Liquid Coolers

In the refrigeration process, heat is transferred from the medium being cooled to the refrigerant as it changes phase in the evaporator. A refrigerant-to-air, or direct expansion (DX), evaporator is typically used with smaller vapor compression systems with a higher-pressure refrigerant, such as HCFC-22, CFC-12, and HFC-134a. The DX evaporator shown in Figure 35-1 is a finned-tube coil over which air is passed and thereby cooled.

Large central air conditioning or process cooling systems with multiple terminal units use a brine or chilled water cooling system because it is impractical to circulate large volumes of refrigerant throughout a facility. In this case, the

Fig. 35-1 Illustration of Fin-Tube Evaporator. chilled water serves Source: The Trane Company as an indirect or

secondary cooling medium that circulates between the loads and a central refrigerant-to-liquid evaporator, or cooler. Upon exiting the cooler, the chilled water is distributed to coils housed in air handling units (AHUs) or to other air conditioning or process heat exchangers.

The liquid cooler is a shell-and-tube heat exchanger of either the flooded or dry type. In a flooded cooler, the refrigerant is vaporized on the outside of bare or augmented surface tubes that are submerged in evaporating liquid refrigerant within a closed shell. Refrigerant may be metered by a float valve or orifice with flooded systems. Evaporators are said to be dry when a portion of the evaporator area is used for superheating the refrigerant. Refrigerant flow to the evaporator is controlled by a thermal expansion valve in response to superheat in the return line from the evaporator coil. Typically, this is set at about 10°F (6°C) of superheat to assure gas inlet to the compressor to avoid compressor damage due to liquid

Fig. 35-2 Direct-Expansion Shell-and-Tube. Source: The Trane Company
Fig. 35-3 Cooler (Evaporator) for Centrifugal Chiller. Source: Carrier Corp.

carryover. Figure 35-2 shows a direct-expansion shell-and-tube evaporator.

Figure 35-3 is a cutaway schematic of an evaporator (or cooler) used with a centrifugal chiller. Notice the eliminator which, in this case, is a series of parallel plates used to trap liquid droplets ahead of the compressor. Also notice the marine water box, which is sometimes used to allow for easier inspection and cleaning of heat exchanger tubes.

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