I just saw a piece on MSNBC about a Toronto hospital that has at least one “negative pressure” room that it has available for SARS cases. The report said that air is always entering the room, but never leaving it. And the reporter said that he could feel an inward draft as he stood just inside the door. How is this possible? Isn’t there a limit to the volume of air a room can contain?
In a nutshell they suck air out of the room through a ventilation system that will kill and or trap any nasty bugs. so if you open a door or window air always flows in.
Maintaining proper pressure differentials is also important in many industries. As an example, I used to work at a nuclear weapons production facility. With all the plutonium and tritium around, maintaining proper pressure differentials between work areas was taken very seriously…
Okay. So the assertion that air never left the room was incorrect, right?
On the other hand, clean rooms use positive pressure. A ventilation system filters the air and pumps it into the room, maintaining a positive pressure. This prevents dust from entering through doors and other small openings that may exist in the room.
Normally air is fed into a room through a controlled supply box from the building’s HVAC system and exhausted through a controlled exhaust box to maintain a balanced static pressure in all areas of a building. When negative or positive pressure is required the boxes are controlled to produce the desired effect. In an instance like quarantine room, the exhausted air passes through a hepa filter then a separate isolated exhaust fan rather than recirculated. When positive pressure is required such as in a clean room environment, the supply air is filtered and the exhaust air goes to the main exhaust fan. A portion can be recirculated for temperature, efficiency etc.
Yes, air leaves the room but not through any windows or doors. It all goes through a filter system that cleans it. The idea is that air from the building is assumed free of SARS(carona?? virus). Air in the special room is assumed to be infected with it. You do not want infected air to come out of the room into the rest of the building, so you setup the ventilation system so that the special room has a slightly lower presure than the surrounding areas. They call this negative pressure only because the pressure is lower than rest of building. It is semantics, not scientific.
So clean air gets sucked in through through all the doors, windows, cracks, what ever and leaves the room through a single vent. In the vent, it gets filtered, cleaned, whatever and then probably pumped outside. The virus can’t swim against the air moving in through the doors and windows so it can’t leave the room to infect the rest of the building.
At UCLA 25 years ago, I was told by a biochem prof that the evacuated air from their negative pressure rooms went thru the equivalent of a jet after burner. I would think that I would have heard that in the science quad.
My understanding is that ‘pressure’ is just the net effect of the motion of individual air molecules, so a room with negative pressure simply has more air molecules entering than leaving (except through the germ-killing exhaust vent).
Air molecules are constantly moving in every direction. When you create a vacuum at the south end of a tube, the net movement of air is south, through the tube toward the vacuum, but the vacuum doesn’t cause the air to move south; it only prevents air from moving north out of the tube. If you create just a partial vacuum, the net movement will still be south, but some of the individual molecules remaining at the south end of the tube will still be able to move north, right?
In that case, if the room still contains air, what prevents individual air molecules and germs from leaving through the door and infecting the hospital staff?
Libertarian said:
It’s sloppy wording on the part of the description. Of course air leaves, it just doesn’t leave via the doors, windows, cracks in the wall, etc. It only leaves via the exhaust port (which cleans it before dumping it).
Mr2001, your understanding is somewhat confused. Yes, pressure is the net effect of the movement of air molecules. Negative pressure just means lower pressure than the surroundings. (Redefine surroundings as your reference zero.)
But the part about vacuum not causing air to move is wrong. Or muddled. Or something. Vacuum is a lowered pressure (with a “pure vacuum” being no molecules whatsoever, a theoretical state only approximated by interstellar space). If you have a room with three walls, and the fourth a membrane, the air molecules are all bouncing off each other and the walls and jostling each other like a crowded subway. Now pop the membrane, with a vacuum on the other side to the far wall, and suddenly you have an empty space with nothing for the air molecules to bounce off. All the molecules already moving that direction hit nothing, so keep going that direction. The molecules moving the other direction bounce off the molecules still in the way or going in that direction, and so they start moving that direction. The effect moves across the whole room until the pressure equalizes, and all the molecules have the same amount of room. Apply a continuous vacuum and all the molecules will be sucked out until there is so much room that they rarely get knocked into the exhaust port.
So what keeps the molecules from leaving a negative pressure room, if they can still be moving away from the exhaust? They run into the molecules that are trying to get into the room. Since the pressure is lower, more molecules are trying to get in than those trying to get out, and that causes an inward flow. All the molecules in the room hit incoming molecules before they get out.
Rooms with pressure control typically use either an “airlock” or an air curtain to aid the control. An airlock is two sets of doors with a stretch of space between, so there’s not a direct flow straight through from outside to inside. This makes sure there’s plenty of incoming air molecules to hit all the outgoing molecules before they get out of the room. An air curtain is a pumped in flow of clean air that makes a pressure wall, so all the molecules attempting to leave run into this wall of air and bounce back. The negative pressure aspect is mostly to protect against leakage around the edges of the door and such, with more positive controls used when the doors are opened to allow people through.
Denver International Airport has such a room for smokers.
This is similar.
I worked in the petrochemical industry. We had control and electrical switch gear rooms located inside areas that could have combustible gas leaks. The rooms were maintained at a slight positive pressure with blowers pulling clean air from high intakes to keep any gas from coming inside through the doors or through underground conduits.
I work in one of these. It’s not as cool as it sounds.
Similar setups are widely used in containments built for asbestos abatement operations. Air movers equipped with HEPA exhaust filters – commonly called “negative air machines” in the vernacular – insure that any air flowing at a crack or hole in the containment flows into the containment, reducing the possibility of fiber-contaminated air escaping. The larger machines I’ve seen are about the size of a clothes washer, and can move one helluva lot of air.
If you want nothing (virus, dust, whatever) leaving the room then you just ahve a duct puliing air out and tereating it to get rid of the bad stuff. Any cracks will be letting air in and no air will escape that way as the pressure inside is a bit lower than outside.
If you do not want any outside air from entering, then you do the opposite: you blow clen air in and this means that the air will escape through any cracks to the outside and outside air will not enter. This is what is done in clean rooms. Even further, the clean air enters from the ceiling and it exits from the floor. This means any dust or particles in the air are immediately removed.