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?

 

[RandomUserName]

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

6.2.5.2 Establishes ventilation efficiency from Table 6-3. Basically the higher the fraction of OA from 6.2.5.1, the lower your efficiency:

If Zp = 0.22, then Ev = 0.9

6.2.5.3 is the basic uncorrected outdoor air rate (number of people times people ventilation rate, plus square footage ventilation rate times square footage).

6.2.5.4 Takes your Vou (uncorrected outdoor air) answer and accounts for the efficiency of the ventilation system found in 6.2.5.2 to give you your corrected outdoor air flow rate:

Vot = Vou / Ev

for you:

Vot = Vou / 0.9


To answer your question about maintaining individual VAV box OA rates, you do not worry about this. Set up your AHU to handle Vot and allow it to distribute in your building as loads change. OA is held constant at 500 CFM no matter if your AHU is supping your minimum 2250 CFM or the maximum 9000 CFM. We can do this a variety of ways. but most common is directly measuring OA flow rate with a OA measuring station and modulating the OA and RA dampers to maintain a constant OA, reliving at excess airflow to exhaust.

For example in your maximum airflow rate, we need 500 OA and 9000 SA. RA = SA = 9000. If we mix that 9000 RA with 500 OA, we are above our requirement, so we exhaust 500 EA. The OA measuring station figures out the velocity pressure required for 500 CFM at this max flow rate and sets its dampers, along with the RA dampers, to maintain this.

In minimum airflow, we need 500 OA and 2250 SA. Same thing happens, we return 2250, and exhaust 500 before mixing the 1750 back in with 500 OA. As the demand in the building lowers from a SA of 9000 to 2250, VFDs on the supply fan reduce airflow, which in turn reduces velocity and static pressure. The reduction in pressure is measured at the OA measuring station as lower, along with a reduction in OA supplied. The measuring station is trying to maintain a constant VP across its surface (therefore constant OA), and it will modulate the OA damper in this case to open more and increase airflow to the required 500 CFM. It can also close the RA damper a little bit to help, forcing the excess pressure to be relived via exhaust.

EDIT: For more information on controls, please read the ASHRAE HVAC Systems and Equipment chapter on Air Handling and Distribution. Specifically, air distribution controls.

 

[317069]

Thank you randoomusername for your reply, it is correct, but my concern was not the full load or minimum load cases,
I mentioned in my post that with full /minimum load, each zone will get its OA requirement, my concern was when two zones were in minimum and the third one was in full load.

 

[ssdd09]

Hi 317069,

As pointed out by randomusername, regardless of SA requirement (full or minimum) the OA (as held constant) would mix with your RA at the AHU. A schematic of air balance would help.

 

[317069]

Thank you ssdd09, correct me please.
All zone at full load the ratio of OA is 500/9000 = 0.055
zone A 2000*0.055 = 111 cfm of OA
zone B 3000*0.055 = 165 cfm of OA
zone C 4000*0.055 = 220 cfm of OA

All zone at minimum load: the ratio of OA is 500/2250 = 0.222
zone A 500*0.222 = 111 cfm of OA
zone B 750*0.222 = 166.5 cfm of OA
zone C 1000*0.222 = 220 cfm of OA

Zones A&B are in minimum load, Zone C is in full load: the ratio of OA is 500/5250 = 0.095 of OA
zone A 500*0.095 = 47 cfm of OA
zone B 750*0.095 = 71 cfm of OA
zone C 4000*0.095 = 380 cfm of OA

I understand the OA will occupy all the volume of the SA, but I am talking about the OA concentration that is coming to each zone will very as the operation mode very.
when we say " in VAV systems, when a zone reach its set point, the VAV box damper will close to a minimum position that ensure ventilation requirement", the question here is: what does mean ventilation requirement her,?
does it mean zoen OA requirement, or just any amount of SA we can introduce it into a zone just to keep air circulation,? usually we use 25% of full load zone SA.
Am I reading the standards in a wrong way?

 

[RandomUserName]

317069,

Quoting you:

"I understand the OA will occupy all the volume of the SA, but I am talking about the OA concentration that is coming to each zone will very as the operation mode very."

