30079_45a.pdf

CHAPTER 45

FURNACES

Carroll Cone

Toledo, Ohio

45.1 SCOPE AND INTENT 14 50

45.9 FLUID FLOW 1485

45.9.1 Preferred Velocities 1485

45.9.2 Centrifugal Fan

Characteristics 1486

45.9.3 Laminar and Turbulent

Flows

45.2 STANDARD CONDITIONS 14 50

45.2.1 Probable Errors 14 50

1487

45.3 FURNACE TYPES 14 50

45.4 FURNACE CONSTRUCTION 1453

45.10 BURNER AND CONTROL

EQUIPMENT 1488

45.10.1 Burner Types 1489

45.10.2 Burner Ports 1494

45.10.3 Combustion Control

Equipment 1494

45.10.4 Air Pollution Control 1496

45.5 FUELS AND COMBUSTION 1454

45.6 OXYGEN ENRICHMENT OF

COMBUSTION AIR

14 59

45.7 THERMAL PROPERTIES OF

MATERIALS 14 60

45.8 HEAT TRANSFER 14 62

45.8.1 Solid-State Radiation 14 64

45.8.2 Emissivity-Absorptivity 1465

45.8.3 Radiation Charts 14 65

45.8.4 View Factors for

Solid-State Radiation 1465

45.8.5 Gas Radiation 14 66

45.8.6 Evaluation of Mean

Emissivity-Absorptivity 1 47 1

45.8.7 Combined Radiation

Factors 14 72

45.8.8 Steady-State Conduction 1472

45.8.9 Non-Steady-State

Conduction

45.11 WASTE HEAT RECOVERY

SYSTEMS

1496

45 . 1 1 . 1 Regenerative Air

Preheating 1496

45. 1 1 .2 Recuperator Systems 1497

45 . 1 1 . 3 Recuperator

Combinations 1498

45.12 FURNACE COMPONENTS IN

COMPLEX THERMAL

PROCESSES

1499

45.13 FURNACE CAPACITY 1501

45.14 FURNACE TEMPERATURE

PROFILES

14 74

1501

45.8.10 Heat Transfer with

Negligible Load Thermal

Resistance 14 77

45.8. 1 1 Newman Method 14 77

45.8.12 Furnace Temperature

Profiles

45.15 REPRESENTATIVE HEATING

RATES

1501

45.16 SELECTING NUMBER OF

FURNACE MODULES 1502

14 79

45.8.13 Equivalent Furnace

Temperature Profiles 14 80

45.8.14 Convection Heat

Transfer

45.17 FURNACE ECONOMICS 1502

45.17.1 Operating Schedule 1503

45.17.2 Investment in

Fuel-Saving

Improvements 1503

14 81

45.8.15 Fluidized-Bed Heat

Transfer 14 83

45.8.16 Combined Heat-Transfer

Coefficients

14 83

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

45.1 SCOPE AND INTENT

This chapter has been prepared for the use of engineers with access to an electronic calculator and

to standard engineering reference books, but not necessarily to a computer terminal. The intent is to

provide information needed for the solution of furnace engineering problems in areas of design,

performance analysis, construction and operating cost estimates, and improvement programs.

In selecting charts and formulas for problem solutions, some allowance has been made for prob-

able error, where errors in calculations will be minor compared with errors in the assumptions on

which calculations are based. Conscientious engineers are inclined to carry calculations to a far greater

degree of accuracy than can be justified by probable errors in data assumed. Approximations have

accordingly been allowed to save time and effort without adding to probable margins for error. The

symbols and abbreviations used in this chapter are given in Table 45.1.

45.2 STANDARD CONDITIONS

Assuming that the user will be using English rather than metric units, calculations have been based

on pounds, feet, Btu's, and degrees Fahrenheit, with conversion to metric units provided in the

following text (see Table 45.2).

Assumed standard conditions include: ambient temperature for initial temperature of loads, for

heat losses from furnace walls or open cooling of furnace loads—70°F.

Condition of air entering system for combustion or convection cooling: temperature, 70°F; ab-

solute pressure, 14.7 psia; relative humidity, 60% at 70°F, for a water vapor content of about 1.4%

by volume.

