Driving force, thermosyphon circulation

The total condensation of a vapor to a liquid is best illustrated by the condensation of steam to water. Figure 13.1 is a rather accurate reproduction of the radiator that heated my apartment in Brooklyn. Steam flowed from the boiler in the basement. The steam condensed inside the radiator, and flowed back into the boiler, through the condensate drain line. This is a form of thermosyphon circulation. The driving force for the circulation is the differential density between the water in the condensate drain line and the steam supply line to the radiator (see Chap. 5, discussion of thermosyphon reboilers). 

Thermosyphon circulation (or natural circulation) Partially vaporizing a liquid causes it to flow to a higher elevation. The driving force is the density difference between the denser liquid feed and the less dense, partly vaporized effluent. 

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