Correct: OA concentration that is coming to each zone will vary as the operation mode varies, but will always equal 500 CFM.

"VAV systems, when a zone reach its set point, the VAV box damper will close to a minimum position that ensure ventilation requirement", the question here is: what does mean ventilation requirement her,?"

What? End of question does not make sense. Please rephrase or restate.

"does it mean zoen OA requirement, or just any amount of SA we can introduce it into a zone just to keep air circulation,? usually we use 25% of full load zone SA.
Am I reading the standards in a wrong way?"


Having trouble following your thought process.

Again, we really do not care where the 500 CFM goes as long as it is introduced into the system at minimum fan speed thru maximum fan speed. (This next sentence is an exaggeration, but it gets the point across If 499 CFM goes into a electrical closet and 1 CFM goes to a 5000 sq. ft. office, we really don't care. The OA will distribute itself how it likes. If you are really intent on making sure every room gets its EXACT OA as calculated, you would have to use a DOAS and duct OA to each room separate of the SA.

EDIT:

"Zones A&B are in minimum load, Zone C is in full load: the ratio of OA is 500/5250 = 0.095 of OA
zone A 500*0.095 = 47 cfm of OA
zone B 750*0.095 = 71 cfm of OA
zone C 4000*0.095 = 380 cfm of OA"

In this scenario, you will supply 5250 SA, of which 500 will be OA.

We really don't care that Zone A and Zone B have lower than required minimum zone OA rates. As long as the cumulative total OA of every zone is introduced into the air stream (500 CFM), we satisfy 62.1.

 

[317069]

RandomUserName
"VAV systems, when a zone reach its set point, the VAV box damper will close to a minimum position that ensure ventilation requirement", the question here is: what does mean ventilation requirement her,?"
I mean what the meaning of ventilation if it is not the OA that should come to a zone. hop this is clear.
I agree building will receive 500 cfm of OA under any operation mode and you idea is very logic for this point, but if we use this approach and we do not care about zones level, then why do we do zoning at first place.
as an example, an office in office building could stay at minimum position for whole occupation hours and this zone has 5 employee need 50 cfm of OA (example) now if this zone in minimum position receives only 25 cfm for whole occupation period, does this zone comply with the standard?

 

[ssdd09]

Hi 317069,

ASHRAE has already considered this issue with VAV system by introducing the "system ventilation efficiency" as discussed by randomusername. This is at the system level and not on the zone level.

 

[RandomUserName]

"I mean what the meaning of ventilation if it is not the OA that should come to a zone. hop this is clear."

Like I said before, we get the ventilation number by square footage and estimated average population. Once we know that number, we get it in the air handler. How it distributes itself, we really do not care.


"I agree building will receive 500 cfm of OA under any operation mode and you idea is very logic for this point, but if we use this approach and we do not care about zones level, then why do we do zoning at first place."

For individual comfort in each zone (individual thermostat per zone), or possibly room pressurization.


"as an example, an office in office building could stay at minimum position for whole occupation hours and this zone has 5 employee need 50 cfm of OA (example) now if this zone in minimum position receives only 25 cfm for whole occupation period, does this zone comply with the standard?"

Yes.

I see where you are going with this. You are wanting exact, specific control over every single OA CFM that goes to every zone. This is typically not feasible. At that point - like I have mentioned before - you are talking a DOAS system. And if you are wanting even more control - like the ability to change each rooms OA based on active population - now you are basically talking DOAS with VAV boxes. This would be very expensive for something that really is not that big of a deal.

 

[317069]

"as an example, an office in office building could stay at minimum position for whole occupation hours and this zone has 5 employee need 50 cfm of OA (example) now if this zone in minimum position receives only 25 cfm for whole occupation period, does this zone comply with the standard?"

Yes.

Can we support this opinion by the standard.?

 

[RandomUserName]

It's is not an opinion, I have already explained it two or three times in different replys just in this thread. User ssdd09 has also explained it.

Read 6.2.5. Multiple-Zone Recirculating Systems.

