Normally each chiller has its own cooling tower sized, like the chiller, to satisfy the peak building load. When the chiller is at low load, the condenser water is overcooled by the tower. Controls can be designed to maintain a constant return-water temperature under variable-load conditions. This keeps the system from wasting pump and fan energy to over cool the condenser water.
A tower with a two-speed fan has three cooling conditions: maximum cooling with the fan on high speed, moderate cooling with the fan on low speed, and a minimum of cooling with the fan off completely. A bypass valve permits variations within each of these modes, by controlling flow through a line between the supply and return lines. Part is sent to the tower and cooled, the rest is mixed with the colder return water to achieve the proper fluid temperature for the condensing side of the chiller. This tempering process is done by the bypass valve, whose position is set by a thermostat in the main return line to the chiller.
In multiple-cooling tower installations, in addition to the bypass feature, the fan for each unit can be controlled independently to reject heat in varying amounts from the water distributed to each tower.
Even more elaborate controls can manage the condenser-water loop for several chillers at a central plant. One arrangement is a primary secondary loop for condenser water from all the chillers, to maintain a fixed-temperature return water for each chiller. This is a common arrangement for many large systems that can also be used at smaller installations. There can be an even lower fan power usage with two-speed fans. In addition, chiller efficiency is enhanced because of better control of the condenser water.
These three staging projects are situated early in the listing because many installations may already have the piping and valving to take advantage of some of the processes, or be able to do so with a minimum of modifications. In such circumstances, they are inexpensive energy-saving opportunities.
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Excerpt from Energy Conservation Projects.