VAV Systems and Air Changes 2

[cwigg66]

I am the HVAC designer for my engineering firm, and we frequently come across VAV systems in medical office buildings that pose a unique problem. Based on heat loads and standard practices, we supply enough flowrate to each room to cool the space using the 400 CFM per ton rule of thumb. However, based on ASHRAE standards, office spaces require a minimum of 4 air changes per hour. When we are dealing with a VAV system that is only capable of supplying 57 degree 50% humidity air, how does one design for the correct amount of air changes without over cooling the space? Also, how do you handle the outside air requirements given you can only provide air from the main loop. IS it a safe assumption to exhaust the amount of O.A. you need and make sure the AHU for the entire floor is pulling in enough O.A. to cover the difference? Thanks for the help!

Chris W.

 

[operatingeng]

Every VAV system I've come across seems to have the same flaw. Because of the minimum damper postion in the VAV box certain areas will always be overcooled. Office buildings move people around all the time, so areas that were designed for ten people may have only two which just compounds the problem.
I'm only an operating engineer, but every VAV air handler i've come across seems to have the minimum OA damper sized based on the total amount of air that unit moves.

 

[tys90]

Where do you get the 4 ACH for offices number from? According to ASHRAE 62.1-2007 an office room is 5 CFM/person plus 0.06 cfm/sq. ft.

To not over cool spaces you provide hot water or electric reheat on your VAV boxes to bring space temperature up in the event of an over cool. Or you can use fan powered boxes to mix return air with primary air to achieve a higher discharge air temperature. There are examples of control sequences around on the internet.

Third, you set your minimum OA based on your ASHRAE calcs or in the case of a medical office building we use AIA guidelines. You have to watch your exhaust rates, if you are starting to become neutral or negative it's time to increase your OA and do energy recovery on the exhaust side.

Lastly, unless your are in an early design phase we never use rules of thumb for cooling loads. Always do proper load calculations.

 

[mauricestoker]

The 4 ACH would be typical for office function and is different than OA requirement.

A standard procedure for doing this is given by 62.1-2007. OA should be estimated for normal (max VAV) and lowest flow. Lowest flow (if you have reheat) is limited by ASHRAE 90.1. ASHRAE has a guideline specifically for outside air intake.

Use of a dedicated minimum OA intake, with static set at minimum flow, would be one way of assuring minimum OA. Using an IAQ outside air damper would be another.

Operatingeng hit the nail on the head; one day a room will be a housekeeping closet, the next day it will be an office. Conference rooms always seem to become a large office for whomever has clout.

 

[sridhar1312]

Room Air changes within room in an air conditioned space is in the range of 13 to 15 .
Fresh air change shall be 5 cfm /person and 0.06 cfm per square feet or as per ASHRAE standard depending on the usage of the area.
Wherever VAV is used it is recommended to take care of the additional load due to occupancy /equipment etc which may vary through the VAV and base fabric load separately to avoid any disturbed conditions

 

[mauricestoker]

I have to say that I've never seen 13-15 ACH for office space. Please let me know the basis for that, it's a new one to me.

If you are supplying air at 57*F without reheat and only one or two rooms are getting overcooled, then you are lucky.

I'd recommend verifying what criteria you are designing for. I think you'll see a large difference between ASHRAE Handbook, ASHRAE Design Manual for Hospitals and Clinics, ASHRAE 170, AIA, 62.1, and governmental (DoD, VA) standards, and which edition by year.

If you don't want to own the IAQ requirements, I'd recommend a reheat.

 

[TOMechEng]

As others have noted the best way to prevent overcooling a space is to use some form of reheat. The issue is obviously that you may be cooling air only to have to heat it up again, which is not so good for energy efficiency. A way to reduce the amount of reheat required is to use a supply air temperature reset based on outdoor air temperature(or OA WB if you monitor it).

