Solution Manual Of Process Heat Transfer By D Q Kern Hit [EXCLUSIVE]

One of the mechanisms for gravity induced fouling is the deposition of colloidal or particulate matter on the heat transfer surfaces. Such deposition can be the result of the drop in local gravity due to the higher curvilinear flow rate at the downstream end of the barrier, which can accelerate the accumulation and agglomeration of particles if they are in the process flow, or cause particles suspended in the process water to become trapped above the level at which gravitational settling takes place. As the particles rise, they may be carried over to the cooling surface, where they will deposit and, just like a sediment, they can cause considerable local problems in the heat exchanger. This is particularly the case where the cooler surface is relatively hot and the upstream temperature distribution may not be uniform, so carrying sections of the pipeline, for example, to the cooler section, will lead to temperature cycling and associated de-maturation of the process.

Fortunately, colloids may be precipitated at a selected temperature (called the lower critical temperature) so that they are effectively removed by the impingement of liquid water droplets that are formed at the heat exchange surface. The water droplets may be brought to the surface either by strong local differences in the heat transfer surface temperature (for example due to a hot or cold spot or wall) or when the thin film of liquid water on the surface of the heat transfer section undergoes thermodynaic convection, allowing the induced heat flux to be dissipated by water droplet entrainment. This is a mechanism that can be triggered by thermocyclic fluctuations induced, for example, by liquid turbulence and is also called nucleation boiling. If this process is appropriately controlled the efficiency of heat transfer is greatly enhanced because the heat required to evaporate the water is carried away by the droplets rather than being dissipated locally by the heat rejection surface, as in the case of spottiness. The water is also taken from the process normally not used in condensing heat exchangers, and this is not pure so that the evaporative heat rejection is somewhat diluted, but thorough and extended clean-ups have shown that it is still much more efficient than local heat rejection. A further advantage is that the kinetic energy associated with the thermocyclic fluctuations can be utilised to the advantage of the steam generator without the need to modify the power plant. d2c66b5586