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Thread: Positive air pressure system
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27-11-2006, 11:34 AM #1
Positive air pressure system
Hello all
We have been called in by one of our food production customers to look at the possibilty of installing a Positive air pressure system within the food area.
Anybody got any ideas as the area has extraction system for the cooking ovens, may pose a problem in generating a positive pressure.
Any ideas greatly appreciated.
Kind regards
Lrac
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27-11-2006, 12:47 PM #2
Re: Positive air pressure system
Well you have to overpower the exhaust rate with the fresh air rate.
As a starting point look at supplying air at a rate 10% higher than what is being 'extracted'.
Then look at how much humidity you have to add to that air int he winter and how much humidity you need to remove from that same air in the summer.
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27-11-2006, 01:24 PM #3
Re: Positive air pressure system
Positive pressure system is just making sure that the supply air is a little more than the exhaust air leaving a certain space. If there are a lot of doors and exhaust fans that are being put on & off, the supply air volume should vary.
Varying the supply air could be accomplished by adding a pressure sensor in the room that varies the supply fan speed thru a VFD or using multiple motor speeds.
For food production areas, some facility requires a filtered air and customer sometimes require a Hepa filter. Fresh air supply air also varies depending on the required volume per person, equipment requirement, etc.
The customer should realize that a positive pressure system will incur additional energy cost.
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27-11-2006, 02:56 PM #4
Re: Positive air pressure system
Originally Posted by winfred.dela
The second point was mentioned above. 100% outdoor air ventilation is not cheap to operate, and require some very fine control tuning to work properly.
The exhaust fans are only a part of the problem. The supply fans need to be able to ramp up and down depending on the pressure difference without over-shooting.
And the volume of make-up air is the volume of exhaust air (by the fans) plus the interior requirements (people, etc.).
If you have to use HEPA filters, I would also consider some pre-filters on these to reduce the HEPA filter changeout intervals.
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27-11-2006, 03:22 PM #5
Re: Positive air pressure system
if you want some help with this drop me a line, we are based in the UK and do this all the time.
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27-11-2006, 03:34 PM #6
Re: Positive air pressure system
Originally Posted by clivet
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27-11-2006, 03:40 PM #7
Re: Positive air pressure system
I spent four years operating a Clean Room for the High Tech Industry. I think as Iceman says it will introduce a lot of complication to the system.
All the air flowing into the room will have to be conditioned. In order to maintain that positive pressure you have to overcome any air loss. Even if the space is designed for it, it still takes a substantial amount of air.
In this case we also had to control humidity (I assume this is not the case in this application). But if it were, you would either be cooling the air to dehumidify, then reheating, or adding steam to humidify when conditions called for it. This is enormously expensive.
When we first started the space we thought about controlling the fan speed to maintain the room pressure. The design engineer discouraged this. He said his past attempts to do this had been failures. The VFDs could not ramp up and down quickly enough to maintain pressure without going into oscillation.
Again your application requirements are probably not this critical.
I did once do some calculations for a nationally known food processor I worked for. We wanted to blow filtered air into a oven area. The plant was closed before we implemented it, but it was practical as we only wanted to introduce outside air. The space never required heating. If this is the case you may have a practical project.
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27-11-2006, 03:40 PM #8
Re: Positive air pressure system
http://www.clivetaircon.co.uk/index.php
Coincidence?
.
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27-11-2006, 04:25 PM #9
Re: Positive air pressure system
The big difference on the ventialtion cost is the amount of air to exceed the exhaust, not the air to equal the exhaust. That air has to be made up some how either mechaninaclly or by infitlration.
Either way a negative or positive pressure ventilation scheme means you would not have to worry about 'natural infiltration' in the loads, the natural effects are over powered mechanically.
Negative scheme accelerates infiltration to work as a partial make up air, positive pressure has air 'exfiltrating' out the same openings infiltration uses as an inlet.
In a cold climate, positive pressure will drive moisture into the building envelope.
Pretty tricky to ramp blowers up and down to maintain a certain level of pressure, wind blowing on sensor can reak hvac.
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27-11-2006, 06:55 PM #10
Re: Positive air pressure system
My two penny worth..
Instead of VFD on the fans you should consider using pressure relief dampers around the space equal to at least one door opening. Also fit air locks to all outside door access points to preserve as much pressure as possible.
It might be prudent to get a specialist designer involved, by the customer, so that you can 'pass the buck' if need be.Brian - Newton Abbot, Devon, UK
Retired March 2015
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27-11-2006, 07:00 PM #11
Re: Positive air pressure system
Ok then so not only are we looking at filtration of the incoming air, also the possibilty of cooling/heating humidification and de-humidification. Also the amount of flow and the extraction rate to exhaust are all going to play a part in this design.