45.2.1 Probable Errors

Conscientious furnace engineers are inclined to carry calculations to a far greater degree of accuracy

than can be justified by uncertainties in basic assumptions such as thermal properties of materials,

system temperatures and pressures, radiation view factors and convection coefficients. Calculation

procedures recommended in this chapter will, accordingly, include some approximations, identified

in the text, that will result in probable errors much smaller than those introduced by basic assump-

tions, where such approximations will expedite problem solutions.

45.3 FURNACE TYPES

Furnaces may be grouped into two general types:

1. As a source of energy to be used elsewhere, as in firing steam boilers to supply process

steam, or steam for electric power generation, or for space heating of buildings or open space

2. As a source of energy for industrial processes, other than for electric power

The primary concern of this chapter will be the design, operation, and economics of industrial

furnaces, which may be classified in several ways:

By function:

Heating for forming in solid state (rolling, forging)

Melting metals or glass

Heat treatment to improve physical properties

Preheating for high-temperature coating processes, galvanizing, vitreous enameling, other coatings

Smelting for reduction of metallic ores

Firing of ceramic materials

Incineration

By method of load handling:

Batch furnaces for cyclic heating, including forge furnaces arranged to heat one end of a bar or

billet inserted through a wall opening, side door, stationary-hearth-type car bottom designs

Continuous furnaces with loads pushed through or carried by a conveyor

Tilting-type furnace

To avoid the problem of door warpage or leakage in large batch-type furnaces, the furnace can

be a refractory-lined box with an associated firing system, mounted above a stationary hearth, and

arranged to be tilted around one edge of the hearth for loading and unloading by manual handling,

forklift trucks, or overhead crane manipulators.

Table 45.1 Symbols and Abbreviations

A area in ft2

a absorptivity for radiation, as fraction of black body factor for receiver temperature:

ag combustion gases

aw furnace walls

as load surface

am combined emissivity-absorptivity factor for source and receiver

C specific heat in Btu/lb • °F or cal/g • °C

cfm cubic feet per minute

D diameter in ft or thermal diffusivity (k/dC)

d density in lb/ft3

e emissivity for radiation as fraction of black-body factor for source temperature, with

subscripts as for a above

F factor in equations as defined in text

fpm velocity in ft/min

G mass velocity in lb/ft2 • hr

g acceleration by gravity (32.16 ft/sec2)

H heat-transfer coefficient (Btu/hr • ft2 • °F)

Hr for radiation

Hc for convection

Ht for combined Hr + Hc

HHV higher heating value of fuel

h pressure head in units as defined

k thermal conductivity (Btu/hr • ft • °F)

L length in ft, as in effective beam length for radiation, decimal rather than feet and inches

LHV lower heating value of fuel

In logarithm to base e

MTD log mean temperature difference

a constant as defined in text

psi

pressure in lb/in2

psig, pressure above atmospheric

psia, absolute pressure

Prandtl number (jxC/A:)

heat flux in Btu/hr

thermal resistance (r/k) or ratio of external to internal thermal resistance (k/rH)

Reynolds number (DGI\L)

radius or depth of heat penetration in ft

temperature in °F, except for radiation calculations where °S = (°F + 460) II00

Tg, combustion gas temperature

jTw, furnace wall temperature

Ts, heated load surface

Tc, core or unheated surface of load

time in hr

IJL

viscosity in Ib/hr • ft

inches of water column as a measure of pressure

volume in ft3

velocity in ft/sec

weight in Ib

time factor for nonsteady heat transfer (tD/r2)

horizontal coordinate

vertical coordinate

coordinate perpendicular to plane xy

For handling heavy loads by overhead crane, without door problems, the furnace can be a portable

cover unit with integral firing and temperature control. Consider a cover-type furnace for annealing

steel strip coils in a controlled atmosphere. The load is a stack of coils with a common vertical axis,

surrounded by a protective inner cover and an external heating cover. To improve heat transfer parallel

to coil laminations, they are loaded with open coil separators between them, with heat transferred

from the inner cover to coil ends by a recirculating fan. To start the cooling cycle, the heating cover

