Stump the Control Experts - Application Challenge

[BillLodato]

Got a bit of a head scratcher application for a retrospective commissioning project.

The challenges are:


- Return Fan Volume Control
– Economizer Damper Control



Application:

System is variable air volume with CFM Tracking at the zone level (i.e. Pressure independent VAV boxes on both supply and return …… The return VAV is controlled to maintain the same volume as the supply VAV … The supply VAV operates in a traditional cooling with Hot Water Reheat application).

Minimum outside air volume control sensed by a flow station in the outside air intake duct.

The central system configuration is a supply HVAC unit with mixing plenum dampers (OA/RA) and an in-line axial return fan.
Spill damper is located in the duct downstream of the axial return fan and upstream of the return damper.

Challenge:


What is the strategy for the return fan volume control?
What is the strategy for the economizer damper control (i.e. Return/Spill dampers)?

[azzad]

Can you do a quick sketch of the layout please including all major items.

Thanks
Dazza

[xarralu]

When you said "spill" damper, is this a damper that is going to the exterior of the building to relieve excess return air when in economizer mode or high building pressure?

As Azzad has mentioned the mechanical lay-out would help alot.

[BillLodato]

Feel a bit silly about posting to Walgreens, but the best i could think of quickly.

http://photo1.walgreens.com/walgreens/share/p=338281303999055559/l=2169798001/g=15778681/cobrandOid=1009/otsc=SYE/otsi=SANR

The spill damper relieve to ambient, in this case the mechanical space.

What is causing the challenge is the fact that the distribution system has pressure independent VAV boxes on the supply and return. THe return VAV boxes control to match the supply volume (the air into and out of each zone is the same).

Since the Return distribution system has pressure independent VAV boxes, what is the best strategy to control the Return fan speed?

With varying supply/return volumes and an outdoor air quantity that is controlled to maintain at least a minimum outsood air CFM .... What is the best strategy to control the economizer dampers (Spill/Return)?

[dracula]

Seems that your terminology may be a bit out of the norm.

From what I see, this is a very typical control application.

What is called the "spill damper" is just the exhaust air damper.

The spill damper control is best served as a stand alone control loop with the input being that of the space pressure in the conditioned space being served by the supply fan (the zones), and a damper actuator to open it as the pressure exceeds setpoint.

Economizer dampers are a seperate control loop with the setpoint being an attempt to maintain supply air setpoint (when OAT is usable) by modulating the outside intake damper and the return air damper. The control inputs would be supply air temperature, outside air temperature, (and others to make it a fancier control loop if you wanted to).

The return damper control and the spill (exhaust damper) control are seperate control loops. Not good practice to try to do "poor mans" economizer control of all three sets of dampers from one control loop.

Since it doesn't appear that the return fan is designed as a pure "exhaust fan", you could easily do supply fan tracking with an adjustable "plus or minus" tuning factor for system commissioning to get it to balance correctly.

[BillLodato]

Thanks for the response ...................

I may have tried to post tooooooo quickly this morning ,, I took some time to elaborate on the sketch.

Since the individual zones are providing zone level flow tracking, I was concerned that the flow tracking at the zone level, and the flow tracking at the central system level might fight each other.

Building pressure control might be the way to go on the exhaust damper (Actually it does "spill" to the mechanical room), but sunce the return volume is going to be limited by the zone return VAV boxes, I'm not sure that controlling the exhaust damper on building pressure will accomplish anything.

Also, the outside air damper needs a separate control signal, since it controls to a minimum outside air volume.

Here's what I was thinking ..................


Control the return fan speed from return duct static, much like supply fan volume control. This will assure the duct static required for the return pressure independent VAV boxes.

Control the outside air damper for minimum volume, with an override for economizer.

During economizer mode when the outside air damper delivers more than minimum volume, control the exhaust/spill from static pressure in the return plenum - just upstream of the return damper. This will maintain a constant static in the return plenum as the outside air damper opens for economizer operation.

As I said, it is a bit of a head scratcher .............

[BillLodato]

attached

[sysint]

whose idea is it to control the unit this way?
What's the objective?

I think you have controlled medium in the wrong places. I'd probably relocate two pressure transducers, and relocate the flow station to the exhaust. Of course then I'd be changing how you are currently controlling your OA, Dampers, Exhaust and Return Fan..... because I don't think your control scheme connections will work. Probably a good idea to contact an engineer.

[mechdorn]

These types of systems rarely work correctly, too many PIDs interacting with one another. Get rid of the return/exhaust fan. Install a dedicated exhaust fan to control the building pressure.

[sysint]

I tend to agree on the exhaust. I'm wondering what the purpose of it is, the potential overage of OA and what other exhaust is in the building.