This is going to be a good one then, as i say the first one is the hardest and as long as we call in the right people to assist with the design and equipment selection it should be a nice project.
Your further input and past input is appreciated.
Kind regards
Lrac
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27-11-2006, 07:33 PM #12
Re: Positive air pressure system
Brian & Abby Normal raise a good point on the VFD's. After I saw the comments the use of barometric relief dampers does make more sense.
You can tell this is something I don't normally do. I should have stayed silent.
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27-11-2006, 07:53 PM #13
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27-11-2006, 07:57 PM #14
Re: Positive air pressure system
[quote=US Iceman You can tell this is something I don't normally do. I should have stayed silent.[/quote]
For once I disagree with you Iceman. This is a forum where ideas need to be thrown out. If one does not venture a opinion, and occasionally be shown to be incorrect, how will we learn? I for one do not being corrected (gently) when I am in error.
In the case of the Clean Room I referred to in my earlier post if just one of the hundreds of Electricians, Pipefitters, Carpenters, or other Tradesman who came through the jobsite has noticed a defect in the ceiling design and spoke up we would have been able to correct a major source of air leaks before it was too late.
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27-11-2006, 08:02 PM #15
Re: Positive air pressure system
Well you can be sure I don't normally have a problem asking questions or opening my mouth!
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27-11-2006, 08:10 PM #16
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27-11-2006, 08:23 PM #17
Re: Positive air pressure system
OK, I'll go out on a limb. I did use the VFD routine on one of the two jobs I designed. It was for an egg breaking room that was isolated from the outside. In fact it was built in the main storage cooler. The unit had 100% outdoor air and the differential pressure was measured by a contoller that ramped the VFD speed, depending on the room pressure.
I used some relief ports (not barometric dampers) cut into the wall to dump the air. Two of the ports were for the conveyor entrances. If I remember correctly, the owner only cared about temperature control and air filtration (we used HEPA filters) and trying to maintain a positive pressure in the room.
By all accounts it seemed to work OK. This was almost 20 years ago, so the details have gotten a little fuzzy.
I can tell you, these sytems are involved especially when you get into the control systems with humidity control.
I did find out later the barometric dampers would probably have been a better choice. Live and learn...Last edited by US Iceman; 27-11-2006 at 08:24 PM. Reason: editing
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27-11-2006, 08:57 PM #18
Re: Positive air pressure system
If you can't fix it leave it that no one else will:rolleyes:
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27-11-2006, 10:37 PM #19
Re: Positive air pressure system
Hi Abby,
- Does this mean that we just need to infuse the amount of air to exceed the exhaust?
- Then design the 3 stage fresh air filtration system based on this amount of air (to exceed the exhaust) which will be introduced to the system.
- How about the infiltration? it is not filtered.
Hope you would answer above question. Have designed and built HVAC system for microlab and tissue culture labs almost 10 years ago. I may have to change my ways next time. . .
Regards
Winfred
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27-11-2006, 11:22 PM #20
Re: Positive air pressure system
Winfred
As you stated, they have to supply more air to the area they want positive then they exhaust to the outside or draw return air from.
If something was to exhaust to the outside only without a dediacted make up air system, air will infiltrate in where ever it can and the HVAC system has to deal with that.
If we are pressurizing, then we are bringing in air at a rate that exceeds what is being exhausted. So the HVAC has to handle the portion that makes up (same as if it was all infiltration) plus the air required to exceed the exhaust to create the positive pressure.
The extra energy penalty is the amount of air that exceeds the exhaust. If we were not pressurizing we still have the energy penalty of making up the exhaust to the outside is my point.
If the space is pressurized, there will not be any infiltration, there will be air leaving the space to relief pressure as long as the building envelope does not have the integrity of Swiss cheese
If it was going to a class 100,000 or better clean room then you could have 3 levels of filtration. Perhaps 30% pleated prefilters, then a good cartridge filter like a camfil farr riga-flo, then perhaps HEPA down stream of the blower even.
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27-11-2006, 11:25 PM #21
Re: Positive air pressure system
You have to start somewhere, 10% is as good a guess as any. The building has to be tight in the first place though.
A stronger rule of thumb would be perhaps 0.5 CFM per square foot of floor space excess. 2.5 l/s per square meter above the exhaust rate maybe if you were trying for clean room.
Best rule of thumb is the one about the size of a stick.
I am in a humid climate down here, and the pressure keeps the humidity out. Does not bother me to make the general contractors go and tighten up their door closersLast edited by Abby Normal; 27-11-2006 at 11:30 PM.
