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Research Papers

A Resistance–Capacitance Model for Real-Time Calculation of Cooling Load in HVAC-R Systems

[+] Author and Article Information
M. A. Fayazbakhsh

Laboratory for Alternative Energy Conversion,
School of Mechatronic Systems Engineering,
Simon Fraser University,
250-13450 102 Avenue,
Surrey, BC V3T 0A3, Canada
e-mail: mfayazba@sfu.ca

F. Bagheri, M. Bahrami

Laboratory for Alternative Energy Conversion,
School of Mechatronic Systems Engineering,
Simon Fraser University,
250-13450 102 Avenue,
Surrey, BC V3T 0A3, Canada

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received February 14, 2015; final manuscript received May 12, 2015; published online June 23, 2015. Assoc. Editor: Zahid Ayub.

J. Thermal Sci. Eng. Appl 7(4), 041008 (Dec 01, 2015) (9 pages) Paper No: TSEA-15-1041; doi: 10.1115/1.4030640 History: Received February 14, 2015; Revised May 12, 2015; Online June 23, 2015

Simulating the real-time thermal behavior of rooms subject to air conditioning (AC) and refrigeration is a key to cooling load calculations. A well-established resistance–capacitance (RC) model is employed that utilizes a representative network of electric resistors and capacitors to simulate the thermal behavior of such systems. A freezer room of a restaurant is studied during its operation, and temperature measurements are used for model validation. Parametric study is performed on different properties of the system. It is shown that a reduction of 20% in the walls thermal resistivity can increase the energy consumption rate by 15%. The effect of set points on the number of compressor starts/stops is also studied, and it is shown that narrow set points can result in a steady temperature pattern in exchange for a high number of compressor starts/stops per hour. The proposed technique provides an effective tool for facilitating the thermal modeling of air conditioned and refrigerated rooms. Using this approach, engineering calculations of cooling load can be performed with outstanding simplicity and accuracy.

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References

Figures

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Fig. 1

Analogous 3R1C electric circuit of a wall heat balance equation in RC modeling

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Fig. 2

(a) Walk-in freezer room. (b) Freezer schematic with inner room dimensions. The front wall, left wall, and roof are omitted for clarity.

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Fig. 3

Measured freezer temperature during 2000 min of its operation. Arrows show temperature swings, circles show temperature spikes, and the bracket shows door opening events.

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Fig. 4

RC model of the refrigeration system. 3R1C and 1C models are used for the walls and the room, respectively.

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Fig. 5

RC model results compared to measured freezer temperature during 200 min of its operation. Maximum discrepancy of less than between the measurements and the present model is observed.

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Fig. 6

Effect of wall thermal conductivity k on temperature swings in the freezer room

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Fig. 7

Effect of set points on temperature swings in the freezer room

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