Re: [EnergyPlus_Support] e+ calculation of AHU minimum damper position in multizone system with VAV termi

On 23 November 2012 15:37, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Dear Jean,
I am not quite certain that I understand your question ? but I?ll give an answer and you can tell me if I?ve helped?

·         As you know the amount of outdoor air is calculated by the E+ sizing objects and can also be manually entered.

·         As in real life, if you provide a specific amount of outdoor air to a single air system which serves several zones, the fraction of outdoor air which is part of the air loop (e.g., 10%) at the fan is the same fraction delivered to every zone. In this example, 10% of the airflow to every zone is outdoor air.

·         If you truly need to have very different outdoor air fractions in each zone, you must define a separate air loop for these zones with the correct amount of outdoor air in each zone.

·         Alternatively, a DOAS is commonly used to provide the minimum outdoor air flow to each zone (plus a zone heating / cooling component such as radiator, fan coil, etc.)

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Jean marais
Sent: Friday, November 23, 2012 3:54 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] e+ calculation of AHU minimum damper position in multizone system with VAV termi

Dear Group,
How does e+ calculate the minimum ODA damper position for a multizone VAV system during autosizing?

I have the following scenario. Big zone requires lots of air (say 30,000 m3/h) for heating, but zero ODA. Small zone requires no air for heating because it has a radiator, but requires 80 m3/h min ODA. They are served by a common AHU.

If the AHU sets damper position as 80/(80+30000)=0.0026, then the small zone will require more air to get the minODA, i.e. 80/0.0026=30080 m3/h.
This would set the total demand for the AHU = 30080+30000. This obviously is unacceptable.

Does it then open the ODA damper such as to minimize the total m3/h? It may end up with the damper at 50% so that the small zone now receives 160m3/h and the total AHU is 30160. But this would mean the big zone is eating energy as it gets 30000m3/h with 50% ODA fraction...lots of ODA.

Any ideas?

Jean

PS. this example has been exagerated to show the principle