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27-11-2006, 11:27 PM #22
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28-11-2006, 12:58 AM #23
Re: Positive air pressure system
If someone wants to do a search on Amazon.com or some other used book seller you might want to look for a book titled "Cleanroom Design" by W. Whyte from the University of Glasgow.
ISBN: 0 471 94204 9
It's good book for some of the details and requirements and provides some good descriptions of the systems.
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28-11-2006, 01:01 AM #24
Re: Positive air pressure system
Whyte is the god of clean rooms
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28-11-2006, 01:24 AM #25
Re: Positive air pressure system
That's good to know. I thought the book was well written, I just have not read it in quite some time.
I don't remember what I was looking for one day, but happened to find it by accident and thought, what the heck?
I think one of the most common places where we should be seeing this type of technology is for food/meat processing rooms. To me it makes a lot of sense. It has to be several orders better than using ceiling hung units or ductwork.
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28-11-2006, 04:04 AM #26
Re: Positive air pressure system
We are sort of straying from food processing into clean rooms here. Maybe that is not quite the challenge the OP was up against.
A common scenario I get is restuarants here. Over all the building needs to be slightly positive to combat humidity and the kitchens negative with repsect to the dining areas.
Have the exhaust out from the ktichen hoods, so much air getting exhausted from the washrooms, so much air brought in by make up air units and so much air brought in by the HVAC system.
Have to start adding up the ins and subtracting the outs, supplies into a space, the fresh air in, the exhaust air getting blown outside (extract to you on the wrong side of the Atlantic) and the return drawn from the spaces.
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28-11-2006, 07:03 AM #27
Re: Positive air pressure system
How do you measure the integerity of a area for keeping the area pressurised,is there a method to measure the area's capability to keep a positive pressure(air tightness).
I need to take this one step at a time and the above is important as the first point to conquer.
If the above cannot be tested then the rest will fail completely.
The area to be pressurised contains food make up area to inlude in this area are 1 x blast freezer, 1 x blast chiller, 1 x changing room, cook off area all with plaster board ceilings and dual discharge evaps for temp control of area.
LracLast edited by LRAC; 28-11-2006 at 07:07 AM. Reason: added more info
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28-11-2006, 07:33 AM #28
Re: Positive air pressure system
Because it involves design time, if you get it wrong unfortunately the repercussions are significant and the cost will kill a non specilaist. I've been doing these projects for a few years and I wouldnt want the responsibility of givimg someone bad advice just because "thats the way we did it last time"
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28-11-2006, 07:40 AM #29
Re: Positive air pressure system
Speaking from experience if you are dealing with a company that is dealing with a supermarket dont **** about with it wiothout specialist advice. An M & S supplier took a contractor to court 18 months ago over the positive prssure issue and it broke them. Employ a specialist to design it for you then its no longer your problem.
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28-11-2006, 08:00 AM #30
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28-11-2006, 11:08 AM #31
Re: Positive air pressure system
Hi Forum
My experience of pressurised areas comes from the offshore oil industry and the food processing industry. An offshore accommodation module requires pressurisation incase of gas leaks outside the module, in the food industry it is to prevent air passing from a low care area to a high care area.
It has been achieved in many ways, but the easiest method to install and balance is to over-pressure the supply air to the area by 60 pascals and using slow opening pressure relief dampers the room pressure can be regulated. The supply filters need to to be checked regularly has a reduced airflow lowers the supply pressure resulting in a lower room pressure. The area to be pressurised needs to be completely leak tight i.e wall panels sealed, airlocks or sealed doors otherwise a larger supply fan is required to achieve the over pressure. Also it is also worth checking the structure of the area to be pressurised, I have seen doors and cold store wall panels blow out even at an over-pressure of 80 pascals.
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28-11-2006, 11:52 AM #32
Re: Positive air pressure system
Why do one has to give bad advice. You don't mean sabotage right? Your are right to advise user/contractor to engage specialist to do the job because it will be very very costly to end up with mistakes.
The thread starter wants to learn more about positive pressure cleanroom. I think it is great to share some fundamentals about cleanroom in this forum. As a specialist you are in a better position to share some pointers.
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28-11-2006, 12:28 PM #33
Re: Positive air pressure system
To know whether the room pressure is positive, you use a 'Dwyer' Magnehelic Differential Pressure Gauge which measures the differential pressures inside & outside the room. The integrity of the cleanroom is determined by conducting Particles Count Test & Bacteria/Fungi Count Test.
Of course you got to have a properly designed and built cleanroom, good cleanroom housekeeping, educate the operators/users on the cleanroom behaviours, dos & don'ts and ensure they practise them all the times. These are some of the things to know.
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28-11-2006, 12:38 PM #34
Re: Positive air pressure system
A common way on this side of the Atlantic is a blower door test.
