Industrial Food Processing : Room Pressurization 1

[Hamdog]

I'm trying to figure out how to calculate how much cfm is required to maintain an interior room at 5 pascals positive pressure.

The room is a "ready to eat" (RTE) room of a barbecue ribs processing plant. The issue being that the adjacent kettle and steam cooker rooms that have a total of 4 exhaust fans each at approximately 2,000CFM. When the fans are running they are drawing air through the RTE room (contaminants, fumes from mechanical room, etc). The fans make-up air are transfer ducts into the ceiling space above the RTE room.

The RTE room has (2) 4ftx4ft conveyor belt openings, one pvc plastic curtain door, one freezer door, and a metal entrance door. I recommended that they put plastic strip door on the conveyor openings, and a door sweep on metal door. The room is maintained at 38-40F with DX cooling racks.

What the owner would like a make-up unit to maintain the room at a positive pressure of 5 pascals. The unit to be control by a VFD and pressure sensor located in the space and adjacent control space, location ideas needed!!!! The unit will have filter media that meets code requirements of FDA, etc etc. The 5 pascals was reccomended after an in inspection was performed, I'm 95% sure this meets the requirements of FDA.

So how would I calculate the CFM and static pressure of the fan needed to maintain this space at a positive pressure of 5 pascals? Thanks!!!!!!!!!!!!!!!

 

[MintJulep]

How high is high?

There are lot's of threads here on room pressurization.

But the bottom line is that the amount of air you need is proportional to the leakiness of the room.

And it sounds like your room is plenty leaky.

It will be far easier for you to get a testing contractor to do a blower door test. You'll be able to identify all the leaks - including the ones you don't know about, and try out some different ways to seal the holes than need to be there.

 

[Compositepro]

If you want to keep a room clean you should not be sucking a lot of air through it in order to supply air to another room. First supply make-up air to the room with the large exhaust flows. Then supply a much smaller amount of filtered air to pressurize your room. The bottom-line you really can't pressurize an unsealed room. High air flow can also sweep the dirt off the floor when people walk by.

 

[Hamdog]

After thinking about it through the night I came to a conclusion to install two make up air units. One to supply 8,000CFM to the kettle/oven area where the main exhaust fans are and one to maintain the positive pressure in the RTE room. Thanks!

 

[Drazen]

As far as I know there is no calculation which could provide preciseness, and in projects I worked on it was always solved with VFD fans and pressure sensors.

 

[317069]

there must be a way to calculate this issue, what about constant voleum systems, and what about hydronic systems?

 

[VerneE]

5 pascals is about 0.02 inches water column. If the room is at that pressure, air will flow through the openings at a speed that will match the velocity pressure of the room pressure. For 0.02 "w.c. that is about 560 feet per minute, i.e. the velocity pressure of air moving at 560 fpm is 0.02"w.c. This means that each square foot of opening will lose about 560 cfm.

We used pressure measurement to control the damper on the return air - if someone opened the door to the room, the return air would be shut off to maintain pressure.

For a 4'x4' opening, that is 16 sq. ft. so the air flowing out will be about 9,000 cfm. You need to make the openings as small as possible, plastic strips will help somewhat.

 

[Hamdog]

after visiting the job site again on wednesday I found more openings in the room. Its hard going off memory especially when you cannot take pictures of the space.

Anyways I calculated the total leakage area to approximately be 6730in^2 or 47ft^2, this is a pretty good assumption. I brought this to the owners attention and explained that we need to seal the room the best we can and add strip doors over the conveyor opening. His response was we are not doing anything to the room just pump in as much air as it takes to maintain a positive pressure. On top of that the owner stated most of the time the doors are left open, well they will need to make sure they remain close.

So I went ahead and used figure 7 (flow rate through leakage area under differential pressure) located on page 18.6 in the 2011 ASHRAE Handbook - HVAC Applications. I came up witht he leakage area calcualted above apprximately will need 17,500 CFM which is around 4CFM/SF. Now if I use VerneE method it would be around 26,000 CFM, either way a lot of air, which equals large unit.

This just seems ridicuously high, but I guess you can't blow up a balloon with holes in it unless you blow in a large volume of air.

Thanks guys!

 

[317069]

- first you have to check the process before any solution you think about, the solution should not interupt the production process, you design HVAC for a factory, not factory for HVAC system.
- I am not sure about cfm numbers provided above but like to say if you apply Torchilly equation you might get reasonable answar.