Two-phase pressure drop and overall heat transfer coefficients have be
en measured in a model horizontal thermosyphon reboiler operating at a
tmospheric pressure and using water and aqueous solutions of methanol
and ethylene glycol. The shell and riser were of glass so that flow pa
tterns could be observed. The aqueous solutions produced much smaller
bubbles, but neither the pressure drops nor the overall heat transfer
coefficients varied substantially from system to system. Comparison of
the pressure drop measurements with available correlations highlighte
d areas, particularly bends and nozzles, where further study is needed
to improve predictions for reboiler design. Overall heat transfer coe
fficients had the expected order of magnitude (2.5-3.0 kW m(-2) K-1) b
ut did not vary as expected in the solutions. Flow instabilities were
noted at low qualities and explored in the water system.