COVER PAGE
STUDY OF DESIGN AND STANDARDIZATION OF
FIRED HEATER FOR INDUSTRIAL APPLICATIONS
Report submitted in partial fulfillment of the requirements for the
B. Tech. degree in Mechanical Engineering
By
SHAH VRUND BHAVINKUMAR
(17BME099)
Under the supervision
Of
Dr. VIVEK KUMAR
SCHOOL OF TECHNOLOGY
PANDIT DEENDAYAL ENERGY UNIVERSITY
GANDHINAGAR, GUJARAT, INDIA
May 2021
INSIDE TITLE PAGE
STUDY OF DESIGN AND STANDARDIZATION OF
FIRED HEATER FOR INDUSTRIAL APPLICATIONS
Report submitted in partial fulfillment of the requirements for the B.
Tech. degree in Mechanical Engineering
By
SHAH VRUND BHAVINKUMAR
(17BME099)
Under the supervision
Of
Dr. VIVEK KUMAR
SCHOOL OF TECHNOLOGY
PANDIT DEENDAYAL ENERGY UNIVERSITY
GANDHINAGAR, GUJARAT, INDIA
May 2021
CERTIFICATE
This is to certify that the report on
“study of design and standardization of fired heater for
industrial applications” submitted by the students, as a requirement for the degree in Bachelor of
Technology (B. Tech) in Mechanical Engineering, under my guidance and supervision for the
session 2020-2021.
Name of the student
SHAH VRUND
BHAVINKUMAR
Date: 22 May, 2021
Place: GANDHINAGAR
Roll No.
Signature
17BME099
Signature of the Supervisor
Dr. VIVEK KUMAR
CONTENT
LIST OF TABLES
Table 1 Mean beam length
Table 2 Effect of pollutant on excess air
LIST OF GRAPHS
Graph 1 alpha factor vs (center to center)/(tube dia)
Graph 2 F factor vs Gas emissivity
Graph 3 Gas emissivity vs PL
Graph 4 enthalpy vs temperature
Graph 5 theta vs fin efficiency
Graph 6 Thermal conductivity vs temperature
Graph 7 Return bend equivalent length curve
Graph 8 Return bend equivalent length Nre correction
LIST OF FIGURES
Fig 1 Typical fired heater
Fig 2A Components of fired heater
Fig 2B Components of fired heater
Fig 2C Components of fired heater
Fig 3A Vertical cylindrical fired heater
Fig 3B Cabin fired heater
Fig 3C Box fired heater
Fig 4 Classification on basis of radiant tube coil configuration
Fig 5 Classification on basis of burner arrangement
Fig 6 Classification on basis of draft
Fig 7 Cold plane area calculation
Fig 8 Direct radiation in shield section
Fig 9 Indirect, non-luminous radiation in shield section
Fig 10 Draft in heater
Fig 11 Air preheat system
Fig 12A Regenerative air heater
Fig 12B Rotor
Fig 12C Basket
Fig 13 Recuperative, Tubular Air heater
Fig 14 Recuperative, Cast Tube Air heater
Fig 15 Heating medium air heater
Fig 16 Retractable soot blower
Fig 17 Fixed rotary soot blower
Fig 18 Stack Damper
Fig 19 tab 1 input sheet
Fig 20 Tab 2 fired heater 2D diagram
Fig 21 Tab 3 load calculation
Fig 22 total weight calculation
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my mentors Dr.Vivek kumar , Assistant
Professor, Department of Mechanical engineering , Pandit Deendayal Petroleum University,
Mr.Manoj Kumar Prasad , Head of Mechanical static Department, Larsen and Toubro, Chiyoda
and Mr.Amit Kumar , Manager in Mechanical static Department, Larsen and Toubro, Chiyoda
for continually guiding me in the right direction in the face of any impediment in the course of
this project. I would also like to extend my thanks to all who directly or indirectly succour me in
bringing this project to this stage.
Name of Student
1. SHAH VRUND BHAVINKUMAR (17BME099).
Signature of Student
List of table
4
List of graphs
4
List of figures
5
Acknowledgement
6
Chapter 1 Introduction
9
Chapter 2 Objective
10
Chapter 3 Fired heater
11
3.1 Radiant section
11
3.2 Convection section
12
3.3 Stack
13
Chapter 4 Components of fired heater
14
Chapter 5 classification of fired heater
15
5.1 Structural classification
15
5.2 Radiant tube coil configuration
16
5.3 Burner arrangement
16
5.4 Draft design
17
Chapter 6 Heat transfer mechanism in fired heater
18
6.1 Direct radiation in the radiant section
18
6.2 Convective heat transfer in the radiant section
22
6.3 Total radiant heat absorption in the radiant section
23
6.4 heat balance in the radiant section
24
6.5 Direct radiation in the shield section
24
6.6 In-direct, non-luminous radiation in the shield section
26
6.7 Convection transfer in the convection section , bare tubes
27
6.8 Convection transfer in the convection section , fin tubes
28
6.9 Convection transfer in the convection section , stud tubes
33
6.10 Thermal conductivity of metals
35
6.11 Tube wall temperature calculation
35
Chapter 7 Variation in properties
37
7.1 Single phase fluids
37
7.2 Mixed phase fluids
37
7.3 Heat transfer coefficient
38
7.4 In tube pressure drop
39
7.5 Gas side pressure drop across tube
40
Chapter 8 Heater stack Draft analysis
41
Chapter 9 ducting pressure loss
47
Chapter 10 Burner type and selection
51
10.1 Draft
51
10.2 Flame stability
51
10.3 Design excess air
52
10.4 Combustion air preheat
53
10.5 Turbine exhaust gas
53
10.6 Combustion air adjustment
54
Chapter 11 Auxiliary equipment
55
11.1 Air preheat system
55
11.2 Soot blower
56
11.3 Fans and blower
58
Chapter 12 heater design and engineering
61
12.1 Process/thermal engineering
61
12.2 Mechanical engineering
61
12.3 Structural /civil engineering
61
12.4 Environmental engineering
61
12.5 Interface engineering
61
Chapter 13 Load data sheet
62
Time line of the project
63
References
64
CHAPTER 1 INTRODUCTION
Since the unabated rise in the increase for petrochemicals it becomes extremely important to
study a technology that helps in obtaining them. There is no doubt in stating that petrochemical
and refining industries are one of the Earth’s most important energy resources. Hydrocarbon
mixtures are produced in the refineries by distillation as well as thermal cracking of higher
boiling hydrocarbon fractions.
