  7. #7
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    On AAON packaged units the refrigerant flows from the reheat coil to the condenser coil
    http://www.aaon.com/Documents/Featur...ary_071012.pdf

    and on the split systems the refrigerant flows from the reheat coil to the liquid line
    http://www.aaon.com/Documents/Featur...lit_071012.pdf

  8. #8
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    Quote Originally Posted by The Doctor View Post
    ...
    Meanwhile, do the specs call for the outer limits on the specific refrigerant used? (E.G. using 22 for a space spec'd for 50*DB and 55%RH. )
    Spec only calls for Hot Gas Reheat. I'm making several assumptions, such as my lowest temp off the evap is 55 deg and I only need to reheat to 72, which is what I used to size reheat coil.

    It appears that units will be used as 100% outdoor air, with no return. Refrigerant is R22.

  9. #9
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    we typically see it routed to the inlet of the condenser and a check valve between that point and the evap outlet. exactly where it is located depends upon piping access. the bigger issue we find is the routing of the pilot lines for the solenoid confuse some guys. we have hadto make several changesto these pilot lines at startup to get these bigass reheat valves to work correctly. we generally only see these on systems over 150 tons.

  10. #10
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    Thanks everyone. I ended up routing it to the condenser with a checkvalve. I'm also using an oil separator on hot gas line going to the coil. Hopefully that will prevent any future issues resulting from oil being collected in the HGRH coil. Time will tell.

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