[Bldg-sim] Appendix G - Exhaust Air Energy Recovery/Number ofBaseline Fan Systems

[Julia Beabout]

Appendix G, Addendum u, provides direction on determining the number of fan systems for the baseline building.
For convenience, it states:
"For systems 1, 2, 3 & 4, each thermal block shall be modeled with its own HVAC system.  For systems 5, 6,7 and 8, each floor shall be modeled with a separate HVAC system.  Floors with identical thermal blocks can be grouped for modeling purposes.

There are a few CIRs out there also regarding how the above gets tempered with reality when the rules above generate a fan system size that is too large to be met by a single unit in reality.  As I recall, LEED didn't really seem to care about that and directed the modeller to just use the App G, Add u rules for sizing.  If you run into modelling limitations though, that seems to be a different story and is an acceptable reason to then use multiple units.  But, my memory could be off.  Best to check the CIRs on line.




________________________________
From: Ulrik Welle-Strand Horn <ulrik.horn at silpainc.com>
To: James V. Dirkes II P.E. <jvd2pe at tds.net>
Cc: bldg-sim at lists.onebuilding.org
Sent: Tuesday, January 13, 2009 1:39:13 AM
Subject: Re: [Bldg-sim] Appendix G - Exhaust Air Energy Recovery/Number ofBaseline Fan Systems

Dear Mr. James V. Dirkes II:

Maybe we are using two different simulation programs? I am using Trace 700. I have confirmed by experiments that the size of the system, or rather the number of systems, has a significant impact on annual energy consumption. For certain system types, this should not even be that case, but somehow, in the calculation procedures of the software, the number of systems somehow enters the calculations and affects the results. I know it should not be this way and I have warned the engineers at Trane about the behavior.

Regarding the sizes of coils and fans in air handlers, I was really looking for a specific value. In a 40 story office building, for example, what are typical ventilation fan sizes? Does 10,000 cfm sound like a reasonable fan sizes?

Thank you for helping me out!

Sincerely yours,


Ulrik Welle-Strand Horn 
Sustainability Engineer


Silpa Inc.
s i m p l e  .  s c i e n t i f i c  .  s u s t a i n a b l e
AMERICAS | MIDDLE EAST | ASIA
www.silpainc.com

India: +91.90080.96083


On Mon, Jan 12, 2009 at 9:06 PM, James V. Dirkes II P.E. <jvd2pe at tds.net> wrote:

p.s.,
 
You can get a custom air handler and coils with almost any capacity you can dream of.  In the case of coils, multiple coils can be used in a single air handler. It's not so common to use very large air handlers because of space restraints and distribution issues, but there are plenty of them in use.  Once again, the model doesn't care about physical size, only physics :)


The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
 



________________________________
From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Ulrik Welle-Strand Horn
Sent: Saturday, January 10, 2009 5:15 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Appendix G - Exhaust Air Energy Recovery/Number ofBaseline Fan Systems


Dear All,

I am curious how many systems or fan systems most of you usually create when you model baseline systems? 

I am particularly interested because under point G3.1.2.10 - Exhaust Air Energy Recovery, one only has to apply an energy recovery device if a single fan system is larger than 5000 cfm. I have not seen anywhere (please direct me if there is indeed prescriptions for this in an ASHRAE publication) guidelines for how many fan systems one has to model in a baseline building? It seems that if I have a building with let's say 15000 cfm supply air, I am free to model this building's baseline system with either 2 or 4 fan systems? Then, if I model this building with 4 fan systems (each system sized equally at 3750 cfm), I would not have to use energy recovery in the baseline building, while if I choose to model the baseline with 2 fan systems (each sized equally at 7500 cfm), then I must use energy recovery.

I would also appreciate input on a typical size of a fan system. If I have a building that has a supply design air flow requirement of 100,000 cfm, how many fan systems would typically be modeled? I am not too familiar with HVAC design, and I tried looking for clues at manufacturers' websites but only Carrier indicated an upper threshold of about 150 tons for their coils. Does anyone have an experience with a typical airflow for a very hot climate over a 150 ton coil? I am looking at System 8 for my baseline building, and I imagine that for a large building (35 stories), several coils will be installed. I am using Trace to do the energy model, and for some reason, the more systems I model the higher the energy usage, keeping everything else constant. Therefore I am interested in modeling several fan systems, if that is common design practice.

Lastly, I would like to confirm that the design of the baseline system beyond what is stipulated in Appendix G should comply with common practice? In particular, numbers and sizes of systems does not seem to be much discussed in Appendix G. Please let me know if you share my approach in applying common design practices to size and locate equipment beyond Appendix G prescriptions.

I appreciate any feedback you might have on these issues.

Faithfully yours,


Ulrik Welle-Strand Horn 
Sustainability Engineer


Silpa Inc.
s i m p l e  .  s c i e n t i f i c  .  s u s t a i n a b l e
AMERICAS | MIDDLE EAST | ASIA
www.silpainc.com

India: +91.90080.96083
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