Thanks Alan, >rom what I recall, the theory is / was that the water in contact with the >wall would bubble as it boiled, and the circulation of the water would >replace it by some more. This suggests that the important point must be the water circulation, but it also suggests that a turbulent, rather than a laminar flow, is required. Peter Chadwick MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.