Liquid metals, heat transfer scale

If steam condenses on a surface, there is no boundary layer the resistance to heat flow is due to scale, metal thickness, and the condensed liquid layer, resulting in a high heat transfer factor. A thin layer of air or other noncondensing gas forms at the surface through which the steam diffuses. The heat transfer factor diminishes rapidly but is considerably higher than in dry convection. 

In practice, we must consider the heat transfer resistance of the dirt or scale which has been deposited on the metal surface, except when values of U are small, as in the case of gas heater or cooler. Usually, we use the so-called fouling factor h(, which is the reciprocal of the dirt resistance and hence has the same dimension as the film coefficient h. The dirt resistance sometimes becomes controlling, when U without dirt is very large - as in the case of liquid boiler heated by saturated steam. Thus, in case the dirt resistance is not negligible, the overall resistance for heat transfer l/Ll is given by the following equation ... 

Industrial Problems. Problems have also been encountered in attempts to commercialize various homogeneous catalytic reactions. These, in addition to the highly corrosive nature of many metal solution systems involve general problems of large scale handling of liquid systems such as mass and heat transfer and the isolation of products from solution. Moreover, the recovery and/or regeneration of metals often presents difficulties. 

The bulk fluid at its boiling point but agitation is good and few bubbles form on the metal surfaces. Instead, homogeneous nucleation occurs near the surface of the liquid where the temperature exceeds the boiling point. The liquid head suppresses boiling deep within the vessel, and the bubble volume is typically a few percent of the liquid volume. This form of heat transfer also scales as... 

Similar equipment is becoming available commercially on an increasingly broad scale, and is flnding application in many nonnuclear processes in which liquid metals are used. Although liquid metals are used in the atomic fleld, usually only as heat transfer fluids, the special features in equipment developed for th service can be advantageously applied in processes in which liquid metals are used in stoichiometric quantities. 

In many cases of heat transfer involving either a liquid or a gas, convection is an important factor. In the majority of heat-transfer cases met in industrial practice, heat is being transferred from one fluid through a solid wall to another fluid. Assume a hot fluid at a temperature ti flowing past one side of a metal wall and a cold fluid at t flowing past the other side to which a scale of thickness x, adheres. In such a case, the conditions obtaining at a given section are illustrated dia-... 

The overall heat-transfer coefficient U in an evaporator is composed of the steam-side condensing coefficient, which has a value of about 5700 W/m K (1000 btu/h ft °F) the metal wall, which has a high thermal conductivity and usually has a negligible resistance the resistance of the scale on the liquid side and the liquid film coefficient,... 

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