Plant Air Pressure / Make Up Air

CaseyK

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Feb 2004
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In the cornfields, on the prarie, outside Chi-Town
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Anybody invoved in Plant Engineering?

I have been involved with a lot of projects with fresh air in factories (primarily). Getting smoke out, fresh air in, and having a slight positive pressure.

PLC's seem a natural for this, 'specially with remote I/O.

First, anybody familiar with a pressure switch to maintain slight positive pressure, so the doors aren't ajar, or slamming shut?

Second, anybody doing this now? Since this would require several intake fans on a large facility, do you turn some off, or have one or more of the larger ones on a VFD system?

With public smoking laws, I have also considered a smaller version of this for bars and restaurants. Though, only one or two exhausr fans would be needed. Again, some sort of air pressure sensor would be handy to have.

Any thoughts?

regards.....kc





Feel free to submit twenty or thirty rungs of logic by ten minutes from now, and include some free RSLogix or Cimplicity, or AutoCAD E2006, and a stoplight program.
 
Some facility I know (mostly drug co) uses PLC or even DCS but most places uses somekind of BMS (Building Management System) for large scale manufacturing building.

A slight positive pressure is not hard to do and no, you do not need to really "control" the pressure as long as the place is fairly large. Door to the outside swing outward and with a door closer there is no problem. In other words, if you balance the air volume correctly, you may not need a "control system" at all. This is actually easier than you think, since air is quite compressable, a little extra incoming air won't cause much problem.
 
harryting said:
Some facility I know (mostly drug co) uses PLC or even DCS but most places uses somekind of BMS (Building Management System) for large scale manufacturing building.

Yep, a BMS conversing with some small PLCs that are reading pressure sensors/switches, inside and outside, (could be differential switches I guess) and controlling VFDs in the air houses is one method I've seen work successfully.
 
In a past career - I have done such a monitoring project. Mainly due to factory workers leaving truck doors open so they could see the sunlight. This created a slight negative pressure thereby allowing dust and dirt to enter the plant.

The plant HVAC units were balanced appropriately for normal conditions. I used the plant air pressure sensing equipment in the door control logic. Would have been a large undertaking to retrofit the HVAC logic to utilize plant pressure and fan speed. There were 108 HVAC units that would have been re-coded vs 18 door controllers.
 
I've been in equipment rooms that didn't have balanced air flow, and either the doors wouldn't close (too high pressure) or would slam shut with a vengeance (too low pressure).

If your fans are running continuously a switch might be problematic - cycling between excess or deficit on air flow. It would take some tricky tuning to get it right. The way it is usually done is by adjusting manual dampers and having only exhaust or intake fans. If you have both the should cycle together.

You could use VFDs on the fans, and tie fans speed to a pressure transmitter. Dwyer has inexpensive transmitters as well as switches. Small VFDs are pretty cheap nowadays.
 
I was involved in some very precise climate control for motor testing (freezing conditions) they had so much difficulties that they ended up requiring the doors to all be shut during tests (many hours).

The way I have seen it done is to control the positive pressure with dampers and have excess air in "normal conditions" and not give a darn the rest of the time.

Its not such a simple task for the load variation is fast and cycling is always present.
 
Hmmm, ever wonder why revolving doors are so popular in Chicago?
revolverslg.jpg
 
It is a shame you can put revolving doors in a plant. Maybe in a 10-20,000 foot facility.

But when you get to a 1/4 mile square, to some of those that are 1/2 mile or longer.....ouch!

Dampers may be a partial solution.

For sensors, I was thinking of an old furnace damper mounted with proxes.

I will look in to the Dwyer unit.

regards.....kc
 
I've been on gas production platforms that use positive pressure in the accommodation module. Leave the door open, and the power goes off, accompanied by much swearing. I'd have thought there must be a standard bit of HVAC kit for this.
 
jstolaruk said:
Hmmm, ever wonder why revolving doors are so popular in Chicago?

I saw a show on the Sears Tower a few months back and they said that air pressure is a major consideration. In big buildings like that do to the fact that hot air rises you can some serious airflow from the bottom floors going up when doors are open so they use the revolving doors. In the shipping/recieving area they had these big airlocks where only one door could be opened at a time.
 
I've worked in the meat processing industry where the plant was not only pressurized, but all air was sterilized. We had all motors on VFD's with basic pressure transmitters 0-100"H2O or something like that. Be sure to keep in mind you will need to run a line outside for a reference pressure.

Believe it or not, one of our biggest problems was in a freezer where temps were about -10C. When the air was very humid (in Nebraska that's 99 percent of the time) we would actually have water dripping from conduit fittings as the warm, humid air would migrate in from engine rooms, MCC rooms, offices etc.. and condense inside the conduit leaving small puddles of ice on the floor.
 

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