Here is some interesting info. Just thought I would throw it out there.http://www.paragoncontrols.com/Airfl...nSequence.aspx
Phoenix uses their valve style because they have a variable compression rate spring. So, it compensates automatically for pressure changes and also regardless of how bad you have run the duct to it will even out the flow.
In the system you should use parallel blade dampers rather than opposed as they are much more linear in nature. From the same article:
"Figure 2 graphs flow across a parallel-blade damper at various damper positions. (Parallel-blade dampers can provide better mixing than opposed-blade dampers and, therefore, are preferred for mixing-box applications.) "
Here is some interesting info. Just thought I would throw it out there.http://www.paragoncontrols.com/Airfl...nSequence.aspx
That's under ideal conditions, which are virtually never seen in the field, unless one is fortunate enough to be able to design and spec and install everything concerned. Size and geometry of mixing box, ducting, size and placement of actual OA intake box and its louvers, etc, etc.
Over 90% of the time we're stuck with just doing the controls, somebody else did the rest. And I've NEVER seen an installation with anything even close to that sort of linear performance.
Nor does it take into account such things as outdoor wind effects.
Just my two cents.
A site where I stash some stuff that might be interesting to some folks.
http://cid-0554c074ec47c396.office.l...e.aspx/.Public
Do you roll around in a cow pen before work and get all crapped up before stepping on the job? Probably not because you like people and want to start the day as clean as possible.
Therefore, starting out on a job with installation variables with a parallel blade damper set is better than opposed blade. Especially if you are trying to make a constant negative in the mixing box. Of course you can even spend all sorts of money preventing the conditions you describe. That's not the point. The point is how good can things perform. The ability to perform with linearity is greater with the parallel blade. - Unless of course you may like to argue otherwise with a different method being better. I'm all for hearing/debating about this. Just something new to learn for me.
No, my point was that control of the amount of OA being taken in by controlling mixing box pressure, in less than ideal real world installations, isn't as simple as or foolproof or as accurate as it might seem at first blush.
Now, don't take that as my indicating some absolute position that it can't be done with some approximation of success and accuracy. I'm only indicating its not so simple as just slapping a DP sensor sensor and controlling to that.
As mentioned in the article you linked to, errors on the part of the sensor can have an effect. i.e. In the example, they mention sensor indicating .25"WC when real pressure is .20"WC in the mixing box. And they point out that one still achieves 89% of design.
Sounds good, enough. But what is not mentioned are several other factors. Besides offset in sensor absolute accuracy on a test bench, are things like drift over time. Fouling of pickup tubes. Variations of OA mass density. Outdoor wind effects. Selection of sensor itself, how good is it's repeatability (more important than absolute accuracy). Are you using single point measurement? If so, is it in a good spot? Or have you an array of pickups, well placed, so you can get a good average. Wouldn't matter much if its a small cross sectional area, can matter a lot in a BIG mixing box. Then one needs to consider, carefully, full range selection of sensor selected. And resolution of controller input.
Wind effects. You are referencing one side of the DP instrument to something, right? A 10 mph wind can induce approx a .04"WC error. Either way. How about a 20 mph wind? Gusting conditions? Etc. How much "hunting" might this induce into this control scheme?
In short, while such a scheme has it's merits, isn't worth much in accuracy unless one pays attention to details. (As one would have to do with air flow measuring stations)
And add that due to all the variables, to be SURE, reasonably sure, one still would need some other method of verifying adequate OA flow. Air flow station, or perhaps if differential between OA and RA temps is adequate one could double check by calculating percentage using resulting MA box temp. But yah really need a second check method to be reasonably sure.
Just my opinion. Nothing else. I don't know a darn thing more than anyone else.
As concerns my comments about the linearity of parallel blade performance, read this ...
http://www.mcquay.com/mcquaybiz/mark...gNews/0101.pdf
Ignore their obvious marketing hype touting their own product, or not, your choice. I'm not in the business of touting McQuay, not the reason for pointing out the doc.
The pertinent info, to my point, is their discussion about just how linear ARE parallel blade dampers. And, as they point out it depends upon a number of factors. Most of which, in most cases, the controls folks have no control of when it comes to planning and installation.
A site where I stash some stuff that might be interesting to some folks.
http://cid-0554c074ec47c396.office.l...e.aspx/.Public
You don't throw out the baby with the bath water. So, I agree but this doesn't mean you do not target parallel blade dampers for mixing. It simply means more importance on mitigating external issues. Which, I need to add has just proved extremely helpful to me for a completely different issue so thanks again.
Took me a bit, but I figured out what bothered me about the damper volume graph. Google found a better description of the opposed and parallel blade issue with more graphs.
http://buildingefficiency.labworks.org/modules.stm
Look at section 8.
Just like valves the pressure drop across the damper in relation to the total pressure drop of the system has a huge effect on the linear performance of the damper. Also isolating the the pressure measurements would make getting performance like the graph sysint provided difficult in the field. To deal with this the system needs calibration at the airflow you wish to provide. This is done while balancing.
The old guy that explained the choice of parallel dampers to me said mixing boxes always use parallel to help direct the two airflows into each other to aid mixing.
To get back on this threads original topic, a good way to control the flow of outdoor air in a mixed air AHU with a return fan is to hold the OA damper a a fixed position and control the pressure drop across the damper. OA to MA. It will take some adjustment on startup to determine the pressure setpoint and the damper position to give you the design CFM value of OA. (Work with the balancer to determine the CFM. Or learn more than I will write here.) Once these field settings are determined the system will provide the CFM you set up for reliably. To control the Delta p, the return air volume into the mixed air plenum must be controlled with either the return fan speed or the return damper position. The choice is based on criteria outside the mixing plenum. In this case the return fan must run at a speed to produce the proper pressure drop across the return VAV boxes. So now the return damper modulates to maintain the DP in the MA. The Exhaust air damper can just be controlled by RA damper signal as it will need to relieve whatever excess air the Return fan brings back.
Bill
Yes but ...................................
I already have an airflow measuring station in the outside air intake that appears to be in a suitable location (i.e. straight duct).
If the minimum outside air volume (non-economizer) is maintained via the outside air damper and we control the return/spill damper on return plenum pressure, we are theoretically maintaining a delta P across the return damper.
As economizer begins to operate we keep the same operating and still maintain (theoretically) the delta P across the return damper.