TABLE OF CONTENTS In reboilers and vaporizers, boiling usually takes place either on the exterior surface of submerged tubes or on the interior surface as the fluid flows through the tubes. In the former case the tubes are oriented horizontally, while in the latter they may be either horizontal or vertical, but the most common configuration is vertical with the fluid flowing upward through the tubes. In both situations, the rate of heat transfer is normally much higher than in ordinary forced convection, and different correlations are required to predict heat-transfer coefficients in boiling systems.
A boiling fluid consists of a two-phase mixture of vapor and liquid. When such a fluid flows through a tube, a number of distinct flow regimes can occur depending on the flow rate and the relative amounts of vapor and liquid present. Two-phase flow is thus more complex than single-phase flow, and special methods are needed to calculate the pressure drop in equipment handling boiling fluids.
Correlations used in the thermal and hydraulic analysis of reboilers and vaporizers are presented in this chapter. Equipment design is covered in Chapter 10.
Pool boiling refers to vaporization that takes place at a solid surface submerged in a quiescent liquid. When the temperature, T s, of the solid surface exceeds the saturation temperature, T sat, of the liquid, vapor bubbles form at nucleation sites on the surface, grow and subsequently detach from the surface. The driving force for heat transfer is ?T e
