Lab Pressure Control 1

[SAK9]

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?

 

[mdonner]

Temp control (not on/off I presume) is optimized at full airflow capacity, therefore it is recommended not to very it. This means negative pressure control via exhaust 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.

 

[ChasBean1]

SAK9,

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!)

 

[SAK9]

Chasebean,

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 ?

 

[ChasBean1]

It's possible to initially achieve it but difficult to maintain it. The more important thing is to maintain the right airflow direction so I very much prefer offset air volume tracking. For CAV, keep supply inflow at a fixed differential from the exhaust airflow, or vice versa.

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.

 

[grim900]

Its a good point regarding damaged doors messing up our carefully designed airflows. In clean rooms Pressure stabilisers are commonly used which are weighted stainless steel flaps mounted within the wall, often above the door. These are totally mechanical and will adjust to maintain the pressure when door gaps are not exactly as designed (i guess this would replace the door sweep?). Only problem is that to make them effective you need to pass as much air through them as you would expect to pass through door leakage. They also provide a good visual indication that airflow is occuring in the right direction giving the user immediate confidence.

 

[briand2]

I use pressure stabilisers not so much to account for door gaps being different from my design assumptions, but more to account for the door being closed most of the time. When the door opens, the room pressure changes and the stabiliser flap tends to close, thus letting air flow via the open door, in order to (try to) maintain clean-to-dirty air flow direction.

Brian

 

[lilliput1]

Differential CFM control and staged pressurization using airlocks that will allow only (1) door to open at a time (except in case of emergency). Plan relative pressurization by marking rooms with +++, ++,+, 0, -, --, --- etc with design pressurization differential between each to be 0.05"wg or 0.025" wg to limit the max & minimum differential such that it will not take more than 10 lb pull to open door. Calculate differential pressure in inches wg as equal to a factor times the velocity pressure of air through the opening. The most obvious opening being the gaps around the door. Design for typical gap through door but require door sweeps. There are brush like sweeps that can be attached to the door bottom that do a good job. The sweeps would be your safety factor. Require gypsum board ceiling. Have electrical require gaskets at lights, receptacles. Have architect seal where bottom of wall meets the floor, behind the baseboard. Seal all penetrations & access panel doors. Start of with minimum 200 CFM differential for office size room with one door, to maintain 0.025"wg. Use supply & exhaust air boxes or valves with good air measuring stations. Provide straight run of duct at airflow measuring stations. Do not use airflow straighteners if possible. They are lint catchers particularly if the exhaust is not filtered. Provide access door to air measuring stations to allow cleaning/ inspection. Provide safety margin in design so there is headroom for pressurization to reduce as room gets old, without setting off pressurization alarms. Require commisioning of contriols & coordination between Testing & Balancing Contractor & DDC Controls for setting up of calibrated CFM set points. Air measuring station have "K" factors that must be considered in calculating the actual CFM flow.

 

[ChasBean1]

grim, you bring up a good point. I haven't seen these spec'd on any of my jobs yet, but it seems like a simple stabilization control. I would still design to maintain the flow rate offset but then adjust the flap to the given pressure...

 

[grim900]

CB
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.

 

[SAK9]

Grim900,

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.

 

[grim900]

SAK9

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.

 

[lilliput1]

Grim900,
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.

 

[grim900]

I used a leakage through the stabiliser of around 0.15m3/s (300 cfm) to maintain a 15Pa pressure drop. You're right about the weight. The unit consists of a stainless steel or powder coated wall sleeve with a stainless steel blade. The blade has a circular adjustable weight in the centre. Have a look at

http://www.apreco.co.uk

which is one of the companies we've bought from in the past.

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!