Experimental investigation of presence of air on performance of steam heating process

Abstract

A Presence of air and other non-condensable gasses poses a serious nuisance for the steam network in a process industry. From a reduction in the process throughput to an acceleration in corrosion effects, presence of air & non-condensable can hamper the productivity as well as life of a process plant. For instance, the thermal conductivity (in W/mk) of air is 0.000049; in contrast with 0.002 for water, 0.20 for iron, and 0.96 for copper. Thus, even a thin layer of air can increase the resistance to heat flow significantly. Even the convective heat transfer coefficient for air is quite low (∼15 W/m2K) as compared to condensing steam (∼2000 W/m2K); so that even if the air convicts heat, it is likely to do so poorly.The objective of this paper is to show experimentally, how air affects the heat transfer process. We investigate the heat transfer performance in terms of the rate of temperature rise (ΔT/Δt) for a water-bath-type indirect heat exchanger. The bath is filled with 100 Litre of water each time and heated with steam; where an air vent is bypassed or taken in-line alternatively, while maintaining the initial conditions constant as far as feasible. The water bath is heated from 25 to 30℃ (room temperature) to 90-95℃ and the time taken for heating is noted in each case. The experimental set-up is equipped with electronic sensors for various temperatures, pressures, steam flow rates etc. Over a set of 10 such experiments, we have found that the presence of the air vent in the steam line consistently improves the rate of temperature rise (ΔT/Δt) with uniform heating in all sections of the heat exchanger. Furthermore, when the air vent is bypassed, the different sections of the heat exchanger are heated at different rates leading to distinct hot & cold spots in the heat exchanger sections.