[Equest-users] What's in an Air Wall?

[Fleming, Joe]

My take on the air partitions is that when the  software is calculating the load for each zone it uses the air partition as the boundary for that zone, any heat transferred into or out of that zone through the boundary will come relatively easily from the adjacent zone, depending on the conditions in the adjacent zone.  I think it is worth it to test your questions/scenarios, but with that in mind one could assume the following responses could be true.

1.  The zones will rapidly transfer energy, but it is finite and the zones should not fight each other to the extent that neither zone is satisfied.  (Unless the equipment isn't sized to handle the heat transfer from a hot adjacent zone).  The cold zone will steal some energy from the hot zone and the hot zone will lose some energy to the cold zone, based on thermal transfer through the shared air wall.
2.  The air wall is just another avenue for heat transfer between each zone.  The System A cooling load should be larger than that of System B.
3.  If System A is not big enough than Zone A will be hotter and transfer more energy to Zone B, which would make Zone B warmer.  So maybe next hours iteration would have less energy transferred because the del T is smaller?
4.  The larger space will have a larger air wall than the smaller spaces so it will transfer more energy through the air wall in its own world.  You can only select one zone for the "next to" zone, this will keep from complicating the heat transfer calc.

This is just my interpretation.

Joe Fleming
E.I., LEED AP BD+C, BEMP
Mechanical Engineer I

TLC Engineering for Architecture
Your 2030 Challenge Partner

800 Fairway Drive, Suite 250
Deerfield Beach, FL 33441-1816

phone:

954-418-9096

fax:

954-418-9296

direct:

954-418-4591

website:

www.tlc-engineers.com<http://www.tlc-engineers.com>


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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Wednesday, January 26, 2011 12:49 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] What's in an Air Wall?

Hi everyone,

A discussion on [bldg-sim] prompted me to bring up a topic that's been bugging me in the "eQuest fundamentals" department...

I have a general understanding that eQuest does not fundamentally model airflow (specifically, convection of internal loads) between zones.

-          The DOE-2 entry for INT-WALL-TYPE says an internal "air" partition " ...designates a non-physical interior surface with no mass (i.e., an opening between spaces) across which convection can take place."

-          A wizard-generated "air" internal partition has a construction with U-factor of 2.7... very conductive.

-           To draw a conclusion - two zones connected with an "air" partition are "connected" thermally.  In practice, the internal loads in one are "combined" with the other.

-          This means heat in one zone should travel to the other in a rapid fashion during the hourly simulation, until the space temperatures are identical between the two.

I hope my understanding thus far is correct, because from here I have some questions that dig at what's going on under the hood:

1.       Imagine an air partition "connects" zones A and B.  These zones have separate systems and separate thermostats with different setpoints.  If zone A's thermostat wants to be much warmer than zone B, is it possible the systems will "fight" each other and cause mutual unmet hours?

2.       In the same setup, if Zone A is identical in geometry to Zone B, but has 2x the internal/external loads, does it follow that the system for System A will handle 2x the internal loads as System B, or are they summed and applied equally to the two systems on an hourly basis?

3.       Is the "distribution of loads behavior" affected if Systems A & B are specified with different capacities and/or airflows?

4.       If one space is larger in area/volume than the other, does that affect how the collective loads are distributed to the corresponding systems?

I have "exploited" air partition behavior in the past to get around the "one system per zone" rule (need two RTU's serving that space?  Just make an imaginary air wall!).  However I want to be sure before I continue this practice or advise others to do the same that there aren't any major potential pitfalls in how the loads/systems are distributed/affected...

~Nick

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NICK CATON, E.I.T.
PROJECT ENGINEER
Smith & Boucher Engineers
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
www.smithboucher.com

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