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64/65 design space temp

Need some help in laying out a cooling system for a local health club. In their aerobics room they would like to maintain a temp of 64 to 65. In other locations designers used standard design and when they try to go that low invariably they freeze the coil. What I'm thinking is up sizing the AHU with a condensing unit sized for the heat loss. With this type of set up wil we be able to get lower space temps, what about humidity, icing coil etc. Any help would be appreciated

Comments

  • Brad White
    Brad White Member Posts: 2,393
    Be careful what they wish for.....

    Aerobic breathing in that environment will show rapid humidity changes with every "feel the burn!"...

    I think humidity control is key so let's assume your people load both sensible and latent reflects athletics.

    If you up-size the AC unit you lower your coil face velocity which will start the freeze-up cycle quite nicely, thank you.

    If you are stuck producing 54-55 degree air you simply need twice as much of it. But your humidity control will suffer especially if you do not have the sensible load to keep the equipment running.

    My first suggestion is to up-size as you say but have a hot gas reheat coil in place both for dehumidification and as a false-load. Energy-painful but it will work. At least the reheat is free, being rejection-bound anyway.

    What I did for a night club (Rave type not Copacabana type) was to install a couple of low-temperature AC units emitting 39-40 degree air as the principal dehumidification means. Sensible heat was taken up with more conventional equipment.

    My second suggestion is to use two separate parallel units, one low temperature properly sized for the application as a base-line (will do some serious sensible cooling too). Fill in the rest of the sensible load with something more conventional.

    I would talk with the Owner if they really need those colder temperatures or could settle on an equivalent slightly warmer but drier environment. Maybe 68 degrees and 40-45% RH? You can reach that RH but will need air cooled to 42 degrees. Cutting it close!
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Jim M
    Jim M Member Posts: 29


    Brad thanks for the reply. Your spot on about the humidity, in my experience (service only) I had to fight to keep the thermostats no lower than 68, they want it cold.

    I'll be using split system dx and was thinking about up sizing the AHU with the cond unit sized for the load (latent & sensible) this way face velocity on the coil will be appropriate (limit freeze). But that would lower my sensible cooling ratio to the point that it may not go below dew point.

    Your idea of a parralel ahu is interesting, what type of unit did you use, did you consider plate type air to air hx, that way we could just dump the latent energy.

    Again thanks for the help

    Jim
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Parallel Unitverses

    Jim, Hi.

    The night club setup was an amalgam of several older systems. I was brought in to help get the humidity down and better ventilate it. So to separate the targeted approach from the whole let me say this:

    The dehumidification units were double-wall special made by McQuay I believe. Higher quality than your garden variety DX FCU and ran about 1500-1800 CFM if I recall. We wanted to keep them below the 2000 CFM NFPA-90A smoke detector limit.

    These were entirely recirculating and the refrigerant tech had his work cut out for him, balancing head pressures, superheat, all of that at a technical level I wish I knew better.

    We got the discharge air off the coil at about 38 degrees withhout frosting over. By recirculating versus outside air going through these we could keep things pretty much steady-state. The other units served OA introduction and conditioning. The condensing units were between 7.5 and 10 tons, I am thinking 10 tons in the end.

    Any colder and we would have been looking at dessicant dehumidifiers and wanted to avoid those due to cost and contractor familiarity.

    Face and bypass dampers were used to balance the coil and fine-tune the load, also to adjust for head pressure. This was a critical feature. Important that the bypass path have enough restriction -a damper- so the air has to make a choice versus going through a wet coil if that makes sense. No damper and the path of least resistance is a no-brainer for the airflow.
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