Building Static pressure
I need to install a differential static pressure senser to control building static 1st part of my question, does the high pressure pitot tube go to the outside and low pressure pitot goes on the inside space. part 2 of my question, where is the most ideal location for the low and high pressure pitot tubes any direction would be great.......... THANKS
High pressure tube should go inside the duct. Low is open to the space or ceiling plenum.
2/3 down the duct (at least that's what all the literature will specify).
For building pressure control - Hi press to space and Lo press to outdoors setpoint is typically .05" WC. control parts houses typically supply a space pressure reference wall plate with a barb fitting for Lo side connection and an outdoor atmospheric referencer with barb fitting for Hi side connection.
As far as best location it depends on the application. IE 2 story versus 1 story offices, lab, hospital etc.. need more info on app and Im sure we can help
Thanks for the response, it's a 2 story with about 40 vav's trying to keep space slightly positive, depending on time of year interier/exterier door's stay slightly open and as summer is approaching this seems to happen more often this time of year.
Chilled water & hot water with AHU w/economizer, RTU w/economizer per floor & associated VAV's??
Originally Posted by westendmechanical
1/10th of InWc is all you will need here
This is a hot and chilled water AHU has mix air temp control with only one output for all dampers but that is going to change, one output for fresh and return dampers and one output for exhaust dampers I'm changing the wire in/output for all dampers, it also has an ABB drive on it controlling duct static.We will have one output for the return fan for building static setpoint at 14pa controlling exhaust dampers. This is a 2 story school building most vav's are on the first floor.
Last edited by westendmechanical; 05-14-2011 at 09:21 PM.
So you will have 1 building press DP per AHU, with the high press sensing tube in the main entry area of 1st floor and in hallway of 2nd floor. Im assuming this is an office building. Not sure what you mean by 14pa, but industry standard is like you said slight positive which is +0.05 inches of water column. (Air pressure Differential transmitter range around (-1.0" to +1.0")
Originally Posted by westendmechanical
As emcontrols said high to inside and low to outside cause you always want your pressure inside higher than outside.
Personally I don't like anything above 0.05" w.c as too often I've found it to be a touch too high. 0.03" w.c is my preferred setting with a +/- 0.02" w.c dead zone, at least in my experience w/buildings 6 stories and less. Also very slow response times. I didn't read the article completely, but briefing it there are good points brought up. Depending on the building location as well will really depend on how critical the placement of your outside reference is. I've yet to find a solution for major wind gusts that just show up, short of very slow response times and surge dampeners. I've had sensors that were mounted on the side a building on the non-prevailing wind side and it worked very well. These buildings were all 'boxed in' per say as they were protected by other buildings. Similar building but on the 'plains' and not too many gusts but the consistent blowing wind, when blowing would play hell on the sensor. Tried various outdoor references and surge dampeners and pneumatic capacitors, but found relocation to be key. If at the edge of a building you want the sensor at least 10' above the paraphet wall or any major structure. This is to minimize the Bernoulli principle effects. What I've found is if you have a flat roof then somewhere in the middle of the roof 5-10' above any significant wind blocking structure seems to be the most ideal for the outdoor reference.
The ideal inside location will be on the first floor in a common hallway area, not too close to exterior doors. A conference/waiting area works as well as long as the room has plenty of 'air leakage' to the hallway that leads to the exterior doors. This will allow you to sense the pressure the exterior doors will sense from the inside and and with that pressure value knowledge you can help keep the doors closed. If you put the sensor right near the exterior door then every time someone goes through the door the sensor will pick it up too fast.
Here's a few styles -
For ceiling tiles and even drywall I like this indoor pickup reference the best, as long as the hole isn't bored too big - http://veris.com/Item/AA05.aspx
For outdoor either style as depicted above, just depends on what it is in view of. The box type device on the right above blends pretty well with some of the dome style security cameras, vs. the 'red bird.'
They also make wall mount indoor pickups - http://www.bapihvac.com/products/pre...ts-and-probes/ or http://www.dwyer-inst.com/Product/Mi...nsors/Ordering
So the next question is how are you going to control the building pressure now that you know where to place the sensors?
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14 PA = 0.056+ in. w.c.
Also I like http://veris.com/Item/PXPLX01S.aspx with a uni-directional setting of 0.5" w.c aka 100 Pa or less.
Also to clarify on the slow response time, I don't want it so slow that the building over/under pressurizes, but I don't want it so fast that just cause somebody opened the exterior door and maybe even held it open for a couple of people that the building pressure control ramps down the exhaust. A heavy filter on the control input....
Good advice, I've been using the Veris pressure sensors maybe 2-3 years. Great units, flexible, work well, and nicely priced.
Originally Posted by crab master
Originally Posted by crab master
Placement of pickup points can make all the difference in the world.
As can slowing down response and damping/filtering inputs, as you mention.
A few additional observations I've made over time.
Make darn sure you're sensing SPACE pressure of your selected area.
i.e. In the past when troubleshooting issues I've found interior pickup points that were actually referencing overhead plenum pressure (above the ceiling tiles), pressure inside wall cavities, etc.
And in a few instances someone had put in nice space pressure sensing plate in ceiling tile ... in itself an okay idea ... but had the plate located right next to supply air diffuser or RA grill. Close enough so air movement velocity at that point influenced reading.
Next. Have found many a time where there was a problem caused by kinking of pneumatic tubing between sensor and either outdoor or indoor reference. That occurred either during installation, or some point later.
This is particularly a problem when using poly. A frequent enough issue so that some designers specify copper to be used. Myself, I think poly is fine as long as it's well and carefully run to prevent the issue.
In short, pay attention to those nitpicky little things ... they can and will bite you in the a**. You're dealing with some very slight pressure differentials you're trying to measure.
Lastly, be judicious in selecting the pressure range of the sensor you'll be using.
i.e. In the past I've seen a lot of installations that used fixed -1.0 to +1.0 "WC sensors. Even with a quality sensor selected that has a guaranteed 1% of full range accuracy you've got a problem. This sensor selection leaves you with what is, in effect a 2.0 "WC full range.
That means at best you've got a possible error of .02 "WC. One way or the other. Not real good when you're trying to control to .05 "WC. I mean, it CAN work. If yah tweak stuff enough. And everything else is good. But essentially you're looking at a possible error that's 2/5ths of your control range.
Personally I prefer a smaller full range selection for this application. The particular sensors mentioned by Crab Master, if yah get the -01 model, are switch selectable down to 0.1 "WC full range. That would put your control setpoint right in the middle of the instrument range. With a resultant accuracy of within .001 "WC (1% of full range). Or if yah don't like that a .25 "WC selection still gives yah only a .0025 "WC possible error.
As concerns bi-directional sensing, being able to detect negative pressures as well as positive, I've seen it spec'd and insisted upon. And if yah have to you have to. But I don't go that way unless forced to.
After all, in most cases the control sequences and available equipment being controlled don't actually allow me to do anything more about a -0.02 condition than I could do about a +0.0 condition. Either way the system is going to be doing everything it can do to increase building pressure back up.
I suppose being able to see the negative numbers out curiosity has its information value. But does nothing more control wise. Unless you've got additional control schemes and methods you can implement upon seeing those negative numbers. i.e. Typically by the time building pressure is down to .01 "WC relief/exhaust dampers are closed and fans ... if yah got em ... are slowed as far as possible, etc. What now? Boost OA intake? Bring in even more air that needs conditioning and spend the energy on that?
Might be worthwhile if you've got a good energy recovery system. But otherwise ....
Just some thoughts ... to consider or ignore. I don't claim to be an expert in these things.