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

[Nick Caton]

I think the “disparate-zone-instability” concern goes away because the “gyp equivalent” air constructions will only permit a finite amount of heat transfer each hour, and this transfer rate/direction is based directly upon the hourly conditioned temperature difference (as calculated from the previous hour), not directly from the amount of internal heat loads generated in the present hour to be handled by the associated systems.

 

Replacing air constructions (which use the U-factor input method) with a frame partition or similar (layer method) will only fundamentally add mass/thermal-lag to the equation (and will likely have a higher R-factor).  I suppose adding mass to the partitions would be a mitigating remedy if you did have crazy swings of this nature causing “instability” with regard to temperature throttling setpoints, but again I don’t think in the course of a normal model one would encounter this problem.  You’d have to almost be trying to create the problem – i.e. making a an air partition with a monstrous area or re-defining the u-factor of the air construction to be 9999 instead of 2.7 between two disparate spaces…  

 

Kudos again to everyone helping me keep my head on straight =)!

 

~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: Arpan Bakshi [mailto:arpanbakshi at gmail.com] 
Sent: Wednesday, January 26, 2011 6:09 PM
To: Nick Caton
Cc: Fleming, Joe; David Eldridge; <equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] What's in an Air Wall?

 

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 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

	 

	<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> <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

	 

	<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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