Heat and Flow

This is the most scientific chapter in the entire book. Combustion, heat transfer, and fluid flow are the subjects on which boiler technology is built. These disciplines involve myriad applications. Only certain facets of these subjects are applicable to boilers. This chapter tries to capture those relevant portions for a quick reference and recapitulation. Many figures and tables are included for reference and preliminary calculations. How­ever, for detailed calculations of boiler designers a much larger body of data is needed. Readers in search of more theoretical background should refer to books on specific subjects.

The thermal design of a boiler is largely based on the subjects covered in this chap­ter. Computerization of thermal calculations has greatly aided in enhancing the speed, accuracy, repeatability, complexity, and volume of calculations. Because of the facility of computer-aided calculation, many alternatives with several fuels at different loads are routinely worked out by boiler designers with no appreciable fatigue. While the modern designer is an expert user of the programs and an innovative interpreter of results, he or she is far removed from the fundamentals. This chapter seeks to remedy this disconnect by providing the theoretical background succinctly.

The same principle applies to practicing boiler engineers.

In this chapter the following topics are covered as briefly as possible with an intent to provide a ready refresher material for a practicing engineer. All the key words are itali­cized for emphasis.

1. Steam and water properties

2. Heat transfer

3. Fluid flow

4. Circulation

5. Combustion

6. Thermodynamic cycles

Steam and Water Properties

Water, along with its vapor phase—steam, is the working fluid of the boiler. When subcooled water is heated at a certain pressure in a closed space, its temperature rises until it reaches its boiling point and becomes saturated water. (Water below the boiling temperature is called subcooled water.) Further addition of heat at that pressure only produces steam bubbles that escape into the space above, but the temperature does not increase. This process is termed boiling, which is the addition of heat at constant pressure and constant tempera­ture. This heat, called latent heat, turns out progressively more steam, and the dryness of the wet steam keeps increasing until all the water is converted into steam. When the dryness reaches 100%, it becomes saturated steam. Further addition of heat raises the steam tempera­ture to make superheated steam. These stages are shown in Figure 2.1.

The latent heat progressively reduces with the increase in steam pressures until it becomes zero at the critical point. At this pressure of 221.2 bar/225.5 kg/cm2 abs (3206.2 psia) with the corresponding steam temperature of 374.1°C (705.4°F), the water turns into steam directly without the intermediate stage of evaporation. These values are based on ASME steam tables, which can be at a fractional variance from other tables. These are criti­cal pressure and critical temperature, and higher conditions are supercritical (SC) conditions. In the industry it is normal to refer to conditions >250 bar as ultra-supercritical.

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