[crab master]

Took me back a bit, but your setup is very similar to this post, Supply and Return Fan VFD's? (http://hvac-talk.com/vbb/showthread.php?t=102593) short of you having airflow stations.

In another setup I had the same thing short of the outdoor airflow station. That's the one where I ended up turning off the return fan completely when OAT was below 20 deg (as mentioned in post 75 of the thread). Too bad it looks like the link mentioned by sysint in the last post has been moved as the link is invalid...

[sysint]

Flow stations stink. If you can maintain a constant negative pressure in the MA plenum you will always can know your OA level. In this system there is a guess that with these RF VAVs (?) they return the same as supply. Well, I think probably this is some BS because there is building leakage and general exhaust. If you can overcome that then possibly you are going to return as much as you supply. So, run the RF off the MA pressure sensor. If the RA damper closes completely switch the RF signal to Building pressure and control the exhaust damper.

If this is some sort of wackydoodle system with tons of OA (I see no HRV) then probably better to relocated the exhaust elsewhere.

The thing about that RF --- it is simply there to overcome the duct resistance in the return duct if the system was designed correctly.

[billcontrols]

Lots of options suggested. You have to decide what plan you are going to use. When you are all done and the thing is running make sure the exhaust air is going out, the outdoor air is going in, and there is enough flow for the VAV boxes downstream.

What does this mean? MA section must be negative to allow OA in. RF discharge must be positive for exhaust to go out. SF discharge must be at a pressure down stream to work at the boxes. Add the economizer damper position changing the pressure conditions must all stay true.

Now you said at the beginning the RF and SF volumes need to track. To do this you need to measure volume. The supply volume will change over time (variable Volume Right?) so the supply volume becomes the setpoint for the return fan volume. You also said the OA airflow would be measured and we will control the OA damper from that at minimum position. On most controllers A PI loop set up with a large P and a slow I will adjust the minimum position value slowly. Setup reasonable limits and a starting bias and turn off the I adjustment during economizing. The return damper I would control to maintain the MA section at a negative static in order for the OA damper system to work. Now control the exhaust damper so the return fan discharge remains positive.

Add economizer operation all you should need to do is override the OA damper open from the cooling PID. This should raise the MA Pressure so the RA damper closes so the RF pressure rises so the EA damper opens.

The hardest part of all this is measuring the volumes. Spend a little there for good quality flow measuring.

Bill

PS IF you want to save money move the Return fan to exhaust and control it from space pressure. Then just link OA and RA dampers. Do not worry about flow measurement on Return and Supply. Just measure the OA as it seems to be the LEED thing to do now.

[sysint]

...Now you said at the beginning the RF and SF volumes need to track. To do this you need to measure volume. Not really, no. I argue against flow stations on just about everything but especially Return Fans.

[billcontrols]

Not really, no. I argue against flow stations on just about everything but especially Return Fans.

I'll take the bait. I will agree there are plenty of control schemes that will maintain space conditions with out measuring the airflow. On the other hand I took the Quote "The return VAV is controlled to maintain the same volume as the supply VAV …" Unquote, to be a design criteria that was specified outside BillLodato's control. Do you have a method of satisfying this condition without measuring volume?

Bill

[BillLodato]

I thought about controlling the return to mixed plenum pressure, but was concerned about the minimum outside air flow control loop and return damper control fighting each other.

I thought if we could keep the return plenum at a constant static, by default, we would be controlling the differential across the return air damper.

The trick will be return plenum setpoint selection.

I gotta put some thought into how to set this thing up and determine control setpoint for the exhaust/return dampers. Perhaps I will end up decoupling the exhaust and Return dampers and putting on separate control loops.

Thanks again !!!!!!!!!

[osiyo]

I'll take the bait. I will agree there are plenty of control schemes that will maintain space conditions with out measuring the airflow. On the other hand I took the Quote "The return VAV is controlled to maintain the same volume as the supply VAV …" Unquote, to be a design criteria that was specified outside BillLodato's control. Do you have a method of satisfying this condition without measuring volume?

Bill

I don't know about SysInt. But in my case, we do it the way the customer (or customer's designer) insists. Come up with the best solution to match the requirement we can. And make sure customer knows we can always change things later and why we think customer MAY want to.

[sysint]

I don't know about SysInt. But in my case, we do it the way the customer (or customer's designer) insists....we think customer MAY want to.I think you need to investigate this a little deeper to know why I stated what I did. I think we are dealing "with the customer"... and the "designer".


I'll take the bait. I will agree there are plenty of control schemes that will maintain space conditions with out measuring the airflow. On the other hand I took the Quote "The return VAV is controlled to maintain the same volume as the supply VAV …" Unquote, to be a design criteria that was specified outside BillLodato's control. Do you have a method of satisfying this condition without measuring volume? BillSee above when quoting requirements. No building in the world that I know of is perfectly tight with perfectly tight ductwork. No building legally here gets away without exhaust unless it is considered a process space. These RF VAV's are interesting so what's the application?

