Chiller efficiency is also dependent on condenser-water temperature. The equipment usually performs best at a steady, low condenser-water temperature. This is accomplished by several means: two-speed or variable-speed cooling-tower fans, cooling-tower cell sequencing, or use of a bypass valve. For a given flow rate, the condenser-water controls can order any number of options to the cooling-tower water circuit:
1. Fan off, water circulating through a minimum number of cells
2. Bypass valve used to control return temperature, until fully closed
3. Water distributed through additional cells
4. Single fan turned on low speed
5. Additional fan(s) turned on low speed
6. One fan turned on high speed
7. All fans turned on high speed
The bypass valve returns a portion of the water from the chiller condenser without circulating it through the cooling tower(s). This is a simple way to maintain a fixed return temperature to the chiller under a variety of operating conditions. Keeping a constant condenser-supply- water temperature also allows the chilled-water-supply temperature to be reset to the optimum temperature for the chiller efficiency.
A variation on this theme is available when the temperature is cold and dry enough so that all the condenser-water heat can be rejected from the chiller by the cooling tower alone. The only energy, in this instance, needed to cool the chilled water is the pump energy to the towers and the forced-air fans on the towers. In this application, the chiller acts as nothing more than a heat exchanger, the refrigerant working to transfer energy between the chilled-water and condenser-water streams using a bare minimum of energy. In some climate,s it’s economical to install a flat-plate heat exchanger in parallel with the chiller to reject building heat directly to the cooling-tower circulating loop.
Overall, the condenser/chiller loops are quite complex, and they must be analyzed carefully, being attentive to the efficiencies and energy-use characteristics of each component. There are breakeven points at every stage that must be monitored by the central computer, which follows a set sequence of instructions when this setpoint is exceeded, then advances the system and tracks the next breakeven point, and so forth.
All of this may seem like a lot of trouble to go through, just for a degree or two of temperature advantage or a few percentage points of efficiency. It sometimes is, unless the EMS system uses control points and monitoring devices justified by other projects. For example, all of the condenser-water controls can be justified by simply making the cooling towers operate optimally. Using the EMS to supervise this operation by varying the condenser-water return temperature then becomes a simple matter easily justified by the nominal extra cost.
__________
Excerpt from Energy Conservation Projects.