fire heaters

8/10/2019 Fire Heaters

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The primary function of a fired heater is to supply all heat required by theprocess in one form or another. A fired heater utilizes gaseous or liquid fuels

often produced as a by-product. The normal process function is raising the

process stream to its required temperature for distillation, catalytic reaction,

etc. Sizes of heater vary considerably, dependent upon the type of duty and

throughput


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There are two general basic designs or types of fired heaters:

Box type

Vertical type.

Either of which may be forced or natural draft.


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A box type heater is considered to be any heater in which the tubes are

horizontal. In this type of heater it is possible to have locations or zones of

different heat densities. The zone of highest heat density is the "radiant

section". The tubes in this section are called "radiant tubes".

The heat pickup in the radiant tubes is mainly by direct radiation from theheating flame. In some heater designs shield tubes are used between

radiant and convection section. The zone of lower heat density is the

"convection section". The tubes in this section are called "convection

tubes". This heat pickup in the convection section is obtained from the

combustion gases primarily by convection.

Box type heaters may be up-fired or down-fired with gas or oil fired burnerslocated in the end or sidewalls, floor, roof or any combination thereof.


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Box-type furnaces are best suited for large capacities and large heat duties.This makes such furnaces particularly suitable for topping units where it is

possible to increase tube length in both the radiation and convection

sections reducing thereby the required number of headers or return bends.

It is possible in such furnaces to open the headers for cleaning, something

which makes cleaning the tubes in the radiation section easier; this is

especially significant in furnaces used to heat easily-coking oils


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In the horizontal up-fired heater, products of combination in the radiationchamber pass upward through banks of roof tubes and a fire brick diffuser

into a plenum or collecting chamber. From the plenum chamber flue gases

are passed through an overhead convection section and then to an

overhead stack. Such heaters may be fired vertically upward by panels,

mounted in the heater floor or hearth, the heater floor being elevated to

provide headroom beneath. Alternatively, these heaters may also be fired

horizontally by burners mounted in the heater-end walls, in which case the

heater floor is only elevated above grade to provide air cooling convection

to the heater foundations. This type of heater may contain single or multiple

radiation chambers discharging flue gases to a common convection section

and stack.


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Vertical heaters are either cylindrical or rectangular. They may have radiantsection only or convection and radiant sections. The radiant section tubes

will usually be vertical, but some cylindrical heaters have helical coils. Theconvection section can be either vertical of horizontal.

Cylindrical heaters with vertical tubes are commonly used in hot oilservices and other processes where the duties are usually small, butlarger units, 100 million kJ/hr and higher, are not uncommon.

Cylindrical heaters are often preferred to box-type heaters. This ismainly due to the more uniform heating rate in cylindrical heatersand higher thermal efficiency. Furthermore, cylindrical heatersrequire smaller foundations and construction areas and their

construction cost is less. High chimneys are not essential incylindrical furnaces because they normally produce sufficientdraught.


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In the usual practice, the process fluid is first heated in the convection

section preheat coil which is followed by further heating in the radiant

section. In both sections heat is transferred by both mechanisms of heat

transfer, viz. radiation and convection, where radiation is the dominant

mode of heat transfer at the high temperatures prevalent in the radiant

section and convection predominates in the convection section where the

average temperature is much lower. Most of the heat transferred to the

process fluid takes place in the radiant section while the convection section

serves only to make up the difference between the heat duty of the furnace

and the part absorbed in the radiant section. Roughly speaking, about 45-

55% of the total heat release in the furnace is transferred to the processfluid in the radiant section, leaving about 25-45% of the total heat release to

be either transferred to the process fluid in the convection section or carried

by the flue gases through the stack and is lost.


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Generally speaking, the chief mechanism by which heat is transferred to theprocess fluid is radiactive heat transfer. In the radiant section the process

fluids are mainly heated by direct radiation from the flame. About 70-90 %

of the heat absorbed by the tubes in the radiant and shield sections of a

furnace is transferred by radiation. This clearly indicates that heat transfer

by radiation is of particular significance for the thermal analysis and design

of fired heaters in general. Furthermore, the fundamental understanding ofheat transfer by radiation is essential for a better appraisal of the expected

effects of any modifications of existing heaters or suggestions for possible

future developments. Heat transfer by radiation can not however be

considered in isolation, but it must be considered in conjunction with other

mechanisms of heat transfer such as conduction and convection, in addition

to heat liberation by such chemical reactions as may take place inside

heater tubes.


