Liquid metals, heat transfer examples

EXAMPLE 4J-3. Liquid-Metal Heat Transfer Inside a Tube A liquid metal flows at a rate of 4.00 kg/s through a tube having an inside diameter of 0.05 m. The liquid enters at 500 K and is heated to 505 K in the tube. The tube wall is maintained at a temperature of 30 K above the fluid bulk temperature and constant heat flux is maintained. Calculate the required tube length. The average physical properties are as follows n =... 

Special correlations have also been developed for liquid metals, used in recent years in the nuclear industry with the aim of reducing the volume of fluid in the heat transfer circuits. Such fluids have high thermal conductivities, though in terms of heat capacity per unit volume, liquid sodium, for example, which finds relatively widespread application, has a value of Cpp of only 1275 k.l/ni1 K. 

The overall heat transfer coefficient is normally made up of several terms arising from the various resistances to the flow of heat. In the simple example mentioned above, there will be terms for heat transfer through the liquid by conduction and convection, for conduction through the metal wall of the tank and through any layers of insulating material and for heat loss from the outer skin to the surrounding air. 

Heat exchangers may be termed, for example, a condenser (where condensing a process vapor) or an evaporator (where vaporizing a liquid), and they are, of course, designed with a variety of metals and alloys having high heat-transfer coefficient U values, such as copper-based alloys. 

There are three means of heat transfer that apply to drying processes. These are conduction, convection, and radiation. Conduction is the transfer of heat from one body to another part of the same body, or from one body to another body in direct physical contact with it. This transfer of heat must occur without significant displacement of particles of the body other than atomic or molecular vibrations. Conductive heat transfer is analogous to electrical flow and can be described by similar terms such as potential and resistance. Some examples of conduction would include heating of metal pipes by a hot liquid inside of them, or heat supplied to a solids bed via a metal shelf. 

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