Otherwise, with leak, exhaust and building pressure you take in more air than you return. If the intent is to exhaust large quantities of air then the OA and EX should be coupled in an ERV elsewhere.

Anyway, back to the flow station talk. Would you agree at X pressure with X duct size volume is set? Well, if you agree to that then you can setup something like OA without a flow station. So, you have this great opportunity with the OA if you can maintain a constant pressure that at X opening you get consistent OA without the mess of a flow station. Really, flow is is what calculation?

FPM = (4005)(√ Δ P) - Then you run a calculation to get CFM = (FPM)(duct size)

Don't you think its easier to maintain a SP level with a PID rather than put together multiple flow stations? The amount of error possibilities is much less with the pressure transducer. Do the math and see what I'm talking about.

Further, what's the purpose of a Return Fan? Just to overcome duct loss to get the air back to the supply fan. Otherwise, you have something called a Supply Air Fan that sucks air in from the other side of it.

Think of a CV RTU with barometric relief. The relief can be wide open but that single SF is sucking in air.

I think when you have multiple flow stations and loops the system is going to have huge potential to get all jacked up and out of control. The method I suggest utilizes three loops with low potential for massive swings and drift because the controlled variables are more stable and don't need to really interact with each other.

[osiyo]

I think you need to investigate this a little deeper to know why I stated what I did. I think we are dealing "with the customer"... and the "designer".

I'm not quite sure as to your meaning above.

I have not taken issue with your stance on the subject. Except in that if customer and/or customer's hired designer insists upon something done a certain way, despite our best efforts to convince them otherwise ... we'll go ahead and do it as they wish. While being clear that if it does not work in satisfactory fashion ... that's their problem. To then change things, will cost them extra.

We're not exactly virgins at this sort of situation. BTDT, more than a couple times. And not just with the sort of problem the OP faces. Gad, I've lost count of the number of times I've faced a REQUIRED SOO, that original designer would not budge from, that I knew wouldn't work in satisfactory fashion no matter how well we implemented it. Because the scheme was fundamentally flawed from the start.

Might look good on paper, might've looked good in that well known and respected engineering magazine article, might work in lab tests and simulations, but in a real world implementation ... sucks.

<Shrug> Depends on customer and design engineer. Some will listen, some won't.

I've no particular nit to pick with air flow stations. Tho the cheap ones aren't worth even their low price and the time and labor to install them. Yah might as well just install a quality DP sensor and do the math in the controller. You'll be money ahead and the results at least as good.

I do have issues with the way air flow stations are sometimes used, or required to be used.

i.e. I've done a number of projects where design engineer required schemes similar to what the OP faces.

Each was a real PIA, and has never performed to expectations.

We've come close on a couple or three projects. But those were special cases. High value lab areas and such. Where ducting was TIGHT, extreme low leakage, no expense spared as concerns quality and ability of air flow stations, etc, etc. And customer's budget included plenty for extra time and attention to detail on the part of balancer and controls guys. AND, in each case we had separate control of building pressure. i.e. Multiple space pressure sensors and VFD driven exhaust fans.

In 3 other cases I personally worked on with similar schemes, where budget was tight and not the best equipment installed and not the best possible ducting work, and other issues ... didn't work worth heck, not even close. And I know of another half dozen such projects others in my team worked on who saw similar results.

In those less than close, much less successful, projects customers eventually called us back (we'd pre-warned them of our worries and concerns) eventually. Desperate for a solution. And we modified operation to a workable scheme.

In the case of the high value projects where we got "close", we finally convinced customer that while all the numbers on the screen didn't add up as envisioned by design engineer ... live with it and ignore it. Its working. We just fudged control scheme with enough slack in controls (deadbands, etc) that performance was something they could live with.

The numbers never add up with those sorts of schemes.

For reasons I'm sure you're aware of.

You can measure supply air at the air handler all day long with the best equipment possible, and even have good numbers validated by independent measurement. But ... that tells yah nothing about how much of that air actually makes it's way to the VAV's. (Think duct leakage)

VAV air flow measurement ... well not all air flow sensors of all VAV controls are equal as concerns accuracy or repeatability. Add, that typically the balancing job done on them and subsequent adjustment of their correction factors ... isn't always the best. Good balancer will do an adequate job, as concerns space temperature control. Things will work in that regard. But few bid the job for, or expect to do multiple checks at varying damper positions and shoot for best possible results within the low and high expected ranges. That takes more time than most will spend at it. And when total balancing job is done, and base operating duct pressure setting is determined, should be redone. To check at THAT supply pressure. Results may, or may not, be the same. From experience I'd say that some percentage of the VAV air flows stations will require re-check/re-calibration at that point because inevitably some are not connected to optimally run supply ducting, and while they may have looked good at higher original duct pressure, they're now sucking. I'm not guessing ... have seen this time and time again.

