HEAT TRANSFER IN BOILERS BASIC INFORMATION AND TUTORIALS


The heat liberated by the combustion of the fuel in the furnace is immediately absorbed, partly in heating the fresh fuel, but mainly by the gaseous products of combustion, causing a rise in their temperatures.

The heat evolved and contained in the gaseous products of combustion is transferred through the gas filled space and then transmitted through the heating plates or tubes into the boiler water. The process of transmission takes place in three distinct ways.

Heat is first imparted to the dry surface of the heating plates or tubes in two ways: first, by radiation from the hot fuel bed, furnace walls, and the flames; and second, by convection from the hot moving gaseous products of combustion.

When the heat reaches the dry surface, it passes through the soot, metal, and scale to the wet surface purely by conduction. From the wet surface of the plate or tube, the heat is carried into the boiler water mainly by convection (but also by some conduction).


The metal of the plate or pipe is covered with a layer of soot on the gas side and a layer of scale on the water side. In addition, a layer of motionless gas is entrapped in the soot while a layer of water and steam adheres to the scale, or, if the boiler is clean, to the metal.

In practically all boilers, only a small portion of the heating surface is so exposed to radiation from the fuel bed, flames, and furnace walls, as to receive heat both by radiation and convection. By far the greater part of the surface receives heat only by convection from the moving gaseous products of combustion.

The greatest resistance to the fl ow of heat, that is, the greatest drop in the temperature of the gases, takes place before the hot gases reach the dry surface of the heating metal plate or pipe. If a boiler is even moderately clean, the resistance of the metal itself to the flow of heat (the drop in temperature) through it is very small.

The resistance to the passage of heat from the metal into the water (loss of temperature) is also very small. Hence, practically all of the heat imparted to the dry surfaces is transmitted to the boiler water. Increasing the rate at which heat is imparted to the dry surface of the heating metal plate or pipe increases the rate of steam production in the same proportion.

If the initial temperature of the moving gases remains constant, an increase in the velocity with which they pass over the heating metal plate or pipe increases, in an almost direct ratio, the rate at which heat is imparted to the dry surface and, therefore, increase almost directly the rate at which steam is produced.

To increase the capacity of any boiler more gases are passed over its heating surfaces. A boiler that has its heating surfaces so arranged that the gas passage are long and of small cross-section is more efficient than a boiler in which the gas passages are short and of large cross-section.

To increase the efficiency of water tube boilers, the pipes are bent to increase their length and baffles are inserted in such a way that the heating surfaces are arranged in series with reference to the gas flow, thus making the gas passage longer.

Boilers are rated in boiler-horsepower (BHP), one BHP comprising 10 square feet of the boiler heating surfaces; for example, a boiler with 5000 square feet of heating surface would be rated at 500 BHP.

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1 comment:

albina N muro said...

The heat liberated by the combustion of the fuel in the furnace is immediately absorbed, partly in heating the fresh fuel, but mainly by the gaseous products of combustion, causing a rise in their temperatures. heating engineer Birmingham

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