Trane SYS-APM001-EN manual Condenser-water temperature control, System Controls

System Controls

The flow reduction options include:

• Cooling tower bypass

• Chiller bypass

• One or two throttling valves in the condenser-water pipe with the pump riding its curve

• A variable-speed condenser water pump

After the minimum-pressure differential is reached, the flow may be increased as long as that minimum-pressure difference is maintained. Some designers and operators become concerned with possible fouling of condenser-water tubes during these start-up conditions. There is little to fear due to the short duration of reduced flow operation and the limited occurrences. The advantages and disadvantages of these options are discussed in a variety of publications.36,37

Condenser-water temperature control

Cooling-tower-fan control

Cooling towers operate to produce a desired sump water temperature. As the heat rejection load and ambient wet-bulb temperature change, the cooling tower fans must move more or less air to produce the desired water temperature.

Cycling a single fan. Cycling a single fan on and off is one method to maintain rough water temperature control. Since airflow changes greatly between fan speeds, so does heat rejection. Temperature swings of 7°F to 10°F [3.9°C to 5.6°C] are not uncommon. Some chillers, especially older chillers with pneumatic controls, may operate poorly in response to these changing temperatures. Make sure that the fan does not cycle too often and damage the motor, drive, or fan assembly.

Two-speed fans. The installation of two-speed cooling tower fans is an option that reduces temperature swings. Typically, the low fan speed is between 50 and 70 percent of full speed. Since the heat rejection changes roughly in proportion with the fan speed, the temperature swing will be only 50 to 75 percent of cycling a single fan. Again, take care not to cycle too often between speeds—or the gear box may incur excessive wear and fail. A distinct advantage of two-speed fans is that at low speed, the fan power is greatly reduced. Since the fan power varies with the cube of the speed (approximately) the power at half speed is about 15 percent of full speed.

“Pony” motors. Another option offered by cooling-tower manufacturers is to have two separate motors available to drive the fan. The smaller motor is referred to as a “pony” motor. It operates at two-thirds of the full speed and uses about 30 percent of the full speed power. While controlling the tower, it is important to minimize cycling between speeds.

Variable-speed drives. Because the cost of variable-speed drives for cooling- tower fans has decreased, variable-speed drives have become more

90 Chiller System Design and Control SYS-APM001-EN