[Bldg-sim] Modeling chillers at non-standard operatingconditions as per Appendix G

Fred Porter FPorter at archenergy.com
Mon Jun 8 05:40:22 PDT 2009


All;
I'm not sure this is a huge deal. Somehow folks manage to use the PRM
and simulate air-cooled DX equipment (rated/baseline condensing drybulb
T 95F) in Las Vegas and Phoenix (design drybulb condensing T 110F). 
 
The program will decrease the baseline/rated chiller or compressor
efficiency and capacity at the higher hourly condensing temps, be those
water or air, during the simulated year compared to the  baseline/rated
temperatures. I don't think any rating authority would argue with
increasing the rated chiller capacity in the baseline to offset the loss
of capacity at actual/simulated conditions. (And there is a 15% safety
factor built in somewhere, for baselines modeled to the PRM.) One might
want to chart the COP and capacity vs. ECWT curves for centrifugal
chillers to make sure they extrapolate reasonably to the high ECWT
temps. This would be an issue even if the baseline conditions were
adjusted, and whatever program is used. 
 
Fred Porter
AEC

>>> "Matutinovic, Luka" <LMatutinovic at halsall.com> 06/05/09 2:06 PM
>>>
David,
 
Thanks for your suggestions.  We are planning to submit a CIR and also
raise this issue with the 90.1 committee as Bill suggested.  I thought
about using the same COP / same conditions in both models to demonstrate
a more realistic comparison.  One approach would be to adjust my own
chiller COP to match the operating conditions of App G.  However, there
may be potential hiccups in eQuest if the software is smart enough to
realize it cannot generate the ECWT specified by App G using the actual
weather data.  So the other approach is to use the actual operating
conditions in both models, but to adjust the reference building COP.
 
Luka

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of David S
Eldridge
Sent: Friday, June 05, 2009 3:48 PM
To: bldg-sim at lists.onebuilding.org; bldg-rate at lists.onebuilding.org 
Subject: Re: [Bldg-sim] Modeling chillers at non-standard operating
conditions as per Appendix G


I think your (pretty strong) argument is that your Appendix G cooling
tower will have difficulty producing 85F ECWT if the wetbulb is 86F. 
Your design is 9F difference, there must be some acceptable value
between -1F and 10F for the baseline building approach.
 
I speculate that the wording of G3.1.3.11 was to make the baseline use
a colder ECWT below 85F as standard where available, not to penalize
hot-humid projects.
 
You may want to submit a CIR to establish a reasonable ECWT schedule,
and/or review the 90.1 addenda so that you can move forward.
 
Given that we can establish that both proposed and baseline chillers
operate outside of non-standard conditions in Table 6.8.1J regarding the
efficiency of the chiller, in most cases where something is excluded
from the standard the approach is to use the same value in both models –
so not penalizing you, but not a place for efficiency gains either. 
This may be the approach rather than your extrapolated efficiency – it
is either covered by the standard, or not covered, so the same
performance curves could be used in both models.
 
Unfortunate that this would be such a potentially large energy end-use.
 Still there might be opportunities since that would only apply to the
mandatory efficiency of the chiller at the non-standard conditions.  The
number of chillers, heat exchanger pressure drop/pumping power, water
temperatures and temperature reset schedules could all vary – focus in
these areas for efficiency opportunities in your proposed design.
 
I hope this helps, and please others chime in if you agree or disagree
with this approach.
 
David
____________________________________________________________________________________________________________________
 
David Eldridge, PE
LEED® AP, HBDP

Grumman/Butkus Associates| 820 Davis Street, STE 300 | Evanston, IL
60201 | Ph: (847) 328-3555, ext 224 | Fax: (847) 328-4550
 
Energy Consultants and Design Engineers
____________________________________________________________________________________________________________________
 
 
 
 

From:bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of
Matutinovic, Luka
Sent: Thursday, June 04, 2009 12:52 PM
To: bldg-sim at lists.onebuilding.org; bldg-rate at lists.onebuilding.org 
Subject: Re: [Bldg-sim] Modelling chillers at non-standard operating
conditions as per Appendix G

 

Thanks to Linda and Bill for helping me out with this issue.  As far as
App G is concerned, there is a specific set of condenser and chiller
temperatures and condenser flows that must be used in the reference
building that represent ARI standard conditions for which the COP is 6.1
(for centrifugal chillers > 300 tons).  Therefore the reference building
always operates at standard conditions, while the proposed building's
performance varies depending on specific operating conditions.

 

While this is tantamount to asking someone to model the proposed
building with a Chicago weather file, compare it to a reference building
with a Miami weather file and require you to reduce cooling use, the
rules are at present clear.  Unreasonable, but clear.

 

So, my question to the design/simulation community is this:  

Has anyone been able to spec a large centrifugal chiller (900-1500
tons) with a full load COP matching or at least approaching 6.1, for a
Zone 1 climate?  (Specifically my design conditions are 122F db, 86F wb,
42/57F CHW, 95/105 CW)  If so, if you could please post the spec sheet
that would be fantastic.  Unfortunately the manufacturers don't post
this kind of performance data so a call to the local rep is needed to
produce project specific data.

 

From:Matutinovic, Luka 
Sent: Tuesday, June 02, 2009 7:28 PM
To: bldg-sim at lists.onebuilding.org; 'bldg-rate at lists.onebuilding.org'
Subject: Modelling chillers at non-standard operating conditions as per
Appendix G

Hello All,

 

I have a question regarding the minimum centrifugal chiller
efficiencies for conditions not covered by ASHRAE 90.1-2004 section
6.4.1.2.  Specifically, I'm trying to model a chiller for a building in
a hot humid climate.  The specified operating conditions for the
condenser EWT are 95ºF (35ºC), which is beyond the range of conditions
covered by ASHRAE.

 

In this case, what should I use for the minimum chiller efficiency in
my reference building?  Do I still have to follow Appendix G section
G3.1.3.11 which states that my condenser supply temp shall be the lower
of 84ºF (29ºC) or 10ºF (5.6ºC) approach to design wet-bulb.  The
wet-bulb in this climate is 86ºF (30ºC), so the 10ºF approach is
therefore 96ºF, which means my condenser supply temp (EWT) shall be 84ºF
(29ºC), the lower of the two.  At this condenser EWT, the minimum ASHRAE
efficiencies from Table 6.8.1J vary depending on condenser flow.  Since
condenser flow is not specified by Appendix G, should I simply used the
Kadj equations at the bottom of that table?  If that's the case, the
condenser DT is 15ºF (8ºC) and the Lift is 41ºF (22.7ºC) and I arrive at
an adjustment factor Kadj of 0.88.  Applied to my standard COP of 6.1, I
get a COP of 5.4.  

 

My issue with this approach is that this COP is very difficult to
achieve at our operating conditions.  In my opinion, this is an unfair
comparison for chillers in very hot humid climates since Appendix G is
prescribing operating conditions which are considerably more "temperate"
than the extreme conditions under which our chillers will operate.  We
can realistically achieve a COP of 5.1.  In this climate, cooling is a
dominant load, yet due to Appendix G "rules", it will be very difficult
to meet the LEED energy prerequisite, since our chiller are worse than
reference.

 

Finally, I'm assuming that since ASHRAE does not cover this operating
range, I don't have to worry about meeting the mandatory provisions for
efficiency listed in Tables 6.8.1H, I and J in my proposed building, ie
there is no minimum chiller efficiency for this range.

 

Any help in this matter would be much appreciated.  If I've missed
something in my calculations or wasn't clear enough, please let me know
because this issue is a big concern to our project.

 
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