Fired Heaters, often referred to as furnaces (direct fired heaters), are pieces of equipment often
used in processing facilities to heat gases or liquids up to a desired temperature as mentioned
above (for distillation or thermal cracking). Fired heaters in the petrochemical and refining
industry are critical pieces of equipment that can have a major impact on process unit safety,
reliability, and economics. They are complex pieces of equipment, where tubes and other
pressure boundary components might fail due to relatively short periods of upset conditions.
The project gave me opportunities to observe the designing of fired heaters first hand and
importance of following standards while designing and also engineering used in designing the
heater. It was completed as a fulfilment for the CP project.
CHAPTER 2. OBJECTIVE
The objective of this project was to teach me a plethora of skills and provide me firsthand
experience which is supposed to help me in my future venture as an engineer working in a
professional environment. I learnt the importance of teamwork, punctuality, professional
etiquettes, time management, etc. It also involved the use of professional application software
and hence giving me a hands on experience to that.
CHAPTER 3. FIRED HEATER
Fired heaters sometimes referred to as process heaters or process furnaces are heat transfer
units designed to heat petroleum products, chemicals, and other liquids and gases flowing
through tubes. The heating is done to raise the temperature of the fluid for further processing
downstream or to promote chemical reactions in the tubes, often in the presence of a catalyst.
Fired heaters may carry liquids at temperatures as high as 1500°F (810°C) and pressures up to
1600 psig (110 barg). The primary modes of heat transfer in process heaters are radiation and
convection. The initial part of the fluid heating is done in the convection section of the furnace,
while the latter heating is done in the radiant section. The fluids flow through an array of tubes
located inside a furnace or heater. The tubes are heated by direct-fired burners that often use
fuels that are by-products from processes in the plant and that vary widely in composition. Using
tubes to contain the load is somewhat unique compared to the other types of industrial
combustion applications.
Heating the fluids in tubes has many
advantages over heating them in the shell of
a furnace. Advantages include better
suitability for continuous operation, better
controllability, higher heating rates, more
flexibility, less chance of fire, and more
compact equipment.A fired heater is defined
as an exchanger that transfers heat from the
combustion of fuel to fluids contained in
tubular coils within an internally insulated
enclosure(API 560).A typical fired heater
consists of three main components namely
radiant section, convection section and stack
Fig 1. Typical fired heater
3.1 RADIANT SECTION
Fuel flows into the burner and is burnt with air provided from an air blower. 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, here the heat is transferred mainly by radiation to tubes around the fire in the
chamber. The heating fluid passes through the tubes and is thus heated to the desired
temperature. Tubes can be vertical or horizontal, placed along the refractory wall, in the middle,
etc., or arranged in cells. Tube guides which are located at the top, middle and bottom hold the
tubes in place. The radiant zone with its refractory lining is the costliest part of the heater and
most of the heat is gained there.
3.2 CONVECTION SECTION
The gases from the combustion of fuel are known as flue gas. After the flue gas leaves the
firebox, most furnace designs include a convection section where more heat is recovered before
venting to the atmosphere through the flue gas stack. Convection section is cooler than radiant
section to recover additional heat. Heat transfer takes place by convection here, and the tubes
are finned to increase heat transfer. The first two 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, 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. . Crossover is the term used to
describe the tube that connects from the convection section outlet to the radiant section inlet.
3.3 STACK
The flue gas stack is a cylindrical structure at the top of all the heat transfer chambers. The
transition from the convection section to the stack is called the breeching. The breeching
directly below it collects the flue gas and brings it up high into the atmosphere where it will not
endanger personnel. By the time the flue gas exits the stack, most of the heat is recovered and the
temperature is less.
CHAPTER 4. COMPONENTS OF FIRED HEATER
Burners: they are critical devices
installed in a heater that generate the
energy absorbed by the fluid moving
through the radiant and convection coils.
Radiant coil: it carries the process fluid
through the radiant section. As radiant
heat is the primary energy transfer
mechanism in this heater section, these
coils have no extended surfaces.
convection coil: it carries the fluid(s)
being heated through the convection
section. A key difference between
convection and radiant coils is that most
tubes that make up the convection coil
have extended surface
Fig 2A components of fired heater
Refractory : insulating material that
lines the interior of the heater. Its
purpose is to reduce energy loss, while
providing lower casing temperatures to
protect operators.
Stack: The stack collects the flue gas and
discharges it to the atmosphere.
