Condensation heat transfer coefficient with noncondensable gases inside near horizontal tubes

An experimental investigation on the role of noncondensable gases during condensation of steam inside horizontal/slightly inclined tubes is presented. In a condenser of a thermal desalination unit, the noncondensable gases flowing within the gas phase cause reduction of performance and efficiency. Many researchers have investigated in-tube condensation for horizontal heat exchangers, but very few works have been performed to study the condensation in slightly inclined tubes with noncondensable gases. Inclination of tubes may be requested to achieve suitable and reliable draining. Experiments were conducted in the following conditions: tube internal diameter 12.6 mm, 20 mm and 26.8 mm; tube inclination 7°; inlet noncondensable gas mass fraction ωin = 5%–42%; inlet mixture Reynolds number Rem,in = 5000–20,000; local noncondensable gas mass fraction ωin = 5%–60%; local mixture Reynolds number Rem = 500–20,000; saturated steam at atmospheric pressure, gravity controlled with stratified flow regime, in which the condensate is collected mainly in the bottom part of the tube due to gravity and it is drawn out by its own momentum. A correlation of mixed gas heat transfer coefficient along a slightly inclined tube, in a gravity controlled flow regime, has been developed, showing a good agreement with experimental results.

► Condensation of steam–air mixed gas has been experimentally investigated.
► Experimental tests in a slightly inclined tube (angle = 7°) have been carried out.
► A correlation of mixed heat transfer coefficient, in good agreement with experimental results, has been developed.