Calculating chiller selection

An analysis on an office building in Milwaukee shows a dramatic difference in chiller operating costs for a building based on chiller selection.


Figure 7: This comparison of chiller operating costs uses a load calculation program and NPLV for a 1,000-ton water-cooled centrifugal chiller plant for an office building. Courtesy: Ring & DuChateauTo display the variations in operating costs for chillers, an analysis was completed for a 300,000-sq-ft office building using an energy simulation calculation program with weather data from Milwaukee for a 1,000-ton water-cooled chilled water plant to determine the chiller with the lowest operational costs. 

Multiple chiller selections were made varying compressor types, chilled and condenser water temperature ranges, air-cooled versus water-cooled heat exchangers, and variable frequency drives to determine the best combination of variables. Chiller selections were made with two or four chillers in parallel, each sized at equal capacity in a variable primary flow configuration, which is common for a new building with multiple cooling coils and chillers. 

To focus on the variation in operating costs, each chiller was calculated with an unloading curve in 10% increments and used in the energy simulation program to determine the annualized operating costs. An analysis was also completed using a simplified approach to see if a quick analysis could be completed with a relatively small degree of error without using an energy simulation program. For the simplified method, IPLV was used for determining chiller operating costs by multiplying the IPLV kW/ton value, the chiller plant tonnage, the chiller plant operating hours provided by the load calculation program, and the electrical consumption rate. 

For all calculations a typical office building schedule was used with operation from 8 a.m. to 5 p.m., Monday through Friday. An electrical rate of $0.12/kWh was used for on-peak electrical consumption and $0.10/kWh for off-peak electrical consumption with an on-peak period from 9 a.m. to 9 p.m. 

The results of the analysis, as well as the chiller selection options, are shown in Figure 7. There is a dramatic difference in chiller operating costs for a building based on chiller selection. Furthermore, the figure shows the resulting error in the simplified method using IPLV for chiller energy consumption as a viable option to calculate operating costs, which ranges from 3 to 5 times the calculated operating cost based on an energy simulation program. The operating cost for each option is provided, with subtitle A for the simplified method and subtitle B for the energy simulation method. In all cases, the IPLV method is dramatically more expensive in determining chiller plant energy consumption. This error can be directly attributed to calculating operating costs from using actual weather data and a true building load profile in lieu of predetermined AHRI operating points. 

David Grassl is a mechanical engineer at Ring & DuChateau, and an adjunct professor in the Civil & Architectural Engineering & Construction Management Department at the Milwaukee School of Engineering. He has analyzed and designed approximately 10,000 tons of chilled water systems for plants ranging from small, individual systems for office buildings to large, complex central plants for universities.

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