Sample Ports: Nozzles near the stack
exit where sampling probes or other
sensors are inserted to measure flue gas
composition, pressure, and/or
temperature.
Stack Damper: A “valve” located inside
the stack used to control flow of flue gas
and adjust draft.
Breeching: The section of duct after the
last row of convection tubes where flue
gases are collected for transmission to the
stack or exhaust ducts.
Fig 2B components of fired heater
Corbels: Protrusions from the
refractory lining the convection
section. These protrusions redirect
flue gases travelling along the walls
of the convection section back toward
convection coil.
Header Box: A refractory lined box
covering the return bends and
separated from the flue gas flow.
Return Bends: U-shaped fittings
connecting tubes in the convection
coil.
Tube Sheet: Support sheet for
convection tubes passing through
it. There are both end tube sheets and
intermediate tube sheets.
Fig 2C components of fired heater
Bridgewall : sometimes called the arch is the area where
flue gas leaves the radiant section.
Coil Supports: Brackets installed in the radiant and
convection section to keep the associated coil in the proper
position while allowing for thermal growth.
Observation Doors: Visual inspection ports that allow
operators to view what is happening inside the furnace.
Tube Skin Thermocouples (TSTCs): Sensors installed
directly on the heater coils to monitor the coil’s skin
temperature. These critical sensors are used to ensure that
the allowable tube metal temperature is not exceeded
Expansion Rows: Space left in the
convection section for the future
addition of rows. Tube sheets are
also designed to accommodate these
rows with the associated holes
plugged to prevent flue gas from
leaking into the header boxes.
Crossover Piping: Interconnecting
piping between the convection coil
and radiant coil.
CHAPTER 5 CLASSIFICATION OF FIRED HEATER
Fired heater can be classified on basis of structural configurations, radiant tube coil
configuration, burner arrangement and draft design
5.1 STRUCTURAL CLASSIFICATION
VERTICAL CYLINDRICAL
It is usually used for quite low power. These heaters can be “all radiant” type or
they can have both radiant and convection section. In the first case the coil is
composed of vertical tubes located around a circle, at the base of the heater
there are the burners. While in the other case convection section, has horizontal
axis and is located above the radiant section. The heater with radiant and
convection section can reach higher power and allow higher efficiency.
Fig 3A Vertical cylindrical
fired heater
CABIN HEATER
It usually has a radiant section in the lower part and a convection section in the
upper one. Fluid to be heated run through the convection section and then
through the radiant one, to have a higher thermal head. Coil are composed of
many tubes in parallel and connected with manifold at the end. Burners can be
located on the floor with vertical axis in one or more rows or they can be
located on the walls with horizontal axis. Tubes in convection section have
triangular pitch.
Fig 3B Cabin fired heater
BOX HEATER
It mainly composed of one or more chambers with rectangular section, the
tubes are usually in horizontal position arranged on one on more rows along the
wall of the section. Burners can be located on the floor or on the wall of the
radiant section. Flue gases run through the convection section and exit from the
stack in the atmosphere
Fig 3C Box fired heater
5.2 RADIANT TUBE COIL CONFIGURATION
Cylindrical heater
with vertical coils
Cylindrical heater
with helical coils
Box heater with
vertical tube coil
Box heater with
arbor coils
Cabin heater with
horizontal coils
Fig 4. classification on basis of radiant tube coil configuration
5.3 BURNER ARRANGEMENT
The installation location and orientation of burners is known as the burner arrangement
Burners can be installed in the floor, walls, or roof of the radiant section and can be positioned to
be up fired, side fired, or down fired. Horizontally fired, wall mounted burners are further
described as being end wall or side wall fired. End wall burners are fired horizontally across the
length of the radiant section. Side wall fired burners direct flames across the radiant section’s
width.
Up fired
End wall fired
Side wall fired
Fig 5 classification on basis of burner arrangement
Side wall fired multi‐
level
5.4 DRAFT DESIGN
According to API 560, draft can be defined as the negative pressure or vacuum of the air and/or
flue gas at any point in the heater
Natural draft
Fig 6 classification on basis of draft
These heaters use the suction generated by the “stack effect” to create negative pressure inside
the heater, pulling ambient air through the burner(s) and into the combustion zone.
Forced draft
Forced Draft Heaters use a fan known as an “FD fan” to push combustion air through the
burner(s) and into the combustion zone.
Induced draft
Induced Draft Heaters use a fan known as an “ID fan” to remove flue gas from the heater and
pull air through the burner(s) and into the combustion zone.
Balanced draft
Balanced draft heaters use both FD and ID fans to push and pull combustion air through the
burner and into the combustion zone
CHAPTER 6 HEAT TRANSFER MECHANISM IN THE
FIRED HEATER
6.1 DIRECT RADIATION IN RADIANT SECTION
Direct radiation in the radiant section of a direct fired heater can be described by the equation
shown below.
qr = saAcpF(Tg4 - Tw4)
Where,
qr = Radiant heat transfer, Btu/hr
s = Stefan-Boltzman constant, 0.173E-8 Btu/ft2-hr-R4
a = Relative effectiveness factor of the tube bank
Acp = Cold plane area of the tube bank, ft2
F = Exchange factor
Tg = Effective gas temperature in firebox, °R
Tw = Average tube wall temperature, °R
Cold Plane Area, Acp :
The normal heat-absorbing surfaces in a fired heater consist of several parallel tubes. In the case
of a fired heater design where the tubes are fired from one side only, the tubes are normally
positioned in front of a refractory wall. Part of the radiation from the hot gas strikes the tubes
directly, while the rest passes through and is radiated back into the chamber, where part is
absorbed by the tubes.In the case of tubes fired from both sides, as when the tubes are positioned
in the center of the chamber, the tubes absorb direct radiation from both sides. Expressing the
tube area as an equivalent plane area simplifies this calculation.