Normally, not a big concern. But if you're using a control scheme that relies upon accurate summing up of all flows ... can be a big issue.

Then, of course, you have the issue of the air handler air flow stations. You can use the best available and still be defeated by lack of a good darn place to install them. BTDT, more times than I can remember. Don't know how many times we were just stuck, screwed. And I explained to customer, "It is what it is, look at that arrangement. There isn't a good place to put anything. We've done best we can. At damper position between such and such, you're getting maybe 3% error. Anything outside that range? You'll be lucky to have only 25% error." Even fan inlet air flow sensor wasn't gonna be an improvement. Bigger minimum error (much), spread across wider range of operation.

Add to all this, the fact of normal, acceptable error rate. i.e. Typical such project looks for "within 10%" on part of the balancer. Yah just hope you end up with a result that's, on average, as far off in one direction as the other when all gets totaled up to all errors more or less cancel out.

But it seems it never quite works that way with the schemes that rely heavily on accurate air flow measurement of many locations, summing them up, and calculations based upon the results.

That's not to mention other influences. Leaky building envelop, outdoor pressure relative to inside pressure, effects of wind, so on and so forth.

And let's not forget the obvious. Many air flow sensors produce a time averaged result. In short, you're dealing with an average as versus a real current value. Then there is whatever delay there is before that data makes it's way over a network to whatever device is being used to actually do something with said data. Like do calculations. There is the controller's internal delay as it does all the other things it is tasked with before it gets to actually doing the calculations. Then there is the delay as commands are sent out to actuators or VFDs to do something ... and delay in time it takes them to actually assume new position or commanded value.

In short, you're always responding well after the event, the actual time when condition was such and such. By the time damper or VFD has responded, things might be much different. And you really don't want them to change position or speed just all that fast. Or your system is gonna go into wild oscillations at any significant change, MTBF of equipment will decrease sharply, etc.

And even when yah go back over and over everything, tweaking and re-tweaking, adjusting and re-adjusting, re-measuring ... then doing it all over again ... and again. And yah finally have things operating acceptably well.

You get calls of complaint again, a year or two down the line. Because, guess what? Typically things, some of them, have now wandered off a bit this way or that, enough so to produce noticeable effects on part of owners/occupants. They're complaining of building pressurization problems. And you spend HOURS trying to isolate the cause.

I know of one building with such a similar control scheme that's had a total, building wide re-balancing done 3 times in the first 6 years of its existence. Cheap air flow sensors, less than best ducting job I've seen, building envelop leakier than my wife's sieve used for draining noodles, etc. They finally got tired of it and had us change control scheme, to the one we recommended in the first place.

[sysint]

Osiyo, can I just assume we are in agreement or do I need to read this twice?

[xarralu]

::DD:

No disrespect meant to anyone, but that was funny!

[exwtk]

FPM = (4005)(√ Δ P) - Then you run a calculation to get CFM = (FPM)(duct size)



I disagree with your formula... not ΔP dictates the flow, the velocity pressure does !!
Think of a filter bank ... dirty filters will have a higher pressure drop than clean filters.
According your formula dirty filters have more airflow than clean filters

[emcontrols]

Just a few opinions:

Supply VAV - standard control
Return VAV - CFM tracking from supply VAV
AHU supply fan VFD - supply duct static
Return fan VFD - return VAV CFM total becomes the return fan CFM setpoint
Relief damper actuator separate DDC loop-space pressure control .05"
OA/RA damper actuator - Minimum CFM setpoint(no economize) mixed air temp setpoint(economize)

[BillLodato]

Each Supply VAV Zone (Except the Corridors) have a corresponding Return VAV box that tracks the Zone Supply VAV CFM. So ...... Supply/Return Volume tracking is done at the zone level. (Note that the Corridor will be constant CFM to supply the make-up air for the building pressurization)

If we control the central return fan based on the central supply fan volume, I'm not sure how the central AHU volume tracking and individual VAV zone volume tracking would work together (or not).

Would it be better to control the return fan volume from duct static??? In this way we maintain enough static in the return duct for the return VAV boxes to operate (i.e. similar concept to traditional a traditional supply VAV system, but on the return side).

[osiyo]

I disagree with your formula... not ΔP dictates the flow, the velocity pressure does !!
Think of a filter bank ... dirty filters will have a higher pressure drop than clean filters.
According your formula dirty filters have more airflow than clean filters

Ummmm, unless my memory is bad, and it may well be ....

