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 *
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 :)
James V. Dirkes II, P.E., LEED AP
Dear Ulrik,
My understanding of G3.1.2.10 is this:
If the actual fan system exceeds 5K CFM (and the other criteria listed in
that paragraph), you must use energy recovery in your energy model for the
baseline.
You might choose to model four 5K systems as a single system as long as the
occupancy, internal loads, etc. are essentially the same. This will not
affect the actual system, of course, but may save modeling time, and will
not affect calculation results.
I combine HVAC system in my models when the operational and other criteria
are the same. There is no accuracy benefit to splitting them and it costs
valuable computational time (mine, too!) For example, if a high rise
building has a separate air handler on each of 20 floors, and all floors are
office occupancy with the same lighting, computers and schedule ... I model
it as a single air handler. Typically, the ground floor and top (roof)
floor are kept separate.
Of course, one of the conveniences of modeling a building before it's in
actual operation is that you can make these types of simplifying
assumptions. That doesn't work when you must match the modeling
calculations to actual energy bills!
James V. Dirkes II, P.E., LEED AP
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 *
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.