Lab Pressure Control
Lab Pressure Control
(OP)
1.Which is the better way to achieve negative pressure in a biosfety(BSL2/BSL3) lab,varying the supply air quantity keeping the exhuast constant or varying the exhaust air quantity keeping supply volume constant?
2.If supply air volume is varied,will temperature control be compromised?
3.What should be the minimum infiltration or offset air quantity for stable control?What is the best path for the infiltration air to come into the lab, door undercut,door grille or leakage through door gaps?
2.If supply air volume is varied,will temperature control be compromised?
3.What should be the minimum infiltration or offset air quantity for stable control?What is the best path for the infiltration air to come into the lab, door undercut,door grille or leakage through door gaps?





RE: Lab Pressure Control
Door undercut or grille is acceptable if the infiltration air quality is not important.
Use HEPA filters on exhaust if potential environment contamination exists.
RE: Lab Pressure Control
The exhaust is often dictated by hoods and is not variable. Some designs use a variable general exhaust so that when hoods are closed, the general exhaust opens to achieve the same net exhaust flow. Supply VAV for temperature control can also be incorporated, so long as the minimum VAV position accommodates the lab's air exchange needs.
Example: The Lab supplies 2,000 cfm during full cooling. The hood exhausts 1,000 cfm (hood fully-open) and 1,200 cfm is exhausted via general exhaust grilles in the room, so the lab is net negative by 200 cfm. Now the room gets too cold, so the supply modulates to minimum setpoint (say 1,200 cfm) and reheat comes on (if available). Now the general exhaust will modulate closed to 400 cfm to maintain the 200 cfm offset for the lab.
In this application, the hood flow is the critical, non-variable parameter and the general exhaust is variable. The supply and general exhaust will change to keep the space thermally happy and to maintain the right pressure.
For the same lab controlled at constant volume (e.g., for BSL3 you usually don't play around with VAV), the room exhaust volume should be set to maintain a constant flow and the supply volume should be set to a fixed differential from exhaust. Now if exhaust dies for some reason, the supply flow senses the loss of flow and likewise shuts off.
That's 1 and 2. For 3, tough to say exactly, but about 50-100 cfm offset per 100 ft2 sounds good. in elevated BSL applications (2+ or 3) seal the room as best as possible, put gaskets at doors and adjustable sweeps at the bottoms of the doors for room pressure adjustment. 0.05 in. w.c. is a good target to shoot for across the door.
(tried to keep this simple but too many words came out!)
RE: Lab Pressure Control
Thanks a million for a very helpful post!I am keen to know your opinion on the two methods of lab pressure control ie active pressure control and offset voulme tracking.Which in your opininon is better and more accurate? Is it possible to achieve a targetted differential pressure and keep it at that value when offset volume tracking method is used ?
RE: Lab Pressure Control
Let's say you instead control air volumes to maintain pressure across a certain door at 0.05 inches. Maintaining this value is totally dependent on room leakage. At initial commissioning, you adjust a room to stay negative by 0.05 in. w.c. To do this, you seal the space well and adjust the door sweep just right to nail your 0.05. Corresponding airflows (hypothetically) are supply = 500 cfm and exhaust = 623 cfm (Bad exhaust number? Yes - keep in mind the space doesn't care about air flow, just pressure). Now after the commissioning guy and balancer get paid and leave, someone bangs a door once or twice with an animal cage rack just returned from the wash room. The door gets a little twisted and the rubber seal slips off the sweep, now the same room maintains the 500 cfm as a fixed value that was decided to be needed for cooling (again, hypothetically), but exhuaust is now 771 cfm to maintain the 0.05 inches.
Do you see where I'm going with this? What happens when people routinely prop doors open? Pressure control messes up central system sizing and changes flow requirements drastically as the space gets bumps and bruises with age.
Space pressure is a result, not a control criterion. Don't try to control airflow to maintain a space pressure unless distinct limits are incorporated. Using flow control instead controls and maintains system demand (so central system requirements are always known) while still maintaining the right flow direction.
RE: Lab Pressure Control
RE: Lab Pressure Control
Brian
RE: Lab Pressure Control
RE: Lab Pressure Control
RE: Lab Pressure Control
Thats right. the units are usually preset at the factory although adjustment can be made on site. They are very effective where the customer doesn't want the expense of active pressure control, although they do result in larger supply / extract volumes being required. This is not usually too excessive unless you have many airlocks on a project. You can then find your total supply volume increasing dramatically together with associated heating / cooling loads etc. Obviously, it would come to a point where an active system would be more commercially attractive.
Briand's point regarding the flap closing as air passes through an open doorway is particularly relevant to operating theatres where the quantity of air supplied to the OR is large enough to protect a single open doorway from backflow of air.(this is according to NHS guides within UK, not sure if the same principles apply in other countries). This volume is far greater than that required merely for pressurisation over a closed door and the surplus is relieved through the pressure stabilisers.
Cheers.
RE: Lab Pressure Control
I have used pressure stabilisers for clean rooms under positive pressure.Have they been successfully used in labs under negative pressure?I believe they would need to open inwards by the negative pressure.
RE: Lab Pressure Control
I used them some time ago on a clean room facility used for powder filling. The filling rooms (6no) were served by two common corridors (one on each side)and were under negative pressure with respect to the adjacent corridors. I basically sized the airflows based on door leakage from the corridor into the fill room at a pressure difference of 15Pa. I then allowed the same (or possibly slightly more airflow) to pass over the pressure stabiliser to allow for any differences in the actual door gap to the designed figure, and to compensate for any future damage etc to doors. The system worked fine and as i say, there is immediate visual indication that air was flowing the right way.Obviously supply volumes into the corridors, and extracts from the filling rooms were increased to allow for this extra airflow.
RE: Lab Pressure Control
What CFM, pressure drop & flap size did you use. What pressure differential was maintained? I have not seen metal flap. Is this like a barometric damper that may need counterweight for adjustment? I had seen rectangular panels made of clear plastic that you can slide and set the plastic cover over the rectangular opening so you vary the opening size.
RE: Lab Pressure Control
For a volume like this the unit is about 300mm wide x 150mm high but there's a range of sizes available. Larger volumes are usually dealt with by a bank of stabilizers.When installed they look the part. I sound like I'm selling these, I'm really not but maybe should be on commission!