Condensation forced convection

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Two condensers, or a condenser assembly having two separate refrigerant circuits and permitting rapair to one circuit while the other is working. If there is one assembly with forced convection, there are at least two fans. 

The term dT/dHt can be evaluated from the condensation curve h c from the single component correlations and h g from correlations for forced convection. 

Non-condensables >70 per cent assume the heat transfer is by forced convection only. Use the correlations for forced convection to calculate the heat-transfer coefficient, but include the latent heat of condensation in the total heat load transferred. 

Kast, W., 1964, Significance of Nucleating and Non-stationary Heat Transfer in the Heat Exchanger during Bubble Vaporization and Droplet Condensation, Chem. Eng. Tech. 36(9) 933-940. (2) Katto, Y., 1981, General Features of CHF of Forced Convection Boiling in Uniformly Heated Rectangular Channels, Ini. J. Heat Mass Transfer 24.14131419. (5)... 

No, H. C., and M. S. Kazimi, 1982, Wall Heat Transfer Coefficients for Condensation Boiling in Forced Convection of Sodium, Nuclear Sci. Eng. 57 319-324. (4)... 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

Forced convection in air Forced convection in water Forced convection in liquid metals Free convection in air Free convection in water Boiling in water Condensing in steam... 

Our development in this chapter is primarily analytical in character and is concerned only with forced-convection flow systems. Subsequent chapters will present empirical relations for calculating forced-convection heat transfer and will also treat the subjects of natural convection and boiling and condensation heat transfer. 

When a plate on which condensation occurs is sufficiently large or there is a sufficient amount of condensate flow, turbulence may appear in the condensate film. This turbulence results in higher heat-transfer rates. As in forced-convection flow problems, the criterion for determining whether the flow is laminar or turbulent is the Reynolds number, and for the condensation system it is defined as... 

Our discussion of film condensation so far has been limited to exterior surfaces, where the vapor and liquid condensate flows are not restricted by some overall flow-channel dimensions. Condensation inside tubes is of considerable practical interest because of applications to condensers in refrigeration and air-conditioning systems, but unfortunately these phenomena are quite complicated and not amenable to a simple analytical treatment. The overall flow rate of vapor strongly influences the heat-transfer rate in the forced convection-condensation system, and this in turn is influenced by the rate of liquid accumulation on the walls. Because of the complicated flow phenomena involved we shall present only two empirical relations for heat transfer and refer the reader t.o Rohsenow [37] for more complete information. 

Boiling Mechanisms Vaporization of liquids may result from various mechanisms of heat transfer, singly or combinations thereof. For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of liquid or as a result of heat absorbed by natural convection from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse of condensation) or from the flashing of liquids superheated by forced convection under pressure. 

As was the case in forced convection involving a single phase, Iteat transfer ill condensation also depends on whether the condensate flow is laminar or turbulent. Again the criterion for the flow regime is provided by the Reynolds number, which is defined as... 

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