Exhaust Blower Sizing for Small Fume Hood

I would size the motor slightly larger than any of the calculations suggest.

I would size the motor slightly larger than any of the calculations suggest.

OK, so I'm going to buy three blowers? One calc was 1/4 HP, the second was 1/12 HP. Mine was 1/8 HP.

The calculations that I made were for the high-end flowrate, so in the calculations, the motor is already oversized (as if I didn't know to allow for that).

What I really want someone to check, is if my calculations make sense. If you don't know how (or care to) go through them, don't bother replying.

I'm sorry to be so abrupt, but I took time to include enough detail for someone who knows something about this stuff to answer my question.

How many questions are posted here that have almost no detail?

I'm sorry I bothered replying. I just posted what I would do. Since you're dealing with toxic, cancer causing solvents, I wouldn't do any calculations. I would get the largest fan that fit that particular hood, and be done with it.

Sometime, something that seems so apparent to you may not be to someone else.

I am planning on a rooftop unit, not something that is attached to the hood. Ostensibly, we could get a blower that would be complete overkill.

Should we get a 20HP unit?

No worries dude. I don't know a whole lot about anything. I'm just a wrong answer waiting for a bad question. All is well.

I would go for the 1/3 HP unit.

I'm sorry for my abrupt manner. I appreciate most of your posts and your take on the current US admin.

Why the 1/3 HP unit? I think 1/3 HP would be overkill.

Did you check out the calculation page on the site I provided in my OP?

Be careful to not work above the Lower Explosion Level. I assume that you bring the solvents also under the hood. If the bottle breaks, you'll have a different scenario, also with spills. Chose the fan carefully - use a metal one with good ground and static discharge lead and brushless. Consider working in hotte (box with frontal sliding door)

A hood will suck too much omnidirectional. The products you use work addictive but are cancer evoking. The velocity and concentration tell me nothing unless you describe the exhaust system. The concentration will be function of work procedure, spill control, application dose and room temperature. And where your nose is.

The company I worked for lost 80% of their lab technicians to cancer.

I wrote: I have estimated that the maximum content of solvent in the air stream is less than 0.01% (100ppm).

Maybe I included too much detail in my post. You must have missed the above statement. Do you know ANY FLAMMABLE SOLVENT that has an LFL below 0.1%, let alone 0.01% (which I padded by more than an order of magnitude?)?

The company I worked for lost 80% of their lab technicians to cancer.

That's a wild, unsubstantiated statement. Do you have evidence to support this? I'll bet it wasn't due to any standard solvents.

I burried them all, one by one. This company started in the 70's and stopped in the 90's. APPLIED SOLVENTS WERE TOLUENE, XYLENE AND ALCOHOLS - all on a LAB Scale. And the lab had for those days good exhaust systems.

Free to air. The production plant was the worst: we used 70.000 liters toluene per month and this was flashed of the furnaces for about 8 years in a row.

Yhe first job I did there was designing a solvent recovery system with active carbon cylinders, that recovered to 92% of the solvents to reuse again and again.

My brother was a lucky one - he just lost the capability to smell.

I worked in a different division and my exposure was significantly less. I quit after 4 years.

It all depends how much toluene you will use and how accurate your vent works.

More: through what opening(s) size in sqFt or parts of Square Meters you attract the air to build up your exhaust stream.

Again: calculate to avoid disaster by accident.

When you smell it, it sucks already.

Why not a bigger fan system, say 1/2 HP with speed control?

At least you can play with openings, adapt the noise and work safe.

I agree that toluene is the most toxic solvent that we work with, but when you are talking less than 50 mL a day usage at most, and half of that gets thrown away with the paper towels into the enclosed flammable can, you really have very little solvent to get rid of.

More: through what opening(s) size in sqFt or parts of Square Meters you attract the air to build up your exhaust stream.

The hood opening is 24" wide by 18" high, as I already said. Did you really read my first post?

I burried them all, one by one. This company started in the 70's and stopped in the 90's. APPLIED SOLVENTS WERE TOLUENE, XYLENE AND ALCOHOLS - all on a LAB Scale. And the lab had for those days good exhaust systems.

Sounds like some lawyers made a lot of money. I'm sorry, but I don't believe they died of solvent-induced cancer. IF THIS REALLY HAPPENED, IT WAS CAUSED BY SOME OTHER REALLY NASTY COMPOUNDS - SOLVENTS THAT WE ARE TALKING ABOUT ARE NOT BAD ENOUGH TO CAUSE WHAT YOU ARE DESCRIBING.

Where did this take place, anyway? General area is OK.

I'm sorry about your friends, but your scenario doesn't make chemical/biological sense.

I need help with engineering and math, not chemical/greenie paranoia.

Hi pritam,

It's late here, and I'm about for bed. I took in enough of your post to appreciate your answer. I will take a look tomorrow and reply with how it helped!

Thanks, and GA!

Mike

A small correction in my comment #7.

Value of velocity pressure is (density x velocity**2)/ 2g ( please note the correction).

Check with your standards for fume cabinets. The last time i had anything to do with them was in 1992 and the Australian standard had changed so that there could not be any discharge to atmosphere. This also applied to cyto toxic cabinets. The rationale is that innocent people in the neighbourhood should not be able under any circumstance to be subjected to your waste product.

