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.4ransfer of energy to or from the atmosphere. The design 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.

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,[10] that is, where there is no motion other than the random motion 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 о 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.

Steam is used for space heating, in manufacturing processes, and for power generation. Except for hydroelectric power plants, practically all the central-station generating capacity is in the form of steam turbines. Because of the mag­nitude of the load and the economies that .are effected through the use of the smallest possible number of largest machines, most central-station turbines now being built are in the size range of 1000,000 to 600,000 kw. It is standard practice to install one steam-generating unit per turbine. Consequently, these, turbines require steam-generating units in the capacity range of 750,000 to over 3,000,000 lb of steam per hr.

The steam boiler is a pressure vessel in which feedwater can be converted into saturated steam of high quality at some desired pressure. When other heat-transfer surfaces such as superheater, air heater, or economizer surfaces are combined with boiler surface into a unified installation, the name steam-generating unit is applied to the complete unit.

Boilers in which the water is inside the tubes are called water-tube boilers, whereas boilers that have the hot products of combustion in the tubes and the water outside the tubes are called fire-tube boilers. Boiler heating surface is defined as that surface which receives heat from the flame or hot gases and is in contact with water. The area is based on the surface receiving the heat, that is, the outside area5 of water tubes and the inside area of fire tubes.