Room Pressurization

[lindalee]

At a facility, where all rooms are required to be maintained at varying negative pressures, I need to come up with a ventilation system design to confine contamination and to control room temperatures. This is a push-pull cascade airflow system; i.e., building supply fan delivers 100% outside air only to clean rooms, and the exhaust fan draws air only from the highest-contamination rooms, which are connected in series to lesser-contamination rooms. There are multiple chains of rooms, which are parallel airflow paths. Pressures range from.25" w.g. (cleanest) to 1.5" w.g. (most-contaminated). Has anyone had experience with a similar system? What kind of pitfalls?

Any comments will be appreciated.

 

[quark]

Your main problem will be in mainaining 1.5" WC, which I never heard of. What type of a plant is yours? I worked in almost all sorts of pharma plants and yet to find such a higher pressure requirement.

How are you planning to create air flow from one zone to another zone? (through hatches or doors or anything else?)

It is hard to comment without knowing heat load of your air supply rooms and air return rooms but my gut feeling is that you may be unable to maintain required temperature in both the areas simultaneously.

Give us specifics.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[lindalee]

quark,
1.5" wc will be required in the last room in the chain because that is where the building exhaust fan draws air, and there may be 3-6 rooms in the chain. I anticipate the pressure to be this high to be able to overcome pressure losses in the transfer openings or the inter-connecting ducts. This is a nuclear waste processing facility. All the rooms in a chain will be connected by the transfer openings in the adjacent walls between connected rooms, and by ductwork when the rooms are remote from each other. Walls are required to be made of poured-in concrete. Do you think the exhaust fan can pull thru the chains of rooms to develop varying levels of negative pressures in the individual rooms? Can I assume the rooms will act like large expanded pieces of ducts in each chain?
Regards...

 

[remp]

Lindalle,

I think you would be better of ducting the exhaust from each room directly. i.e. have a common exhaust duct with a brnach take off to each room.

Deliver the right amount of supply air as per design air flow and then balance the exhasut air system to achieve the required pressure profile. using transfer grilles between rooms on door/wall is a cheap and smart method but is very hard to balance right.

Its good the room are made of concrete, little or no leakage wihich will help with the pressurisation.

Good luck

 

[lindalee]

billyq,
Ducting the exhaust from each room directly; i.e, have a common exhaust duct with a branch take off to each room may not be allowed. Rooms with the highest potential for contamination are only accessed thru airlocks with as high as 55 air changes per hour required. We are told to bring the air into a contamination room thru the airlock to maintain the required air capture velocities when a door is opened. Also, it would be cost-prohibitive to throw away all that air from the airlocks where there is very little cooling load. Why not re-use it in the adjacent contamination zone where all the heat-generating process equipment are located.
Thanks...

 

[remp]

Lindalee,

If you supply only through the air locks your transfer grille (thats if you use one) is going to be hugh especially if you need 55 air changes per hour. Also you mention maintaining velocities when doors are open, what happens when doors are closed, no a/c in contamination room? Doesnt make sence? Why dont you have a dedicate supply and exhasut from each area if you are afraid of cross contamination from common system? if all areas have seperate systems it is easuier to control,otherwise a door opening in one area is going to effect another area unless the ventilation systems are seperated

 

[dxla]

lindalee,
I'm working on a nuclear waste processing facility also for the first time. We have 3 areas that are classified according to radioactive contamination potential(1 being least contaminated, 3 the most contaminated). We supply 100% OA to area 1. Air is then cascaded thru grilles to area 2 then thru HEPA filter assemblies to area 3. Air is exhausted from area 3. We also have dedicated exhaust for area 1 and area 2 like Billyq suggested. We maintain certain minimum velocities thru door openings also. Our pressures are about the same.

 

[lindalee]

billyq & dxla,

Your suggestions have merit; I will study the effects of the individual supplies and exhausts for individual rooms as much as possible, while considering the client's concerns for cross contamination. Thanks for your thoughts.