The ΔP SysInt has in his formula represents velocity pressure.

Typically velocity pressure measurement is achieved by feeding input from a total pressure pickup tube to the high side of an instrument, and the input from a static pressure pickup tube to the low side of the same instrument. And the differential (ΔP) between the two IS the velocity pressure.

The constant (4005) in his formula is just a factor that indicates units are inches of water column, standard air conditions (70ºF and sea level, at which point air will have a mass of .075 pounds per cubic foot).

So, near as I can tell and know, his formula is correct. Of course it does not include corrections for not being at sea level, air temperatures significantly different than the assumption, etc.

And, of course, if the flow cross sectional area is large one would want to be able to measure the DP at numerous points and be able to average them.

[osiyo]

Osiyo, can I just assume we are in agreement or do I need to read this twice?

I do believe I indicated we are pretty much in agreement in the first couple lines of my response.

The rest of the response was aimed at providing info for those who may not have faced having to implement such a control scheme as to what the problems and issues might be. My assumption was that YOU probably already knew those factors.

As would several others in this forum.

However, not everyone has faced such issues, thought about them, or had experience with them.

It was my thought that those of the last group might be interested in the info I provided. And, if they were, might read on.

I fully expected that anyone already knowledgeable in this area, would have just skipped reading it all.

Its kinda like anything else, SysInt. I have been an instructor. When presenting students with material to be learned, the material will include stuff that some of the students are already familiar with.

Those persons will just scan the material looking for key information that might be new to them and skip over the rest. The less knowledgeable/experienced will read more of it, because there is more which ... to them ... is new.

It never bothers me that the more knowledgeable and experienced skip over some or most of the info presented. I do this myself when reading technical articles, text books, etc. Skip quickly over stuff I already know, looking for what I do not know.

My lengthy (too lengthy) response was not some jab at you, I presumed you already were familiar with the points I was attempting to make.

The response was aimed at anyone interested enough to bother to read it. Those who might not have faced this sort of issue. Those who may not have thought about or experienced the sort of problems that crop up with this sort of control scheme.

Anyone who does not wish to read my posts, who do not find any interesting enough to read .... feel free to skip them. I'll not be offended, not in the least. I hardly think that I or anything I type is so important and weighty that its a "must read". Nor do I ever think that my thoughts or opinions are just all that important or good.

Read, or not, your choice.

[MikeSands02]

You can't make this work, relocate the exhaust fan and shoot the engineer.

[emcontrols]

Each Supply VAV Zone (Except the Corridors) have a corresponding Return VAV box that tracks the Zone Supply VAV CFM. So ...... Supply/Return Volume tracking is done at the zone level. (Note that the Corridor will be constant CFM to supply the make-up air for the building pressurization)
OK, nothing out of the ordinary here. Engineer just wants a constant volume box. Adjust CFM minimums that will corespond to engineers required cfm setpoint.

If we control the central return fan based on the central supply fan volume, I'm not sure how the central AHU volume tracking and individual VAV zone volume tracking would work together (or not).
Yes they would, set up the controllers (IF control manufacture is capable) in a config called master/slave operation. If supply box (Master) is needing X CFM it will open up damper to required cfm setpoint via flow ring, and the return box(slave) will open its damper to required cfm via its flow ring and wal Lah, now you have a pre meditated return fan CFM value for controlling your return fan VFD via its AMS.
Would it be better to control the return fan volume from duct static??? In this way we maintain enough static in the return duct for the return VAV boxes to operate (i.e. similar concept to traditional a traditional supply VAV system, but on the return side).
NO. If you were going to bring your current design down a notch by eliminating the AMS, then you would either add mixed air static sensor and maintain AHU manufacture required pressure drop, or even another notch down, 10 % less signal voltage from supply fan VFD DDC loop to return fan VFD DDC loop

The reason the AMS (Air measuring stations) are there is for precisely controlling the amount of CFM in this application. So use them. The reason there is a return VAV, is to prove the amount of CFM leaving the space or another words "prove the amount of air changes" Sounds like a fun project with the best toys to play with!!!! What is the application, clean room, surgery or what?



