VAV system min outside air control 4

[remp]

Hi

I have just installated a new VAV system in an office area.
We have an AHU with supply air fan on a VSD
The mixing box at the back of the AHU has a modulating return air dameper taking air from the ceiling void and outside air damper taking fresh air in from the roof. The outside air damper is split in 2. Part 1 has a fixed blade with a flow measuring station across it and was set into position during commissioning to ensure minimum outsdie air at full load. Part 2 is modulating and has a damper actuator attached ( to be used for econnomy cycle)

To ensure minimum outsdie air at all times during part and full load when the VSD is ramping up/down the return air damper modulates to maintain the min o/a set point. So e.g. when the VSd ramps down the amount of o/a will reduce belwo s.p. and hence the return air damper will close off to bring more air in thro the o/a damper to achieve s.p again. I must say it works well.

My problem is when we go into economy cycle. How do I control min o/a and ensure we do not go below min o/a when the VSD is running at part load . Will I leave the r/a damper control routine in operation as explained above. I think I have to.
e.g.
At the moment the load is minimum so the VSD is runnig at 50%. The r/a damper is 30% open to maintian min o/a s.p, economy cycle is OFF so the mod. o/a damper is 0% open.

During the morning the economy cycle is on and still at 50% VSD speed. If for free colling the mod. o/a damper is opened, the amount coming througn the fixed o/a damper reduces. I need to start closing the r/a damper at this stage The r/a damper will close to try and achieve the min o/a s.p .... is this OK

 

[trashcanman]

In the economy mode, isn't the supply air 100% outside air? Isn't that enough?

 

[RossABQ]

Is there a separate return/relief air fan, or is there just a damper for relief?

 

[engair]

trashcanman is correct. If you are in economy mode you should be bringing in 100% outside air, but the system should only be in economy mode when the O/A temperature suits the needs of the space.

 

[ChasBean1]

Economizer mode won’t necessarily bring in 100% OA. It will just modulate the dampers as necessary to maintain a mixed air or discharge air temperature.

There are things I’d do differently with this system’s control but that is not pertinent to your point. Your point seems to be that the OA flow reading reduces when in economizer and you are unsure what to do with the minimum OA dampers. The OA flow station is only measuring inflow across the minimum OA dampers. When you start to economize, the OA flow station reading is no longer valid because an un-quantified amount of air is now coming through the modulating OA dampers (and less through the minimum dampers, although more overall).

Moral of the story: when in economizer mode due to favorable OA conditions, DISABLE the control that modulates the return (and hopefully also relief) dampers to maintain minimum OA flow. The control can’t work with an unreliable OA flow reading plus you’re now above that minimum value of concern.

CB

 

[mauricestoker]

Would that cause a conflict with ASHRAE 90.1? An air economizer shall be capable of modulating OA and return dampers to provide up to 100% of design supply as OA for cooling.

If the minimum OA was set at balance for design load, and the AO damper linkage was adjusted so that the damper position was then considered 100% open, and dP was set for min OA 100% open, no minimum OA problem should occur. The theory is that airflow is constant across a fixed orifice (open damper) at fixed dP. With economizer lockout, the same dP is maintained, hence same OA.

An economizer doesn't have to bring in 100% OA, only 100% of cooling load. I would recommend opening the economizer damper full along with full open return as first step of sequence, then modulating the return damper shut, which would not give 100% outside air at initation.
A dual setpoint dP may also work, but would not have as good control.

ASHRAE covers this is Guideline 16.

 

[remp]

I have set up the control so that the r/a damper is always working to achieve the s.p min o/a even in economizer as follows and it is like this say today

During the day: The AHU VFD was runnig at at 70% (35hz). O/A modulating damper 0% open and r/a damper was at 30% open. The min O/A quantity across the min o/a damper was 2000L/s ( design) - EVERYBODY HAPPY!

In the morning: Economy cycle ON. The o/a modulating damper opens to take advantage of free cooling, the quantity of air in thro the min o/a damper reduced but the over all o/a quantity had to go up since the modulating o/a damper is also open. The r/a damper then closed off so as to try and get more air in thro the min o/a damper (even though it doesn need to). The beauty of this is that if in economy cycle the load in the space goes up say from 70% Hz to 90% Hz the min/oa quantity will go up and the r/a damper will have to open up to get it to set point.

By-the way the relief air fans are on the room working off room Diff pressure and works fine - 4 relief air fans sucking from the r/a ceiling void.

 

[remp]

Chasbean/Maurice,

I agree with everything ye said and thank God ye understand what I am on about, however disabling the r/a in economy cycle (Chasbean) or opening up the o/a and r/a fully and closing the r/a damper (Maurice stokers) sounds good and yes I did consider something along those line but if im the middle of economizer cycle and the AHU goes from say 25 Hz to 50Hz (unlikely I know) due to load changes your min outside air will be all over the place and no guarantee of having minimum. Your description is OK if the fan speed is constant and stays at say 25 hz, if it changes I need to ensure the min o/a is ok and I need the r/a damper to make sure of that. What you reckon??

 

[willard3]

The rock-solid method for OA on an VAV system I have used is to have a small fan blowing into the plenum balanced to the required OA volume. Turn it on in occupied mode, off in unoccupied mode.

AHU fan and pressure modulation don't effect this.

 

[remp]

Willard

I would if I had the space and money, plantroom too small, but yes I agree its the way to go.

REMP

 

[trashcanman]

What Willard3 said! Sure fire solution.

 

[ChasBean1]

It would be easier with a supply/return fan combination—return tracks supply at a fixed Hz offset, then all dampers have to do is maintain temperature. But apparently you don’t have a return fan.

One side note on this: there shouldn't be a two-part (minimum and modulating) outdoor damper set in an AHU with no return fan. It's a misapplied concept.

What if you set all the terminal boxes to minimum flow, figured out the minimum OA cfm to meet your 20 cfm per person in the building, then established the min damper command for OA at that value? Your drive would be at the lowest it would ever be, AHU dampers would have a fixed minimum command, and you wouldn’t have to worry about the minimum OA flow reading (whether in economizer mode or not).

Two other notes worth considering:

• OA flow station readings are usually iffy at best, especially with wind. You’re bending over backwards to try to maintain a steady set point in what is usually a very imprecise application.

• Coil design. If you’re adamant about maintaining a constant OA flow even when the drive reduces to 25 Hz as you mentioned, you could find during winter design conditions that you exceed the coils’ limitations by bringing in too high a percentage of outdoor air.