Heat transfer

Introduction

Heat transfer is probably the most common unit operation within chemical as well as within food processing industries. Equipment such as sterilizers and pasteurizers in the food industry and incinerators and distillation columns in the chemical industry involve cooling as well as heating of the process fluids. Heating media are hot water or steam and cooling media are tower water, tap water or ice water.

The control of the flow of heat at the desired rate is therefore of prime importance in the design and operation of heat transfer equipment in the processing industries.

Given a temperature difference heat transfer will occur, according to one or more of three different mechanisms.

Heat transfer mechanisms
Heat transfer depends on three different mechanisms:
Calculation of overall heat transfer coefficients

In most heat transferring processes heat passes through a series of different layers before reaching the heat absorbing fluid. The layers are normally of different thicknesses and with different thermal conductivities. In a steam heated pasteurizer, for instance, heat is given away by the condensing steam to the metal wall which in turn gives away the same amount of heat to the laminar sub layer on the process fluid side of the heat exchanger. From the laminar sub layer heat is transferred to the bulk of the fluid.

In some cases deposits are formed, e.g. from the denaturation of proteins in a milk sterilizer, or from burning-on from a chemical process fluid. These deposits contribute to the total heat transfer resistance across the heat exchanger surface.

Consequently, knowledge of the overall heat transfer coefficient is essential in the design of heat transferring equipment:

Calculation of individual heat transfer coefficients

For calculation of the overall heat transfer coefficient knowledge of the individual heat transfer coefficients is necessary. The calculation and estimation of the individual coefficients normally involves some dimensionless numbers, such as the Nusselt number, the Prandtl number, and the Reynolds number:

Heat transfer pioneers

The by far most famous scientists within the field of heat transfer are Osborne Reynolds, 1842 - 1912, Wilhelm Nusselt, 1882 - 1957, Ludwig Prandtl, 1875 - 1953 and Franz Grashof, 1826 - 1893.

Their contributions to the understanding of heat transfer were developed in some cases about 100 years ago but are still valid and widely used in today´s heat transfer calculations.

Heat exchanger calculations

With known overall heat transfer coefficient the required heat transfer area is calculated by an integrated energy balance across the heat exchanger. The integrated value of the local temperature difference between the two fluids is called the logarithmic mean temperature difference.

Heat exchanger types

Heat exchangers are available in a number of various designs. The most common types are the tubular heat exchanger (THE), the plate heat exchanger (PHE), and the scraped surface heat exchanger (SSHE). Depending on application choice of construction materials differ. In the food industry the predominant materials are stainless or acid proof steel or even more exotic materials like titanium, the latter typically for fluids containing chlorides. In other industries heat exchangers made out of mild steel may be sufficient.

Plate heat exchangers are often used on low-viscous applications with moderate demands on operating temperatures and pressures, typically below 150°C and 25 bars. Gasket material is chosen to withstand the operating temperature at hand and the constituents of the processing fluid. In the food industry PHEs are typically used for milk and juice pasteurisers operating at temperatures below 100°C and pressures below 15 bars.

Tubular heat exchangers are used on applications where the demands on high temperatures and pressures are significant. Also, tubular heat exchangers are employed when the fluid contains particles that would block the channels of a plate heat exchanger. In the food industry THEs are typically used for milk and juice sterilisers operating at temperatures up to 150°C. THEs are also used for moderate to high-viscous and particulate products, e.g. tomato salsa sauce, tomato paste and rice puddings. In some of these cases the operating pressure can exceed 100 bars. Particles up to 10 - 15 mm in size can be treated in a THE without problems.

Scraped surface heat exchangers are used on applications where the viscosity is very high, where big lumps are part of the fluid or where fouling problems are severe. In the food industry SSHEs are used e.g. on products like strawberry jam with whole strawberries present. The treatment in the heat exchanger is so gentle and the pressure drop so low that the berries will pass the system with only very little damage. The SSHE is, however, the most expensive solution and is therefore used only when PHEs and THEs would not perform adequately.

Heat transfer literature

A vast number of books within various aspects of heat transfer is available in normal or specialized book-shops and libraries. The below list just shows some examples of various engineering as well as specialized textbooks.

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