Y

[emcontrols]

Each Supply VAV Zone (Except the Corridors) have a corresponding Return VAV box that tracks the Zone Supply VAV CFM. So ...... Supply/Return Volume tracking is done at the zone level. (Note that the Corridor will be constant CFM to supply the make-up air for the building pressurization)
OK, nothing out of the ordinary here. Engineer just wants a constant volume box. Adjust CFM minimums that will corespond to engineers required cfm setpoint.If we control the central return fan based on the central supply fan volume, I'm not sure how the central AHU volume tracking and individual VAV zone volume tracking would work together (or not).
Yes they would, set up the controllers (IF control manufacture is capable) in a config called master/slave operation. If supply box (Master) is needing X CFM it will open up damper to required cfm setpoint via flow ring, and the return box(slave) will open its damper to required cfm via its flow ring and wal Lah, now you have a pre meditated return fan CFM value for controlling your return fan VFD via its AMS. Would it be better to control the return fan volume from duct static??? In this way we maintain enough static in the return duct for the return VAV boxes to operate (i.e. similar concept to traditional a traditional supply VAV system, but on the return side).
NO. If you were going to bring your current design down a notch by eliminating the AMS, then you would either add mixed air static sensor and maintain AHU manufacture required pressure drop, or even another notch down, 10 % less signal voltage from supply fan VFD DDC loop to return fan VFD DDC loop

The reason the AMS (Air measuring stations) are there is for precisely controlling the amount of CFM in this application. So use them. The reason there is a return VAV, is to prove the amount of CFM leaving the space or another words "prove the amount of air changes" Sounds like a fun project with the best toys to play with!!!! What is the application, clean room, surgery or what?

Y

[osiyo]

Just a few opinions:

Supply VAV - standard control
Return VAV - CFM tracking from supply VAV
AHU supply fan VFD - supply duct static
Return fan VFD - return VAV CFM total becomes the return fan CFM setpoint
Relief damper actuator separate DDC loop-space pressure control .05"
OA/RA damper actuator - Minimum CFM setpoint(no economize) mixed air temp setpoint(economize)

Just some thoughts ...


Return VAV - CFM tracking from supply VAV

Typically one might not want the return VAV to remove EXACTLY the same amount of air as the supply VAV. Usually you remove a bit less, in order to maintain the space at a slight positive. You also need to account for any exhaust fans serving the space.


Return fan VFD - return VAV CFM total becomes the return fan CFM setpoint

Let's think about this. One of the issues is the expected error, accumulated error, one will get from tallying up all those VAV cfms. Typical specs are a balancer is only required to get within 10% on each sensor/VAV. In a group of such VAVs, did the result of all accumulated errors more or less balance out? Or are they slanted to on side or the other (low or high)?

Next, this method does not take into account things like duct leakage. For instance, its pretty typical for the supply fan to be actually moving more air than what reaches and passes thru the supply VAVs, and is then measured and reported. This varies a lot of installation to installation. And can be quite considerable. Its possible this can cause over pressure conditions on the building interior. And in any event, numbers aren't likely to add up right. All this has to be checked, and a little fudge factor arrived at and introduced into the equation.

Add another issue, if the return fan attempts to move only, and just only what the return VAVs are summing up in their reporting it is possible to at times reach a point at which VP at one or more return VAV sensor pickup tube's location that the sensor's accuracy just goes to heck. Compounding the issue.

In any event, I think the whole thing stinks. Would be MUCH more workable and easy if there were separate damper PLUS variable speed exhaust fan dedicated to building pressure control.

A lot of lack of adequate information here. As to precise building layout, etc OP is dealing with. Not to mention, a simple goal statement as to what design engineer wants to achieve with his scheme.

Is that exhaust/relief damper that is dumping into machinery room, just dumping into closed room? Is their some sort of relief damper in it, hopefully, maybe a powered exhaust fan?

[sysint]

I do believe I indicated we are pretty much in agreement in the first couple lines of my response....Relax. I was injecting a little humor into the discussion. At least somebody got it. Really, I read everything you post as completely as I can.

[emcontrols]

Just some thoughts ...



Typically one might not want the return VAV to remove EXACTLY the same amount of air as the supply VAV. Usually you remove a bit less, in order to maintain the space at a slight positive. You also need to account for any exhaust fans serving the space.

Agreed, If there is not a make-up air unit already providing the required pre-conditioned air for the mystery exhaust fans (Hoods) if exists. Should be separate system! As far as space pressure thats what the space press sensor and releif damper would be for. Also agreed you can fine tune the correction factor for balancing purposes.

Let's think about this. One of the issues is the expected error, accumulated error, one will get from tallying up all those VAV cfms. Typical specs are a balancer is only required to get within 10% on each sensor/VAV. In a group of such VAVs, did the result of all accumulated errors more or less balance out? Or are they slanted to on side or the other (low or high)?

Agreed, thats the beauty of DDC loops, they learn.

Next, this method does not take into account things like duct leakage. For instance, its pretty typical for the supply fan to be actually moving more air than what reaches and passes thru the supply VAVs, and is then measured and reported. This varies a lot of installation to installation. And can be quite considerable. Its possible this can cause over pressure conditions on the building interior. And in any event, numbers aren't likely to add up right. All this has to be checked, and a little fudge factor arrived at and introduced into the equation.

