School Science Labs: Ventilation Question

You have two tuff questions. Let me make a stab at it. If you vent, you really don't want to vent the entire room? That is not practical and yes, that would over work you heating and cooling systems. Think about it this way, what would happen if you removed the windows?

Same result, but people think that if we funnel the air into a duct and suck it out with a fan then it's not as bad on the systems as opening a window.

OK, off my soap box now. You have two rooms that have there own systems, not attached to the central system. My first thought is if you have to vent the entire room, the take the returns and set them up with a filter system that can handle the fumes. This way your using the existing system to everyone advantage. (win-win) to me.

If I was pushed into having a vent, then I create one or two hoods at either end of the room and have the teacher use only those tables and have the hoods vent the fumes out from there. But again, it's just like opening a window and sucking out your heat and or your air conditioning.

Please write back to this thread so we can all see if our advice is being heard, all too often guests write in and ask and we never hear back on weather they think we are right or just full of hot air.

Hope it helps and good luck

Two questions for J.T.

1. Is the Lab on an exterior wall of the school (does it have windows)?

2. Can you get the ventilation schematic for your section of the school?

This really shouldn't turn into a huge project. Why not stop by Duluth HS and see how they are venting their labs? Whatever they are doing must be working.

Layguy has it. Have the tables set up with hoods that run to a fume, chemical extractor\filtration unit like a Smoghog, then returns the clean air. Keeps the air clean and easy on the HVAC costs. Graingers and McMaster-Carr carries them I believe.

The other option is a downdraft table, perhaps with a hood over, depends on your application. You can filter the air, maybe with a wet filter if you have to catch fumes as well as dust, then return the air to the room.

Most specimen packing companies don't use formaldehyde any more inform the school and or teacher to research for alcohol packed specimens. That would save you and the school a lot of time and money on a vent system.

Make sure you vent to the outside. Just to pull it out of the room is not good enough. You will need to get a HVAC engineer involved. It could throw off the ballance of the entire HVAC system. There are building codes you must adhere to. I am not an HVAC engineer, but I use to work for an architect who designed 1/3 of the schools in our state. I have been involved with several schools that have had simular problems. Please at least get the local building inspector involved.

Some thoughts

Classrooms have a relatively high outdoor air ventilation requirement. It was and may still be for your state, 15 cfm per per person per classroom (recent guidelines by Ashrae lowered the requirement some, but local code has the final say). Typical classroom size is usually dictated at 28 students so 420 cfm of ventilation. Science room requirements may be higher -- perhaps 20 cfm per person. Note also that in my state and others I have worked in, all classrooms are required to have windows.

Those exterior wall mounted units are about the lowest cost method of conditioning a classroom. When the classroom is occupied the unit should run continuously with the OA damper open (it may be a manual and not an automatic damper). I have seen schools where the unit cycles on and off based on load during occupied times. This is not compliant with the ventilation code, but will solve high humidity and mold problems that can occur when such units are bringing in humid outside air with the compressor cycled off because the space temperature is satisfied. A rule of thumb is that if the outside air is more than 15% - 20% of the total airflow from the AC unit, you will have humidity problems in humid climates, such as Georgia, if the outside air is not conditioned separately.

Most commonly these units are 3 tons (check the model number, a Bard unit will end in 036 for 3 tons, 048 for 4 tons etc). 420 cfm of outside air on a design day is about a 2.5 ton load. This would be about 40% of the total airflow from a 3 ton unit. So usually the damper is adjusted so they bring in much less outside air (below code requirements) as otherwise they cannot maintain temperature.

The air that is brought in has to vent out somewhere. Usually the A/C unit has no relief capability. The ventilation air migrates to the hall through the doorway and probably transfer vents in the ceiling or wall. From there the air moves to the lavatories where it is sucked out of the school by exhaust fans. Also it takes about .05 cfm per sq-ft to properly pressurize a school building. So a lot of air will just leak out of the school through cracks and joints.

When the students are dissecting it will probably be on the science room tables -- I doubt the school would have invested in proper exhaust hoods, especially considering the A/C units. Exhaust hood are expensive items. You could only get about 3 students per hood, ergo the classroom would need a lot of hoods.

The best you could probably do would be to install an exhaust fan that would be ducted to exhaust grills, one located above each table. To keep concentrations of formaldehyde fumes to a minimum the fan would pull air from the space and exhaust it somewhere more than 20 feet from an intake. Then you need to have make-up air when this fan was running. There are a number of companies that manufacture small 100% make-up air units. It will have to have both heating and cooling and ideally would use the exhaust air for preheating/pre-cooling.

Alternately of course, the dissecting could be scheduled when ambient conditions are pleasant and the classroom windows can be opened. An exhaust fan would still be a good idea, but no outdoor make-up unit would be required.

Such a random post in a 13 year old discussion.

You need not only an exhaust, but a fresh air supply, and possibly a larger A/C unit, or auxiliary unit added...The room should be kept under negative pressure to control odors....If they can work with a work surface fume hood that's the best way to go...

This then exhausts the work area...cuts the area to be exhausted down to a minimum....

Professional labs have 100% fresh air....In this situation slightly more air is exhausted than supplied keeps the lab in negative air pressure so no odors can escape...

https://www.flinnsci.com/laboratory-ventilation-and-use-of-fume-hoods-middle-school-lab-safety/vsc0588/