Heat Transfer Theory
Conduction and Convection are generally the methods of heat transfer used for valve jackets.
Bolt-On jackets conduct heat from the jacket surface to the valve surface. Forced convection, by way of pumping hot media through the valve jacket, allows the jacket to conduct heat to the valve.
Full Weld-On and Partial jackets use forced convection to heat the valve and allow for best heat transfer.
Jacketed pipe uses forced convection by way of the heated media being pumped through the annular area of the pipe.
Flow Trace heats pipe through conduction and the trace is heated through forced convection as the heated media travels through the center of the trace.
Convection is the transfer of thermal energy from one place to another by the movement of fluids or gases. Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convection describes the combined effects of conduction and fluid flow or mass exchange.
Two types of convective heat transfer may be distinguished:
- Free or natural convection: when fluid motion is caused by buoyancy forces that result from the density variations due to variations of temperature in the fluid. In the absence of an external source, when the fluid is in contact with a hot surface, its molecules separate and scatter, causing the fluid to be less dense. As a consequence, the fluid is displaced while the cooler fluid gets denser and the fluid sinks. Thus, the hotter volume transfers heat towards the cooler volume of that fluid. Familiar examples are the upward flow of air due to a fire or hot object and the circulation of water in a pot that is heated from below.
- Forced convection: when a fluid is forced to flow over the surface by an external source such as fans, by stirring, and pumps, creating an artificially induced convection current.
Internal and external flow can also classify convection. Internal flow occurs when a fluid is enclosed by a solid boundary such when flowing through a pipe. An external flow occurs when a fluid extends indefinitely without encountering a solid surface. Both of these types of convection, either natural or forced, can be internal or external because they are independent of each other. The bulk temperature, or the average fluid temperature, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes and ducts.
For a visual experience of natural convection, a glass filled with hot water and some red food dye may be placed inside a fish tank with cold, clear water. The convection currents of the red liquid may be seen to rise and fall in different regions, then eventually settle, illustrating the process as heat gradients are dissipated.
Heat transfer coefficient of pipe wall
The resistance to the flow of heat by the material of pipe wall can be expressed as a "heat transfer coefficient of the pipe wall". However, one needs to select if the heat flux is based on the pipe inner or the outer diameter.
where k is the effective thermal conductivity of the wall material and x is the wall thickness.
If the above assumption does not hold, then the wall heat transfer coefficient can be calculated using the following expression:
where di and do are the inner and outer diameters of the pipe, respectively.
The thermal conductivity of the tube material usually depends on temperature; the mean thermal conductivity is often used.
Overall heat transfer coefficient
The overall heat transfer coefficient is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in heat exchangers, but can be applied equally well to other problems.
For the case of a heat exchanger, can be used to determine the total heat transfer between the two streams in the heat exchanger by the following relationship:
- = heat transfer rate (W)
- = overall heat transfer coefficient (W/(m²·K))
- = heat transfer surface area (m2)
- = log mean temperature difference (K)
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