Trane SYS-APM001-EN manual Common, Campus, Tertiary or distributed

System Configurations

Elevated return-water temperatures. Because unused chilled water does not bypass the cooling coils (two-way, rather than three-way, control valves), all water that is returned accomplishes some cooling. Theoretically, the return- water temperature will always be at least as high as it is at full load. From a practical standpoint, this is not always possible, but it is closely approached in a properly operating system. In fact, at most part-load conditions, water returns from properly functioning air-conditioning coils at higher-than-design temperatures. In systems that use counterflow cooling coils, this occurs because the water leaving the coil tries to approach the temperature of the entering airstream.

Warm return water provides advantages in system design. It permits “preferential” loading of chillers, for example. Warmer return water is useful with all systems, but particularly so with heat recovery and free cooling applications. For more information, refer to “Chilled-Water System Variations” on page 70.

Pumping arrangements

Common

Various distribution system arrangements are possible. A single, large pumping station, as shown in Figure 29, can be used. The station may consist of single or multiplexed pumps sequenced on or off.

Figure 32. Campus pumping arrangement

Campus

Alternatively, each of several secondary distribution systems can be piped in parallel. For example, Figure 32 shows separate distribution systems for each of three loads. Clearly, this arrangement lends itself to the possibility of plant expansion by simply adding secondary distribution pumps to the existing plant.

Tertiary or distributed

Secondary Distribution Pumps

Tertiary pumping is an extension of primary–secondary pumping when the distribution or secondary pump must overcome diverse and severe pumping requirements.

System loads can also be decoupled from the secondary distribution system. This is frequently done with very large systems. Figure 33 shows one method of providing “tertiary pumping” at the loads. A “load” may be something as large as an entire building, or as small as an individual cooling coil. When one or more loads have extreme head requirements, the degree of range ability of the distribution pump is severely curtailed. Tertiary pumping allows the excess pumping requirements to be placed on a third pumping system thus shielding the distribution pump from divergent pressure requirements.

Most importantly, the loads must be controlled so that only the water needed to perform cooling is taken from the distribution loop. Water must not be allowed to flow into the return piping until it has sustained a specific temperature rise. Tertiary pumps can be either constant flow or variable flow, to best meet the terminal load requirements.

SYS-APM001-EN Chiller System Design and Control 49