Trane SYS-APM001-EN manual Primary System Components, Effect of chilled-water temperature

Primary System Components

•In a direct-expansion (DX) shell-and-tube evaporator (Figure 3), warmer water fills the shell while the cool, lower-pressure liquid refrigerant flows through the tubes.

Figure 3. Direct-expansion evaporator cut-away

Chilled Water

Supply

Chilled

BafflesWater

Return

Refrigerant

Vapor

Liquid

Refrigerant

Tube Bundle

In either design, there is an approach temperature, which is the temperature difference between the refrigerant and exit water stream temperatures. The approach temperature is a measure of the heat transfer efficiency of the evaporator.

Effect of chilled-water temperature

For a given chiller, as the leaving chilled-water temperature drops, the refrigerant temperature and pressure must also drop. Conversely, as the leaving chilled-water temperature rises, so do the refrigerant temperature and pressure. When the leaving chilled-water temperature changes, the work a compressor must do also changes. The effect of leaving chilled-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 only the chiller. It is important to remember that although reducing leaving chilled-water temperature penalizes the chiller, it may reduce the overall system energy because less water is pumped through the system. System interactions are covered in more detail in “System Design Options” on page 27.

Effect of chilled-water flow rate and variation

The evaporator is sensitive to the water flow rate. Excessive flow may result in high water velocity, erosion, vibration, or noise. Insufficient flow reduces heat-transfer efficiency and causes poor chiller performance, which might cause the chiller controls to invoke safeties. Some designers have concerns over low flow rates causing fouling. Generally, as Webb and Li1 noted, these concerns are unwarranted since the chilled-water loop is a closed system, thus reducing the chances of materials entering the system and causing fouling. Chilled-water flow through the evaporator must be kept within specific minimum and maximum limits. Contact the manufacturer for these limits.

SYS-APM001-EN Chiller System Design and Control 3