[Bldg-rate] Modelling chillers at non-standard operating conditions as per Appendix G

Tue Jun 2 16:27:56 PDT 2009

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.

Many thanks,

Luka Matutinovic, B.A.Sc., LEED® AP

Green Building Consultant

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