Firebox


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firebox

[′fīr‚bäks]
(mechanical engineering)
The furnace of a locomotive or similar type of fire-tube boiler.

Firebox

 

(or combustion chamber; the term “furnace” is used in some applications), a device for the combustion of fossil fuel in order to produce hot flue gases; the heat of the gases is converted into electricity or mechanical energy or is used in technological processes.

In the general case, a firebox is a chamber into which solid, liquid, or gaseous fuel is introduced along with an oxidant, usually air. In boiler units, the combustion products release their heat to a heat-transfer agent, such as water or steam, which circulates through pipes located in the walls of the firebox. In furnace fireboxes, the heat of the flue gases is used in the working chamber of the furnace for the heat treatment of materials or articles or for heating.

The limiting temperature Ta of the flue gases in a firebox—the theoretical combustion temperature, or heat value of the fuel—is determined from the formula

where QT is the heat of combustion of the fuel, α is the excess air ratio, L0 is the theoretically required air consumption, and cT is the average heat capacity of the flue gases. The actual temperature in a firebox is lower than Ta because of heat losses from chemically incomplete combustion of fuel and the external radiation of the firebox chamber. The combustion temperature may be raised by preheating the air or fuel. For more complete fuel utilization, the process is carried out with an excess of air—that is, the amount of air actually introduced into the furnace is greater than the amount theoretically required for combustion. Combustion is accelerated by enrichment of the air with oxygen.

The main characteristics that determine the efficiency and economy of firebox operation are the forcing, or heat stress of a cross section of the furnace, Q/F, where Q is the heat released by complete combustion of the fuel and F is the cross-sectional area (for a laminar combustion firebox, F is the surface area of the burning fuel bed), and also the heat stress Q/V of the heating space, where V is the volume of the chamber.

Three main groups of fireboxes—laminar combustion fireboxes and air-jet and vortex furnaces—are distinguished according to design. The first boiler-unit fireboxes were of the laminar combustion type, which were designed for the combustion of solid fuel in a bed. Laminar combustion fireboxes were long the main devices for the combustion of large amounts of fuel and were commonly used for boilers with a steam output of 20–30 tons/hr. In the late 1920’s, a firebox was developed for the air-jet combustion of pulverized solid fuel, which made possible the use of low-quality fuel with good reliability and economy and a significant increase in the unit output of boiler units. Prior to introduction into an air-jet furnace, the fuel is purified, ground, and dried in a pulverized-fuel preparation system (seePULVERIZED-FUEL FURNACE). Airjet furnaces are very convenient for the combustion of gaseous and liquid fuel; gaseous fuel does not require pre-treatment, whereas liquid fuel must be atomized in nozzles.

Vortex, or cyclone, furnaces became common in the 1950’s. In this type of firebox, solid fuel with a particle size of up to several dozen millimeters is almost completely burned in a precombustion chamber, where a gas-air vortex is created. Air-jet and vortex furnaces form the general class of chamber furnaces; they are used in boiler systems with moderate and high steam output (2,000 tons/hr and higher).

In pulverized-fuel furnaces, in contrast to gas and mazut furnaces, the temperature of the combustion products must be kept lower than the melting point of the slag; this is done to avoid the formation of slag on the convective heating surfaces. This is accomplished by completely covering the walls of the furnace with furnace shields. Chimneys and exhaust fans are used to remove gaseous combustion products from fireboxes. If the firebox has gastight shielding, the movement of the flue gases is provided by fans, as in supercharged boiler units; in this case, the furnace chamber is under a pressure of 3–5 kilonewtons per sq m (kN/m2), or 0.03–0.05 kilograms-force per sq cm (kgf/cm2). Significantly higher pressures—0.6–2.5 MN/m2 (6–25 kgf/cm2)—are used in high-pressure steam generators of steam-gas turbine installations.

The main characteristics of fireboxes and furnaces (1975) are presented in Table 1.

REFERENCES

Knorre, G. F. Topochnye protsessy, 2nd ed. Moscow-Leningrad, 1959.
Marshak, lu. L. Topochnye ustroistva s vertikalnymi tsiklonnymi predtopkami. Moscow-Leningrad, 1966.
Murzakov, V. V. Osnovy teorii i praktiki szhiganiia gaza v parovykh kotlakh, 2nd ed. Moscow, 1969.
Speisher, V. A., and A. D. Gorbanenko. Povyshenie effektivnosti is-pol’zovaniia gaza i mazuta v energeticheskikh ustanovkakh. Moscow, 1974.

I. N. ROZENGAUZ

Table 1. Main characteristics of fireboxes and furnaces for boiler units with steam output of at least 75 tons/hr
ClassTypeFuelExcess air ratioUndercombustion (percent)Forcing Q/F Gcal/(m2-hr)Heat stress of firebox chamber Q/V Mcal/(m3-hr)
* Together with afterburner in the studded zone
Laminar combustionWith pneumatic spreader and stationary gratePoorly caking coals1.45.50.8–1200–300
 With chain grateHigh-grade anthracite1.5100.8–1250–400
 Shaft-chainLump peat1.331.5–1.9250–400
  Coal1.21–1.52–2.5150
Air-jet. . . . . . . . . .With burners and drydeslaggingAnthracite1 .2–1 .254.62–2.5120
  Mazut1.030.52–2.5250
  Natural gas1.10.52–2.5300–400
 With shaft millsBrown coal1.20.5–12–2.5160
  Milled peat1.20.5–12–2.5140
 Slag-tapCoal1.20.5Up to 800
  Crushed coal1.1–1.21.512–141,100*
Vortex. . . . . . . . . .With horizontal cyclonesCoarse pulverized coal1.1–1.21.510–121,100*
 With precombustion chambers designed by the All-Union Heat Engineering InstituteCoarse pulverized coal1.1–1.20.516650–750*
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