THE MISMATCH BETWEEN LABORATORY BOILING HEAT-TRANSFER DATA AND INDUSTRIAL-REQUIREMENTS

There is a large mis-match between published boiling heat transfer dat a and the operation of industrial cryogenic plant. Published data are generally obtained on small laboratory test-rigs at uniform heat flux with vertical channel heights of the order of 50mm. Operation of indus trial plant demands a constant wall temperature and vertical channel h eights of up to 2000mm (2m). Using a 2.7m long cryostat with forced ci rculation of liquid nitrogen to promote an isothermal bath temperature , results show that boiling heat transfer at constant wall temperature T-w (as in a reboiler-condenser) is strongly affected by local sub-co oling of the liquid. Indeed, no boiling takes place until T-w exceeds the local boiling temperature determined by the pressure head of the l iquid. For high thermodynamic efficiency, boiling heat transfer with s mall temperature difference T-w-T-0 (where T-0 is the surface saturati on temperature) there is a depth of liquid below which there is no boi ling.