The calculated cold plane area is the area of a plane through the tube center lines, whether they
are in a curved plane, such as in a cylindrical pattern or in a row side-by-side. For most tube
panels, the width would be equal to the center/center spacing of the tubes times the number of
tubes. The length is the length of tube exposed to the radiation. In the case of tubes penetrating a
tube sheet it is the length between tube sheets. But for tubes with the return bends inside the
firebox, the length may be taken as the distance from the centerline of the return on one end to
the centerline of the return on the other end.
For a firebox with the tubes down the center, or other pattern which results in the tubes being
fired from both sides, the cold plane area would be twice the projected area.
For single sided firing:
Acp = Ntube*Stube*Ltube
For double sided firing:
Acp = Ntube*Stube*Ltube * 2
Where,
Ntube = Number of tubes wide
Stube = Tube spacing, ft
Ltube = Effective tube length, ft
Fig 7 cold plane area calculation
Relative Effectiveness Factor, a :
Because the tube bank does not absorb all the heat radiated to the cold plane, an absorption
effectiveness factor, a, can be used to correct the cold plane area, depending on the arrangement
of the tubes. The relative effectiveness factor can be described by the following curves:
Graph 1 aplha factor vs (center to center)/(tube dia)
Exchange Factor, F :
Because the flue gas in the firebox is a poor radiator, the equation must be corrected using an
exchange factor which is dependent on the emissivity of the gas and the ratio of refractory area
to cold plane area. Since the radiant heat is reflected into the firebox, by the refractory, a heater
having a larger ratio of refractory surface relative to the tube surface, will absorb more heat.
Since the tubes themselves are not perfect absorbers, the curves are based on a tube-surface
absorptivity of 0.9.
Graph 2 F factor vs Gas emissivity
Where,
Aw/aAcp :
The equivalent cold plane area, aAcp, is the product of the effectiveness factor and the cold plane
area as described above. The Aw can be described as follows,
Aw = Ar - aAcp
and,
Aw = Effective refractory area, ft2
Ar = Total refractory area, ft2
aAcp = Equivalent cold plane area, ft2
The total refractory area, Ar, is simply the total of the refractory area exposed to the radiant
section of the heater.
Flue Gas Emissivity :
The tube wall temperature has only a minor effect. Therefore, the emissivity can be correlated as
a function of PL product and the gas temperature, Tg. Variations in tube wall temperatures
between 600 and 1200°F cause less than 1% deviation from these curves.
Graph 3 Gas emissivity vs PL
And,
PL = Product of the Partial Pressure of the carbon dioxide
and water times the Beam Length, in atm-ft.
The only constituents normally in the flue gas that contribute significantly to the radiant emission
are the carbon dioxide and the water, the sum of these are all that are considered. The Partial
pressure of a gas component in atm's is the mole volume fraction percent of that component.
Mean beam length
For Box Type Heaters
Dimension Ratio
Mean Beam Length
1-1-1 to 1-1-3
2/3(Furnace Volume)1/3
1-2-1 to 1-2-4
1-1-4 to 1-1-inf
1 x Smallest Dimension
1-2-5 to 1-2-inf
1.3 x Smallest Dimension
1-3-3 to 1-inf-inf
1.8 x Smallest Dimension
With the box dimensions, length, width, and
height being in any order
For Vertical Cylindrical Heaters
Length/Diameter < 2 (((L/D)-1)*0.33 + 0.67)*D
Length/Diameter >= 2 Diameter
Table 1 Mean beam length
Effective gas temperature in firebox, Tg
For a radiant section that is considered "well mixed", this temperature is assumed to be equal to
the temperature leaving the radiant section, i.e., the bridgewall temperature. But in a high
temperature heater with a tall narrow firebox and wall firing, the Tg controling radiant transfer
may be 200 to 300 °F higher than the exit temperature.
Average tube wall temperature, Tw
Tube wall temperature depends on the temperature of the process fluid and its transfer coefficient
inside the tube, the thermal resistance of the tube wall, the heat flux, and the fouling. The
average tube wall temperature may be one of either the average temperature of the front 180°
face of the tube, or the overall average for the full circumference.
6.2 CONVECTIVE HEAT TRANSFER IN THE RADIANT SECTION
Even though most of the heat exchanged in the radiant section is from radiant heat transfer, the
convective heat transfer cannot be ignored. The heat exchanged by convection can be described
with the following equation:
qc = hcAt(Tg - Tw)
Where,
qc = Convection heat transfer, Btu/hr
hc = Film heat transfer coefficient, Btu/hr-ft2- °R
At = Area of the tubes in bank, ft2
Tg = Effective gas temperature in firebox, °R
Tw = Average tube wall temperature, °R
Film heat transfer coefficient, hc
The arrangement of the tubes as well as the firebox design contributes to this factor. For
horizontal tube, cabin type heater, which is normally small, this coefficient might = 1.5, where
on large box heaters with multiple tube cells, it may be as high as 2.8. Vertical heaters with an
L/D less than 2 would normally be designed with hc = 2, where for an L/D greater than 2.0, you
could use 3.0.
