Shell and tube type heat exchangers (Figure 3.20) are the more common form of evaporation units for water cooling and chilling applications. These are utilized to cool liquids, which can be used as the secondary refrigerant or to cool the final products directly. In practice, these types of heat exchangers are known as liquid coolers or chillers.
Some example applications in food and refrigeration industry are:
• chilling of drinkable water,
• chilling of water for air conditioning coils,
• chilling of milk after pasteurization, and
• process cooling operations.
Chilled water systems can use either a flooded evaporator or a direct-expansion evaporator which are typically shell and tube type heat exchangers. In a flooded evaporator, refrigerant floods the shell side of the heat exchanger and is controlled by a level valve. Water being chilled passes through the tubes. Conversely, in a direct-expansion evaporator, water is carried in the shell and refrigerant is boiled inside the tubes. The rate of refrigerant flow is throttled to insure that only refrigerant gas exits the evaporator. Copper tubes mounted within a carbon steel shell is the most common construction used for chilled water evaporators.
It is important to note that if the refrigerant vaporizes on the outside surface of the tubes the evaporator is a flooded cooler; if it vaporizes inside the tubes the evaporator is a dry cooler (note that in this more common type, the mixture of liquid and vapor is evaporated completely, usually with some degree of superheating (Hewitt et al., 1994). In a flooded cooler the water or brine is circulated through the tubes, which are usually finned to provide an increment in the heat transfer rate and a decrease in the evaporator size. In a dry cooler the liquid refrigerant is contained within the tubes, and water or brine is circulated through the shell of the cooler, which serves as an evaporator. Flooded coolers are often specified for applications where shell-side vaporization of refrigerant of other liquids is desirable. Due to rapid boiling in the shell, in order to obtain high purity vapors, a vapor disengagement vessel is often welded to the main shell. Flooded coolers are particularly employed in multiple compressor systems.