THERMODYNAMICS OF BOILER

Thermodynamics of Boiler

Thermodynamics of Boiler

Introduction

Boiler is a device for generating steam. It consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. A steam engine is driven by steam generated under pressure in a boiler. The amount of steam that can be generated per hour depends upon the rate of combustion of the fuel in the furnace and upon the efficiency of heat transfer to the boiler proper. Since the rate of combustion of the fuel in a furnace is largely dependent upon the quantity of air available, i.e., upon the draft, a sufficient supply of air is an important consideration in boiler construction. In some large installations the incoming air is preheated by the waste heat of the flue gases, and in order to increase the speed of combustion a forced draft (air at higher than atmospheric pressure) is often used. Two types of boilers are most common—fire-tube boilers, containing long steel tubes through which the hot gases from the furnace pass and around which the water to be changed to steam circulates, and water-tube boilers, in which the conditions are reversed. Water is changed to steam in these continuous circuits and also is super-heated in transit. This additional heating of the steam increases the efficiency of the power-generating cycle. The safety valve is used to prevent explosions by releasing steam if the pressure becomes too great. The construction of boilers in the United States is governed by the American Society of Mechanical Engineers' Boiler Construction Code. Progress in boiler design and performance have been governed by the continuous development of improved materials.

Discussion

Heat recovery is a general term used to denote the process of utilizing some of the heat energy contained in the output of a process or an effluent and generally indicates that the heat would be otherwise unused and dissipated into the immediate environment. The prevalence of heat processes in energy systems, such as for space heating in the domestic sector, for generating motive power in the transport sector, and for producing electricity for use in all sectors, means that heat recovery has a wide area of potential application. In general, energy savings of up to 30 percent can be possible through heat recovery measures. Sources of waste heat are ubiquitous because the heat loss is inevitable, but waste heat is not always suitable for heat recovery. There are various technological options available for heat recovery, although economic or technical constraints sometimes preclude their successful application.

In many heat- and/or electricity-generating processes, the heat available for recovery is a by-product of the process itself; the heat demand of the process is met, and any excess or waste heat is exhausted from the process. The laws of thermodynamics indicate that heat transfer will naturally occur from higher to lower temperatures. The temperature of a process's waste heat is thus inevitably lower than the temperature of the process ...