![Industrial furnace](https://upload.wikimedia.org/wikipedia/commons/7/76/Industrial_furnace.jpg)
An industrial furnace, also known as a direct heater or a direct fired heater, is a device used to provide heat for an industrial process, typically higher than 400 degrees Celsius.
They are used to provide heat for a process or can serve as
reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. Heat is generated by an industrial furnace by mixing fuel with air or oxygen, or from
electrical energy. The residual heat will exit the furnace as
flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produc ...
.
These are designed as per international codes and standards the most common of which are ISO 13705 (Petroleum and natural gas industries — Fired heaters for general refinery service) /
American Petroleum Institute
The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in production, refinement, distribution, and many other aspects of the ...
(API) Standard 560 (Fired Heater for General Refinery Service). Types of industrial furnaces include
batch ovens,
vacuum furnace
A vacuum furnace is a type of furnace in which the product in the furnace is surrounded by a vacuum during processing. The absence of air or other gases prevents oxidation, heat loss from the product through convection, and removes a source of co ...
s, and
solar furnace
A solar furnace is a structure that uses concentrated solar power to produce high temperatures, usually for industry. Parabolic mirrors or heliostats concentrate light ( Insolation) onto a focal point. The temperature at the focal point may ...
s. Industrial furnaces are used in applications such as
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s,
cremation
Cremation is a method of final disposition of a dead body through burning.
Cremation may serve as a funeral or post-funeral rite and as an alternative to burial. In some countries, including India and Nepal, cremation on an open-air pyre is ...
,
oil refining, and
glasswork.
Overview
![furnace2-en](https://upload.wikimedia.org/wikipedia/commons/1/1a/Furnace2-en.svg)
Fuel flows into the
burner
Burner may refer to:
* Gas burner, coal burner or oil burner, a mechanical device that burns a gas or liquid fuel in a controlled manner
** Laboratory gas burners:
*** Bunsen burner
*** Meker–Fisher burner
*** Teclu burner
** Hot-air balloon de ...
and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section or
firebox
Firebox may refer to:
*Firebox (steam engine), the area where the fuel is burned in a steam engine
*Firebox (architecture), the part of a fireplace where fuel is combusted
*Firebox Records
Firebox Records was a Finnish record label based in S ...
. In this chamber where combustion takes place, the heat is transferred mainly by
radiation to tubes around the
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
At a certain point in the combustion reaction, called the ignition point, flames a ...
in the chamber.
The fluid to be heated passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as
flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produc ...
. After the flue gas leaves the firebox, most furnace designs include a
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
section where more heat is recovered before venting to the
atmosphere through the
flue gas stack
A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue g ...
. (HTF=Heat Transfer Fluid. Industries also use their furnaces to heat a secondary fluid with special additives like anti-
rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
and high heat transfer efficiency. This heated fluid is then circulated round the whole plant to heat exchangers to be used wherever heat is needed instead of directly heating the product line as the product or material may be volatile or prone to
cracking at the furnace temperature.)
Components
Radiant section
The radiant section is where the tubes receive almost all its heat by
radiation from the flame. In a vertical, cylindrical furnace, the tubes are vertical. Tubes can be vertical or horizontal, placed along the
refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
wall, in the middle, etc., or arranged in cells. Studs are used to hold the
insulation together and on the wall of the furnace. They are placed about 1 ft (300 mm) apart in this picture of the inside of a furnace.
The tubes, shown below, which are reddish brown from
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
, are
carbon steel tubes and run the height of the radiant section. The tubes are a distance away from the insulation so radiation can be reflected to the back of the tubes to maintain a uniform tube wall temperature. Tube guides at the top, middle and bottom hold the tubes in place.
Convection section
The convection section is located above the radiant section where it is cooler to recover additional heat.
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
takes place by
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
here, and the tubes are finned to increase heat transfer. The first three tube rows in the bottom of the convection section and at the top of the radiant section is an area of bare tubes (without fins) and are known as the shield section ("shock tubes"), so named because they are still exposed to plenty of radiation from the firebox and they also act to shield the convection section tubes, which are normally of less resistant material from the high temperatures in the firebox.
The area of the radiant section just before flue gas enters the shield section and into the convection section called the bridgezone. A crossover is the tube that connects from the convection section outlet to the radiant section inlet. The crossover piping is normally located outside so that the temperature can be monitored and the efficiency of the convection section can be calculated. The sightglass at the top allows personnel to see the flame shape and pattern from above and visually inspect if flame impingement is occurring. Flame impingement happens when the flame touches the tubes and causes small isolated spots of very high temperature.
Radiant coil
This is a series of tubes horizontal/ vertical hairpin type connected at ends (with 180° bends) or helical in construction. The radiant coil absorbs heat through radiation. They can be single pass or multi pass depending upon the process-side pressure drop allowed. The radiant coils and bends are housed in the radiant box. Radiant coil materials vary from carbon steel for low temperature services to high alloy steels for high temperature services. These are supported from the radiant side walls or hanging from the radiant roof. Material of these supports is generally high alloy steel. While designing the radiant coil, care is taken so that provision for expansion (in hot conditions) is kept.
Burner
![Furnace Burner labeled](https://upload.wikimedia.org/wikipedia/commons/f/f1/Furnace_Burner_labeled.jpg)
The burner in the vertical, cylindrical furnace as above, is located in the floor and fires upward. Some furnaces have side fired burners, such as in train
locomotives
A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the ...
