Exhaust fan sizing

Consider also:

• Containing the odour inside the process equipment
• Venting the odour to the outside of the room using suitable ducting
• Opening the windows
• Demolishing the room walls.

Damn! Evicted again! I was getting used to dossing in there.

When utilizing exhaust ventilation to remove odorous (non-hazardous) contaminants, it is always a good idea to have the exhaust fan's intake as close to the point of odor emanation as possible, ideally, enclosing the process itself inside of a "hood" (Think college chem class). Alternatively, if the process is taking place in a vessel or container, then a slotted intake should be configured, to increase the air velocity at the intake point, and improve capture efficiency. (A good reference manual is Industrial Ventilation: A Manual of Recommended Practice, by the ACGIH. )

Be aware, running general exhaust ventilation at 4 air changes per hour will be costly with respect to the energy required to "condition" the makeup air (that replaces the odorous air being exhausted.)

That being said, based upon your room dimensions, you are looking for an exhaust system that can move 900 M³/hour, or 15 M³/minute.

That's about 530 CFM (cubic feet per minute). In order to keep the velocity up, you will probably want to utilize ductwork < 8" in diameter.

As a lab designer, I can tell you that 4 AC per hour will not remove an odor. Base minimum design is 6 AC / hr when all odor is captured in a hood. We regularly go as high as 12 AC/hr for certain circumstances.

Best practice is to contain the odor and use 100 FPM velocity across any opening in the enclosure.

This is a pretty complex design issue and should be addressed by a professional if the odor is offensive or hazardous.

RDMJR

"I can tell you that 4 AC per hour will not remove an odor."

That is a pretty strong blanket statement...

I believe that a few factors would need to be considered, such as the odor threshold of the substance in question, the volume of substance released, and its rate of release/evaporation/volatization.

By the way, 4 ACH is considered the "standard" when calculating exhaust ventilation requirements for safe asbestos removal, in accordance with US regulations.

Don't forget to install an intake duct of at least the same diameter.

(Sorry I didn't read all the posts before I had to spout off.)

Indeed, don't forget the make-up air! Without the make-up air a vacuum will be created in the room and you may not be able to open the door to the room. Make sure the suction side of the make-up air is not close to the discharge side of the exhaust fan.

All good points above, but you should also characterize the odor causing substance to see if it generally wants to go up or down (specific density relative to air).

I completed a similar design for a pesticide company recently and we used less than 2 air changes per hour, but we located all of the inlet ducts in a way that the source was well protected and local air velocities were high enough to keep the fumes in the air ducts. We also utilized heavy plastic curtains to enclose some of the equipment. The problem here was a filling operation where the chemical was filling plastic jugs, so we "enclosed" the filling stations with the plastic curtains.

The largest problem on that project was that it is located in Montana and the conditioned air had to be heated (a lot!) in the winter. The return air issue can be significant as someone else has already pointed out.

Stephan

You have 225 cubic meters of air to displace. It all depends on what chemical you are using as to removing the smell. Is it oil based, water based, alcohol based, then you have to determine what residual volume you have left in the room, and then how much volume of air flowing over it will entrain it in the air stream. It is a simple question, but not a simple answer. Will you have to entrain water in your airflow to lift the offending chemical? Not an easy answer.

I don't know if you have hoods over the process you have mentioned. Hoods have different design criteria. Depending of what kind of exhaust system you have, the size of fan is dependent of the volume of air being exhausted. It also depends on whether the entire room is air conditioned or not. If it is air conditioned, it complicates the matter a little bit as you would not want to be exhausting conditioned air. This would be expensive. Consider a system like a kitchen hood exhaust. There is the exhaust and the make up air. The capture velocity is critical here, otherwise you won't be able to control the odor. There would be always odor migration to the other parts of the room or even outside of the room especially if the fresh air is greater that the exhaust air. Consider a negative pressure of around ten percent of exhaust air. For fan motor sizing, compute the pressure losses imposed by the system and size the motor adequately for the require air flow.