Shell-side boiling of water at sub-atmospheric pressures

Boiling was found to occur at all heat fluxes at a pressure of 850 mbar, with the measured heat-transfer coefficients shown to be in broad agreement with nucleate boiling correlations available in the open literature. However, they were also consistent with a flow boiling process involving natural convection and nucleation, where the convection was driven by variations in liquid temperature on the walls of the tubes. This natural convection relies on an interaction between the tubes that produces mass fluxes in the range 46-87 kg/m2 s, based on the approach area to the tube bundle. Boiling occurred only at the higher heat fluxes during the low level tests at a pressure of 50 mbar, with interactive natural convection being the dominant heat-transfer mechanism. The mass fluxes produced were in the range 28-70 kg/m2 s. Boiling also occurred only at the higher heat fluxes during the high level tests at a pressure of 50 mbar. However, the convective heat transfer was more compatible with little interaction between the tubes, although some evidence suggests that the evaporator oscillates between interactive and isolated tube behaviour.