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L.
Wang,
P.
Haves
&
F.
Buhl
Lawrence Berkeley National
Laboratory Berkeley, CA
An Improved Simple Chilled
Water
Cooling
Coil
Model
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Motivation
and
History
Purpose:
•Energy simulation
•Model‐
based
FDDIssues:
•Model ‘equations’: scope, approximations …
•Input data: physical or rating point
NTU- / LMTD Holmes (1982)
Elmhardy & Mitalas (1977)CCDET
Braun (1988)CCSIM
Brandemuehl (1993)
Chillar&Liesen (2004)
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Overall
Thermal
Resistance
– (ratio?)
Rn AUA row facetotholmes /=
Holmes
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Dry and Wet Sections
where: C = UAtot,des / UAtot, holmes
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Three Modes of Operation
Completely dry
EDB, ERH (EWB)LDB,
LRH
EWTLWT
Partially wet
EDB, ERH (EWB)
LWT
LDB, LRH
EWT
IF tDewPt (Entering air dew point)< tSurfExt (Surface temperature at air exit) THEN
ELSE Assume fully wet coil
IF
tDewPt (Entering air dew point)>tSurfEnt(Surface temperature at air entrance) THEN
ELSE
Completely wet
EDB, ERH (EWB)
LWT EWT
LDB, LRH
1. Assume dry coil:
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Algorithm
DesignOperation
Input Rating
Condition,
OperationConditions
Calculate:
LWTdes, DewPtdes,
UAholmes,des
Calculate:
UAtot,des, C
Determine coil
operation mode
Calculate: LWT
,Qsen
Calculate:
UAext,des / UAint,des
UAext,des, UAint,des
UAext, UAint
,
LDB LW Qtot ,Fwet
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Validation using Purdue Measurements
Zhou, X. , (2005). Dynamic modeling of chilled water cooling coils. PhD thesis, Purdue University.
Cooling coil specification:
•8-row, 8 tubes/row, 8 circuits
•ID: 0.0119 m
•Face area: 0.3716 m2
Data Set:
•32 cases (16 dry, 16 wet)
•Measurements:
• EDB, ERH, LDB, LRH, ma
• EWT, mw
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Comparison of Proposed Model and Chillar and Liesen
Model with Purdue Measurements (Dry Coil)
Maximum Fractional Error
Proposed model: 9.3%
CL model: 16.3%
Mean Fractional Error
Proposed model: 4.3%
CL model: 5.6%
Chillar,R. and Liesen, R. (2004). Improvement of the ASHRAE secondary HVAC toolkit simple cooling coil model for simulation. SimBuild 2004, IBPSA-USA
National Conference Boulder, CO, August 4-6, 2004. (CL model)
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Maximum Fractional Error
Proposed model: 7.7%
CL model: 24.8%
Mean Fractional Error
Proposed model: 3.4%
CL model: 13.3%
Comparison of Proposed Model and Chillar and Liesen
Model with Purdue Measurements (Wet Coil)
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Maximum fractional error
Proposed model : 4.7 %
Mean fractional error
Proposed model : 2.3 %
Heat transfer rate imbalance for wet
cases measurement : 0.1%-6.2%
Wet Coil Comparison using Adjusted RH Measurements
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Sensitivity to Number of Coil Circuits
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100% water flow rate 30% water flow rate
100% air flow rate 1 3
30% air flow rate 4 2
Model
Simplification
using
Fixed
Ratio
of
UAext to
UAint
• Operating condition: inlet air temperature 27˚C, inlet water temperature 7˚C
• Design velocities: va= 2.5 m.s-1 v
w= 1.4 m.s-1
• High Fin Spacing: UAint :UA
ext= 5.15
• Low Fin Spacing: UAint :UA
ext= 3.45
Proposed default ratio UAint :UA
ext= 4.3
Max diff: 3% duty
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Conclusions
• Use of Holmes’ empirical model allows configuration of a partly wet coil model from a single rating point and
limited geometrical
data:
– Face area
– Tube diameter
– Number of rows
– (Number of circuits)
• Relatively small accuracy degradation from using 4.3 as
the default value of the ratio of the water‐side to the
air‐side
overall
heat
transfer
coefficient
– no
geometrical data required, just a single rating point.
Method especially useful for existing buildings where
manufacturer’s data
difficult
to
obtain