The mechanism of refrigerant boiling inside a tube of an evaporator is complex. There are more than 4000 technical papers on the subject, so a person could make a career of studying boiling heat-transfer. Even the prediction of heat transfer coefficients for a given refrigerant in a certain size tube with a specified flow rate is difficult. Fortunately, the designer and manufacturer of the evaporator take over that task, and the engineer who selects or uses the evaporator normally does not need to. However, the application engineer should understand what occurs during the boiling process.
One evaporator concept is direct expansion, discussed further in Sec. 6.27. The direct-expansion evaporator receives refrigerant from the expansion valve with a small fraction of vapor as shown in Fig. 6.8.
As the warm tube adds heat to the refrigerant, progressively more refrigerant evaporates, and the velocity increases until the refrigerant leaves the evaporator saturated or superheated. Figure 6.8 also shows typical boiling heat-transfer coefficients corresponding to the position along.the evaporator tube. The changes in the heat-transfer coefficient are associated with differing patterns of flow as the fraction of vapor and the velocity change along the tube. At the entering section of the evaporator, bubbles and plugs of vapor flow along with the liquid.
Further along the tube, the flow becomes annular with high-velocity vapor rushing through the center and the liquid clinging to the inside surface of the tube. Still later in the evaporator, the flow converts to a mist and eventually there could be a nonequilibrium mixture of superheated vapor and liquid until all the liquid finally evaporates.
The benefit of being aware of a distribution of heat-transfer coefficients as in Fig. 6.8 is to understand why such concepts as liquid recirculation (Sec. 6.7) have some heat-transfer advantages and to be able to diagnose operating problems attributable to the refrigerant-side heat transfer.