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

[Nick Caton]

Thanks everyone for the replies - from this collective advice I've
identified an assumption related to my third "bullet" that led me astray
(and also makes all 4 "cases" seem like silly questions in hindsight):  

 

Air partitions are not "thermal superconductors."  The wizard-generated
(and DOE-2 help files suggested) U-2.7 value for an Air wall
construction is comparable in thermal resistance to a single layer of
3/8" Gyp, without the mass.  I had the picture in my mind's eye that
these constructions were by default a few orders of magnitude higher in
conductivity, permitting any delta-T to be "instantly" resolved between
spaces, effectively tying the two spaces into one (thermally).  From
that, I was concerned with how that might lead to setpoint-related
instability in the model or unpredictable behavior in how loads would be
distributed between in-equal systems and so forth... 

 

I'm also concluding the practice of defining air walls to have two
systems working together in the same zone is sound, however the
temperatures between the divided spaces will not "instantly equalize" as
I assumed.  

 

I think I'm on much more solid footing now - thanks fellas!

 

~Nick

 

 

 

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 

 

From: Fleming, Joe [mailto:joe.fleming at tlc-eng.com] 
Sent: Wednesday, January 26, 2011 2:43 PM
To: David Eldridge; equest-users at lists.onebuilding.org
Cc: Nick Caton
Subject: RE: [Equest-users] What's in an Air Wall?

 

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/> 

 

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

 

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] On Behalf Of Nick
Caton
Sent: Wednesday, January 26, 2011 11:49 AM
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

 



 

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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