VAV minimum fresh air V free cooling

[remp]

I have a VAV system which has a pressure sensor in the return air pelnum. The pressure sensor is set during commissioning to say -20pa to achieve the minimum fresh air. When the VAV system slows down the plenum pressure reduces and therfore so as to increase it again the return air damper closes off so as to draw more air in thro the fresh air damper so as to maintein min. fresh air. this works fine if the supply air fan is not ramping up and down and we are in economy cycle.

What happens when you go into economy cycle with the fresh air damper modulating bewteen min and 100% open. and hence the fresh air damper is open anyway. Do you forget about trying to maintain the plenum pressure..what happens then if the VAV system speeds up/slows down what happens the fresh air damper? Please help>>

 

[flyrfan]

With an economizer you should also have a mixed air temp sensor that also modulates RA & OA to achieve a MA temp. The control logic should modulate the RA & OA dampers in response to the variable (SP or MAT) that is furthest from setpoint.

 

[flyrfan]

Oh, depending on where the job is located, you may need to preheat the OA since the MAT with could trip your low limit freeze stat during low return air volumes & low OA temps.

 

[AbbyNormal]

I have never had much luck with modulating dampers on VAV to control how much outside air you get. I prefer a dedicated fan to do this

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChasBean1]

Remp, this is a late reply but this system is has some GCEs (gross conceptual errors). The first is controlling to a return plenum pressure while you have an economizer. If this is still open, please reply back with more details. Otherwise, sorry for the late reply... CB

 

[remp]

ChasBean

I would still like to hear you opinions. I think I have to make it that when in economy cycle the return pressure control if OFF.

Remp

 

[flyrfan]

I would be interested in what you have to say as well ChasBean.

I have used OAI plenum pressure control successfully in the past. I am curious why you think it is a CGE.

Thx.

 

[ChasBean1]

So many times I see something like this misapplied because it doesn't account for the system that's attached. Remp, could you descibe the system on the supply side downstream of the AHU and on the return side upstream of the AHU?

Economizer should be enabled any time the OA enthalpy is less than RA enthalpy. So (usually) for most of the year, depending on where you are, the dampers should be modulating to maintain temperature. Striving to maintain a steady return fan outlet pressure will result in no (or at least poor) correlation between the supply fan flow and the return fan flow which is what is needed to maintain steady building pressurization and ensure a fixed minimum outdoor air volume.

The control operates on pressures that are near neutral and can tend to push the return fan speed much higher than supply at minimum OA (when the relief damper is closed and return is full open) and then ramp the fan back down during economizer.

Down the road, people will wonder why the building is sometimes very negative and other times very positive.

If you get a moment, could you post more details about the attached system?

Thanks, CB

 

[flyrfan]

CB...I think that your're confusing return fan control w/ min. OA control.

A strategy that can be used in maintaining a fixed OA flow employs the used of a static pressure sensor located in the mixing box of an AHU. A controller modulates the RA dampers to maintain a fixed SP in the MB. The theory being that the flow rate of a damper (fixed orifice) is proportional to the DP across it. SO if the SP is maintained in the MB, the OA flow can be controlled independent of supply airflow rate changes.


This strategy can be used whether you have a RAF or economizer.

I agree that RAF control should not be through RA duct or plenum SP. Typically, RAF control is through differential flow tracking or building pressure control.

 

[ChasBean1]

I’m not confusing anything with anything.

This control violates economizer. Economizer wants to see energy savings. It does this by modulating mixing dampers to maintain the AHU temp needed to condition the building before using mechanical cooling or heating.

The economizer (preferably an enthalpy based one) proportions relief air, return air, and outdoor air to give the leaving AHU conditions needed.

Now let’s say we want to either change the return fan speed or return damper position to maintain a fixed mixing plenum pressure. Changing the return fan speed will affect building pressure and changing the return damper position will negate the economizer.

Both are methods that seem to be accepted in some circles but are methods I’d discourage.

CB

 

[remp]

CB
I understand what you are talking about and this was my question at the start in fact. However here is the situation that can probably work.

1. Use static pressure in the mixing box to maintain minimum fresh air when not in economizer mode

2. When in economizer mode forget about maintaining minimum frehs air and that control routine and open/close the dampers to maintain the required supply air temps.

The return air and supply air fans may ramp up and down to maintain pressures in the supply duct and space but once you go into economizer mode the fresh air damper is open beyond minimum so so worries about min. fresh air. Well thats what I think anyway.

 

[flyrfan]

CB -

I know what an economizer does & how it works. Are you familiar w/ ASHRAE 62.1? If so, you know that in a VAV system you must maintain a minimum airflow rate, regardless of supply airflow. I fail to see how maintaining a minimum OA flow rate, that is independent of supply airflow, has anything to do w/ the economizer.

I said nothing about changing fan speed to maintain plenum pressure, in fact I agreed w/ you in the fact that RAF (return air fan) control is not done through static pressure...go back & read my post.

Since you disagree w/ this strategy, I'd be interested in hearing how you would design VAV system having an economiser in order to comply w/ ASHRAE 62.1.

I am all ears!!

 

[ChasBean1]

A lot of these new design criteria (from the document you mention, green initiatives, etc.) fails to see the same and seem to be driving people to design in performance problems.