6.3 TOTAL RADIANT HEAT ABSORPTION IN THE RADIANT SECTION
The total heat absorbed by the radiant section tubes, now can be expressed by the following
equation.
qR = qr + qc
Where,
qR = Total heat transfered to radiant tubes, Btu/hr
qr = Radiant heat transfer, Btu/hr
qc = Convective heat transfer, Btu/hr
6.4 HEAT BALANCE IN THE RADIANT SECTION
There are three primary sources of heat input to the radiant section, the burner release, qrls, the
sensible heat of the combustion air, qair, and the sensible heat of the fuel and any atomizing
medium, qother. Heat is taken out of the radiant section by the two heat transfer methods, qR and
qS, and by losses through the casing, qloss, and sensible heat of the exiting flue gas, qout.
qrls + qair + qother = qR + qS + qloss + qout
Where,
qrls = Heat released by burners, Btu/hr
qair = Heat in the combustion air, Btu/hr
qother = Heat in other items, Btu/hr
qR = Heat absorbed by radiant tubes, Btu/hr
qS = Radiant heat to shield tubes, Btu/hr
qloss = Heat loss through setting, Btu/hr
qout = Heat in gas leaving radiant section, Btu/hr
qrls = Heat release by burners, Btu/hr
The burner release can be easily calculated for a gas when we know the composition of the fuel
and the heating values of the various components. For liquid fuels, the heating values are
obtained by a calorimeter test. The heating values normally used in fired heater design are
the LHV, lower heating values.
qair = Heat in the combustion air, Btu/hr
The heat available in the combustion air, such as from preheated air, or using Gas Turbine
Exhaust, etc., is taken as the heat content above 60 °F, since that is the design datum temperature
for fired heaters.
qother = Heat in other items, Btu/hr
The heat available in other items would include such things as the fuel when it is above 60 °F,
atomizing air or steam, etc.
qloss = Heat loss through setting, Btu/hr
These losses, referred to as Setting Loss or Radiation Loss are usually not calculated during
heater rating calculations. They are normally accounted for by allowances, such as a percent of
burner release or a percent of heat absorbed.
qout = Sensible heat in flue gas leaving radiant section, Btu/hr
From the flue gas composition, we can calculate the overall enthalpy of the flue gas, at a specific
temperature, by adding the proportion each of the components contribute to the total. These
enthalpies can be obtained from the following curves:
Graph 4 enthalpy vs temperature
6.5 DIRECT RADIATION IN THE SHIELD SECTION
As already stated , Shield section normally refers to the first several rows in the convection
section, which "shield" the remaining tubes from the direct radiation occurring in the radiant
section. The shield section normally consists of two to three rows of bare tubes, but the
arrangement varies widely for the many different heater designs.
In a heater similar to this one, the lower rows are directly
exposed to the hot gasses and flame in the radiant
section. To calculate the heat transfered to these tubes by
radiation, we use the same formula that we did in the
radiant section.
Fig 8 direct radiation in shield section
qS = saAcpF(Tg4 - Tw4)
Where,
qS = Radiant heat transfer to shield, Btu/hr
s = Stefan-Boltzman constant, 0.173E-8 Btu/ft2-hr-R4
a = Relative effectiveness factor of the tube bank
Acp = Cold plane area of the tube bank, ft2
F = Exchange factor
Tg = Effective gas temperature in firebox, °R
Tw = Average tube wall temperature, °R
Cold Plane Area, Acp :
The cold plane area for the shield section is equal to the cold plane area of the first row of tubes.
Acp = Ntube*Stube*Ltube
Where,
Ntube = Number of tubes wide
Stube = Tube spacing, ft
Ltube = Tube length, ft
Relative Effectiveness Factor, a :
Since all the heat directed toward this bank of tubes, leaves the radiant section and is absorbed by
the tubes, the relative absorption effectiveness factor, a, for the shield tubes can be taken to be
equal to one.
Exchange Factor, F :
Effective gas temperature in firebox, Tg
Average tube wall temperature, Tw
The values used for these factors are the same as the values to be used in the radiant section. The
difference is, when there is a shield section, which is receiving direct radiation, the aAcp for the
radiant and the shield are calculated independently, then added together to calculate the exchange
factor, F.
So the equation for Aw, becomes:
Aw = Ar - ((aAcp)rad + (aAcp)shld)
And for Aw/aAcp,
Aw/aAcp = Aw/ ((aAcp)rad + (aAcp)shld)
Where,
Aw = Effective refractory area, ft2
Ar = Total refractory area, ft2
(aAcp)rad = Equivalent cold plane area of radiant tubes, ft2
(aAcp)shld = Equivalent cold plane area of shield tubes, ft2
Therefore, the corrected formula for the radiant heat transfer in the radiant section, when a shield
section is present becomes,
qtot-rad = s(aAcp)radF(Tg4 - Tw4) + s(aAcp)shldF(Tg4 - Tw4)
And total transfer to radiant tubes,
qR = s(aAcp)radF(Tg4 - Tw4) + qc
And radiant only transfer to shield tubes,
qS = s(aAcp)shldF(Tg4 - Tw4)
It should be noted here, that the convective transfer, qc, for the radiant section remains as it was
described and it does not apply to the shield tubes. The convection transfer to the shield tubes is
calculated the same as for other convection tubes. It is also, important to remember that the
Tg applies to the gas temperature after the shield radiant heat is removed, but before the shield
convection heat is removed.
6.6 IN-DIRECT, NON-LUMINOUS RADIATION IN THE SHIELD SECTION
In a heater similar to
this one, the lower
rows are not directly
exposed to the hot
gasses and flame in the
radiant section.