Table 45.2 Conversion of Metric to English Units

Length

1 m - 3.281 ft

1 cm - 0.394 in

1 m2 - 10.765 ft2

1 m3 - 35.32 ft3

1 kg = 2.205 Ib

1 g/cm3 - 62.43 lb/ft2

1 g/cm2 = 2.048 lb/ft2 - 0.0142 psi

1 kcal - 3.968 Btu

1 kwh - 3413 Btu

1 cal/g - 1.8 Btu/lb

1 kcal/m2 - 0.1123 Btu/ft3

1 W/cm2 - 3170 Btu/hr • ft2

1 cal 242 Btu

sec cm °C hr ft °F

1 cal 7373 Btu

sec cm2 °C hr ft2 °F

1 cal/sec • cm • °C 3.874 Btu/hr • ft • °F

C • g/cm3

Area

Volume

Weight

Density

Pressure

Heat

Heat content

Heat flux

Thermal conductivity

Heat transfer

Thermal diffusivity

C • lb/ft3

is removed by an overhead crane, while atmosphere circulation by the base fan continues. Cooling

may be enhanced by air-blast cooling of the inner cover surface.

For heating heavy loads of other types, such as weldments, castings, or forgings, car bottom

furnaces may be used with some associated door maintenance problems. The furnace hearth is a

movable car, to allow load handling by an overhead traveling crane. In one type of furnace, the door

is suspended from a lifting mechanism. To avoid interference with an overhead crane, and to achieve

some economy in construction, the door may be mounted on one end of the car and opened as the

car is withdrawn. This arrangement may impose some handicaps in access for loading and unloading.

Loads such as steel ingots can be heated in pit-type furnaces, preferably with units of load

separated to allow radiating heating from all sides except the bottom. Such a furnace would have a

cover displaced by a mechanical carriage and would have a compound metal and refractory recu-

perator arrangement. Loads are handled by overhead crane equipped with suitable gripping tongs.

Continuous-Type Furnaces

The simplest type of continuous furnace is the hearth-type pusher furnace. Pieces of rectangular cross

section are loaded side by side on a charge table and pushed through the furnace by an external

mechanism. In the design shown, the furnace is fired from one end, counterflow to load travel, and

is discharged through a side door by an auxiliary pusher lined up by the operator.

Furnace length is limited by thickness of the load and alignment of abutting edges, to avoid

buckling up from the hearth.

A more complex design would provide multiple zone firing above and below the hearth, with

recuperative air preheating.

Long loads can be conveyed in the direction of their length in a roller-hearth-type furnace. Loads

can be bars, tubes, or plates of limited width, heated by direct firing, by radiant tubes, or by electric-

resistor-controlled atmosphere, and conveyed at uniform speed or at alternating high and low speeds

for quenching in line.

Sequential heat treatment can be accomplished with a series of chain or belt conveyors. Small

parts can be loaded through an atmosphere seal, heated in a controlled atmosphere on a chain belt

conveyor, discharged into an oil quench, and conveyed through a washer and tempering furnace by

a series of mesh belts without intermediate handling.

Except for pusher-type furnaces, continuous furnaces can be self-emptying. To secure the same

advantage in heating slabs or billets for rolling and to avoid scale loss during interrupted operation,

loads can be conveyed by a walking-beam mechanism. Such a walking-beam-type slab heating fur-

nace would have loads supported on water-cooled rails for over- and underfiring, and would have an

overhead recuperator.

Thin strip materials, joined in continuous strand form, can be conveyed horizontally or the strands

can be conveyed in a series of vertical passes by driven support rolls. Furnaces of this type can be

incorporated in continuous galvanizing lines.

Unit loads can be individually suspended from an overhead conveyor, through a slot in the furnace

roof, and can be quenched in line by lowering a section of the conveyor.