The cabinet has to have inbuilt filtration with the onus being placed on the operator to check the efficacy of same.

Dear friend

I have calculated using your figures v=300 acfm (= 8.5 m3/min) and TDH = 0.57 inch water (= 142.3 Pa). Assuming 60% efficiency.

Power = (8.5/60) x 142.3/0.6 = 33.6 W or 0.045 HP

My result is very close to the vendor figure of 1/12 HP. Therefore, to answer your question, your figure of 1/8 is very-very reasonable.

Hope the above helps.

P.s. Have you included the entrance and exit loss in your pressure drop calculation?

Your calculation is based on figures of OP and formula given by pritam at #7. Formula is right but figures of OP is un-believable. See:

100 ft/min ave face velocity at the 1.5ft x 2ft opening, I need a 300 ACFM flow rate. I have used an on-line calculator that gave me 0.426 inH₂O pressure drop and 0.145 inH₂O velocity pressure.

100 ft/min velocity is too small for blowers and fans, based on which he has back calculated flow rate requirement. Velocity should be somewhere 30 to 60 ft/s. Pressure rise taken is also too small. It is based on pressure drop calculation, and not at static pressure requirement at discharge which shall be around 4 inH2O. So, OP has to check once again the requirement. I strongly agree with#7, that velocity pressure is negligible compared to static pressure.

I won't go through the design stages for building a discharge fan system for a lab booth here. Much to complicated for a short paragraph or two.

There is one manual that I kind of use for the bible to the standards required here in the states by OSHA. It is the ACGIH Industrial Ventilation, A manual of Recommended Practice For Design. A copy of which you should obtain and read before committing to building a booth of any kind.

Everyone, thank you for the replies. Maybe this will clear some things up. This is how I have it pictured:

The box itself is an off-the-shelf unit. We are not building it ourselves.

If this clears things up for anyone, please feel free to comment.

My main reason for posting here is to double-check vendor info. One of them came up with a 1/4HP blower. Another came up with 1/12HP for the blower. Seems like a pretty wide spread.

I used Pritam's calculation and got 1/20HP. I will likely go with the 1/12HP model we were quoted.

Have you consider elevation in your pressure drop calculation? the fan needs also to overcome the force due to gravity.

If I assume that the height of the stack is 10 m and adding to the pressure drop given earlier, the HP required will be 0.08 HP which is very close to 1/12HP quoted by the vendor.

GA to prdp2344 for pointing out important issues pertaining to calculations.

"100 ft/min velocity is too small" and "It is based on pressure drop calculation"

Mikerho, please check these issues. Decide static pressure and velocity at discharge, refer some standard. Pressure rise and velocity both selected by you are extremly on lower side.

Hi pritam,

Thanks for the concern. This is from WIKI; this excerpt is derived from Occupational Safety and Health Administration (OSHA) standards:

Maintenance

Fume hood maintenance can involve daily, periodic, and annual inspections.

• Daily fume hood inspection
• The fume hood area is visually inspected for storage of material and other visible blockages.
• If hood function indicating devices are not a part of the fume hood, a 1-inch (25 mm) by 6-inch (150 mm) piece of soft tissue paper should be placed at the hood opening and observed for appropriate directional flow into the hood.
• Periodic fume hood function inspection
• Capture or face velocity is typically measured with a velometer or anemometer. Hoods for most common chemicals must have an average face velocity of 100 feet (30 m) per minute at sash opening of 18 inches (460 mm) or higher. Face velocity readings should not vary by more than 20%. A minimum of six readings shall be used determine average face velocity.
• Other local exhaust devices shall be smoke tested to determine if the contaminants they are designed to remove are being adequately captured by the hood.
• Annual maintenance

I really have done my "homework". So you see, my flow rate is adequate, and, since the calculations indicated a 1/20HP motor, and, since I am upscaling it to 1/12HP, I believe that everything is well in hand.

I appreciate your feedback!

Mike

try this mob http://www.biocabinets.com.au/fume-extraction/fume-cabinets-fume-hoods.html

In my experience we slavishly follow the States in regard to our standards. Please check that you haven't been sold a pup.

Our standards call for the exhaust air to be sent back into the room, after going through appropriate filters.

Well, we could not put in a hood, and let the guy who has to clean the coupons with the solvents breathe the stuff day after day.

Sorry for the sarcasm, but we will be below fugitive emissions amounts.

However, we do have a Regenerative Thermal Oxidizer (RTO) to incinerate the solvent vapors coming from our coater drying oven. It's possible that we could tie in to the oven exhaust duct. The vapor throughput is on the order of 50 lb/hr - the RTO would EASILY handle what comes from the hood.

It is quite a ways away however and the hood would still need a blower to feed to the plenum between the oven and the RTO.

These regulations really piss me off! If someone uses MEK at home to do degreasing and whatnot, capturing the fugitive vapors is not regulated. In a work/production environment however, everything can be scrutinized with an electron microscope.

It might be worth it to feed the RTO to avoid any remotely possible noncompliance.

Hmmm, I think I'm getting paranoid

Thanks for the response!

Mike