detailed air balance calculations 2

[shamu08]

How to make a detailed air balance calculations? (zone air balancing for AHU) . please advise.

 

[dbill74]

Conservation of mass. Σ_CFM-in = Σ_CFM-out

True for room, zone or entire building.

 

[shamu08]

dbill74
thanks for your answer. but can you explain to me the attached air balance schedule .
1-why the zone is pressurized with +2400 cfm
2-if you look at the office room the supply air =600 cfm and the return air =500 cfm
from where he get this return air flow rate?
thanks in advance.

 

[DrRTU]

1. In a conventional system, the zone will be pressurized via outside air introduced into the return air path. This results in the excess air being lost to the structure and thus the zone is at a positive pressure in relation to outside environment. This OA introduction is used to ventilate the space or control directional air flow from a quality point of view.
2. This appears to be a designers schedule to instruct the balancer to setup the system, verify code compliance for ventilation and or owners design objectives. This is a poorly developed table due to the notes do not call out the use of an exhaust fan. Most designers will separate the exhaust flow from a return air path.
3.

http://www.smacna.org/store

has an excellent book on testing and balance of systems.

 

[shamu08]

so, in case we have a positive pressure space, how to calculate the leakage air (transfer air) through the door undercut and air gaps?

 

[DrRTU]

The delta between supply and rtn/exhaust will be pressure. The space will leak (in or out) based upon the integrity of the pressure boundary of the envelope and the DP. You use the term calculate, do you mean measure?

 

[shamu08]

thank you all for your reply

 

[dbill74]

1-why the zone is pressurized with +2400 cfm
2-if you look at the office room the supply air =600 cfm and the return air =500 cfm from where he get this return air flow rate?

1) First thing I'm noticing in the table is no OA. Including OA will change the amount of pressurization.
2) You need to look at each room individually and the entire zone/building simultaneously.

The excess air from the offices and waiting rooms has to go somewhere, similarly the toilets (which are negative) need to get air from somewhere.
You need to consider where and how the air is moving throughout the zone. Not all the excess air in the offices is going to escape to the outside, some of that is going to go to corridors or other adjacent spaces including the toilet exhaust and eventually to the outside.

so, in case we have a positive pressure space, how to calculate the leakage air (transfer air) through the door undercut and air gaps?

Back to my first reply and conservation of mass. The air HAS to go somewhere. There may be a period of time where it doesn't, and the air pressure in the space will increase (like blowing up a balloon), but eventually the air will find a way out, else something eventually breaks. In the long run, the air WILL get out of the space.

When looking at a room in the zone you have SA and RA for air in/out.
Then for the zone you have OA and EA as the air in/out.

 

[lilliput1]

First off the balancer list just reflects the air balance as shown on the HVAC drawings. It is the HVAC engineer or designer that designs the airflow based n the following criteria:

Minimum OA = Air required for ventilation
but OA must also equal Exhaust + Pressurization Air based on what goes in the building = what goes out.
Pressurization offsets infiltration so the building is positive. We usually estimate this to he 0.05 CFM/Square Ft floor area for normal office design. However for clean or dirty areas it would be about 400 CFM per door.

This is based on no leakage through the normally closed relief air damper which would be open during outdoor air economizer cycle.

Actual design minimum OA would be the larger value based on the above.

The AHU air balance would be Supply Air = OA + Return Air - Minimum Relief Air

During variable air operation the return air fan volume is controlled to track the supply fan CFM by CFM differential equal to the design minimum outdoor air.