The system in Figure 3-15 uses two 3-way diverter valves and one 3-way modulating valve. It also uses a SEP4 hydraulic separator to interface the evaporator side of the chiller to the system. This separator allows the chiller to maintain a minimum flow rate when the demand from a zoned distribution system is low. It also provides air, dirt and magnetic particle separation for the system.
In Figure 3-15a, the position of the valves and circulator operation are in the mode where the chiller provides 100 percent of the cooling load. This mode applies when outdoor conditions are warmer than the return water from the cooling distribution system. The water-side economizer is not functioning in this mode.
In Figure 3-15b, the position of the valves and circulator operation allows the plate & frame heat exchanger (e.g., the economizer) to assume 100 percent of the cooling load. This mode applies when the outdoor temperature is below the return temperature from the cooling distribution system — but is still several degrees above freezing. This is also the mode that yields the greatest reduction in operating cost since the chiller is completely off. The only significant electrical power being used is for the circulators.
In Figure 3-15c, the system is using the plate & frame heat exchanger to pre-cool the water returning from the cooling load, and then passing that water to the chiller to further lower its temperature to the desired setpoint. The pre-cooling removes some of the chiller load and lowers its power demand. A modulating 3-way valve controls the portion of flow from the cooling tower to the heat exchanger and the chiller. This mode of operation is appropriate when the outdoor temperature is low enough to allow some pre-cooling, but not enough to drop the chilled water temperature to the desired setpoint. As the outdoor temperature decreases, more of the tower flow is directed through the heat exchanger and less through the chiller, and vice versa.
Cold water from the building is supplied to the sump in the cooling tower through an automatic make-up assembly. A float switch inside the cooling tower monitors the water level in the sump. When the water level drops, the float switch operates a solenoid valve to allow water into the sump. This make-up water passes through a testable backflow preventer and a pressure-reducing valve upstream of the solenoid valve.
The closed cooling tower circuit operates with an antifreeze solution. If the tower will be inactive in winter, its sump water would be drained.
The heat exchanger in the cooling tower and the economizer heat exchanger are protected against dirt that could foul the heat transfer surfaces by DiscalDirtMag® separators. These separators also collect air entrained with the water and eject it from the system.
Water-side economizers are especially well-suited to chiller/tower systems serving with a high percentage of sensible cooling load. Examples include data centers or industrial cooling in arid climates. These loads allow for higher chilled water temperatures, and thus increase the hours where the economizer can provide most, if not all, of the cooling requirement. They are also well-suited for applications where a high percentage of the sensible cooling load is handled by radiant panel cooling or chilled beams, and the latent load is handled by a separate chiller operating at a lower water temperature.