Hello, I am a relatively new engineer and I am trying to design an exhaust fan for a fume hood. I am having trouble figuring out when a "laboratory" exhaust fan is required and if bypass air is required for dilution. Our client will be using Methylene chloride and potassium chloride; I have the data sheets and evaporation rates (relative to butyl acetate) but I can't find information on when a proper laboratory exhaust fan and bypass air is required. I have searched for a few hours online and through ASHRAE HVAC applications and the industrial ventilation manual but I'm having a tough time coming up with answers. If anyone can point me in the right direction, that would be great! Thanks!
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Laboratory Fume Hood Exhaust Fan Information
- Thread starter nuuvox000
- Start date
Since you have double posted this into the Local Exhaust ventilation forum which may be a better place for it, I suggest you either link the two posts or redflag one of them so you are not double posting. the double post dilutes the answers you receive because not everybody looks in both forums.
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B.E.
You are judged not by what you know, but by what you can do.
There's a lot of moving parts to this analysis, and since this sounds like it could have safety implications you need to get solid direction from more senior engineers that you work with.
Since this is a real process specific analysis, I don't know that you'll find a standard with cut and dry direction on this, they usually come with more guideline direction. I think ASHRAE applications has a section on this, ANSI Z9.5, NFPA 45, possibly others. ASHRAE laboratory design guide has a whole section on this topic. There are also specific mechanical code requirements, but while they will direct you on the stack height and separations that are code required, I believe they do provide direction on dilution of flammables as well in the hazardous exhaust section.
The process to give you the information you need would be
- what are my hazardous materials being used, and how are they defined in NFPA 704
- what is the highest concentration my exhaust airstream will have of these materials, based on your fume hood airflow
- are concentrations at these levels a danger (combustible, toxic, etc.)
- where am i discharging this air to, and what is nearby
- to what level of certainty are you that your fan discharge will not get re-entrained into any building in the vicinity
From here you then make a determination on how to minimize this risk - you can dilute the air, increase your discharge height for more separation, increase your overall stack velocity/mass to decrease the likelihood that the wind takes, move the fan to a better location, or any combination of these. Some times engineers will take this to the wind tunnel analysis level, which is the better analytical way of understanding where this exhaust air would end up before ultimately getting diluted in the outside air.
The lab exhaust fan combines the best of both methods as it simultaneously dilutes the exhaust airstream while also increasing the discharge mass and velocity which increases the final height the gases reach in the atmosphere, and it is unaffected by any fume hood airflow turndown since the bypass will just open more.
Since this is a real process specific analysis, I don't know that you'll find a standard with cut and dry direction on this, they usually come with more guideline direction. I think ASHRAE applications has a section on this, ANSI Z9.5, NFPA 45, possibly others. ASHRAE laboratory design guide has a whole section on this topic. There are also specific mechanical code requirements, but while they will direct you on the stack height and separations that are code required, I believe they do provide direction on dilution of flammables as well in the hazardous exhaust section.
The process to give you the information you need would be
- what are my hazardous materials being used, and how are they defined in NFPA 704
- what is the highest concentration my exhaust airstream will have of these materials, based on your fume hood airflow
- are concentrations at these levels a danger (combustible, toxic, etc.)
- where am i discharging this air to, and what is nearby
- to what level of certainty are you that your fan discharge will not get re-entrained into any building in the vicinity
From here you then make a determination on how to minimize this risk - you can dilute the air, increase your discharge height for more separation, increase your overall stack velocity/mass to decrease the likelihood that the wind takes, move the fan to a better location, or any combination of these. Some times engineers will take this to the wind tunnel analysis level, which is the better analytical way of understanding where this exhaust air would end up before ultimately getting diluted in the outside air.
The lab exhaust fan combines the best of both methods as it simultaneously dilutes the exhaust airstream while also increasing the discharge mass and velocity which increases the final height the gases reach in the atmosphere, and it is unaffected by any fume hood airflow turndown since the bypass will just open more.
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