Evaporator Heat Transfer

This chapter is not intended for the engineers of evaporator manufacturers who must decide on the refrigerant circuiting, tube arrangement, and other important details. These designers consider the performance, cost of materials, and ease of manufacturing in deciding the configuration of the evaporator.

Instead the audience of this chapter is intended to be the user of evaporators who should understand the basic performance of the evaporator as a heat exchanger, how to properly select the evaporator from manufacturer’s catalogs, and how to install, operate, and maintain the evaporator properly.

As a heat exchanger, the evaporator follows the rules of heat transfer. In the evaporator of Fig. 6.4 heat flows in series through three resistances—the fluid side, the metal of the tube, and the refrigerant.

Heat-transfer coefficients in an evaporator.

Many engineers visualize a heat-flow process as an analogy to the electric flow process with the correspondence of terms shown in Table 6.1.

Ohm’s law for electricity states that

The corresponding heat-flow equations expressed in terms used in Fig. 6.4 are:

Application of the electrical analogy provides a simple means of deriving an expression for the overall U-value, where U is a term which when multiplied by the overall temperature difference, tf – tr, and the area yields the rate of heat transfer q, W (Btu/hr).

where U=overall heat-transfer coefficient, W/m2·°C (Btu/hr·ft2·°F).

When electrical resistances are connected in series, as in Fig. 6.5, the resistance of the combined circuit is the sum of the individual resistances,

Total electrical resistance is the sum of the individual resistances.

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