If there's a return fan, maintain minimum OA by the supply/return offset (by offset fan sizes/sheaves, VFD offset, or flow offset). The mixing dampers then only exist for the purpose of temperature control.

Think of it: under what condition would you need to maintain a certain pressure in the mixing plenum? If you're at minimum OA, the relief is closed, the return is open, and the OA is partly open. The system WILL bring in the amount of OA that is the difference between the supply and return volumes, correct? If you're economizing, who cares? You're bringing in more OA than you need anyway. Why would you want to go playing with pressure to make the whole thing more complex?

If you have supply and return fan, my opinion is that using mixing dampers to control to a plenum pressure, usually about -.01-.05" wc range (make sure you have the right pressure sensor range) could result in higher conditioning cost or an operating problem. It would have an adverse impact on econimizer, which should control the mixing dampers for temperature optimization, not for any kind of pressure control.

If there's no return fan, I can see that a mixing plenum pressure strategy could be used at minimum OA (only at minimum OA) but would need to be overridden during economizer.

I'd asked about the attached system previously but still haven't heard. That will be important in determining your supply/return fan strategy to maintain min OA.

Best regards, CB

 

[flyrfan]

CB -

I don't mean to be rude, but the more I read your posts the more I am not sure you have a full grasp of AHU control sequencing as your descriptions have GCEs.

The simplest way to do min OA control is to measure it & have a control loop. On smaller AHU's or RTU's where this may be impractical, OA plenum pressure control works and is simple.

Check out the article "Controlling Minimum OA Flow in VAV Systems" Authors: Kettler J PCitation: ASHRAE Journal, May 1998, vol.40, no.5, 45-50, 3 fPages: 45-50Publish Date: May 1999Publisher: ASHRAE

 

[SAK9]

I have used a different version of what flyrfan has suggested above and it did work alright.A differential pressure sensor is installed across a dedicated outside air damper(economiser damper is seperately provided.The differential pressure is measured during TAB for design ventilation airflow.When the supply fan ramps down the ventilation air flow is reduced thereby reducing the DP across the damper.The damper control system modulates the return damper to bring the DP across outside air damper to the set value.This system maintains steady outside air flow rate irrespective of SA fan speed.

Fan tracking in my opinion is superior if you have straight duct runs and large airflows to measure it accurately.By using a high velocity over 8m/s near the measurement section will certainly improve accuracy.Honeycombs upstream of FMS will also greatly help.They all cost money so if a simple pressure sensor can do the job,why not?There is no right or wrong about these things....

 

[remp]

SAK,

I have done your method with dedicated o/a damper. Is it true that when you go into eceonomy cycle that control function is OFF and the economy cycle damper just opens. what happens the r/a damper at that instance?

B

 

[flyrfan]

Sak9 & Remp -

If you're members of ASHRAE, you can download the article referenced above.

FYI, the last paragraph states:

"The bottom line is this: fan-tracking systems
and fixed-area, flow-measuring
stations sized to measure total
outdoor airflow do not work
in practice as they do in theory,
and it is time the engineering
community recognizes this. The
injection-fan method and
plenum-pressure control are better
solutions."

 

[SAK9]

Remp,

Obviously you would need to override the damper differential pressure control in economy cycle.Based on outside air enthalpy measurement,damper pressure control must be disabled and economiser mode activated.In economy cycle return damper will be modulated by the mixed air temperature only.You may leave the dedicated outside air damper untouched so no actuator is required on it.

Flyrfan,
I would treat articles in AHSRAE journal for what it is worth.My experience with VAV systems using FMS for critical applications such as labs has been only good.So I would only judge them based on my first hand experience not based an article written in a journal.One major argument against flow tracking is that you can not measure airflow accuratly.I agree that air flow is difficult to measure and there can errors if the measurement conditions are not good.Then who said it was easy to measure pressure accurately?Any of the factors such as turbulence in the duct,external wind etc that affect FMS reading affect pressure measurement also.Finally what a FMS measures is also pressure only!

 

[ChasBean1]

Flyrfan,

Understood – but I'd tend to agree w/Sak9 regarding treatment of the article. There are many ways to maintain minimum ventilation rate, and those ways the author mentions may in fact be better ways in his experience.

My argument with this however is about using mixing dampers to maintain the minimum OA. They are not for regulating ventilation rate and using this method puts precedent on the wrong thing. When at minimum, the system is trying to recover all return air energy and this control is taking and dumping some of it. If it’s doing that, something isn’t set right.

I’d also never put a flow station in a 200-500 fpm and wind affected OA flow stream—if flow tracking is used it should be done by the difference between supply and return.

Fan tracking can be done very accurately if it’s not done on direct percent. For example, most systems keep things simple and just track return at some value (say 85%) of supply. The control system could very easily track for improved minimum OA, even though the article’s author didn’t see that used. An example would be return = supply minus 6 Hz. Then the fraction of OA would increase some as the VAV decreased during winter. Even more accurate would be a two-point scale based on testing. In other words, at 60 Hz, testing shows target minimum OA with return at 53 Hz, and at 30 Hz, testing shows target minimum OA with return at 20 Hz. Now just scale operation between the two.

My point is that if maintaining absolutely precise minimum OA is critical, there are better ways to do it than using the devices that are there to optimize energy savings.

Hopefully this clarifies my position on this a little. CB