An introduction to Fired Heaters.pdf

Furnaces in Refinery and Petrochemicals
- an Introduction
pramod_dixit@yahoo.com 1

Process furnaces
• Process furnaces are widely used in petroleum
refineries and petrochemicals. These are also called
fired heaters.
• They generate heat by combustion of fuels and
transfer to process fluid which is in side coil.
• The heat duty can be few Thousand MW to few
Million MW.
• Process fluid in side coil may either liquid or gas .

A typical application of Furnace
(Crude distillation unit)
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pramod_dixit@yahoo.com

CDU (Crude distillation Unit) Heater
1. CDU heater is direct fired heater. It converts most of
the hydrocarbon fractions in to vapour phase which
are separated in distillation column.
2. No cracking or reaction takes place in this heater.
3. Flame radiation & hot flue gases of combustion
transfers heat & raise temperature of process fluid.
4. Every heater is tailored / designed for its specific
application.
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Reaction Heaters
1. In reaction heaters , these are filled with catalyst and
perform reaction inside tubes, besides heating.
2. These are called reaction heaters.
e.g. : Hydrogen reformer
Methanol heater
Catalytic reformers
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Reformer Heater
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Heater Performance objectives
• Maximize heat delivery to feed
• Minimal fuel consumption
• Flexibility of operation with varying fuel type/ quality and
also process flow variation in heater tubes.
• Easy to operate with safe start-up, operation and shut down.
• Minimize stack emissions (HC, Nox, Sox also hot flue gases)
• Achieve maximum thermal efficiency.
7

Reasons to save Energy
• Energy costs 60 to 65% of a Refinery / process plant.
Highest energy requirement is in Furnaces .
• Cost of energy going up every day compels plant
operators to make use of waste Off –Gas or oil.
• In today ‘s scenario waste ( Off gases) usage is
maximized saving clean fuels like Natural gas or LPG.
• Due to waste gases of combustion, we have to be more
careful of environment pollution.
8

Heater Types
(based on their shape & geometry)
1. Vertical cylindrical furnaces.
2. Box type with vertical tubes
3. Cabin furnaces with horizontal tubes.
4. Radiant wall furnaces.
5. Double fired furnaces
6. Multi cell
7. All convection

Few Heaters geometry

Heater Sections
• BottomRadiant Section : where Tubes directly see the burners /
radiant wall.
• Coils : Carry process feed to be heated
• ConvectionSection: Packed bundle of bare/ finned tubes
absorbing balance heat of flue gas before it is goes to stack.
• Cross Over Section: Section in-between the radiant & convection
• Stack: Finally dischargesfue gas to atmosphere
• Air Pre-heaters: Used to preheat the fresh air feed to burners by
exchanging with flue gases being discharged
All above sectionsare confinedin a Caron Steelbody structure
designedto take care of all operation,wind and seismic loads
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Heater
Sections
(TOP)
12

Heater
Sections
(Bottom
)
13

Radiant section
1. The radiant section is where the tubes receive
almost all its heat by radiation from the flame
2. The tubes are a distance away from the Refractory
so radiation can be reflected back to tubes to
maintain a uniform tube wall temperature.
3. Tubes are held at one end & guided at another
end to allow them to expand. These can not have
fixed ends.
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Convection section
1. Located above radiant section where Heat transfer takes
place by convection .
2. Tubes are studded or finned to increase heat transfer
surface area.
3. The first two tube rows in the bottom of the convection
section are bare tubes called shield or shock tubes. They
protect the finned tubes from direct radiation.
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Crossover section
1. A crossover is the tube that connects from the convection
section outlet to the radiant section inlet.
2. The crossover piping is normally located outside so that the
temperature can be monitored and the efficiency of the
convection section can be calculated.
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Radiant section- photo from in side
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Vertical Cylindrical heaters
1. Largecylindrical radiant
section.
2. Tubes are oriented vertically
and in a largecircle
3. Burners fire from the floor.
4. Coil may be vertical or
helical coil (radiant and
convection)

Example of vertical cylindrical heater
Vertical Cylindrical with convection Vertical cylindrical with helical coil.
19