Again, the space pressure would tell us this story along with the numerous flow rings. If they had to they could add redundancy to the redundancy to double the chances for no failures. alarm trending is a beautiful option. LOL

Add another issue, if the return fan attempts to move only, and just only what the return VAVs are summing up in their reporting it is possible to at times reach a point at which VP at one or more return VAV sensor pickup tube's location that the sensor's accuracy just goes to heck. Compounding the issue.

I totally agree, hopefully has a good commissioning agent, and a ton of money to ensure proper installation and operation.

In any event, I think the whole thing stinks. Would be MUCH more workable and easy if there were separate damper PLUS variable speed exhaust fan dedicated to building pressure control.

100% agree if added duct to existing releif damper and fan downstream. but of course 1 stage releif will be modulate damper until it hits 100% open and then if we still cant remove the positive pressure bring on the releif fan vfd and modulate its speed. If duct and damper is sized correctly releif fan shouldnt be needed. But who knows like you said below need more info.

A lot of lack of adequate information here. As to precise building layout, etc OP is dealing with. Not to mention, a simple goal statement as to what design engineer wants to achieve with his scheme.

Is that exhaust/relief damper that is dumping into machinery room, just dumping into closed room? Is their some sort of relief damper in it, hopefully, maybe a powered exhaust fan?

y

[billcontrols]

I missed the Return CFM is controlled by boxes per zone. Do not get that config much around here. My take on this configuration is the boxes need a static pressure drop through them to work properly. Look up the Boxes literature. I would use a pressure sensor out on the duct to control the return fan speed for this application. All cfm control is done by the boxes. I agree the return CFM usually needs to be less in the real world. In this application BillLodato stated the extra Supply cfm would come from the corridors where there are supply boxes with no returns.

Now I would say the OA damper would be controlled by the Airflow station if required by spec or set to a certain minimum position determined at system balancing. The pressure drop across the oa damper (OA to MA) needs to be kept constant during minimum airflow periods by modulating the return airflow with the return air damper. You may as well just link the exhaust air damper in the unit with the return air damper because the return fan will push the required airflow and the dampers just direct the air where to go.

In Economizer just let the cooling demand open the OA damper and close the return air damper (opening the linked exhaust damper).

In this application we do not need any Airflow stations unless you need to change the ventilation rate between some specific setpoints. I did say in my previous post airflow stations require money. Properly done they are a duct section that evens out the flow and are installed in straight duct with linear airflow. They are often not properly done. If not properly done their info is not very useful.

This is why "airflow stations suck" according to sysint.

I have to agree that return fans are often not needed where applied. Controlled improperly return fans can really mess up a system and are often best left in the off position. On the other hand it might be difficult to get a Return Box system with economizer to work properly without one.

Bill

[sysint]

Bill, I do think more of these guys need to answer this question in their heads:
What is the purpose is the Return Fan and why is it used in a system?

I think flow stations stink because of the installation parameters and the excessive cost, but mostly because of the jacked up calculation lends itself to higher degree of error in control. I can more accurately control SP directly and not be as off as with a flow station.

So, I suppose some people think that 50 bazillion interacting loops and sequencing is the way to go but I think the really smart guys are the ones that find a way to minimize error with the most simple control possible.

When people design systems they use tested equations and design principles. if designed properly, the system gets in the actual building and the only reason it doesn't work is installation error (duct changes, clearance, leak, actual diversity etc...).

So, theoretically the system has a design point and whatever a guy can do to get there and then minimize changes the better.

For this job it seems clear to me if I can maintain a constant negative in the mixing box with a pressure sensor I'm solving a couple problems with one basic loop with less error than managing multiple flow stations.

For guys that are suggesting otherwise - to me I think they need to spend some time working with a slide rule to understand what I'm talking about.

[BillLodato]

Good Discussion …. Thanks all for the thoughtful responses !!!!!!!!!!!!!!!!

This is a five story building (Plus basement and penthouse Mechanical rooms)

Each of the floors 2 – 5 is about 2/3 Lab Space and 1/3 office space. The Lab and office spaces are served by different AHU’s. Lab spaces are constant volume via zone CAV flow tracking (100% outside air).

The first floor has some general labs and animal labs. The general labs are served by the same AHU serving floors 2-5 and the animal labs are served by yet another separate 100% OA system with CAV zone tracking).

The first floor also has some general office spaces, meeting rooms, etc. served by a separate return air AHU.

In Summary:
Three 100% AHU’s serve a common plenum to provide conditioned air to the general lab spaces on floors 1,2,3,4,5 (CAV Zone Tracking)
One 100% AHU serves animal labs on the first floor (CAV Zone Tracking)
One return air system serves the office space on floors 2-5 (VAV Zone Tracking)
One return air system serves the office and meeting space on the first floor. (VAV Zone Tracking)

The discussion was focused on the two return air AHU’s serving office spaces.

