Chapter II научно-информационный центр санкт-петербургского государственного технологического университета растительных полимеров heat transfer and steam generation

Boilers, superheaters, economizers, condensers, evaporators, coolers, and heaters are types of equipment that are used to transfer energy from one fluid to another through a metal surface that prevents the fluids from mixing. Since most of this equipment operates at temperatures that are considerably different from room temperature, the equipment and interconnecting piping are insulated to prevent transfer of- energy to or from the atmosphere. The designs of the amount of heat- transfer surface and its arrangement and the selection of the insulation to be applied to the equipment are based on the laws of heat transfer and economics.

Modes of heat transfer

Heat has been defined as energy that is being transferred across the boundaries of a system because of a temperature difference. This transfer may occur through the mechanism of conduction, convection, or radiation, either separately or in combination.

Heat is transferred by conduction through a solid, partly as a result of molecular collisions but primarily as a result of a flow of electrons which is induced by a temperature difference. Metals that are good conductors of electricity are also good conductors of heat. Poor conductors (good insulators) are solids that have low density because of the presence of large numbers of small pores or pockets containing air which reduce to a minimum the cross-sectional area of the solid material through which the electrons may flow. Conduction also occurs in liquids and gases at rest, that is, where there is no motion other than the random

•mot ion of the molecules. Since the energy is transferred as a result of random molecular collisions, the conductivity of liquids and gases is low as compared to the conductivity of solids.

Convection occurs when, either because of a difference in density or because of the operation of a fan or pump, a fluid flows across a hot or cold surface and exchanges energy with that surface. The heated or cooled fluid may then flow to some other region. Since convective heat transmission always involves a flowing fluid, the laws governing heat transfer by convection are closely related to the laws of fluid dynamics.

Radiation involves the transfer of energy through space in the form-of electromagnetic waves that are different from light waves only in their length (frequency). Since radiant energy travels in straight lines with the velocity of light and may be absorbed, reflected, or transmitted by the receiving surface in a manner similar to the action of light, the laws o optics are important in the study of radiant- energy transfer.

In general, a heat exchanger consists of a metal wall through which heat flows from one fluid to another. Heat transfer through the wall follows the laws of

conduction. Heat transfer between the moving fluid and the wall involves convection, in addition to which radiation may be important at high temperatures.

НАУЧНО-ИНФОРМАЦИОННЫЙ ЦЕНТР САНКТ-ПЕТЕРБУРГСКОГО ГОСУДАРСТВЕННОГО ТЕХНОЛОГИЧЕСКОГО УНИВЕРСИТЕТА РАСТИТЕЛЬНЫХ ПОЛИМЕРОВ