Trane SYS-APM001-EN manual Primary-Secondary Decoupled Systems, Hydraulic decoupling

System Configurations

percent of the system load. At system loads greater than 50 percent, the upstream chiller is preferentially loaded because it will attempt to produce the design leaving chilled-water temperature. Any portion of the load that remains is directed to the downstream chiller.

If chiller setpoints are staggered (upstream at 49°F [9.4°C] and downstream at 42°F [5.5°C]), the downstream chiller is loaded first. The upstream machine then meets any portion of the system load that the downstream chiller cannot meet. This control strategy offers several benefits. The first is that the upstream chiller is always operating at an elevated temperature. This allows it to operate at a higher efficiency. Also, placing an absorption chiller in the upstream position increases its capacity. As an example, an absorption chiller that can produce 500 tons [1,760 kW] at a leaving chilled-water temperature of 44°F [6.6°C] may produce 600 tons [2,110 kW] at 50°F [10°C]. Centrifugal, helical-rotary, reciprocating, and scroll chillers experience capacity and efficiency changes to a lesser degree. By judicious use of the series configuration, these benefits can provide reduced installed cost and fuel flexibility to the building owner. While not shown, a single manual bypass with proper valving can provide for servicing of chillers.

Equal loading of the two chillers may be accomplished by using a chiller plant management system to dynamically reset the upstream chiller’s setpoint in response to changes in system load.

Primary–Secondary (Decoupled) Systems

The root cause of the difficulties with parallel chiller control in a constant volume system is the fixed relationship between chiller- and system-flow rates. If, instead, we can hydraulically decouple the production (chiller) piping from the distribution (load) piping, it is possible to control them separately. The fixed relationships are then broken apart. The production pumps are typically constant volume, while the distribution pumps are variable volume.

Hydraulic decoupling

Figure 28 shows the basic decoupled system. This strategy is also referred to as a primary–secondary pumping arrangement. Separate pumps are dedicated to production and distribution. While the same water is pumped twice (by different pumps), there is no duplication of pumping energy. This is because the production pumps overcome only the chiller and production- side pressure drop while the distribution pumps overcome only the distribution system pressure drop.

SYS-APM001-EN Chiller System Design and Control 45