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The heat-absorbing surface in both sections of the heater is the outside wallof the tubes mounted inside the heater. The overall thermal efficiency of the

fired heater is dependent to a large extent on the effectiveness of the

recovery of heat from the flue gases, which depends in turn on the size of

the heat exchange surface area in the furnace. In order to increase the heat

transfer area and thus further increase the overall efficiency of the heater,

finned or studded tubes are normally used in the convection section. By thismeans it is often possible to attain heat flux in the convection section

comparable to that in the radiation section. Different methods are available

for the calculations involved in the thermal evaluation and design of fired

heaters and some of these methods may be based in theory on

fundamental considerations, but more usually the calculation of heat

transfer in heaters is based on empirical methods and the construction ofsuch units precedes in general theoretical developments. The validity and

significance of a method is determined to a large extent by its general

application for different furnace designs. Such a method would take into

consideration a larger number of variables that affect the process of heat

transfer.


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Heat transfer by radiation is governed in theory by the Stefan-Boltzman lawfor black body radiation, QR = T4. In practice, the mathematical solution

of radiative heat transfer is more complicated however as it involves the

calculation of heat exchange factors as a function of the furnace geometry

and the calculation of the absorptivity and emissivity of the combustion

gases. There are also other factors to be considered such as the

emissivities of the surface and the effects of re-radiation of the tubes.

There are two primary sources of heat input to the radiant section, the

combustion heat of fuel, Qrls, and the sensible heat of the combustion air,

Qair, fuel, Qfuel and the fuel atomization fluid (for liquid fuel when

applicable), Qfluid. Part of this heat input liberated in the radiant section is

absorbed by the tubes in the radiant QR and shield Qshld sections, whilethe remaining heat is either carried as sensible heat by the exiting flue gas,

Qflue gases or is lost by radiation through the furnace walls or casing,

Qlosses. The temperature of the flue gas can then be calculated by setting

up a heat balance equation.


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Radiation heat losses through furnace walls depend on the size of thefurnace, where greater heat losses are to be expected in small furnaces as

the ratio between the walls area and the volume of the radiation section

decreases with furnace size increase. For furnaces of 10 MW or more, they

range from 2 to 5% of the net calorific value. Apart from furnace size,

however, radiation heat losses depend also on the material composing the

insulating refractory lining and its thickness, and these losses may be lowfor an economically optimum insulating lining.


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Flame and effective gas temperatures are key variables that need to beaccurately determined before analysis of the heat transfer in the radiant

section of fired heaters can be meaningfully undertaken. Flame temperature

is the temperature attained by the combustion of a fuel. This temperature

depends essentially on the calorific value of the fuel. A theoretical or ideal

flame temperature may be calculated assuming complete combustion of the

fuel and perfect mixing. But even when complete combustion is assumed,the actual flame temperature would always be lower than the theoretical

temperature. There are several reasons for this, chiefly the dissociation

reactions of the combustion products at higher temperatures (greater than

about 1370C), which are highly endothermic and absorb an enormous

amount of heat, and heat losses by radiation and convection to the walls of

the combustion chamber.


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Heat transfer in the convection section is composed in general of thefollowing:

1. Direct convection from the combustion gases. A film coefficient based

on pure convection for flue gas flowing normal to a bank of bare tubes

may be estimated

2. Direct radiation from the combustion gases3. Radiation from refractory walls. Re-radiation from the walls of the

convection section usually ranges from 6 to 15% of the total heat transfer

by both direct convection and radiation from the combustion gases. A value

of 10% represents a typical average


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Furnace thermal efficiency is usually defined as the percent ratio of the total

heat absorbed by the process fluid to the total heat input. The total heat

input is the sum of the calorific value of the fuel (gross or net) and the

sensible heat of all incoming streams including combustion air, fuel and

atomization steam (if used). Heat losses calculated from the difference

between the heat input and heat absorbed comprise both stack heat losses

and radiation heat losses through furnace walls.Stack heat losses are sensible and latent heats carried by the hot flue

gases discharged through the stack. They are a function of the flue gas flow

rate and temperature. Of these two factors, flue gas temperature is the

main factor in furnace heat losses, and the thermal efficiency may be

greatly improved if the flue gases are cooled before being discharged from

the chimney. In order to cool the flue gases, a cold fluid must be availablethat needs to be heated. Flue gas cooling is limited, however, by corrosion

problems caused by sulphuric acid condensation due to the presence of

sulphur compounds in the fuels burned.


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If the fluid to be heated is at a temperature that is too high to give asufficiently low flue gas temperature, i.e. a satisfactory thermal efficiency,

either one of these two solutions may be used: (1) Steam production, which

does not reduce fuel consumption, but is advantageous if the steam can be

exploited, (2) Recycling the flue gas and utilizing its heat for preheating the

combustion air. Preheating the combustion air allows a thermal efficiency of

approximately 90% of the net calorific value, but this requires an air blower.While flue gas temperature is the main factor in furnace heat losses, the

effect of flow gas rate is by no means negligible. This depends to a large

extent on the excess air ratio, and for higher efficiency the furnace should

operate with the least possible excess air while ensuring at the same time

complete combustion of the fuel. Operation with too little excess air may

lead to unburned fuel losses that may be greater than the efficiency gained

by reducing the excess air. Incomplete combustion is undesirable not only

on account of calorific value loss, but also because of fouling problems

often associated with the unburned fuel. If the flue gases are cooled,

excess air will no longer have any importance since all the heat transferred

to the excess air will be recovered from it.


