Two-phase pressure drop calculations are especially important for thermosiphon type reboilers because they are necessary to determine flow rates on which heat transfer calculations are based. More detailed descriptions of two-phase pressure drop correlations are covered in other sections. In this section, approximate methods that are useful in reboiler design are summarized.

A. General

Pressure drop in a vaporizing system is composed of three elements: static head ΔP_tps, momentum ΔP_tpm, and friction ΔP_tpf, as follows:

\[\label{eq1} \varDelta P_{tp} =\varDelta P_{tps} +\varDelta P_{tpm} +\varDelta P_{tpf}\tag{1}\]

Normally, the pressure drops are calculated for short-length increments, over which the vapor fraction is assumed constant, and summed. The individual pressure drop elements may be estimated as follows.