"When one air handler supplies a mixture of outdoor air and recirculated return air to more than one zone, the outdoor air intake flow (Vot) shall be determined in accordance with Sections 6.2.5.1 through 6.2.5.4."

I used your scenario and calculated 6.2.5.1 through 6.2.5.4 in my very first response in this thread.

Read 6.2.2.1 Breathing Zone Outdoor Airflow

"The design outdoor airflow required in the breathing zone of the occupiable space or spaces in a zone, i.e., the breathing zone outdoor airflow (Vbz), shall be determined in accordance with equation 6-1."

If you read Chapter 3: Definitions, and look up the definition of Zone

"Zone: One occupied space or several occupied spaces with similar occupancy category (see Table 6-1), occupant density, zone air distribution effectiveness (see Section 6.2.2.2), and zone primary airflow (see Section 6.2.5.1) per unit area.

Note: A ventilation zone is not necessarily an independent thermal control zone; however, spaces than can be combined for load calculations can often be combined into a single zone for ventilation calculations."

Beyond this, I really do not know how much else explaining I can do to help you understand. I guess you can call the 62.1 Technical Committee if you need more explanation.

 

[317069]

it is an opinion, if we consider whole building as a one ventilation zone then we are ok with it, and we do not have to use multiple zone procedure since we have only one zone.
if we have more than one ventilation zone then we have to provide required OA to each breathing zone in each ventilation zone to meet with 6.2.2.1 as you mentioned.

 

[RandomUserName]

Somebody else want to take a stab at explaining this? I do not think I can help anymore.

I am sorry I am not helping you understand. Hopefully someone else can chime in.

 

[317069]

Thank you for your helpful effort, you helped me a lot. Thank you

 

[Drazen]

in your scenario, the only way to ensure minimum outdoor air is to increase fraction of outdoor air.

as there is one supply fan only, outdoor air can be ensured only by damper positions, and outdoor damper would have to tend toward 100% outdoor air to ensure your small zones have enough fresh air, while zone c would receive too much outdoor air and that would be large waste of energy.

therefore, if you want to attain energy efficiency, scrap such system. one vav system should consist of zones with similar load pattern only.

 

[317069]

Drazen the question was: do we need to ensure minimum OA to a zone or it is enough to provide required OA (500 cfm in my example) to the system as a one zone.

 

[Drazen]

Apparently that was not a question, your original post can be re-read, but such, sorry to say, nonsense question is not visible.

If you are a student, you need to understand basics good enough, than go after more complex questions.

 

[317069]

Drazen, if can you understand what are you reading in my original post you would find out that was the question.

 

[ssdd09]

Hi 317069,

There are some conflict with your info.

"Drazen the question was: do we need to ensure minimum OA to a zone or it is enough to provide required OA (500 cfm in my example) to the system as a one zone"

First, you must use different approach on computing single zone and multiple zone OA requirements. Your example has multiple zone, hence you need to introduce the system efficiency factor from ASHRAE (basically it is just increasing the OA fraction as discussed by drazen).

"if we consider whole building as a one ventilation zone then we are ok with it, and we do not have to use multiple zone procedure since we have only one zone"
You can't do that to your example because "ventilation zone" is similar with your "AC zone", zone are controlled by a single thermostat. If you don't have VAV box, and directly introducing the FA to each space then you can consider it as one ventilation zone.

If you are being OCd about it, use the IAQ procedure of ASHRAE, introduce CAV box on OA intake then balance your zone using controls for SA and RA/EA (VAV for SA VV for RA/EA).

 

[317069]

ssdd09, regardless to calculation approach as a single or multiple zones, the main question still the same.
I gave an example for multiple zone and found out that under a certain operation case, some zones will not receive their OA requirments, but whole system is receiving required amount of OA.
Random said it is enough to introduce the calculated OA (500 cfm in our example) to the rooftop to comply to the standard regardless what each zone is receiving and he supported his opinion by the standard.
You said it is a system level not a zone level.
let me ask this in a different way, if we have multiple zones VAV project that need a total of X cfm of OA, we introduced this X cfm to the VAV rooftop unit that serves the project, do we conform to standard 62,Randoom said Yes, what do you think? yes or No.