Mechanisms of Heat Transfer
Approximate Overall Heat Transfer Coefficient
The calculation of overall heat transfer coefficient U using the equations previously presented can be rather tedious. Heat transfer specialists have computer programs to calculate this value. There are some quick approximation techniques. Table 2-8 comes from the Gas Processors Suppliers Association’s Engineering Data Book and gives an approximate value of U for shell and [...]
9Sep2009 | admin | Comments Off | ContinuedOutside Film Coefficient (Shell-and-Tube Exchangers)
For shell-and-tube heat exchangers with shell-side baffles, the shellside fluid flow is perpendicular to the tubes. In this arrangement, the outside film coefficient can be calculated from the following equation:
9Sep2009 | admin | Comments Off | ContinuedOutside Film Coefficient (in a Liquid Bath)
The outside film coefficient for a process coil in a liquid bath heater is the result of natural or free convection. Temperature variations in the fluid cause density variations. These density variations in turn cause the fluid to circulate, which produces the free convective heat transfer. For horizontal pipes and tubes spaced more than one [...]
9Sep2009 | admin | Comments Off | ContinuedInside Film Coefficient
The inside film coefficient represents the resistance to heat flow caused by the change in flow regime from turbulent flow in the center of the tube to laminar flow at the tube surface. The inside film coefficient can be calculated from:
(The viscosity of a fluid in lb/hr-ft is its viscosity in centipoise times 2.41.) The [...]
Overall Heat Transfer Coefficient
The overall heat transfer coefficient is a combination of the internal film coefficient, the tube wall thermal conductivity and thickness, the external film coefficient, and fouling factors. That is, in order for the energy to be transferred through the wall of the tube it has to pass through a film sitting on the inside wall [...]
9Sep2009 | admin | Comments Off | ContinuedOverall Temperature Difference
The temperature difference may not remain constant throughout the flow path. Plots of temperature vs. pipe length for a system of two concentric pipes in which the annular fluid is cooled and the pipe fluid heated are shown in Figures 2-2 and 2-3. When the two fluids travel in opposite directions, as in Figure 2-2, [...]
8Sep2009 | admin | Comments Off | ContinuedMultiple Transfer Mechanisms
Most heat transfer processes used in production facilities involve combinations of conduction and convection transfer processes. For example, in heat exchangers the transfer of heat energy from the hot fluid to the cold fluid involves three steps. First, the heat energy is transferred from the hot fluid to the exchanger tube, then through the exchanger [...]
8Sep2009 | admin | Comments Off | Continued