However they are
exposed to the heat
radiated off the
refractory lined
plenum beneath the
tubes. To calculate the
heat transfered to these
tubes by radiation, we
use the same formula
that we did in the
radiant section.
Fig 9 indirect, non‐luminous radiation in shield section
qn = saAcpF(Tg4 - Tw4)
Where,
qn = Non-luminous radiant heat transfer to the shield, Btu/hr
s = Stefan-Boltzman constant, 0.173E-8 Btu/ft2-hr-R4
a = Relative effectiveness factor of the tube bank
Acp = Cold plane area of the tube bank, ft2
F = Exchange factor
Tg = Effective gas temperature in firebox, °R
Tw = Average tube wall temperature, °R
Relative Effectiveness Factor, a :
Since all the radiant heat directed toward this bank of tubes is absorbed by the tubes in the
convection, the relative absorption effectiveness factor, a, for the shield tubes can be taken to be
equal to one.
Cold Plane Area, Acp :
The cold plane area for the shield section is equal to the cold plane area of the first row of tubes.
Acp = Ntube*Stube*Ltube
Where,
Ntube = Number of tubes wide
Stube = Tube spacing, ft
Ltube = Tube length, ft
Exchange Factor, F :
Effective gas temperature in firebox, Tg
Average tube wall temperature, Tw
The values used for these factors are calculated the same way as was described in the radiant
design section. The Ar factor being the only exception. This factor is the inside area of the
plenum below the tubes. The openings where the flue gas enters are normally ignored, since the
ducting connecting them perform the same reflective purpose.
This type of shield calculation is totally independent of the radiant section where the heat
balance is performed as if there where no shield. The radiant heat from this calculation will
reduce the gas temperature used in the convection transfer calculation.
6.7 CONVECTION TRANSFER IN CONVECTION SECTION , BARE TUBES
Overall Heat Transfer Coefficient, Uo:
Uo = 1/Rto
Where,
Uo = Overall heat transfer coefficient, Btu/hr-ft2-F
Rto = Total outside thermal resistance, hr-ft2-F/Btu
And,
Rto = Ro + Rwo + Rio
Ro = Outside thermal resistance, hr-ft2-F/Btu
Rwo = Tube wall thermal resistance, hr-ft2-F/Btu
Rio = Inside thermal resistance, hr-ft2-F/Btu
And the resistances are computed as,
Ro = 1/he
Rwo = (tw/12*kw)(Ao/Aw)
Rio = ((1/hi)+Rfi)(Ao/Ai)
Where,
he = Effective outside heat transfer coefficient, Btu/hr-ft2-F
hi = Inside film heat transfer coefficient, Btu/hr-ft2-F
tw = Tubewall thickness, in
kw = Tube wall thermal conductivity, Btu/hr-ft-F
Ao = Outside tube surface area, ft2/ft
Aw = Mean area of tube wall, ft2/ft
Ai = Inside tube surface area, ft2/ft
Rfi = Inside fouling resistance, hr-ft2-F/Btu
Effective outside heat transfer coefficient, he
he = 1/(1/(hc+hr)+Rfo)
Where,
hc = Outside heat transfer coefficient, Btu/hr-ft2-F
hr = Outside radiation heat transfer coefficient, Btu/hr-ft2-F
Rfo = Outside fouling resistance, hr-ft2-F/Btu
Outside film heat transfer coefficient, hc:
The bare tube heat transfer film coefficient, hc, can be described by the following equations.
For a staggered tube arrangement,
hc = 0.33*kb(12/do)((cp*mb)/kb)1/3((do/12)(Gn/mb)))0.6
And for an inline tube arrangement,
hc = 0.26*kb(12/do)((cp*mb)/kb)1/3((do/12)(Gn/mb)))0.6
Where,
hc = Convection heat transfer coefficient, Btu/hr-ft2-F
do = Tube outside diameter, in
kb = Gas thermal conductivity, Btu/hr-ft-F
cp = Gas heat capacity, Btu/lb-F
mb = Gas dynamic viscosity, lb/hr-ft
Gn = Mass velocity of gas, lb/hr-ft2
6.8 CONVECTION TRANSFER IN CONVECTION SECTION , FIN TUBES
Overall Heat Transfer Coefficient, Uo:
Uo = 1/Rto
Where,
Uo = Overall heat transfer coefficient, Btu/hr-ft2-F
Rto = Total outside thermal resistance, hr-ft2-F/Btu
And,
Rto = Ro + Rwo + Rio
Ro = Outside thermal resistance, hr-ft2-F/Btu
Rwo = Tube wall thermal resistance, hr-ft2-F/Btu
