Flooded boiling of ammonia with miscible oil outside a horizontal plain tube

Pool boiling tests were conducted with plain steel tubing using pure ammoni a and a mixture of ammonia and a miscible lubricant. By comparing the resul ts, the effect of miscible lubricant on shell-side boiling heat transfer co efficient of ammonia was quantified. The experiment covered saturation temp erature from -23.3 degreesC to 7.2 degreesC (-10 degreesF to 45 degreesF) a nd heat flux up to 60 kW/m(2) [19,000 Btu/(h(.)ft(2)]. Nucleate boiling of ammonia took place outside a plain steel tube of 19.1 mm (0.75 in.) diamete r. The tube was heated by liquid flowing inside to simulate areal heat exch ange device. A polyalkylene glycol lubricant, miscible with ammonia, was te sted at 0%,, 1%, 5%, and 10% mass concentrations. The experimental results showed that under a particular saturation temperature and heat flux, the he at transfer coefficient generally first decreased with an increase in oil c oncentration up to 5%. It was then followed by an insignificant increase in the coefficient with a further increase in oil concentration to 10%. At a low temperature and a high heat flux, the heat transfer coefficient decreas ed with the lubricant concentration and stayed constant. The effect of misc ible lubricant strongly depends on saturation temperature and weakly depend s on heat flux. The largest degradation of hear transfer performance due to the miscible lubricant occurs at the present low temperature of -23.3 degr eesC where the heat transfer coefficient can be reduced by up to 33%. A gen eral correlation was developed for heat transfer coefficients both with and without lubricant effect.