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

[Fleming, Joe]

With these ideas in mind, for assigning 2 systems to one zone.  Maybe you could set your second zone up as being contained entirely within the other zone (set up as best as equest will allow for zone within a zone), this way there is a maximum amount of heat transfer happening between the two zones.
Let's say the goal is to try and see if a VAV box, in a given hour, has enough air to cool both spaces so that a dedicate system doesn't need to run to supplement it.  The VAV minimum would be reached before overcooling began (although you could double the minimum airflow, assuming each zone is half of the space served by 2 systems), and once enough overcooling occurs the reheat will initiate.  So, in this case there won't be much shared load between the two spaces separated by an air wall, and the dedicated system will run as well...
Hmmm...  Equest can't be entirely steady state, because iterations seem to occur to decide if certain parts of a system need to initiate or not.  If equest wanted to decide whether or not to bring on a humidifier, it should first need to run the loads one time to see if the unit, given its airflow and supply temp, would lower the %rh enough to require humidification, before initiating humidification.  It would need to see the final space temp after one iteration right?
If this is the case then there would be a chance to shut off a system in one of the zones if the heat transfer, after iteration #1, is enough to satisfy the load.

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

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 David Eldridge
Sent: Wednesday, January 26, 2011 3:20 PM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] What's in an Air Wall?

Although "highly-conductive" you wouldn't necessarily assume that the space temperatures end up being identical - there is still some resistance in your example, even if very small, and the area of interface is not infinite either.

Your last example with area/volume - the heat transfer will be limited by the size and thermal conductivity of this air wall.  There are also radiant and storage effects from the other surfaces in the zone that might keep the two from being in equilibrium - that said your approach may be fine as you may not have widely differing temperatures/loads.  One possible tweak might be to allocate your internal gains in these two modeled spaces to load the separately modeled HVAC systems along how you think they would actually perform in the real "two-system-one-zone" space.

David



David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, HBDP
Grumman/Butkus Associates


From: equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org> [mailto:equest-users-bounces at lists.onebuilding.org]<mailto:[mailto:equest-users-bounces at lists.onebuilding.org]> On Behalf Of Nick Caton
Sent: Wednesday, January 26, 2011 11:49 AM
To: equest-users at lists.onebuilding.org<mailto: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

[cid:image003.jpg at 01CBBD6D.5C015FF0]

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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