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  1. #1

    Hot Gas Reheat Retrofit Question

    Hey guys,
    I have a client with three Carrier air handling units (a 25 Ton, 15 Ton and 10 Ton). These units cool and dehumidify their zones all year due to high internal loads. During hot summer months, these units consume a significant amount of natural gas to reheat the air after dehumidification. I suspect utilizing hot gas reheat within the units could reduce or even eliminate their summer natural gas consumptions. I have seen hot gas reheat on many newly installed units. However, I do not know if it is possible (or common) to retrofit an existing unit to have hot gas reheat. Is it possible or common? Thanks.

  2. #2
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,720
    Probably not viable could do more harm than good.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  3. #3
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281
    Quote Originally Posted by Kleinhenz View Post
    Hey guys,
    I have a client with three Carrier air handling units (a 25 Ton, 15 Ton and 10 Ton). These units cool and dehumidify their zones all year due to high internal loads. During hot summer months, these units consume a significant amount of natural gas to reheat the air after dehumidification. I suspect utilizing hot gas reheat within the units could reduce or even eliminate their summer natural gas consumptions. I have seen hot gas reheat on many newly installed units. However, I do not know if it is possible (or common) to retrofit an existing unit to have hot gas reheat. Is it possible or common? Thanks.
    A ton of outside air being introduced then? and for what reason?

    Whats the application???

  4. #4
    Join Date
    Mar 2003
    Posts
    7,326
    it can be done with limited success, and lots of attention to detail. Typically hot gas reheat obviously uses discharge gas. You will need a valve to flop the discharge gas to the coil, and back to the condenser. You will need a check valve in the return from the coil to where you tie back into the condenser piping to prevent migration, you will need an oil scavenging line in the reheat coil piping, and obviously a coil sized correctly. You will then need good control of this thing, and carefull attention to oil return to prevent coil logging and compressors going boom.

    Would I attempt this as a service man.....no way man. offer to sell them new units utilizing factory hot gas solution, and sell based upon the benefit of the energy savings as well as act 579. units should almost pay for themselves.

  5. #5
    Actually, there is no outside air being introduced (They don't meet meet code, which is another story). It is a manufacturing facility and these three units cool the production area. There are enough latent and sensible loads internally that cooling and dehumidification is needed even in the winter (with no outside air they also don't economize).
    Despite their lack of of heating loads in the space, they still use a lot of natural gas energy simply to reheat the air after dehumidification. I am trying to help them cost effectively reduce this re-heat energy. I thought hot gas bypass would be ideal, but I suspect it is not a viable option in a retrofit scenario.

  6. #6
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281
    Quote Originally Posted by Kleinhenz View Post
    Actually, there is no outside air being introduced (They don't meet meet code, which is another story). It is a manufacturing facility and these three units cool the production area. There are enough latent and sensible loads internally that cooling and dehumidification is needed even in the winter (with no outside air they also don't economize).
    Despite their lack of of heating loads in the space, they still use a lot of natural gas energy simply to reheat the air after dehumidification. I am trying to help them cost effectively reduce this re-heat energy. I thought hot gas bypass would be ideal, but I suspect it is not a viable option in a retrofit scenario.
    What are they doing that is producing the moisture? Generally when colder its drier then a pop corn fart outside. Meaning free air is mo better in both regards

  7. #7
    It is an injection molding process. I am not completely sure where in the process all of the moisture comes from, but I am sure the many plant workers also contribute. We are definitely recommending that they start bringing in outside air and economizing during winter months.
    Thanks for the opinions and comments guys! Based on the comments, I don't think I will pursue a hot gas reheat retrofit. I will analyze the cost of completely new units versus their current reheat costs though. Maybe that is also a cost effective route.

  8. #8
    Join Date
    Mar 2003
    Posts
    7,326
    you just added more savings to the mix, outside air in the winter verses mechanical cooling. sell them new units, or i will. it will be damn near free.

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