Pressure drop data measured during adiabatic two-phase flow in a plate-and-frame heat exchanger (PHE) are normalized with respect to the single-phase liquid pressure drop to give two-phase multipliers. A curve-fitted equation defines this relationship, which is a strong function of the Lockhart-Martinelli parameter C coefficients are shown to be strong functions of both the Lackhart-Martinelli parameter and liquid viscosity making this correlation unsuited to predictions of pressure drop in PHEs. Interfacial structure, observed during air-water downflow in replica channels (d(C) = 3 mm), is categorized into five flow patterns. These have a number similarities with structures reported for circular and rectangular channels of similarly low hydraulic diameter The transition boundaries between the patterns are shown to be a function of the chevron angle.