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SYS-APM001-EN Chiller System Design and Control 3
• 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
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 Li
1
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.
Chilled Water
Supply
Baffles
Tube Bundle
Chilled
Water
Return
Refrigerant
Vapor
Liquid
Refrigerant