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

[Arpan Bakshi]

I second the setpoint-related zone instability concerns, particularly  
as it pertains to unmet hours. What are everyone's thoughts on  
replacing virtual air walls with constructed partitions along zone-to- 
zone adjacencies, with disperate conditioning requirements?



Arpan Bakshi



On Jan 26, 2011, at 5:47 PM, "Nick Caton" <ncaton at smithboucher.com>  
wrote:

> 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 wa 
> ll construction is comparable in thermal resistance to a single laye 
> r 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 magnitu 
> de 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 th 
> at might lead to setpoint-related instability in the model or unpred 
> ictable 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 s 
> ystems working together in the same zone is sound, however the tempe 
> ratures between the divided spaces will not “instantly equalize”  
> as I assumed.
>
>
>
> I think I’m on much more solid footing now – thanks fellas!
>
>
>
> ~Nick
>
>
>
> <image001.jpg>
>
>
>
> 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
>
>
>
> <image002.gif>
>
>
>
> 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 limit 
> ed by the size and thermal conductivity of this air wall.  There are 
>  also radiant and storage effects from the other surfaces in the zon 
> e that might keep the two from being in equilibrium – that said your 
>  approach may be fine as you may not have widely differing temperatu 
> res/loads.  One possible tweak might be to allocate your internal ga 
> ins 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 be 
> en 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 convecti 
> on 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 diff 
> erent setpoints.  If zone A’s thermostat wants to be much warmer tha 
> n 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 b 
> e sure before I continue this practice or advise others to do the sa 
> me that there aren’t any major potential pitfalls in how the loads/s 
> ystems are distributed/affected…
>
>
>
> ~Nick
>
>
>
> <image001.jpg>
>
>
>
> 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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