©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 3, March 2012.
By Steven T. Taylor, P.E., Fellow ASHRAE
About the Author
Steven T. Taylor, P.E., is a principal at Taylor Engineering in Alameda, Calif.
This is the fourth of a series of articles discussing how to optimize the design and control of chilled water plants. The series will summarize ASHRAE’s 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 articles, and the SDL course upon which it is based, are intended to provide techniques for plant design and control that require little or no added engineering time compared to standard practice but at the same time result in significantly reduced plant life-cycle costs.
A procedure was developed to provide near-optimum plant design for most chiller plants including the following steps:
- Select chilled water distribution system;
- Select chilled water temperatures, flow rate, and primary pipe sizes;
- Select condenser water distribution system;
- Select condenser water temperatures, flow rate, and primary pipe sizes;
- Select cooling tower type, speed control option, efficiency, approach temperature, and make cooling tower selection;
- Select chillers;
- Finalize piping system design, calculate pump head, and select pumps; and
- Develop and optimize control sequences.
Each of these steps is discussed in this series of five articles. This article discusses steps 5 and 6.
Cooling Tower Selection
Cooling tower characteristics the designer must select and define are described in the following paragraphs. Each is briefly discussed, but this article focusses on the last two variables, efficiency and approach temperature.
- Open vs. closed circuit. This discussion is limited to open circuit towers. Closed circuit towers are seldom used for chiller plants due to higher costs and reduced efficiency due to the added approach of the heat exchanger and higher tower fan energy due to the heat exchanger pressure drop.
- Propeller vs. centrifugal fans. Propeller fans are almost always preferred due to much higher efficiency (they use about half the power of centrifugal fans) and lower costs. Centrifugal fans are also limited to systems smaller than about 1,100 gpm (69 L/s) per ASHRAE/IESNA Standard 90.1 prescriptive requirements.
- Draw-through vs. blow-through fan arrangement. Most propeller-fan towers are draw-through with top discharge. This results in a high exit velocity, which reduces the possibility of recirculation of tower effluent, and the tower mass below the fan reduces fan sound transmission into the occupied space that is often below the tower.
- Cross-flow vs. counter-flow arrangement. The flow arrangement describes whether airflow through the tower is sideways across the water flowing downward through the tower fill or upwards, counter to the water flow direction. Counter-flow towers are usually a bit less expensive, but both arrangements are effective. The selection is often driven by the physical constraints of the tower location: cross-flow towers tend to have a low profile but a large footprint, while counter-flow towers are the opposite.
- Single-speed vs. two-speed vs. pony motors vs. variable speed motors. Variable speed drives (VSDs) are the preferred approach to fan control since they minimize energy costs, reduce belt wear due to soft start, and provide the most stable condenser water temperature control compared to other methods. VSD costs are now low enough that they are clearly cost effective, and often even lower cost, than alternatives such as two-speed and pony motors.
- Gear vs. belt drive. Gear drives cost more but reduce maintenance frequency and may reduce maintenance costs compared to belt drives. But the increasing popularity of VSDs has also increased the popularity of belt drives; the belts last longer due to the soft start feature of VSDs, and belt drives allow near zero minimum speeds while gear drives require a minimum speed of approximately 20% to ensure adequate lubrication. Lower minimum speed reduces the wear-and-tear of fan cycling, reduces noise levels and abrupt changes in noise levels, and improves energy efficiency.
- Temperature range. Tower range is the difference between the temperature of the water entering and leaving the tower, also known as condenser water ΔT. Optimum condenser water ΔT was discussed in Part 3 of this series of articles.
- Efficiency. Cooling tower efficiency, expressed in gpm/hp (L/s·kW), is defined by ASHRAE Standard 90.1 as the maximum flow rate in gpm (L/s) that the tower can cool from 95°F (35°C) to 85°F (29.4°C) at 75°F (23.9°C) ambient wet-bulb temperature divided by the tower fan motor horsepower (kW). Optimum tower efficiency is discussed further below.
- Approach temperature. The tower approach temperature is the difference between the temperature of the water leaving the tower and the ambient wet-bulb temperature. Optimum approach is discussed further below.
To determine optimum ΔT, efficiency, and approach temperature, a large office building chilled water plant was analyzed as part of the ASHRAE self-directed learning course that is the basis of this series of articles. Utility costs and life-cycle cost assumptions are those used in the evaluation of energy conservation measures for Standard 90.1-2010 ($0.094/kWh average electricity costs and 14 scalar ratio1 [the scalar ratio is essentially the maximum simple payback period]). The plant was modeled in great detail (including real equipment and piping costs) for three climates: Oakland, Calif., Albuquerque, N.M., and Chicago. Additional analyses for optimum approach temperature were made for Miami, Las Vegas, and Atlanta. The condenser water system was designed, cost estimated, and modeled at all permutations of the following design parameters:
- Condenser water ΔTs of approximately 9°F, 12.5°F and 15°F (5°C, 6.9°C and 8.3°C). In Part 3 of this series, a 15°F (8.3°C) condenser water ΔT was found to be the life-cycle cost optimum for all climate zones, tower sizes, and tower efficiencies analyzed.
- Three ranges of tower efficiencies: “low” was the least efficient available for the cross-flow propeller fan tower series analyzed with efficiencies ranging from 45 to 60 gpm/hp (3.8 to 5.1 L/s·kW); “medium” with efficiencies ranging from 65 to 75 gpm/hp (5.5 to 6.4 L/s·kW); and “high” with efficiencies ranging from 80 to 100 gpm/hp (6.8 to 8.5 L/s·kW). Note that even the “low” efficiency towers are significantly more efficient than the Standard 90.1 minimum of 38.2 gpm/hp (3.2 L/s·kW).
- Tower approach temperatures ranging from 2.5°F to 11°F (1.4°C to 6.1°C) based on actual tower selections for a cross-flow propeller fan tower series.
Citation: ASHRAE Journal, vol. 54, no. 3, March 2012
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