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  1. #1
    Join Date
    Jan 2009
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    Keokuk, IA
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    How Much Ventilation in Wet Location

    We have a building that houses an open Sludge Press for our onsite wastewater treament facility. The building is (I can go get exact measurements) approximately 60' square with 15' ceilings. IT currently has a large upblast exhaust fan (needs to be repaired or replaced, but the roof deck is rotted out so they can't work up there) that I would esimate is about 36" diameter and probably around 15,000-20,000 CFM.

    The walls are cinder block. It's a low pitched roof, that's pitched in one direction. Roof supports are engineered steel trusses (not sure if that's the right term).

    A sludge press is a large roll press that has a fine mesh belt and sludge is squeezed between rollers for dewatering. Below the press is an open pool with warm waste water. THe pool I'd estimate is about 20' square. The press is the same width.

    The roof deck is... or rather was galvanized steel. I'm fixing the roof deck problem by going with a T&G treated lumber (couldn't find T&G marine grade). I'd going to blast and repaint the trusses.

    But I need to resolve the building venting.

    How mch ventilation do I need? Short term after the roof is repaired I'm going ot add a 48" square louvered inlet. But I suspect I'll need a MUA unit for winter to temper the air.

    How do I calcuate the the amount of MUA??? Do I target a certain ACH?

  2. #2
    Join Date
    Jan 2009
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    Keokuk, IA
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    I think I meant to put this in the Pro forums.

  3. #3
    Join Date
    Jun 2006
    Location
    New Jersey
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    4,289
    Quote Originally Posted by motoguy128 View Post
    We have a building that houses an open Sludge Press for our onsite wastewater treament facility. The building is (I can go get exact measurements) approximately 60' square with 15' ceilings. IT currently has a large upblast exhaust fan (needs to be repaired or replaced, but the roof deck is rotted out so they can't work up there) that I would esimate is about 36" diameter and probably around 15,000-20,000 CFM.

    The walls are cinder block. It's a low pitched roof, that's pitched in one direction. Roof supports are engineered steel trusses (not sure if that's the right term).

    A sludge press is a large roll press that has a fine mesh belt and sludge is squeezed between rollers for dewatering. Below the press is an open pool with warm waste water. THe pool I'd estimate is about 20' square. The press is the same width.

    The roof deck is... or rather was galvanized steel. I'm fixing the roof deck problem by going with a T&G treated lumber (couldn't find T&G marine grade). I'd going to blast and repaint the trusses.

    But I need to resolve the building venting.

    How mch ventilation do I need? Short term after the roof is repaired I'm going ot add a 48" square louvered inlet. But I suspect I'll need a MUA unit for winter to temper the air.

    How do I calcuate the the amount of MUA??? Do I target a certain ACH?
    Maybe I'm not understanding the question........you have a exhuast fan the was properly sized (for the application) and the airflow was verified. Then you need no MUA.

    Maybe you mean the exhaust fan is suppose to move "X" amount of cfm and you assume it's not. So your adding a 48"x 48" square inlet louver, but also think you need injected air.

    You need to know these items to answer your questions-
    1) What is the volume of air in this space?
    2) What air exchange per hour and/or minute does it require?
    3) Does the air in the space need to be a certain temp and/or humidity?
    4) Can the space go into a slight negative?
    5) If "no" to #3 and #4, then what size louver do I need to provide to keep your exhuast cfm up.
    6) If "yes to either #3 or #4, then what do I need as far as a MUA to provide set temp/humidity and/or positive pressure to the space.

  4. #4
    Join Date
    Jan 2009
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    Keokuk, IA
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    I doubt it was properly sized, or they simply picked a arbitrary ACH value, used some rule of thumb or bought the largest they could find, or the same size the've boguht in the past. I suspect it was even added after the building was constrcuted since there is no accomidation made for supply air ventilation into the space.

    I need to get exact measurements or find the original drawings, but I'd estiamte the volume at looks to be approximately 14,000 cubic feet. Looking at a aieral view, it's a little smaller than I originally posted, closer to 30'x30'x15'.

    I don't know the ACH required. That's the primary piece of info I was after. WIth a damp location like a indoor pool for example, what ACH do you use if active dehumidificaiton its' being used. (NO AC system to dehumidifier)

    In winter, I'd like to keep it over 50F so I'll need some supplemental heat, but the rest of the year, within 5F of outdoor ambient is fine. There's currently 2 electric unit heaters installed in there. I think they are just 15-20kw each so only enough for freeze protection without any ventilation running.

    This is a unoccupied space without adjacent occupied building and no combustion venting. Negative won't be a problem, but it would be nice if doors could open and close properly. Slightly negative is probably preferred to keep moisture out of the walls and roof structure.

    SO question #3 is year and no. I don't have a specific temp and humidity target, other than non condensing. So I'll need year round ventilation, so I'll need to temper the air in winter by at least 40F at design conditions.

    I don't want to overcomplicate the system, but in winter when outdoor air is dryer, I won't need as much ventilation, so I could use a dewpoint transmitter with and output to cycle the exhaust fan or modulate it speed and install a VFD, then have a MUA unit that targets something like 0.05-0.1" negative building pressure.

    First I just need to know what volume of air I need in mid summer.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    Mayeb I can answer my own question I suppose. The real challenge woudl be determining the actual latent load off of the sludge press. If we assume the building right now has a passive ventilation rate of 0.5ACH and was holding at lets say 80%RH at 85F indoor temp yesterday then that shold give me a latent load.

    Next is determining when my worst case conditions are. I probably should try and keep the indoor <60%RH. Here's where it gets tricky. What time of year is worst case? It's probably spring and fall where I may need supplemental heat to lower the indoor RH. The best woudl be a "little back box" that looks at he indoor RH, the outdoor RH, knowns the spaces volume and calculates the ACH and has a min and max indoor temperature and minimum ACH rate, then determines the minimum airflow and temp rise needed.

    However, I'd really prefer something simple.

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