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Heaters can be fired from any position, i.e. bottom, top, side or end. By far the most common is bottom fired, mainly because it is more

economical. The burner of a bottom fired heater will be located 2.1-2.7 M

above grade at a height which is suitable for an operator to work

underneath. Operating from under the heater is more dangerous than other

types of firing, which is the principle reason certain operating companies will

not install a bottom fired heater.

Heaters are commonly light with an electric ignitor. some refineries use a

propane torch while some still light the burners with a rag soaked in spirits

or kerosene.


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Consideration must be given at the layout stage to accommodate the

additional equipment associated with a forced draft heater. This will usually

comprise an air inlet duct with silencer, forced draft fan and an air

preheater. The inlet duct may require a support structure.


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Snuffing Steam Station

Snuffing steam connections are supplied by the heater manufacturers,

generally in the combustion chamber and header boxes.

The control point or snuffing steam manifold is generally located at least 15

meters away from the heater, is supplied by a live steam header and is

ready for instantaneous use. Smothering lines should be free from low

pockets and should be so arranged as to have all drains grouped near themanifold.

Collected condensate (apart from freezing and blocking lines) can, when

blown into a hot furnace, result in serious damage. (Low points should be

drilled 10 mm diameter or provided with spring opening auto-drain valves).

Monitor Nozzles

Some customers request that turret or monitor nozzles be located around

the heater so that water for fighting a fire is instantly available. Controls for

such nozzles should be located at least 15 meters away from the heater.


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Supply of fuel to individual burners is adjusted by individual valves. These

should be so located that the burners can be operated while observing theflame through peepholes or burner openings.

All burner leads for gas and steam (atomizing) must be taken from the top

of the headers, and fuel gas piping should be so arranged that there are no

pockets in which condensate could collect. The fuel oil header must have

full circulation; under no condition shall it be a dead-end line.

A ring header around the furnace mounted a short distance above the

peepholes, having vertical leads adjacent to the vertical doors to the

burners, provides the greatest degree of visibility from the operators point

of view.

Atomizing steam to be used in conjunction with fuel oil shall be taken the

main steam supply header at or near the heater. Steam traps shall beprovided to drain all low points in the atomizing steam system. Separate

leads to each burner shall be taken from the top of the atomizing steam

subheader. Shutoff valves shall be so located that they can be operated

while observing the flames from the observations ports.


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The internal cleaning of tubes and fittings may be accomplished by severalmethods. One is to circulate gas oil through the coil after the heater has

been shutdown but before the coils are steamed and waterwashed and

prior to the opening and start of inspection work.

This method is effective if deposits in the coil are such that they will be

softened or dissolved by gas oil. When tubes are coked or contain hard

deposit, other methods may be used, such as steam air decoking andmechanical cleaning for coke deposits and chemical cleaning for salt

deposits. Chemical cleaning and steam air decoking are preferable as they

tend to clean the tube to bare metal.. Steam air decoking process consists

of the use steam, the coil. Steam air decoking process consists of the use

of steam, air and heat to remove the coke. The mechanics of decoking are:

a. Shrinking and cracking the coke loose by heating tubes from outside

while steam blows coke from the coil.

b. Chemical reaction of hot coke with steam.

c. Chemical reaction of coke and oxygen in air.


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In some heaters the convection section contains tubes with extendedsurface in the form of either fins or studs. Extended surface tubes are used

to increase the convection heat transfer area at low capital cost. Because of

the tendency of extended surface tubes to foul when burning heavy oils,

sootblowers are usually installed.

Sootblowers employ high pressure steam to clean the tube outer surfaces

of soot and other foreign material. Sootblowers may be either automatic

electric motor operated by a pushbutton at grade, or manual requiring

operation from a platform located at the convection bank level. Care must

be taken that sufficient clearance is allowed for the withdrawal of

sootblowers.

Generally heaters are supplied with sootblowing facilities in the convectionsection although tubes may not be of the extended surface type.


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Stack a. Damper: Mechanical or pneumatically operated to control the draft

through the stack.

b. Draft gauge (P and I)

c. Flue temperature (TI)

d. Orsat (O2CO CO2analyzer). Instruments b., c. and d. are used to access the correct combustion

conditions. Steam is supplied for the Orsat connection in the stack. Water is

supplied to the O2analyzer. Platforming for the access to the stack

instruments is supplied with the heater.

Heater Body

a. Skin thermocouples or tube wall TIs: to indicate overheating of tubes.

b. PIs: to measure the draft pressure through the combustion section of the

heater.


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