The issue of ensuring that you are maintaining your outdoor air rate when the VAV system is at minimum flow is a tricky one. Depending on your system it may be possible to vary your outdoor air damper based on the fan speed so that you provide an increased % of OA at min flow to maintain requirements.

Another way is to seperate your ventilation and cooling/heating system so that the required ventilation is accurately controlled.

 

[SAK9]

Choose your supply air temperature such that you are able to achieve the required air change rates in the space.Also bear in mind your dehumidification needs.

 

[willard3]

An excellent method of getting a known quantity of OA in a VAV system is to install a constant volume fan that blows into the AHU RA plenum.

A DOAS will also obviate the problem with pressure changes in a VAV system.

 

[sridhar1312]

mauricestoker (Mechanical)

Room air changes will be 13 to 15 for any normal air conditioned space. This will increase as we go for Class 10000, class 1000 class 100 etc.

Fresh air change shall be based on ASHRAE 5 cfm per person and 0.06 cfm per square feet of the area, if it is designed for green building we do take additional 30% obn the Fresh air with damper with actuator to work based on CO2 sensor in the return air or room air accordingly fresh air is taken as per requirement.
As far as air quantity for a VAV system it is better to cater to the peak air quantity and minimum air quantity(min opening of damper in VAV) shall be set for minimum load which may occur. In many jobs we normally take 1.5 times the normal air distribution in that area. Still the RH problem may be there depending on the variation of fresh air.

 

[mauricestoker]

sridhar,

I've never designed an office to clean room standards.

 

[RossABQ]

I think the term "room air changes" is being confused with recirculation rate. "Changes" is used in cleanroom work to be the recirculation rate, but in office HVAC it is often the ventilation (fresh air) rate.

 

[mauricestoker]

I'm working on several HVAC projects for medical environment now. OA and ACH are required for offices. For clean rooms, I think it may depend on whether personnel protection is needed, or product, or both. Hospitals include clean rooms under USP 797. For toxic compounding, ACH and OA are the same. For non-toxic, recirc is allowed.

OA requirement and OA damper/ducting depend greatly on unit configuration. For VAV with air side economizer, OA duct and damper would be sized for total air flow, unless (my preference)a separate minimum OA and economizer intakes are included. I think reference to ASHRAE Guideline 16 would be the point of the OP.

For office HVAC, corrected OA calculations should be made, unless it is a single zone system. If the air path is not considered, which is both OA and RA, then calculations per 62.1-2007 are not being done. Some areas will always receive too much cooling or not enough, which is why 90.1-2007 addresses reheat limitations on VAV.

The Advanced VAV Design Guide from PIER is a real good source for understanding and designing VAV for energy efficeincy and IAQ. Link is below:

http://www.energydesignresources.com/Resources/Publications/DesignGuidelines.aspx

 

[jvithayapun]

I have yet to read through everyone else's reply so what I type might either agree or conflict.

But in VAV systems the box can be equipped with a hot water or electric heating coil to heat back up the air that has essentially been 'overcooled' due to air changes.

As far as the outside air rate...first you will have to find out what the outside air percentage is that the unit is providing. Ex. 400 cfm return air mixed with 100 cfm outside air. so you have 20% outside air. Then you need to determine if the room requires an outside air change rate (ex patient rooms require 2 outside air changes)

Example:
Exam room
Volume: 1000 ft^3
air change required: 6 air changes per hour
outside air change required: 2 air changes per hour
AHU outside air % = 20%
total heat gain = 850 btuh
EAT=57
required room temp=75

so you will need
AIR CHANGE RATE
(1000 ft^3 * 6 ACH)/60 = 100 CFM
OUTSIDE AIR CHANGE RATE
(1000 FT^3 * 2)/ ( .2 * 60) = 166 CFM

in this case you would need to supply 166 cfm of air to achieve the outside air requirements (which also satisfies the air change rate requirement). you would need to heat 166 cfm from your 57F to 75, which is roughly 3652 btuh. But since the room already gives off 850 btu, you need a coil that would be able to do 2802 btu.