The office spaces require comfort control. The air balance (i.e. supply/return zone vav tracking with make-up air supplied to corridor) was designed to provide the office comfort via VAV and the make-up air (Lab Make-up Air plus Toilet Exhaust Make-up Air) to the corridors.

The office AHU’s are located in the basement MER which is ventilated and has the means to relieve any air from the office AHU economizer operation.

Thanks again for all responses and some interesting discussion.

[sysint]

Do you know one of the most successful lab flow companies does not use airflow monitoring to control? They have a specially designed box that automatically (mechanically) adjusts for fluctuations in ducting pressure to deliver the right flow to the space. Very accurate. I've seen extremely quick response times.

To me they are successful because they don't measure flow. They meter it instead. Very good idea. Think valves.

Anyway, I take it the labs run slightly negative to the office spaces but the building remains slightly positive? Then, I'd be concerned to run the offices as consistently as possible on delivery maintaining a mixed air box at X" W.C. This way your volume is consistent. Use the office units to control building pressure - ideally to me this is separated to an ERV, but this is not what you have.

Then, you have these labs that are likely tight with duct work (at least on the exhaust). This is easier because you have CAV and can likely track inflow/outflow as a match as you are not providing any mixed air.

This statement is puzzling: The air balance (i.e. supply/return zone vav tracking with make-up air supplied to corridor) was designed to provide the office comfort via VAV and the make-up air (Lab Make-up Air plus Toilet Exhaust Make-up Air) to the corridors.

I want to clarify you aren't using the office VAV's to account for anything in the labs...

[xarralu]

I think flow stations stink because of the installation parameters and the excessive cost, but mostly because of the jacked up calculation lends itself to higher degree of error in control. I can more accurately control SP directly and not be as off as with a flow station.

We have this same situation. The OA duct is 6' X 2' and is used for the min. OA and econ mode. The problem is when in min. OA mode the Dp across the flow meter is so low, it reads about .01"wc for design minimum, and when the wind blows... well you can only imagine!

[emcontrols]

In most situations, not specifically pointing any fingers, DO NOT take this directed towards anyone individual, But most green control techs like to bring the engineers design down to a level they can understand, or the only way they can make it work. I have had several AMS applications that like was said, were improperly sized or most of the time installed incorrectly, the answer to that is fix the installation and make a valliant effort to implement what the engineer designed and the customer paid for. To me it posses an oportunity to find a solution and solve it. It is correct in saying static pressure is a means to control VFD fans, as is air measuring stations, I personally like the fan inlet style of course much more expensive but much more accurate. Every seasoned control tech has different opinions. In the end the control contract will have someones name on it, with an engineers approval or disapproval.:grin2:

[sysint]

This article backs up my claims: HPAC October 2009 (http://hpac.com/ventilation-iaq/controlling-outside-airflow-vav-1009/)

Of note:

WHY IT WORKS

A damper open to a fixed position acts like a fixed orifice. The pressure drop through an orifice (or fixed damper) varies with the square of the flow. As long as the damper does not move, a constant pressure drop across the damper translates to a constant flow. Regardless of the total airflow through the mixing box, if the pressure drop across the outside-air damper is constant, the outside-air quantity will be constant as well.

...The mixed-air-plenum-pressure concept works for systems with return fans. The sole function of a return fan is to overcome pressure in the return duct. For minimum-outside-air and building-pressure control, return fans are problems to be solved, not solutions to a problem. Avoiding return fans saves costs and often makes sense.

[BillLodato]

I think i will stay with my original concept .........
Control the spill/return dampers to maintain a constant return plenum static.

It will be a few months before we get to do any testing, but I intend to post results.

Another issue, is the linearity of typical Opposed Blade dampers (that is why Pheonix uses a venturi style valve).

The control loops get increasingly to difficult to control over wide variable swings as the static pressure setpoint gets lower and the control devices (dampers) get larger. Kinda like trying to fill a coffee cup by controlling the flow out of a fire hose.

If I can keep the return plenum static constant with the spill/return dampers, I think we can make this work.

[sysint]

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:

http://hpac.com/images/VAV-systems-Fig2.gif

"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.) "

[emcontrols]

Here is some interesting info. Just thought I would throw it out there.http://www.paragoncontrols.com/AirflowPressureControl_AboutApplicationSequence.as px

:angel:

[osiyo]

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:

http://hpac.com/images/VAV-systems-Fig2.gif

"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.) "

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.

[sysint]

That's under ideal conditions, which are virtually never seen in the field...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.

[osiyo]

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/marketing_tools/mt_corporate/EngNews/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.

[sysint]

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....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.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.

[billcontrols]

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

[BillLodato]

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.