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  1. #1
    Join Date
    Nov 2009
    Posts
    28

    Cooling system modif

    Hi everybody,

    I want to reuse a 36" main duct originally sized for 14 000 cfm but now with only 7000 cfm. Do the VAVs remaining in the system supplied by this main duct will be able to operate properly?

    Regarding the cold deck supplying this system. Can I resize the coils and the fan for 7000 cfm (originally sized at 14 000 cfm). Is this cause any problem?

    Thanks
    Alex
    Last edited by alupien; 10-24-2010 at 07:48 PM.

  2. #2
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    A typical vav system will accommodate part load conditions, that said, what are asking from it at low load conditions. If your air volume across the coil is less than what the refer system is able to turn down to, provisions must be made. If you have two cooling coils, the second will no longer be activated if the controls are correct. Number of stages and hgbp must be considered. Cycling rates might increase and efficiency will certainly drop.

    Is this 7000 cfm situation permanent or is there an occupancy issue that may reverse the needs?

  3. #3
    Join Date
    Nov 2009
    Posts
    28

    cold deck

    The situation is permanent. The cold deck is now dersserving offices (7000cfm) and a machine shop (7000cfm). The odors from welding, cutting, etc. are drawn by the 7000 cfm return grill in the machine shop. I want to present two options to the client to solven this contamination problem:

    Option 1: dowsizing coils and drawthru fan to 7000 cfm (only one stage with three way valve). Keep existing ductwork for office and add a new independant air handler in the machine shop.

    Option 2: Keep system as existing but add an air cleaning system (eg: filters) but I am not very familiar with this kind of system

    what do you think?

  4. #4
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Considering one system for your app would be a mistake I think. The office won't tolerate the fumes or oders and will wind up being an OSHA issue at the least.
    If you have a chiller and boiler to use, great. Reduce the flow thru the existing with a drive or other means and serve one area, add another ah for the second. You'll still need to filter the machine shop at the point of return to reduce fouling of the duct and coils. The shop will need plenty of OSA and ach so calculate that into the load. Economizers, Exhaust,preheating ,etc are all considerations.

    Scratch option2, give them something else to say NO to.

  5. #5
    Join Date
    Mar 2003
    Posts
    7,321
    Reconfigure the system so that the shop air is exhausted as opposed to returned. Cut the return off from that area, reduct to a heat exchange unit and exhaust it, then bring in the required fresh air to match. Use a flat plate style exchanger to recover the sensble load exhausted.

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