Air balancing in food preparation plants

[athlonus]

I work for a refrigeration company. We design and install industrial refrigeration systems for packing houses, refrigerated warehouses, and processing plants.

It has become a increasing concern in these plants that air-borne particles drift from ”clean” areas of the plant to the “dirty” areas – ( cooked area of food preparation plants to raw, or cut areas toward kill areas of slaughterhouses ). E-Coli and other health emergencies have made air drift in plants a public safety issue that is taken very seriously by our customers.

Our company installs air handling equipment for refrigeration, so we have become involved by default. I am with the electrical department, my primary job functions involve PLC control and instrumentation equipment. We are having difficulties getting accurate measurements of static room pressures. Our problems, I believe are due to the miniscule pressure differentials we are trying to achieve and air movement in the areas we are trying to control. It is hard to get a sample in a room where there is no breeze.
Air is constantly moving and doors opening and closing. We have the same problem with outside air.

Another problem we have encountered seems to be differential transmitter response drift due to ambient temperature of the transmitter.

I am looking for stable instruments and technical advice in these areas.
We have been using air make-up units and exhaust fans – both with and without damper and VFD controls. Our problems are not control of the equipment, but reliable indication of the room pressures.

 

[walkes]

For room pressure control we sometimes use MODUS pressure monitors. See the following link:

http://www.gesensing.com/products/Modus_ModelRPM1.htm?bc=bc_ps+bc_prod_trm_prs

The operating range is 0°C to 50°C.

You will get space fluctuations if you are not able to maintain the necessary airflow offsets to maintain the appropriate pressure differentials. If you are expecting the doors to open and close frequently you may consider putting a vestibule in (ante room essentially) to mitigate sudden changes in pressure difference.

 

[cme]

With my experience in hospital isolation systems and labs, you need tight building construction i.e. caulking around the space where drywall meets the floor, gasketed doors and windows, caulked openings where wires penetrate walls, etc. Even then, 150 cfm differential gives you about .01-.015" differential. Ante rooms are a help in meeting the requirments and would help in your case. Forget about trying to control differential with open doors.

 

[SAK9]

I would suggest pressure differentials of 10~15 pa between clean and dirty areas for maintaining the cleanliness of rooms.This can be achieved by having a differntial between the supply and extract air volumes from the rooms.It will be good to install some sort of a device like a magnehelic gauge to monitor the pressure differential across the rooms.If you use a photohelic gauge ,it is capable of sending alarms to your control/BMS system.

In order to get stable pressure readings ,the reference point locations should be carefully chosen.

 

[friartuck]

Food factories are generally large open areas with simple openings between production spaces etc.

Sometimes plastic vertical sheet doors are used to separate one area from another. It is unusual to get involved in room preesure control to the same extent as in for instance a clean room or theatre.

The same principles are often used, i.e. supply clean air to clean areas and extract dirty air from dirty areas. Encourage airflow in the right direction.

Experienced food factory designers may use the 'protected door' principle whereby you may protect a clean food space from a dirty food space by ensuring that the airflow across the free opening of the dividing doors is maintained at a certain 'control velocity'. (i.e. 0.25m/sec)
This can give high interrelational airflow rates and can be quite difficult to achieve.

For a single door opening you might need for example 0.50 m3/sec airflow through it to maintain a 'clean/dirty' relationship.

Often food factories have huge air handling units with F7 filtration or better.

Do I take it from your question that the regulations in your country require room pressure diffential control with monitoring. If so..good luck...it is very difficult to achieve in large fully operational work spaces.

Any monitoring should have a time delay alarm built in otherwise your panel alarms will be going off all the time

Friar Tuck of Sherwood

 

[HEC]

All the advice above is good, I cannot add much more other than to say the use of air locks or vestibules is a good way to control air pressure diferentials. This can be a common hall way between the areas with mass extraction to ensure there is a positive flow from both of the areas. The sealing of each room is also paramount. Unfortunately most of the control methods require propper thought in the layout of the facility.

Generally the pressure diferentials of 15Pa are adequate with closed doors. The open door velocity requirement should also be applied to ensure no back flow into the clean area. as stated the basic facility layout is key to controlling airborne contaminants. If you have an existing facility to bring up to speck good luck!

Mark Hutton
hutton4eng@picknowl.com.au

 

[Drazen]

I will add to Friartuck to avoid much repeating as many good advices already exist here:
since you probably have rather large rooms it is very easy to have draft even if you don't open door ferquently - temperature differnces in room, obstacles to air flowing in and out of air distibution equipment are sufficiant to cause this.
The only area where I saw pressure measurement practicable is operation and intensive care rooms in hospitals, but only if you have installed equipment that should assure almost laminar flow of air (pereorated walls/ceilings, radiation panels with minimal convection and similar).

This equipment can have cost justification just in such areas and in pharmaceutical industry.

So I beleive that you need different way to control air transmission from room to room.

One solution is to provide little larger difference between make-up and exhaust air. You can even install directional transmission grilles between such rooms, and add adequate filter to the frame if you are still afraid of non-wanted air flow. You can also make oppostite difference in dirtier room (more exhaust than supply).

That is very similar to ordinary toilet venting design, but it is still the most efficient. Pressure sensor in one position in room will give you just indication of pressure on that location, and is not something you can rely on for critical matters.

...and it is not something which fights e-coli. If such thing apperar, you have several weeks shutdown of the whole plant, no matter how good and reliable systems you generally have.

 

[lilliput1]

We rather design by fixed airflow differential of transfer air in & out to maintain design relative pressurization with doors closed than use active pressure controls. Sometimes differential pressure sensors are required but use only for monitoring, not active pressure control. Any door opening would cause hunting in active pressure control unless the control is setup to respond very slowly. Relative room design pressurization is planned by indicating incremental steps of pressurization as indicated by say --, -, 0, +, ++, +++ at each room. Then assigning a pressure difference to each step, say 0.025" wg. Then designing air transfer through closed doors with the more positive rooms transfering air to the less positive or more negative room. The CFM value of the transfer air is correlated to the CFM flow through the closed door crack opening such as to effect the in. wg. pressure differential. Air flow control valves are set for constant flow, constant two position flow (with setback at unoccupied) or active tracking (master is say the exhaust CFM & the supply CFM to the room is set to track the exhaust with a fixed differential).

 

[HEC]

One item I recalled, a rather embarassing situation where a room with a suspended ceiling was pressurised. Due to the room size and light weight ceiling the cieling was blown up! Just be aware of the forces imposed on the building too.

Mark Hutton
hutton4eng@picknowl.com.au