VAV box minimum postion and ventilation equirments 7

[317069]

How do we consider the VAV box minimum position.?
I am reading ASHRAE 62, 2007, multiple zone calculation and trying to apply on an example from my imagination.
three zones, A, B , and C, served by three VAV boxes.
let us say all efficiencies mentioned in the standard equal to 1,
Supply air rates are 2000, 3000, and 4000 cfm with total of 9000 cfm.
outdoor air in breathing zone requirement 100, 150, and 250 cfm with total of 500 cfm.
minimum VAV position 25% each (500, 750, and 1000 cfm)
Standard says ventilation should be available under any operation condition.
Now if all boxes are in minimum, each zone would get his outdoor requirement, but,if zones A&B were in minimum and zone C was in full load, then the total cfm would be 5250 cfm, the outdoor is still 500 cfm
the outdoor coming to zone A in this case would be 500x500/5250 = 47 cfm.
to reach 100 cfm in zone A in this case, we need either a total outdoor air of 1166 cfm, means double of full load requirement, or to increase the minimum position to 1052 cfm instead of 500 cfm, then we have to recalculate again.
this is for three simple zones only, what about 40 zones or more?
am I right, or reading the standard in a wrong way?

 

[ssdd09]

317069,
You are missing the point. The answer to your question is Yes. VAV rooftop unit does not necessarily mean it has a VAV box on the zone. Your example has VAV box which means it has multiple zone. If you would just spend time on analyzing the std. 62, for multiple zone system, your OA might be the same regardless of the SA (if Zp<0.15 Ev=1.0) or it may not. Zp computation is at the system level.

 

[317069]

if VAV system does not has VAV box or mixing boxes, what would have then and why you are using VAV rooftop unit for a building without comfort zoning, anyway this is not our concern for now.
so your answer is "Yes" that was what I would like to know, thank you for your help

 

[ssdd09]

31706,
VAV rooftop (used as DOA) with FCU for zone cooling is an example.

 

[Drazen]

317069, i fully understand that you don't understand the basics. and you will not compensate it with arrogance, engineering is not the place where you can achieve that. if you improve your tone, i will try to repeat what is already stated in possibly more understandable manner, or you find someone patient enough to accept being bitten when trying to help.

 

[317069]

Drazen,
"How do we consider the VAV box minimum position.?" was the first line in my original post.
you said we need to increase OA damper and we may get 100%, I told you this was not the question, you didn't like that and started to attack, anyway thank you for your help and I will try to improve my knowledge.

 

[Bastiat]

Hello RandomUserName,
Your calculation of Zp is incorrect.

RandomUserName (Mechanical) 15 Sep 15 12:54

"Somebody else correct me if I am wrong:

6.2.5.1 Calculates your primary outdoor air fraction. For your job:

Zp = Voz/Vpz

Vpz is the minimum expected primary airflow to the zone from the air handler including recirculated air
Voz is the zone outdoor airflow

Zp = (100+150+250)/(500+750+1000) = 0.22"

There is a Zp value for each zone, so think of Zp as an array of values. The maximum value in the array is what impacts the system outdoor air intake.

Also, the maximum Zp value will occur when a particular zone is at minimum primary airflow during full occupancy. This is not necessarily the VAV box minimum position, but it could be. It depends on the breakdown of internal and external loads for that zone.

All of this can be verified by referencing:
ASHRAE 62.1-2007 section 6.2.5.1 & Table 6-3 notes
ASHRAE 62.1-2010 User Manual Section 6.2.5
IMC-2012 section 403.3.2.3.1

I hope this helps.

 

[EnergyProfessional]

Most designs I see show most engineers don't understand the complex ASHRAE calculations. I think I'm glad my state that has its own simple ventilation requirements (7.5 cfm/person and required ft²/person for different occupancies). With the ASHRAE calculations required for LEED you always can tweak them to meet the code rates

I'd use global return CO2 sensor to reset OA (and EA), and also use CO2 sensors in critical zones, like conference rooms to increase local min. flowrate. Other than that use low min. flowrates. As long as CO2 levels are low, the intent should be met. All these equations don't take every-day use into account, nor accuracy of metering devices etc. It is very simple, if CO2 increases, you need more air. If CO2 is low, you need less.

 

[tys90]

Bastiat's correction is critical to this post and it hinted at the key to determining minimum VAV box positions, which is what I believe was the OP's original question.

The very basic answer is: you can set your VAV box minimum to whatever you want as long as you satisfy the requirements of 62.1 and other applicable codes.

However, if you set your minimums too low, the Primary Outdoor Air Fraction (Zp) becomes very high and in turn your System Ventilation Efficiency (Ev) becomes very low and will increase your Outdoor Air Intake Flow Rate (Vot) at the air handling unit, sometimes making it a 100% Outdoor Air unit.

The key is to go through an iterative process to optimize Zp and Ev while meeting codes. There are numerous engineering judgements to be made with this process as it can even increase overall supply air flows among other things (some zones will essentially become constant volume). Knowing what to optimize and the consequences of every change you make is crucial. This is one of the many reasons an engineer gets paid to do this stuff.