Max air tightness in a lab. (I may be over thinking this.)

[kainkelly]

How air tight is to air tight? I may be over thinking this, but a fume hood works off of negative pressure, so basically its sucking in fresh air and exhausting contaminated air into a scrubber. At at what point would a lab could be to air tight that the fume hood could not generate enough negative pressure to extract the fumes?

Looking at building a small shed type building for a lab most likely 10×14ish I highly doubt it will be that tight but I am curious is pumping fresh air in may be a good idea just in case.

I hope that made sense.

 

[goldandsilver123]

From my experience, you can not seal a room, it should have grates to allow air to follow free.

My 2 HP motor gave me the impression that the windows would pop off when i had the door and windows closed at the same time.

 

[kainkelly]

From my experience, you can not seal a room, it should have grates to allow air to follow free.

My 2 HP motor gave me the impression that the windows would pop off when i had the door and windows closed at the same time.

Yeah thats what I figured, while it will not be sealed I am thinking I may still pump air into to it with a little bathroom fan. Rather safe the sorry....

 

[snail]

No need to pump air in, the vent hood creates negative pressure in the room, an opening to outside air allows the outside air to replace the air pumped out of the room.
The opening needs to be large enough to flow the amount of air the vent hood blows out.

 

[kurtak]

Negative pressure means that air is being "drawn" out of a space (or room) so the room is actually under vacuum which is why it will "suck" an open door shut

Positive pressure means that air is being "pushed" into the space (or room) so the room is actually pressurized (like air in a compressor) which is why it will actually "push" the door open

The reason you want negative pressure in a lab is so as not only to draw the fumes out - but so that also as fumes are being drawn out (with air) the air is also being replaced - from outside the lab --- AND - you want that air replacement (from outside) to be at near to the same rate as the air being drawn out of the lab

In other words if (as example) you are drawing 100 cubic foot air out of the room you want "near" to that 100 CF being replaced from outside --- that's because you want to "fill" the vacuum created by the negative pressure

If the room is to air tight - the in coming air wont "fill" the vacuum of the the out going air --- kind of like the flow of a river - only air (instead of water)

Example; - if your hood is pulling 100 CF per minute - & the only source of in coming air is the crack under the door (call it one cubic foot) --- the 100 CFM draw of the hood is going to create such a vacuum on the one CFM of the door crack to travel - along the floor - at a "high" rate of air speed - like a river of in coming air - at floor level ONLY --- so when that "small" fast moving air reaches the hood its going to hit the hood "face" at the bottom of the hood face (in stead of the full face)

Because no fresh air is coming in at ceiling level the air (at the the ceiling) is slowed WAY DOWN to point of near stagnant - if not even a little positive --- there by allowing any fumes that may (& likely will) escape the "top" of your hood to collect at ceiling level & especially in corners

In other words the in coming air needs to be "balanced" with the out going air - so that the vacuum of negative pressure "fills the vacuum" with in coming air

Other wise - you have a vacuum that is for the most part stagnant - with a "river" of flowing air at the point of the "restricted" in coming air

Kurt

 

[kainkelly]

Negative pressure means that air is being "drawn" out of a space (or room) so the room is actually under vacuum which is why it will "suck" an open door shut

Positive pressure means that air is being "pushed" into the space (or room) so the room is actually pressurized (like air in a compressor) which is why it will actually "push" the door open

The reason you want negative pressure in a lab is so as not only to draw the fumes out - but so that also as fumes are being drawn out (with air) the air is also being replaced - from outside the lab --- AND - you want that air replacement (from outside) to be at near to the same rate as the air being drawn out of the lab

In other words if (as example) you are drawing 100 cubic foot air out of the room you want "near" to that 100 CF being replaced from outside --- that's because you want to "fill" the vacuum created by the negative pressure

If the room is to air tight - the in coming air wont "fill" the vacuum of the the out going air --- kind of like the flow of a river - only air (instead of water)

Example; - if your hood is pulling 100 CF per minute - & the only source of in coming air is the crack under the door (call it one cubic foot) --- the 100 CFM draw of the hood is going to create such a vacuum on the one CFM of the door crack to travel - along the floor - at a "high" rate of air speed - like a river of in coming air - at floor level ONLY --- so when that "small" fast moving air reaches the hood its going to hit the hood "face" at the bottom of the hood face (in stead of the full face)

Because no fresh air is coming in at ceiling level the air (at the the ceiling) is slowed WAY DOWN to point of near stagnant - if not even a little positive --- there by allowing any fumes that may (& likely will) escape the "top" of your hood to collect at ceiling level & especially in corners

In other words the in coming air needs to be "balanced" with the out going air - so that the vacuum of negative pressure "fills the vacuum" with in coming air

Other wise - you have a vacuum that is for the most part stagnant - with a "river" of flowing air at the point of the "restricted" in coming air

Kurt

Thats what I was concerned about would having both an open door and window be enough to prevent that?

 

[kurtak]

Don't have time to post today - will try to post more the weekend

Kurt

 

[kainkelly]

I understand thank you for your time and sharing your knowledge.

 

[kurtak]

kainkelly - you posted -----

would having both an open door and window be enough to prevent that

Now you are over thinking it in the other direction (to open instead of to air tight)

Large openings like a "wide open" door &/or windows can/will cause positive air pressure at those openings causing that positive pressure to "fight" with the negative pressure you are wanting to create with your hood

that positive pressure can cause fumes to be pushed into places like corners - under work benches etc. even though negative pressure is still working in the "area" of your hood

In other words - though you have negative pressure "near" your hood - parts of the room will can/will have positive pressure due to air "pushing" through the "large" openings instead of being "drawn" though the openings

As I said in my first post --- you need to "balance" the incoming air with the out going air

I also said you want that incoming air to be "near" to what the out going air is

in other words the incoming air needs to be "near" - but "somewhat" less then the out going air so that a true vacuum on the room is maintained & you want that vacuum to work on the "whole" room - not just one point of air flow (as explained by the river effect)

That's all I have time for today but will post more more when I get time

Kurt