Trane SYS-APM001-EN manual Preferential loading - sidestream arrangement

Chilled-Water System Variations

One caveat when applying this arrangement is that chillers on the production side of the bypass line will run more often at low part-load conditions. Older chillers or newer chillers with a high cycle point may not have this capability.

Preferential loading - sidestream arrangement

Figure 45. Sidestream preferential loading arrangement

Figure 45 shows a simple modification to the traditional decoupled arrangement. The sidestream arrangement ensures that the chiller piped in the sidestream position still receives the warmest entering-water temperature and can fully load it whenever the chiller plant operates.

Chiller 3

Production

Chiller 2

Bypass Line

Distribution

Chiller 1

Sidestream position receives the warmest return water.

This arrangement is unique because it not only allows preferential loading, but it also permits the cooling device (chiller, heat exchanger, etc.) in the sidestream position to operate at any leaving-water temperature. This configuration precools the system-return water for the chillers downstream, reduces their loads and energy consumption, and decreases the overall operating cost of the chilled-water system.

When cooling devices are located in the return piping of the distribution loop, they do not contribute to system demands for flow. They simply reduce the temperature of return water to the production loop. While this is counterproductive to the principle of striving for the highest possible return water temperature, it is often the best way to obtain free cooling, specialized heat recovery, or reduce the capital cost of ice storage equipment.

Sidestream, decoupled applications are usually most economical when the sidestream chiller is smaller than those on the production side of the bypass line. Since pumping requirements and energy consumption change with modifications to the system arrangement, it is best to use a computerized analysis tool to model the economic effects.

The following are different system configurations that can benefit from the sidestream application.

Sidestream plate-and-frame heat exchanger

A free-cooling heat exchanger may be capable of chilling water to only 48°F [8.9°C] during some periods. Rather than overlook this portion of cooling capacity assistance, the heat exchanger does whatever it can to its portion of the total return stream. Figure 40 on page 70 shows a possible chilled- and condenser-water piping arrangement that allows for simultaneous waterside economizer and chiller operation. Chillers operating downstream can reduce chilled-water temperature further, allowing simultaneous free cooling and mechanical cooling. This configuration increases the hours that the heat exchanger may be used. Since this capacity is brought to bear on the warmest water in the system, it allows the highest heat exchanger effectiveness and has the greatest impact.

74 Chiller System Design and Control SYS-APM001-EN