Heat transfer , condensation and fog formation in crossflow plastic heat exchangers
-In this paper heat transfer of air-water-vapour mixtures in plastic crossflow heat exchangers is studied theoretically and experimentally. First, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations. Subsequently, heat transfer with wall condensation and fog formation are considered in some detail. Separate attention is paid to the heat transfer and condensation of pure steam in the heat exchanger. Finally, the experiments performed are reported and the results compared with the models presented. From this comparison it can be learnt that the models are well able to predict the rate of heat transfer and phenomena such as condensation and fog formation.
Waste heat and water recovery characteristics of heat exchangers for dryer exhaust
ABSTRACT In many industrial drying processes, water is evaporated in the drying section by a large volume of fresh air supply, which contains a large amount of sensible and latent heat. Recovering heat and water from the dryer exhaust is a potential way to realize energy and water conservation in drying systems. In this paper, the experimental and theoretical investigations of the waste heat and water recovery characteristics of heat exchangers for dryer exhaust are presented. A hot air and water vapor mixture was used to simulate dryer exhaust. Convection–condensation heat transfer and water recovery characteristics were first obtained under various water vapor mass fractions, flow velocities, and humid air inlet temperatures through experiments, which revealed that the convection–condensation heat transfer coefficient increased with the increase in water vapor mass fraction and humid air velocity, and decreased with the increase in inlet temperature. The condensed water rate increased with the increase in water vapor mass fraction and humid air velocity, while inlet temperature had almost no effect on it. The condensing ratio decreased with the increase in water vapor mass fraction and humid air velocity, while improving the inlet temperature slightly reduced it. Then, a theoretical calculation model was developed to predict the heat and water recovery characteristics of heat exchangers under a wide working condition. In this model, the pipe and suction effects caused by condensation on the tube wall were considered. The properties of humid air were changed according to the temperature and vapor content instead of regarding them as constant values. The calculation results were compared with the present experimental data and those of previous studies. The good prediction accuracy and the universality of the present model were verified. This study is of great significance to the design of heat exchangers.
Evaluation and Control of Collapsible Soils
Some compacted soils wetted under certain conditions of compaction and loading may exhibit additional settlement (collapse). To investigate the effect of the soil type, compaction water content, initial dry unit weight and applied pressure at wetting on collapse potential, 138 single‐oedometer tests were conducted on eight different soils. The results indicate that well‐graded soils tend to collapse more than poorly graded ones under similar conditions. Furthermore, collapse potential decreases with an increase in (1) The difference between the sand and clay percentages; (2) compaction water content; and (3) initial dry unit weight, while it increases with pressure at wetting. A multiple regression analysis was performed on the laboratory results to provide a collapse‐predictive model that was substantiated by data reported by several researchers. A chart is presented to estimate (1) The collapse potential of soils under various conditions; and (2) the critical water content beyond which no collapse is ex...