. The burner
tile
Tiles are usually thin, square or rectangular coverings manufactured from hard-wearing material such as ceramic, stone, metal, baked clay, or even glass. They are generally fixed in place in an array to cover roofs, floors, walls, edges, or o ...
is made of high temperature refractory and is where the flame is contained. Air registers located below the burner and at the outlet of the air blower are devices with movable flaps or vanes that control the shape and pattern of the flame, whether it spreads out or even swirls around. Flames should not spread out too much, as this will cause flame impingement. Air registers can be classified as primary, secondary and if applicable, tertiary, depending on when their air is introduced.
The primary air register supplies primary air, which is the first to be introduced in the burner. Secondary air is added to supplement primary air. Burners may include a pre-mixer to mix the air and fuel for better combustion before introducing into the burner. Some burners even use steam as premix to preheat the air and create better mixing of the fuel and heated air. The floor of the furnace is mostly made of a different material from that of the wall, typically hard castable refractory to allow technicians to walk on its floor during maintenance.
A furnace can be lit by a small
pilot flame
Merker Tankless water heating, tankless gas-fired water heater from the 1930s, with pilot light clearly visible through the aperture in the front cover. The large opening allowed for the manual lighting of the pilot light by a lit match or taper
...
or in some older models, by hand. Most pilot flames nowadays are lit by an ignition transformer (much like a car's spark plugs). The pilot flame in turn lights up the main flame. The pilot flame uses natural gas while the main flame can use both
diesel
Diesel may refer to:
* Diesel engine, an internal combustion engine where ignition is caused by compression
* Diesel fuel, a liquid fuel used in diesel engines
* Diesel locomotive, a railway locomotive in which the prime mover is a diesel engin ...
and natural gas. When using liquid fuels, an atomizer is used, otherwise, the liquid fuel will simply pour onto the furnace floor and become a hazard. Using a pilot flame for lighting the furnace increases safety and ease compared to using a manual ignition method (like a match).
Sootblower
Sootblowers are found in the convection section. As this section is above the radiant section and air movement is slower because of the fins,
soot
Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...
tends to accumulate here. Sootblowing is normally done when the efficiency of the convection section is decreased. This can be calculated by looking at the temperature change from the crossover piping and at the convection section exit.
Sootblowers utilize flowing media such as water, air or steam to remove deposits from the tubes. This is typically done during maintenance with the air blower turned on. There are several different types of sootblowers used. Wall blowers of the rotary type are mounted on furnace walls protruding between the convection tubes. The
lance
A lance is a spear designed to be used by a mounted warrior or cavalry soldier (lancer). In ancient and medieval warfare, it evolved into the leading weapon in cavalry charges, and was unsuited for throwing or for repeated thrusting, unlike s ...
s are connected to a steam source with holes drilled into it at intervals along its length. When it is turned on, it rotates and blows the soot off the tubes and out through the stack.
Stack
![Stack damper mechanism](https://upload.wikimedia.org/wikipedia/commons/7/7f/Stack_damper_mechanism.svg)
The
flue gas stack
A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue g ...
is a cylindrical structure at the top of all the heat transfer chambers. The
breeching directly below it collects the flue gas and brings it up high into the atmosphere where it will not endanger personnel.
The stack
damper
A damper is a device that deadens, restrains, or depresses. It may refer to:
Music
* Damper pedal, a device that mutes musical tones, particularly in stringed instruments
* A mute for various brass instruments
Structure
* Damper (flow), a mechan ...
contained within works like a
butterfly valve
A butterfly valve is a valve that isolates or regulates the flow of a fluid. The closing mechanism is a disk that rotates.
Principle of operation
Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves ...
and regulates
draft
Draft, The Draft, or Draught may refer to:
Watercraft dimensions
* Draft (hull), the distance from waterline to keel of a vessel
* Draft (sail), degree of curvature in a sail
* Air draft, distance from waterline to the highest point on a vesse ...
(pressure difference between air intake and air exit) in the furnace, which is what pulls the flue gas through the convection section. The stack damper also regulates the heat lost through the stack. As the damper closes, the amount of heat escaping the furnace through the stack decreases, but the
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
or draft in the furnace increases which poses risks to those working around it if there are air leakages in the furnace, the flames can then escape out of the firebox or even explode if the pressure is too great.
Insulation
Insulation is an important part of the furnace because it improves efficiency by minimizing heat escape from the heated chamber. Refractory materials such as
firebrick
A fire brick, firebrick, or refractory is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal con ...
, castable refractories and
ceramic fibre, are used for insulation. The floor of the furnace are normally castable type refractories while those on the walls are nailed or glued in place. Ceramic fibre is commonly used for the roof and wall of the furnace and is graded by its
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
and then its maximum temperature rating. For example, 8# 2,300 °F means 8 lb/ft
3 density with a maximum temperature rating of 2,300 °F. The actual service temperature rating for ceramic fiber is a bit lower than the maximum rated temperature. (i.e. 2300 °F is only good to 2145 °F before permanent linear shrinkage).
Foundations
Concrete pillars are foundation on which the heater is mounted. They can be four nos. for smaller heaters and may be up to 24 nos. for large size heaters. Design of pillars and entire foundation is done based on the load bearing capacity of soil and seismic conditions prevailing in the area. Foundation bolts are grouted in foundation after installation of the heater.
Access doors
The heater body is provided with access doors at various locations. Access doors are to be used only during shutdown of heater. The normal size of the access door is 600x400 mm, which is sufficient for movement of people/ material into and out of the heater. During operation the access doors are properly bolted using leak proof high temperature gaskets.
See also
*
Electric arc furnace
An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.
Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to ...
*
Flued boiler
A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and ...
*
Fire test furnaces
References
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External links
*{{curlie, Business/Industrial_Goods_and_Services/Machinery_and_Tools/Furnaces%2C_Incinerators%2C_Kilns/, Furnaces, Incinerators, Kilns
Industrial furnaces
Industrial processes