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Old 06-20-2012, 02:55 AM   #1
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exhaust fans


Hey guys, I was wondering if any of you have experience with exhaust fans. I need to know how many air changes per hour I should achieve in order to ensure that an odor will not escape the room when the door opens. Im doing a negative pressure room before entering the test area, and I need zero smell escape. Thanks in advance.
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Old 06-20-2012, 07:32 PM   #2
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First, I need to know if you are a terrorist making biological, radiological or chemical WMD.
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Old 06-21-2012, 09:02 AM   #3
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Are you serious dude?
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Old 06-21-2012, 10:37 AM   #4
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Quote:
Originally Posted by thehvacguy View Post
Are you serious dude?

All the negative-pressure rooms I have been involved with are for RG3 biohazard work. What can I say?
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Old 06-21-2012, 12:16 PM   #5
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Grow-op?
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Old 06-21-2012, 08:16 PM   #6
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First, my disclaimer, let me say I have no experience other than allergies and this is only my 2nd forum post.

To address the bomb theory, considering he said oder and not hazardous materials, chemicals, etc, I'd say it's more likely he is probably growing marijuana and not making crystal meth or building bombs or mixing pool supply store chemicals (all of which are tracked by the fed along with piping from home depot and other basic supplies, etc.... just fyi).

But from my end, assuming it is not marijuana and is indeed allergies because I will not help you break the law:
to eliminate a smell give us the size of the room and where you will be venting and the typical temp/humidity in the room.

Without this information I recommend measuring the cubic feet of your room (height, length, width) and purchasing an air filter capable of 4x air changes per hour. 3x air changes per hour is the recommended value, but from personal experience living in a room with mold spores in the ceiling 3x per hour is not enough.

Example(http://www.sylvane.com/learning-center/air-purifier-buying-guide.html):
Here's a step-by-step example for a possible scenario:
  • Find the cubic footage of the space you want to purify by multiplying the length in feet by the width in feet by the ceiling height in feet.
    • 18 feet L x 24 feet W x 8 feet H = 3456 cubic feet
  • Multiply 3456 cubic feet by 4 air changes per hour.
    • 3456 cubic feet x 4 ACH = 13,824
  • Divide the result by 60 minutes to get your recommended cubic feet per minute.
    • 13,824 / 60 = 230.4 CFM

*note* - CFM on box might be the CFM of the fan without filter
*note* - CFM on box will be High/Turbo setting value (sometimes loud)
*note* - Replace the carbon pre-filter about 1x every 3 months (you cannot wash this).

Next weather strip your door (bolt rubber guard to bottom) and caulk the windows/frames. You can follow leaking air by burning incense or with a thin piece of paper. It is unlikely you will create a large pressure differential within your room regardless of the CFM exhaust value unless you have designed this room specifically for this purpose and it is sealed. You will need a controlled air intake pipe to reduce unmonitored leaks.

Vent 2x-3x air changes per hour exhaust. For low noise you need less than 700 feet per minute air speed. This is approximated as below:
Straight returns:
8" = up to 245cfm
10" = 440cfm
12" = 715 cfm
14" = 975cfm
16" = 1270cfm

flex pipe (diameter):
8" = 170cfm
10'" = 315cfm
12" = 510cfm
14" = 755cfm
16" = 990cfm


*note* in really bad smell cases you might need to hang a cut up curtain (strips are cut) by the door like you see in large dairy farmer fridge rooms. A heavy curtain blocks significant air flow near entrances and exits especially when a large temperature differential exists.


Good luck with your allergies.
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Old 06-21-2012, 11:33 PM   #7
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Well, I am glass that someone has a sense of humor. Nice post with great quantitative information. Obviously, the leakier the room, the more air will be needed.
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Old 06-22-2012, 10:50 AM   #8
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Quote:
Originally Posted by hvac benny
Grow-op?
I dont know what you are talking about what is that?
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Old 06-22-2012, 10:54 AM   #9
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Quote:
Originally Posted by LifeIsGood169
First, my disclaimer, let me say I have no experience other than allergies and this is only my 2nd forum post.

To address the bomb theory, considering he said oder and not hazardous materials, chemicals, etc, I'd say it's more likely he is probably growing marijuana and not making crystal meth or building bombs or mixing pool supply store chemicals (all of which are tracked by the fed along with piping from home depot and other basic supplies, etc.... just fyi).

But from my end, assuming it is not marijuana and is indeed allergies because I will not help you break the law:
to eliminate a smell give us the size of the room and where you will be venting and the typical temp/humidity in the room.

Without this information I recommend measuring the cubic feet of your room (height, length, width) and purchasing an air filter capable of 4x air changes per hour. 3x air changes per hour is the recommended value, but from personal experience living in a room with mold spores in the ceiling 3x per hour is not enough.

Example(http://www.sylvane.com/learning-center/air-purifier-buying-guide.html):
Here's a step-by-step example for a possible scenario:

[*]Find the cubic footage of the space you want to purify by multiplying the length in feet by the width in feet by the ceiling height in feet.
[*]18 feet L x 24 feet W x 8 feet H = 3456 cubic feet

[*]Multiply 3456 cubic feet by 4 air changes per hour.
[*]3456 cubic feet x 4 ACH = 13,824

[*]Divide the result by 60 minutes to get your recommended cubic feet per minute.
[*]13,824 / 60 = 230.4 CFM




*note* - CFM on box might be the CFM of the fan without filter
*note* - CFM on box will be High/Turbo setting value (sometimes loud)
*note* - Replace the carbon pre-filter about 1x every 3 months (you cannot wash this).

Next weather strip your door (bolt rubber guard to bottom) and caulk the windows/frames. You can follow leaking air by burning incense or with a thin piece of paper. It is unlikely you will create a large pressure differential within your room regardless of the CFM exhaust value unless you have designed this room specifically for this purpose and it is sealed. You will need a controlled air intake pipe to reduce unmonitored leaks.

Vent 2x-3x air changes per hour exhaust. For low noise you need less than 700 feet per minute air speed. This is approximated as below:
Straight returns:
8" = up to 245cfm
10" = 440cfm
12" = 715 cfm
14" = 975cfm
16" = 1270cfm

flex pipe (diameter):
8" = 170cfm
10'" = 315cfm
12" = 510cfm
14" = 755cfm
16" = 990cfm

*note* in really bad smell cases you might need to hang a cut up curtain (strips are cut) by the door like you see in large dairy farmer fridge rooms. A heavy curtain blocks significant air flow near entrances and exits especially when a large temperature differential exists.

Good luck with your allergies.
Only 4 air changes per hour? Commercial kitchens are at 15... Residential bathrooms are at 9 and my wife can sure smell the poop when I open the door...

A few side notes, the room will be very well sealed as it is a labratory enviornment. And just so you know, right now in california it is legal to grow marajuana for medicinal purposes.
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Old 06-22-2012, 12:50 PM   #10
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Laboratory environment does not always mean well-sealed. The need for that kind of expensive construction is very application-dependent.
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