ASHRAE Journal - June 2012 - 58

pumps are usually not life-cycle cost effective for plants serving office building type loads. Also note in Figure 1 that the cooling towers do not include any isolation valves to shut off flow to allow one tower to operate alone. As discussed in Part 42 of this series, towers generally can be selected with nozzles and dams that allow half flow from one CW pump while still providing full coverage of fill and it is always most efficient to run as many tower cells as possible. So whether one or two CW pumps are operating, both tower cells are enabled and fans are controlled to the same speed.

Determining Optimal Control Sequences
Chilled water plants have many characteristics that make each plant unique so that generalized sequences of control that maximize plant efficiency are not readily determined. Equipment and system variables that affect performance include: Chillers. Each chiller has unique characteristics that affect full-load and part-load efficiency such as compressor design, evaporator and condenser heat transfer characteristics, unloading devices (such as variable speed drives, slide valves, and inlet guide vanes), oil management systems, and internal control logic. Cooling towers. Tower efficiency (gpm/hp) varies significantly by almost an order of magnitude between a compact centrifugal fan tower to an oversized propeller fan tower. Towers can also be selected for a wide range of approach temperatures. Chilled and condenser water pumps. Pumps and piping systems can be selected for a broad range of ΔTs and may or may not include variable speed drives. Pump efficiency also varies by pump type and size and pump head varies significantly depending on physical arrangement and pipe sizing standards. Chilled water distribution systems. Distribution system arrangements, such as primary-secondary vs. primary-only variable flow, significantly affect plant control logic. Weather. Changes in outdoor air conditions affect loads and the ability of cooling towers to reject energy. Load profile. The size and consistency of loads will affect optimum sequences. For instance, control sequences that are optimum for an office building served by air-handling systems with airside economizers may not be optimum for a data center served by systems without economizers. With so many variables, no single control sequence will maximize the plant efficiency of all plants in all climates for all building types. There are a number of papers3,4 on techniques to optimize control sequences for chilled water plants. Almost all require some level of computer modeling of the system and system components, and the associated amount of engineering time that most plant designers do not have. In writing this series of articles and the SDL upon which it is based, significant modeling was performed in an effort to determine generalized control sequences that account for most of the variation in plant
June 2012

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ASHRAE Journal

ASHRAE Journal - June 2012

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