[Equest-users] What's in an Air Wall?
Nick Caton
ncaton at smithboucher.com
Fri Jan 28 06:44:20 PST 2011
Joe/James:
Deleted/No partition - No heat transfer, no mass [Total disconnect]
Adiabatic – No heat transfer, Yes mass
Standard – Yes heat transfer, Yes mass (frame/mass partitions)
Air – Yes heat transfer, No mass
Internal – No heat transfer, Yes mass (another means of specifying thermal mass in a space – I haven’t used this myself)
An adiabatic surface will retain/store heat hour-to-hour because it has mass, but will not transfer heat to or from another space. If you take square room and delete one of the walls, it is thermally a space with 3 walls only, regardless of the geometry.
Air walls connecting separate zones are the closest thing to reality within these options for multiple zones in a single, unpartitioned space. They impose a finite limit on the amount of heat transfer that may occur each hour between spaces. I’m unsure of whether jacking up their conductivity (i.e. U=9999) would be closer to reality or would potentially cause instabilities in the simulation, per discussion below, but the default U-2.7 is recommended in the DOE2 help files.
~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: Thursday, January 27, 2011 11:13 PM
To: James Newman; Nick Caton; Arpan Bakshi
Cc: equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] What's in an Air Wall?
A deleted wall/partition may just be an adiabatic partition. An air wall might be the better choice here. Not 100% sure.
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 James Newman
Sent: Thursday, January 27, 2011 6:00 PM
To: Nick Caton; Arpan Bakshi
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] What's in an Air Wall?
I’m curious as to what people think is better for having a thermal zone that is open to another thermal zone. Is it better to delete the interior wall completely, or is it better to put an air wall in place of the interior wall? Which way models more appropriately?
James M. Newman, EIT, LEED AP
project engineer / energy analyst
M-E Engineers, Inc. <http://www.me-engineers.com/>
10055 west 43rd avenue
wheat ridge, co 80033
office: 303.421.6655
direct: 720.898.3148
james.newman at me-engineers.com <mailto:james.newman at me-engineers.com>
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 7:09 PM
To: Arpan Bakshi
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] What's in an Air Wall?
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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