The test is used to estimate infiltration rates of structures.
It also is used for fire suppression systems similar to Halon such as FM200 where the gas must be retained at certain concentrations to extinguish a fire.
Seems as if inergen and argonite are more popular in the UK and in those systems you need signifigant pressure relief.
In a blower door test, a door with a blower is temporarily installed. The blower is throttled until the stucture is depressurized by 50 Pa.
The flow to produce this level of deprussurization is recorded. While the test is being performed, smoke pencils are used to see where the air is infiltrating in.
In the case of homes, the natural winter infiltration rate would be most likely be in the neighbourhood of the exhaust rate needed to depressurize to 50 Pa divided by about 17Last edited by Abby Normal; 28-11-2006 at 01:01 PM.
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28-11-2006, 12:59 PM #35
Re: Positive air pressure system
A common problem when people 'go cheap' on regular HVAC systems is over pressurization.
Example the structure is fairly tight and a roof top packaged unit with a manual/motorized fresh air damper with no relief is used.
Relief can be barometric, or a powered exhaust fan right in the packaged unit or, the dumbest one, a scheme that uses motorized dampers to try and regulate the pressure.
Without the relief, It likes to blow doors open and in a cold climate it forces moisture out through the building walls. Besides a hard time closing doors another symptom is frozen locks.
This type of pressurization happens with a manual/motorized fresh air damper for upwards to 25% outside air.
In summer time with framed construction 2 Pa pf positive pressure with respect to the outside,tends to reverse infiltration and air leaves the structure. The mechanics of infiltration change with the season, in particular stack effect.
Continuous negative pressures of 2 Pa in humid climates has been documented as causing significant water damage in hotel/motel walls in the Southern United States.
Sometimes minimal fresh air to the space under pressure, and then exhaust being drawn from the areas surrounding the 'positive zone' works as well.
There are 'many ways to skin a cat' and the advice to get a design specialist, and better yet the customer to hire this specialist is good advice.
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28-11-2006, 01:12 PM #36
Re: Positive air pressure system
The simplest positive pressure scheme is the central return grille in the hallway a condo.
Doors are under cut to provide an air transfer from bedrooms to the hall way.
Typically if there is a cooling call and if the bedroom door is slightly ajar, the door slams shut when the AHU blower engages.
This will pressurize a bedroom as it pushes just as hard on exterior walls, window frames and ceilings as it does on that slamming door and air leaves the structure.
However it will depressurize other areas of the home , like the hallway for example, and cause humid air to infiltrate in from the outside.
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28-11-2006, 01:30 PM #37
Re: Positive air pressure system
Is it to be a clean space LRAC?
As in one of those clean spaces, the clean area would be positve with respect to the changing room where the employess change into their 'bunny suits'.
The change room would be more positive than the 'dirty area'. Emplyees enter through the change room, similar to the airlocks Brian mentioned.
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28-11-2006, 01:49 PM #38
Re: Positive air pressure system
Once you get those clean spaces, they have to be concerned with the type of paint/finishes on the wall or the plaster ceilings.
Don't want paint flaking off etc. Its an architectural concern that becomes the mechanical guys problem if he is supposed to be filtering the dust out.
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28-11-2006, 03:52 PM #39
Re: Positive air pressure system
Is anyone aware of a suface finish that supposedly kills bacteria which can be used to coat the interior surfaces of a clean room.
I've heard different stories about a coating that apparently has some kind of suface finish that punctures the bacteria cell wall when the bacteria comes into contact with the wall finish.
Any produt names or manufacturers you can think of?
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28-11-2006, 04:17 PM #40
Re: Positive air pressure system
That would be Sherwin Williams "Micro Iron Maiden", in various shades of red
Last edited by Abby Normal; 28-11-2006 at 05:40 PM.
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01-12-2006, 12:21 AM #41
Re: Positive air pressure system
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17-10-2008, 04:27 AM #42
Re: Positive air pressure system
Hi : Try an RSI pressure controller linked to an $85 variable speed motor control and a positive inline ventilating fan. Cheap(under $8000), effective, and as used for hospital SARS/TB negative pressure room controls with automatic pressure control and alarms. Remember to size the fan for any pressure drop that you will find across Hepa filters you install.
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17-10-2008, 05:29 AM #43
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17-10-2008, 06:27 AM #44
Re: Positive air pressure system
If anyone is still interested in this topic, here is what we used:
TSI Pressura 8630 controller with 4-20 MA output(has visible display of pressure differential from the transducer) an inexpensive variable voltage fan motor controller similar to one available now as "Smartfan"from Control Resources, and a Soler Palau PV-200 Hi static pressure inline vent fan providing 485cfm at .25" static pressure.