In air-cooled and evaporative condensers, the refrigerant condenses inside tubes. The mechanism of condensation is complex and the flow regimes continue to change as the refrigerant passes through the tube. Even though the state of
the refrigerant is superheated vapor on entering the tube, condensation begins immediately and a spray regime develops. Later on the flow converts to annular then stratified with the liquid flowing along the bottom of the tube. Near the end of the condenser tube the flow regime is characterized as slug or plug.
Figure 7.3 shows relative values of the condensing coefficient throughout a tube. At the entrance to the tube with its superheated vapor content the coefficient is low, which is typical of convection heat transfer with a gas. The coefficient increases once surface condensation begins and is usually at its highest value during annular flow. As more and more condensed liquid flows with the vapor, the surface available for condensation decreases. Near the end of the condenser tube the coefficient drops quite low, because the process has approached that of convection heat transfer to a liquid.
The low heat-transfer coefficient near the end of the condenser tube when all or most of the vapor has condensed is pertinent to the plant operator. The reason is that backing liquid into an air-cooled or evaporative condenser shifts some heat-transfer area into the liquid subcooling mode which exhibits low heat-transfer coefficients.