To improve pool boiling heat transfer in an annulus with closed bottoms, the length of an outer tube has been changed between 0.2 m and 0.6 m. A heated tube of 19.1 mm, diameter and the water at atmospheric pressure have been used. Three annular gap sizes of 3.65, 6.35, and 17.95 mm have been investigated. To elucidate effects of the outer tube results of the annulus are compared with the data of a single unrestricted tube. The change in the outer tube length results in much variation in heat transfer. As the outer tube length is much shorter than the heated tube the deterioration point of heat transfer gets moved up to the higher heat fluxes and the possibility of CHF creation is prevented. The major cause of the tendencies is related with the decrease in the intensity of bubble coalescence. (c) 2006 Elsevier Ltd. All rights reserved.