Trane SYS-APM001-EN manual Water-cooled condenser, Effect of condenser-water temperature

Primary System Components

Some chiller controls can accommodate very little flow variation during machine operation.2 Other, more sophisticated, chiller controls allow some flow variation. Some chillers can tolerate flow-rate variations—as much as 50 percent per minute or greater—while others can only tolerate up to 2 percent per minute. It is important that chiller capabilities are matched to system requirements. Contact the chiller manufacturer to determine the allowable rate of flow variation before varying the flow through the evaporator in a chiller. Flow variation is discussed in detail in the section “Variable-Primary- Flow Systems” on page 55.

Water-cooled condenser

To cool a building or process, the transferred heat must ultimately be rejected outdoors or to another system (heat recovery). The total amount of heat rejected includes the sum of the evaporator load, the compressor work, and the motor inefficiency. In a hermetic chiller, where the motor and compressor are in the same housing, these loads are all rejected through the condenser. In an open chiller, where the motor is separate from the compressor and connected by a shaft, the motor heat is rejected directly to the surrounding air. The evaporator load and the compressor work are rejected through the condenser, and the motor heat must be taken care of by the equipment room’s air-conditioning system.

Effect of condenser-water temperature

For a given chiller, as the leaving condenser-water temperature rises, refrigerant temperature and pressure also rise. Conversely, as the leaving condenser-water temperature drops, so do refrigerant temperature and pressure. As the refrigerant pressure and temperature changes, the work a compressor must do also changes. The effect of leaving-condenser-water temperature change on power consumption can be 1.0 to 2.2 percent per degree Fahrenheit [1.8 to 4.0 percent per degree Celsius]. Always consider the energy consumption of the entire system—not just the chiller. It is important to remember that although raising the leaving condenser-water temperature penalizes the chiller energy, it may reduce the energy used by the condenser pumps and cooling tower through the use of reduced flow rates and higher thermal driving-forces on the tower. System interactions are covered in more detail in “System Design Options” beginning on page 27.

Effect of condenser-water flow rate

The condenser is sensitive to the water flow rate. Excessive flow may result in high water velocity, erosion, vibration, or noise, while insufficient flow reduces heat transfer efficiency and causes poor chiller performance. Therefore, condenser-water flow through the chiller should be kept within a specific range of limits, except during transient startup conditions. Contact the manufacturer for these limits. Some chillers may allow extended operation below the selected flow rates.

If water velocity through the condenser tubes is too low for significant periods of time and the water is extremely hard, long-term fouling of the tubes may also occur. Webb and Li1 tested a number of internally-enhanced condenser tubes at low velocity (3.51 ft/s [1.07 m/s]) and high water hardness.

4 Chiller System Design and Control SYS-APM001-EN