Distribution piping, Primary System Components

Figure 9. Pump per chiller

Pump

Load

Figure 10. Manifolded pumps

Manifolded

Pumps

Load

SYS-APM001-EN

Primary System Components

•accommodates the total pressure (static head plus dynamic head) on system components such as the chiller’s evaporator, valves, etc.

Note that the pump heat is added to the water and must be absorbed by the chiller. Generally, this represents a very small temperature increase.

Multiple pumps are often used for redundancy. Depending on the terminal control devices and system configurations, the chilled-water pumps may be either constant- or variable-flow.

As previously stated, pumps may be either on the inlet or the outlet of the chiller, as long as the inlet of the pump experiences an adequate, positive suction pressure. In applications where there is a significant liquid column head (for example, a high-rise building), the pump is often located at the chiller’s outlet so that the evaporator bundle is subject only to the static head (rather than the static head plus the dynamic head added by the pump). The need for high-pressure water boxes on the chiller can be eliminated.

Conversely, an advantage of locating the pump at the chiller’s inlet is that if the pump motor rejects its heat to the water, the heat can be removed directly by the chiller. The chiller does not need to compensate for the pump heat by making colder water.

Pump per chiller

In either a primary–secondary or variable-primary-flow system, using one pump per chiller simplifies system hydraulics (Figure 9). The pump can be selected to produce the flow and pressure drop necessary for the specific chiller. Bringing on additional pumps changes system hydraulics, but only minimally. One drawback of such a system is a lack of redundancy, since the pump and chiller are dedicated to one another. This may be overcome by using a spare pump, pipes, and valves so that the spare pump could work with any chiller during emergency conditions.

Manifolded pumps

In an effort to resolve the redundancy consideration, some designers prefer to manifold pumps and provide n+1 pumps, where n is the number of chillers (Figure 10). Such an arrangement allows any pump to be used with any chiller. However, system hydraulics become more complicated. Unless all piping runs and evaporator pressure drops are equal, the amount of water flowing to each chiller will differ. As discussed in “Moderate ’low ΔT syndrome’" on page 68, manifolded pumps present a control opportunity when low ΔT is experienced.

Either pump configuration can be successful; one pump per chiller simplifies the hydraulics, while manifolded pumps allow redundancy.