The effect of the tube geometry was investigated in a natural circulation single tube reboiler. Two stainless steel tubes of different dimensions (the first being 0.5” i.d, 0.625” o.d. and 4 ft heated length; the second being 0.872” i.d, 1” o.d. and 6 ft heated length) were used. Both tubes were assembled from sections of 8” length, each section was heated by its own steam jacket. Test liquids temperatures were measured by thermocouples along the test tube. A pressure transducer at the inlet of the tube was used to record the static pressure fluctuations using a UV recorder. Distilled water and water–ethylene–glycol solutions were used as test liquids. Heat flux for each section was measured.