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Evaporators
DOI 10.1615/hedhme.a.000400
3.26.4 Evaporators
J. R. Barbosa, Jr. and C. J. L. Hermes
A. Definition
An evaporator can be defined as any heat transfer surface in which a volatile liquid is evaporated in order to extract heat from a cooling medium (Dossat, 1997), which may be the surrounding air, a secondary coolant or another substance. In a refrigeration application, most of the thermal load is removed by latent heat transfer in the evaporator, whose role is to vaporize the liquid refrigerant at low pressure fed by the expansion device in such a way liquid droplets are not carried over into the suction line — the tube fittings that connect the evaporator to the compressor — which may result in performance degradation or even compressor slugging (Gosney, 1982).
In vapour compression refrigeration systems, the evaporator is an indirect-contact heat exchanger (Gosney, 1982). In other words, the evaporator is the interface between the refrigeration cycle and the environment or product to be cooled. Thus, its performance will affect dramatically the cycle COP, which increases exponentially with rising evaporating temperature. Hence, for a given room or product design temperature, evaporators with higher heat transfer conductance can operate with higher evaporating temperatures, requiring less work consumption in the compressor for the same cooling demand.
B. Classification
There are several types of evaporators in use nowadays (see for instance Section 270 of this Handbook), and these may be classified according to the medium to be cooled, the method of refrigerant feed and the type of application (Dossat, 1997). The medium to be cooled may be a gas (generally air), a liquid (generally water, brine or glycol) or a product load (an ice maker, for example, in which ice is produced directly in contact with the heat transfer walls). As depicted in Table 1, Stoecker and Jones (1982) extended this classification for cases in which the refrigerant circulates inside or outside the tubes. It is worth noting that applications in which gas flows inside the tubes are quite rare in refrigeration due to the very high pressure drops which result in prohibitive levels of pumping power.
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