Shell-side boiling of water at sub-atmospheric pressures

Abstract

Experimental data are reported for water boiling at pressures of 850 and 50mbar absolute on the shell-side of a model industrial boiler slice. The boiler test section was 1m high, 0.75m wide and contained 36 electrically heated tubes. The tubes were 28.5mm in diameter and 98mm long. The design of the boiler ensured that the tubes were submerged in a liquid pool. The height of the liquid pool could be varied. The pool height was set to approximately 0.8m for the tests carried out at a pressure of 850mbar, submerging the top of the tube bundle by about 200mm. Two pool heights were used in the tests carried out at a pressure of 50mbar, one at approximately 0.8m and another at approximately 2m. The later submerged the top of the tube bundle by about 1.6m. The heat flux was varied within the range 10–70kW/m2. A near-symmetrical half of the tube bundle contained wall thermocouples. An additional 29 thermocouples were located throughout the liquid pool.The liquid temperature in the pool was found to be reasonably uniform and controlled by the pressure at the free surface. This led to a small amount of subcooling at a pressure of 850mbar, up to 3K, and a significant amount of subcooling at a pressure of 50mbar, up to 16K for the smaller pool height and up to 31K for the larger pool height. The reasonably uniform pool temperature suggests that the liquid re-circulates within it.Boiling was found to occur at all heat fluxes at a pressure of 850mbar, 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 46–87kg/m2s, 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 50mbar, with interactive natural convection being the dominant heat-transfer mechanism. The mass fluxes produced were in the range 28–70kg/m2s. Boiling also occurred only at the higher heat fluxes during the high level tests at a pressure of 50mbar. 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.