ASHRAE Journal - December 2011 - 34

Central Chilled Water Plants Series
This series of articles summarizes the upcoming Self Directed Learning (SDL) course called Fundamentals of Design and Control of Central Chilled Water Plants and the research that was performed to support its development. The series includes five parts. Part One: “Chilled Water Distribution System Selection” was published in July and Part Two: “Condenser Water System Design” was published in September. Parts Four and Five are forthcoming. Chiller and cooling tower selection. This article will address how to select chillers using performance bids and how to select cooling tower type, control devices, tower efficiency, and wet-bulb approach.

Optimized control sequences. The series will conclude with a discussion of how to optimally control chilled water plants, focusing on all-variable speed plants. The intent of the SDL (and these articles) is to provide simple yet accurate advice to help designers and operators of chilled water plants to optimize life-cycle costs without having to perform rigorous and expensive life-cycle cost analyses for every plant. In preparing the SDL, a significant amount of simulation, cost estimating, and life-cycle cost analysis was performed on the most common water-cooled plant configurations to determine how best to design and control them. The result is a set of improved design parameters and techniques that will provide much higher performing chilled water plants than common rules-of-thumb and standard practice.

(including real cooling tower and piping costs) for three climates: Oakland, Calif., Albuquerque, N.M., and Chicago. Figure 4 shows results for Chicago but the trend was the same in all three climate zones: life-cycle costs were minimized at the largest of the three ΔTs analyzed, about 15°F (8.3°C).§ This was true for both ofﬁce buildings and data centers and for both single-stage centrifugal chillers and two-stage centrifugal chillers. It was also true for low, medium, and high approach cooling towers. (The optimum approach temperature will be discussed in the next article in this series but it had no impact on the optimum ΔT). In all cases, pipe, pump, pump motor, and pump variable frequency drive (VFD) sizes reduced as ΔT increased, and these cost differences were the primary driver in life-cycle cost differences as shown in Figure 4. The differences in energy use among the options is not as signiﬁcant since savings in pump and tower energy largely (though not completely) offset the increase in chiller energy use. Other studies have also found that 15°F (8.3°C) condenser water ΔT is optimum and can even reduce annual energy costs.7,8 The plant analyzed (shown schematically in Figure 5) had a relatively short distance between the towers and chillers; high ΔTs would have an even larger life-cycle cost advantage for plants that have a large distance between the two, such as a plant with chillers in the basement and towers on the roof. Based on this analysis, the following procedure is suggested to pick the condenser water ΔT (cooling tower range): 1. Calculate the condenser water ﬂow rate for all pipe sections assuming a range of 15°F (5°C). 2. Pick primary pipe sizes (at pumps, headers, main risers, main branch lines) in the “critical circuit” (that which determines pump head) using Table 2 or LCCA spreadsheet. 3. With pipe sizes selected, use Table 2 or LCCA spreadsheet backwards to ﬁnd the maximum ﬂow for each pipe size and then recalculate the ΔT in each pipe using these ﬂow rates.
§ It

4. The largest calculated ΔT in any pipe segment is the design plant ΔT. Recalculate all ﬂow rates using this ΔT. This procedure attempts to minimize cost by reducing pipe size as much as possible, but then taking full advantage of the resulting pipe size to minimize ΔT to reduce chiller energy. Pump energy will be a bit higher than if a 15°F (8.3°C) ΔT were simply used, but pump energy is small relative to the impact of high ΔT on chiller energy use. An example of this technique is shown in the sidebar on Page 32.

Summary
This article is the third in a series of ﬁve that summarize chilled water plant design techniques intended to help engineers optimize plant design and control with little or no added engineering effort. In this article, optimum pipe sizing and optimum design chilled and condenser water temperature selection were discussed. In the next article, cooling tower and chiller selection will be addressed.

References
1. Taylor, S. and M. McGuire. 2008. “Sizing pipe using life-cycle costs.” ASHRAE Journal 50(10):24 – 32. 2. Taylor Engineering. 2011. “EDR Cool-Tools Pipe Size Optimization Spreadsheet.” Available for free at: www.taylor-engineering.com/ publications/design_guides.shtml. 3. ASHRAE Standard 90.1-2010, Energy Standard for Buildings Except Low-Rise Residential Buildings. 4. Stein, J. 2009. “Waterside economizing in data centers: design and control considerations.” ASHRAE Transactions 115(2). 5. ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality. 6. McBride, M. 1995. “Development of Economic Scalar Ratios for Standard 90.1.” Proceedings of Thermal Performance of the Exterior Envelopes of Buildings VI. Atlanta: ASHRAE. 7. Trane. 2009. Trane Applications Manual, Chiller System Design and Control, SYS-APM001-EN. 8. Trane. 2005. Trane Engineers Newsletter, Volume 34 – 1. January. http://tinyurl.com/7ﬂevx8.

is possible that an even larger ΔT is life-cycle cost optimum—our analysis did not look at ΔTs higher than about 15°F (8.3°C).

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