Small parts or bulk materials can be conveyed by a moving hearth, as in the rotary-hearth-type

or tunnel kiln furnace. For roasting or incineration of bulk materials, the shaft-type furnace provides

a simple and efficient system. Loads are charged through the open top of the shaft and descend by

gravity to a discharge feeder at the bottom. Combustion air can be introduced at the bottom of the

furnace and preheated by contact with the descending load before entering the combustion zone,

where fuel is introduced through sidewalls. Combustion gases are then cooled by contact with the

descending load, above the combustion zone, to preheat the charge and reduce flue gas temperature.

With loads that tend to agglomerate under heat and pressure, as in some ore-roasting operations,

the rotary kiln may be preferable to the shaft-type furnace. The load is advanced by rolling inside

an inclined cylinder. Rotary kilns are in general use for sintering ceramic materials.

Classification by Source of Heat

The classification of furnaces by source of heat is as follows:

Direct-firing with gas or oil fuels

Combustion of material in process, as by incineration with or without supplemental fuel

Internal heating by electrical resistance or induction in conductors, or dielectric heating of

nonconductors

Radiation from electric resistors or radiant tubes, in controlled atmospheres or under vacuum

45.4 FURNACE CONSTRUCTION

The modern industrial furnace design has evolved from a rectangular or cylindrical enclosure, built

up of refractory shapes and held together by a structural steel binding. Combustion air was drawn

in through wall openings by furnace draft, and fuel was introduced through the same openings without

control of fuel/air ratios except by the judgment of the furnace operator. Flue gases were exhausted

through an adjacent stack to provide the required furnace draft.

To reduce air infiltration or outward leakage of combustion gases, steel plate casings have been

added. Fuel economy has been improved by burner designs providing some control of fuel/air ratios,

and automatic controls have been added for furnace temperature and furnace pressure. Completely

sealed furnace enclosures may be required for controlled atmosphere operation, or where outward

leakage of carbon monoxide could be an operating hazard.

With the steadily increasing costs of heat energy, wall structures are being improved to reduce

heat losses or heat demands for cyclic heating. The selection of furnace designs and materials should

be aimed at a minimum overall cost of construction, maintenance, and fuel or power over a projected

service life. Heat losses in existing furnaces can be reduced by adding external insulation or rebuilding

walls with materials of lower thermal conductivity. To reduce losses from intermittent operation, the

existing wall structure can be lined with a material of low heat storage and low conductivity, to

substantially reduce mean wall temperatures for steady operation and cooling rates after interrupted

firing.

Thermal expansion of furnace structures must be considered in design. Furnace walls have been

traditionally built up of prefired refractory shapes with bonded mortar joints. Except for small fur-

naces, expansion joints will be required to accommodate thermal expansion. In sprung arches, lateral

expansion can be accommodated by vertical displacement, with longitudinal expansion taken care of

by lateral slots at intervals in the length of the furnace. Where expansion slots in furnace floors could

be filled by scale, slag, or other debris, they can be packed with a ceramic fiber that will remain

resilient after repeated heating.

Differential expansion of hotter and colder wall surfaces can cause an inward-bulging effect. For

stability in self-supporting walls, thickness must not be less than a critical fraction of height.

Because of these and economic factors, cast or rammed refractories are replacing prefired shapes

for lining many types of large, high-temperature furnaces. Walls can be retained by spaced refractory

shapes anchored to the furnace casing, permitting reduced thickness as compared to brick construc-

tion. Furnace roofs can be suspended by hanger tile at closer spacing, allowing unlimited widths.

Cast or rammed refractories, fired in place, will develop discontinuities during initial shrinkage

that can provide for expansion from subsequent heating, to eliminate the need for expansion joints.

As an alternate to cast or rammed construction, insulating refractory linings can be gunned in

place by jets of compressed air and retained by spaced metal anchors, a construction increasingly

popular for stacks and flues.

Thermal expansion of steel furnace casings and bindings must also be considered. Where the

furnace casing is constructed in sections, with overlapping expansion joints, individual sections can

be separately anchored to building floors or foundations. For gas-tight casings, as required for con-

trolled atmosphere heating, the steel structure can be anchored at one point and left free to expand

elsewhere. In a continuous galvanizing line, for example, the atmosphere furnace and cooling zone

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: