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Why model?
A plant model could be of value in commissioning or troubleshooting a plant. The model would show the efficient performance of the plant for any conditions of wet bulb and site load. The first law of thermodynamics would give the real-system site load, evaporator load, and condenser load, which could be checked against the model. The chiller model and cooling-tower model working in balance would provide the plant-efficiency performance for any condition. Having this capability on a laptop computer would be of value, but having a plant model as part of the central monitoring/control system would provide valuable information for operators and owners.

Summary
Hour-by-hour analysis has been suggested for chiller selection,1 a suggestion this author agrees with in terms of the need to perform a mathematical model of a system. However, a model of the plant, including part-load performance, may be a simpler yet still adequate approach.

The monthly or yearly energy consumption of a system could be estimated using the data in the bottom half of Figure 7. Selecting a site load and wet-bulb temperature consistent with the use and location of the plant would increase the accuracy of the energy-use estimate. Defining the hours of operation at each condition could provide an estimate of monthly and/or annual energy consumption of the plant.

References

1. Geister, W.R., & Thompson, M. (2009, December). A closer look at chiller ratings. ASHRAE Journal, pp. 22-25, 28-30.
2. Hubbard, R.S. (2010, March). Letters. ASHRAE Journal, p. 16.
3. Sauer, H.J., Howell, R.H., & Coad, W.J. (2001). Principles of Heating Ventilating and Air Conditioning. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
4. Nelson, K.P. (2006, December). 7 upgrades to reduce building electrical demand. ASHRAE Journal, pp. 50-52, 54-57.
5. Nelson, K. (2010, March). Central chilled water system modeling. Engineered Systems.
6. Schwedler, M. (1998, July). Take it to the limit ...or just halfway? ASHRAE Journal, pp. 32-33, 36-39.

Kirby Nelson, PE, has been involved in the modeling of HVAC systems since the oil embargo of 1973—first as corporate energy manager for Texas Instruments Inc., then as a consultant. Models he has used include DOE-2, E Cube, and models developed on an analog/digital computer, including models of cleanrooms. A life member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, he has presented numerous papers, led an energy engineering delegation to China, and more recently developed models for district cooling systems, thermal-storage systems, and central plants.

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