Example of Box type heater
Horizontal tubes Horizontal tubes single row Double fired
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Vertical Cylindrical
( Radiant with convection)
1. Vertical hairpin coil in firebox, Horizontal
tubes in convection section
2. Symmetrical & uniform heat absorption
3. Burners in a circle on floor
4. Vertically up firing
5. General use for low heat duty.
– Low cost
– Size small
– Low foot print
– Easy construction
21

Cabin Type Heaters
1. Large Heat duty 60 to 80 MW or more
2. Cabin heaters have sloped arch, usually rectangular
cross section
3. Horizontal Cabin - Large rectangular radiant section
oriented horizontally.
4. Burners : Generally fired from the floor or side
walls.
5. Tubes : May be vertical or horizontal in radiant
section.
6. Convection section : contain multiple horizontal
bank of tubes of single or multiple products.

Cabin Furnace with Horizontal Tubes
1. The generally radiation section has
parallel tubes with the greatest
length being horizontal.
2. Tubes placed horizontally on the
longer side walls.
3. The burners are located on the
floor on the lower part of the
longer side walls or at end walls

Burners
25

Burners
Burners are most important players in heater
performance. Placed in lower radiant section.
Flue gases travel vertically through :
1. Radiant section
2. Convection section
3. Air pre-heater if required
4. Finally through stack.
26

Burners and its accessories
The combustion components include:
• Burner
• Fuel supply Piping
• Flame Safety Supervision & Control System.
• Burner management system
• Low Nox and staged burning is a preferred choice
27

Burners placement & types
Burners positioning
1. Single or double side
2. Floor mounted up-fired
3. Roof mounted or down fired
4. End of side wall fired
5. Multi-level fired
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Positioning of burners
29

A Combination Burner
• A combination burner is
selected if we need to
use both type of fuels
(Gas as well as liquid)

Burner Control Panel
Consistsof :
– Burner managementsystems
– Single loop controllers
– Limit controllers
– NFPA & local regulation requirements.
– The control system provides a pre-purgeand post-purge of the
heater, automatic ignition, low flame start (high turn down
burner), continuous main flame detected by UV sensor.
– Control options !!!!!! virtually limitless !!!!!
31

Role of air pre-heaters in furnace
32

Air Pre-heaters
• The purpose of the air pre-
heater is to recover the heat
from the furnace flue gas.
• This increases the thermal
efficiency of the furnace by
reducing the useful heat lost
in the flue gas.
• This also reduces high
temperature discharge to
atmosphere
• Shown here is glass tube type
APH.

Air flow scheme with APH system

Furnace types based on DRAFTs
Once we have APH in the system, we need to have a
draft control on Flue gas flow.
1.Natural ( no APH possible in this case)
2.Forced
3.Induced
4.Balanced
A word of caution :
Flue gas discharge temperature to atmosphere
should be above DEW POINT, to avoid acid attack
by gas condensation. Which corrodes the stack.
35

Industrial process furnaces- Forced draft
36

Fuel used in heaters
• Refinery waste gas
• Natural gas
• Fuel Oil / waste oil
• Process combustible gases ( like H2 )

Refractory & Insulation
• Since all heater parts are associated with heat , suitable
Refractory is required to protect all CABIN structure as
well as prevent heat loss to atmosphere.
• Refractory radiates back the heat to tubes
• Materials : Fire bricks, insulating & heat resistant cast able,
ceramic fibre etc .
• Ceramic fibre is light and easy to apply, used for roof & wall,
not used for oil fired burners in radiant zone.
38

Applicable Codes for Fired heater design
• API 535 : guidelines for the selection and/or evaluation of burners
installed in fired heaters in general refinery services.
• API 538 : recommendations for design, operation, and maintenance
considerations for industrial fired boilers.
• API 560 : Fired Heaters for General Refinery Service,
recommendations for the design, materials, fabrication, inspection,
testing, preparation for shipment, and erection of fired heaters, air pre-
heaters (APHs), fans etc.
• API 573 : inspection practices for fired boilers and process heaters.
39

Thank you
40

An introduction to Fired Heaters.pdf

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An introduction to Fired Heaters.pdf