Regards...

lindalee

 

[lindalee]

dxla,

I have a couple of questions:

1. Is any of your category 1 or 2 rooms connected to a 3 room with a door in the common wall?

2. What did you mean with your last sentence, "Our pressures are about the same."?

Regards...

lindalee

 

[dxla]

lindalee,
1. Area 3 is highly radioactive so it is totally inaccessible. Therefore, no doors. Air cascades in thru HEPA assemblies only.
2. we try to maintain -0.1" wg in area 1 rel to atm, -0.4" in area 2 and -1.4" in area 3.

 

[lindalee]

dxla,

Thank you very much for the info.

Regards...

 

[quark]

Nuclear faciltiy is definitely not my cup of coffee but I am interested to know how you derived the figure 1.5" wc.

Feb,2003 issue of ASHRAE Journal deals with the room pressurisation problems and it was demonstrated that 12 Pa (or 1.2 mm of WC) is good enough to resist contamination. This is very near to the limits what we use generally (10 to 15 Pa).

I generally don't like the idea of further contaminating the highly contaminated room. It is better if you make HEPA filtered openings in each room to its adjacent room. If the rooms are tight, they sure behave as a large openings in duct work but door openings and other leakage paths make the situation worse. It is better to have individual systems with runaround loops to minimize the energy wastage. (heat recovery wheels are not an option in your case)

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[lindalee]

quark,
1.5" wc is simply a result of adding 0.25" wc for each room in the chain of rooms based on the client's criterion of 0.25" wc for the first room, to which clean air is required to be supplied. This is subject to change (it can be higher or lower, depending on how the rooms in one chain interact with rooms in other chains, etc.). We are told that 0.25" wc criterion may be reduced to as low as 0.10" wc depending on the type of process taking place in any given room. But, to start with we are using 0.25".

You're right, the heat recovery option is not allowed; i.e., once-thru ventilation is required for every room.
Regards...

 

[quark]

Actually I am, still, not convinced with the actual situation described above.

A pressure difference of 0.25" wc creates a velocity of approximately 2000 fpm and subsequent turbulence. This is to be avoided all cases if you are dealing with containment issues. Secondly, these are impractical velocities for HEPA filtered openings(but pressure drop across HEPA will sure help you in maintaining pressures and I suggest you to check actual required flow rates).

Dxla says, they are trying to maintain the pressures but I would like to know how strictly they are being maintained. (just for my knowledge)

If you are not taking return air from each room, then the exhaust from the final room will be very high(added exhaust from all the rooms).

My suggestion would be to take indepedant supply and exhaust from each room and then exhausting the rest what is required for pressure balancing.

For example, if the input flow rate is 1000 cfm then exhaust 1000 cfm from each room and the rest 2000 cfm (considering 1 sq.ft room connections) through the succeeding rooms.

I am interested to know, how you finally design the system.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[lindalee]

quark,
The facility design is in embryo stages; building room layout is changing daily, and process functions are evolving as we speak, etc. All I have for the ventilation system at the present time is a concept perceived from the client's requirements. Room exhaust duct velocity is required to be greater than 2,000 fpm to prevent contamination settling in welded joints, since the client requires the exh ductwork to be all welded pipe. As to HEPA filters on supply-side, this is new to me; but, I will study the concept before going any further with it. Room actual flow rates must be based on two factors; room heat loads, and air changes req'd for contamination removal. There will be 8 rooms receiving supply air, each starting a chain, and 8 exhaust ducts coming off these chains, finally forming a common exhaust duct. Some of these chains of rooms are cross-connected to cut down on the total bldg air. The exhaust duct, goes to the filter farm in the yard (HEPA and charcoal filters), then to the bldg exhaust fan and to the stack. I wish I could tell you more.
Regards...

 

[lilliput1]

If the differential pressure is too high, one may have problem opening the door to escape! Calculate the force to open the door by taking moments about the hinge with the pressure force acting at the center of the door and the pull to open force acting at the door lock handle.

 

[lindalee]

lilliput1,
Thank you for your response. You are absolutely right. Opening the doors against pressures above 0.25" wc will be a problem. I'll have to coordinate special hardware requirements with the architect.
Regards...