Limitations of Simplified Load Calculation Procedures (pdf)
Transcript of Limitations of Simplified Load Calculation Procedures (pdf)
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Limitations of Simplified Load Calculation Procedures
Jeffrey D. Spitler, Ph.D., P.E.Ipseng (Calvin) IuOklahoma State UniversityBuilding and Environmental Thermal Systems Research Group
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Introduction
Limitations – what limitations?Simplified ProceduresASHRAE 942-RPExamples (Primarily with RTS)
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Approximations to the Heat Balance Method
Transfer Function Method (TFM)CLTD/SCL/CLF *TETD/TA *Radiant Time Series MethodAdmittance Method
* Both of these methods use data that are derived from TFM
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Approximations to the Heat Balance Method
In general, simplified methods:Treat radiation and convection heat transfer together (particularly questionable when large glazing areas are involved).For the exterior surface, this involves the use of a sol-air temperature.The interior surfaces are assumed to convect and radiate to the room air temperature.
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Approximations to the Heat Balance Method
In general, simplified methods:Use some form of precalculated response for energy storage/release in the zone.Often simplify treatment of transient conduction heat transfer through walls.Assume all heat gain becomes cooling load; i.e. no heat gain is conducted back out of the space. Tend to overpredict cooling loads.
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Approximations to the Heat Balance Method
Benefits of approximate methods:Simpler to useGive component loads.Tend to overpredict cooling loads. ☺
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ASHRAE 942-RP
Comparison of ASHRAE and CIBSE load calculation procedures.Compared results of procedures for thousands of different zones.
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Sample Results
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ad (
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Heat Balance Peak Load (W /m2)
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k Lo
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Lightweight Zones Heavyweight Zones
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Why does RTS over-predict?
The RTS procedure over-predicts most noticeably for cases with high % of single pane glazing. Two conditions must be present for significant over-prediction
Large amount of high conductance wall or windowLarge amount of radiant heat gain
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Example
Two-story (+ basement) office building located in Chicago. Look at first floor, Northeast zone.Heavyweight walls: brick/concrete blockFloor: Lightweight layer on top of 4” concreteCeiling: 4” concrete, airspace, acoustic tileLightweight partitions: gypsum/studs
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50'-0"
10'-0"
20'-0"
20'-0"
20'-0"
15'-0'' 15'-0"
Office Office
OfficeConferenceRoom
Storage
Corridor
Lobby
Toilet
1 2 3
4 5
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50'-0"
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Example
North wall (150 sq. ft. total) has 40 sq. ft. of double-pane windowsEast wall (200 sq. ft. total) has 60 sq. ft. of double-pane windows.4 people on office schedule1 W/sq. ft. equipment on office schedule1 W/sq. ft. fluorescent lightingNo infiltration.Overly high design condition: high of 96 F, 19 F range
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Base caseBase - Chicago
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1 5 9 13 17 21Time (hrs)
Clg
. Loa
d (B
tu/h
r)
HBMRTS
13% high at peak
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Glazing area and type
More glass: 50%, 100%, 200% more.Change glass from double pane to single pane.
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Mostly glass facade200% Extra Glass Area (DP) -- Chicago
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Clg
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d (B
tu/h
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HBMRTS
19% high at peak
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Single pane glass facadeBase+200 - SP - Chicago
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Clg
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d (B
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HBMRTS
33% high at peak
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Summary - Chicago
Chicago
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Base Base+50% Base+100% Base+200%
Ove
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DPSP
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Move Building to Seattle
Chicago had a high temperature of 96 F, low of 77 FUsing Seattle 2% design conditions: high of 78 F, low of 60 FNote that a similar effect might be obtained for zones with winter peaks.
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Seattle – SP Glass FacadeBase+200 - SP - Seattle
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Time (hrs)
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HBMRTS
34% high at peak
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Seattle SummarySeattle
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Base Base+50% Base+100% Base+200%
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DPSP
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What about TFM?
Difference between RTS and TFM depend primarily on tabulated factors.How close is actual zone to tabulated zones? Weighting factors were grouped, adding another (variable) overprediction.
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626-RP Tabulated WF
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Chicago Base CaseComparison of Three Methods
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Hour
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HBMRTSTFM
+29%
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Conclusions
RTS procedure works well for many zones.RTS procedure may over predict for zones with high amounts of glazing Exacerbating factors include:
Single pane glassLow outdoor design temperatures