Sizing The Liquid/Vapor Return Line

Mixtures of liquid and vapor flow through the tubes of condensers and evaporators and in the return line from the evaporator to the low-pressure receiver in liquid recirculation systems. Sizing the pipe and specifying the pressure drop in evaporators and condensers is under the jurisdiction of the component manufacturer, but the system designer is responsible for the liquid/vapor return lines. In horizontal or near-horizontal liquid/vapor return lines, as in Figs. 9.11a and 9.11b, the pressure drop is influenced by the degree of slope of the pipe. A high velocity of vapor will carry the liquid along in a mist. At lower vapor velocities, and/or if the line has sufficient pitch in the direction of flow, the liquid and vapor may separate, with the liquid flowing along the bottom of the tube.

The designer must select the size of and compute the pressure drop in the liquid/vapor suction lines shown in Fig. 9.11. The classic method for computing the pressure drop of two-phase, liquid/vapor flow in horizontal lines is the Lockhart and Martinelli correlation. When using this method, the pressure gradient of each phase is computed as though it flowed separately in the pipe. Then, a graph provided by Lockhart and Martinelli combines those two gradients to predict the two-phase gradient. Most designers of refrigeration systems do not expend the effort to make this calculation. Designers often select the pipe by first determining the size that gives the desired pressure gradient were the vapor alone flowing in the pipe. Then the next larger pipe size is selected with the extra cross-sectional area of the pipe assumed to provide the area for the flow of liquid.

Liquid/vapor return line that is (a) horizontal and (b) sloped slightly in the direction of flow.

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