Rio = Inside thermal resistance, hr-ft2-F/Btu
And the resistances are computed as,
Ro = 1/he
Rwo = (tw/12*kw)(Ao/Aw)
Rio = ((1/hi)+Rfi)(Ao/Ai)
Where,
he = Effective outside heat transfer coefficient, Btu/hr-ft2-F
hi = Inside film heat transfer coefficient, Btu/hr-ft2-F
tw = Tubewall thickness, in
kw = Tube wall thermal conductivity, Btu/hr-ft-F
Ao = Total outside surface area, ft2/ft
Aw = Mean area of tube wall, ft2/ft
Ai = Inside tube surface area, ft2/ft
Rfi = Inside fouling resistance, hr-ft2-F/Btu
Effective outside heat transfer coefficient, he:
he = ho(E*Afo+Apo)/Ao
Where,
ho = Average outside heat transfer coefficient, Btu/hr-ft2-F
E = Fin efficiency
Ao = Total outside surface area, ft2/ft
Afo = Fin outside surface area, ft2/ft
Apo = Outside tube surface area, ft2/ft
And,
Average outside heat transfer coefficient, ho:
ho = 1/(1/(hc+hr)+Rfo)
Where,
hc = Outside heat transfer coefficient, Btu/hr-ft2-F
hr = Outside radiation heat transfer coefficient, Btu/hr-ft2-F
Rfo = Outside fouling resistance, hr-ft2-F/Btu
Outside film heat transfer coefficient, hc:
hc = j*Gn*cp(kb/(cp*mb))0.67
Where,
j = Colburn heat transfer factor
Gn = Mass velocity based on net free area, lb/hr-ft2
cp = Heat capacity, Btu/lb-F
kb = Gas thermal conductivity, Btu/hr-ft-F
mb = Gas dynamic viscosity, lb/hr-ft
Colburn heat transfer factor, j:
j = C1*C3*C5(df/do)0.5((Tb+460)/(Ts+460))0.25
Where,
C1 = Reynolds number correction
C3 = Geometry correction
C5 = Non-equilateral & row correction
df = Outside diameter of fin, in
do = Outside diameter of tube, in
Tb = Average gas temperature, F
Ts = Average fin temperature, F
Reynolds number correction, C1:
C1 = 0.25*Re-0.35
Where,
Re = Reynolds number
Geometry correction, C3:
For segmented fin tubes arranged in,
a staggered pattern,
C3 = 0.55+0.45*e(-0.35*lf/Sf)
an inline pattern,
C3 = 0.35+0.50*e(-0.35*lf/Sf)
For solid fin tubes arranged in,
a staggered pattern,
C3 = 0.35+0.65*e(-0.25*lf/Sf)
an inline pattern,
C3 = 0.20+0.65*e(-0.25*lf/Sf)
Where,
lf = Fin height, in
sf = Fin spacing, in
Non-equilateral & row correction, C5:
For fin tubes arranged in,
a staggered pattern,
C5 = 0.7+(0.70-0.8*e(-0.15*Nr^2))*e(-1.0*Pl/Pt)
an inline pattern,
C5 = 1.1+(0.75-1.5*e(-0.70*Nr^2))*e(-2.0*Pl/Pt)
Where,
Nr = Number of tube rows
Pl = Longitudinal tube pitch, in
Pt = Transverse tube pitch, in
Mass Velocity, Gn:
Gn = Wg/An
Where,
Wg = Mass gas flow, lb/hr
An = Net free area, ft2
Net Free Area, An:
An = Ad - Ac * Le * Nt
Where,
Ad = Cross sectional area of box, ft2
Ac = Fin tube cross sectional area/ft, ft2/ft
Le = Effective tube length, ft
Nt = Number tubes wide
And,
Ad = Nt * Le * Pt / 12
Ac = (do + 2 * lf * tf * nf) / 12
tf = fin thickness, in
nf = number of fins, fins/in
Surface Area Calculations:
For the prime tube,
Apo = Pi * do (1- nf * tf) / 12
And for solid fins,
Ao = Pi*do(1-nf* tf)/12+Pi*nf(2*lf(do+lf)+tf(do+2*lf))/12
And for segmented fins,
Ao = Pi*do(1-nf* tf)/12+0.4*Pi*nf(do+0.2)/12+Pi*nf (do+0.2)((2*lf-0.4)(wn+tf)+ws*tf)/(12*ws)
And then,
Afo = Ao - Apo
Where,
ws = Width of fin segment, in
Fin Efficiency, E:
For segmented fins,
E = x * (0.9 + 0.1 * x)
And for solid fins,
E = y * (0.45 * ln(df / do) * (y - 1) + 1)
Where,
y = x * (0.7 + 0.3 * x)
And,
x = tanh(m * B) / (m * B)
Where,
B = lf + (tf /2)
For segmented fins,
m = (ho (tf + ws) / (6 * kf * tf * ws))0.5
And for solid fins,
m = (ho / (6 * kf * tf))0.5
Fin Tip Temperature, Ts:
The average fin tip temperature is calculated as follows,
Ts = Tg + (Tw - Tg) * 1/((e1.4142mB+e-1.4142mB)/2)
Maximum Fin Tip Temperature, Tfm:
The maximum fin tip temperature is calculated as follows,
Tsm = Twm + q(Tgm - Twm)
Where,
Tsm = Maximum Fin Tip Temperature, F
Tgm = Maximum Gas Temperature, F
Twm = Maximum Tube Wall Temperature, F
And,
The value for theta, q, can be described by the following curve.
Graph 5 theta vs fin efficiency
6.9 CONVECTION TRANSFER IN CONVECTION SECTION , STUD TUBES
Overall Heat Transfer Coefficient, Uo:
Uo = 1/Rto
Where,
Uo = Overall heat transfer coefficient, Btu/hr-ft2-F
Rto = Total outside thermal resistance, hr-ft2-F/Btu
And,
Rto = Ro + Rwo + Rio
Ro = Outside thermal resistance, hr-ft2-F/Btu
Rwo = Tube wall thermal resistance, hr-ft2-F/Btu
Rio = Inside thermal resistance, hr-ft2-F/Btu
And the resistances are computed as,
Ro = 1/he
Rwo = (tw/(12*kw))(Ao/Aw)
Rio = ((1/hi)+Rfi)(Ao/Ai)
Where,
he = Effective outside heat transfer coefficient, Btu/hr-ft2-F
hi = Inside film heat transfer coefficient, Btu/hr-ft2-F
tw = Tubewall thickness, in
kw = Tube wall thermal conductivity, Btu/hr-ft-F
Ao = Outside surface area, ft2/ft
Aw = Mean area of tube wall, ft2/ft
Ai = Inside tube surface area, ft2/ft
Rfi = Inside fouling resistance, hr-ft2-F/Btu
Effective outside heat transfer coefficient, he:
For staggered and inline pitch,
he = (hso*E*Afo+ht*Apo)/Ao
Where,
ht = Base tube outside heat transfer coefficient, Btu/hr-ft2-F
hso = Stud outside heat transfer coefficient, Btu/hr-ft2-F
Ao = Total outside surface area, ft2/ft
Afo = Stud outside surface area, ft2/ft
Apo = Tube outside surface area, ft2/ft
Inline pitch correction,
he = he*(do/Pl)0.333
Where,
do = Outside tube diameter, in
Pl = Longitudinal pitch of tubes, in
Base tube outside heat transfer coefficient, ht:
ht = (0.717/do0.333)(Gn/1000)0.67(Tb+460)0.3
And the stud coefficient,
hs = 0.936*(Gn/1000)0.67(Tb+460)0.3
With fouling,
hso = 1/(1/hs+Rfo)
Where,
hs = Stud outside heat transfer coefficient, Btu/hr-ft2-F
Gn = Mass velocity of flue gas, lb/hr-ft2
Tb = Average gas temperature, F
Stud efficiency, E:
E = 1/((ex+e-x)/1.950)
Where,
X = Ls/12((2*hso)/(ks*Ds/12))0.5
And,
Ls = Length of stud, in
Ds = Diameter of stud, in
ks = Conductivity of stud, Btu/hr-ft-F
6.10 THERMAL CONDUCTIVITY OF METALS
The thermal conductivity of the tube material and the extended surface is needed for calculating
the heat transfer coefficients.
Graph 6 Thermal conductivity vs temperature
6.11 TUBE WALL TEMPERATURE CALCULATION
The temperature of the tube wall may be calculated using the following equations. This method
does not take coking into account.
Tw = Flux*do/di*Rfi+Flux*do/di*1/hi+Flux*do/(do-tw)*tw/(kw*12)+Tf
Where,
Tw = Tube wall temperature, F
Flux = Flux rate, Btu/hr-ft2 of bare tube
do = Outside tube diameter, in
di = Inside tube diameter, in
tw = Tube wall thickness, in
Rfi = Inside fouling factor, hr-ft2-F/Btu
hi = Fluid film coefficient, Btu/hr-ft2-F
kw = Thermal conductivity of tube wall, Btu/hr-ft-F
Tf = Bulk process fluid temperature, F
CHAPTER 7 VARIATION IN PROPERTIES
7.1 SINGLE PHASE FLUIDS
The thermal properties of the process fluid flowing through the fired heater are extremely
important to the fired heater designer. These properties not only have a direct affect on the
amount of heat transferred, they also are important in predicting the pressure loss and furnace
coking rates, etc.
For single phase fluids, liquid or vapor, the properties can normally be assumed to change on a
straight line basis from the inlet to the outlet of the heater. Therefore, providing the designer with
the properties of the process fluid at the inlet and outlet conditions will normally suffice.
The one exception to this, is the viscosity. The following formula may be used to correct the
viscosity using the two given values.
mnew = A * e(B/Tnew)
And the constants,
A = min * e(-B/Tin)
B = ln(min/mout) / (1/Tin-1/Tout)
Where,
mnew = Corrected viscosity, Cp
min = Inlet viscosity, Cp
mout = Outlet viscosity, Cp
Tnew = Temperature at new condition, °R
Tin = Temperature at inlet, °R
Tout = Temperature at outlet, °R
7.2 MIXED PHASE FLUIDS
For mixed phase process, obtaining the thermal properties of the fluid at the different points in
the fired heater is much more difficult than with the single phase flow. However, for a heater
with mixed phase at the inlet, the thermal heat transfer calculations may be performed using a
straight line approximation similar to that used with single phase, without much loss in reliability
of the results. It should be noted that when a heater has mixed phase at inlet and multiple tube
passes, the actual flow conditions in the various passes may not be equal.
For the more normal situation, where the inlet process is a single phase liquid and vaporization
begins at some unknown point in the heater, it becomes more difficult to estimate the properties.
One way to do this is to set up a grid of the properties based on various pressures and
temperatures.
7.3 HEAT TRANSFER COEFFICIENTS
The inside film coefficient needed for the thermal calculations may be estimated by several
different methods. The API RP530, Appendix C provides the following methods,
For liquid flow with Re =>10,000,
hl = 0.023(k/di)Re0.8*Pr0.33(mb/mw)0.14
And for vapor flow with Re =>15,000,
hv = 0.021(k/di)Re0.8*Pr0.4(Tb/Tw)0.5
Where the Reynolds number is,
Re = di*G/mb
And the Prandtl number is,
Pr = Cp*mb/k
Where,
hl = Heat transfer coefficient, liquid phase, Btu/hr-ft2-°F
k = Thermal conductivity, Btu/hr-ft-°F
di = Inside diameter of tube, ft
mb = Absolute viscosity at bulk temperature, lb/ft-hr
mw = Absolute viscosity at wall temperature, lb/ft-hr
hv = Heat transfer coefficient, vapor phase, Btu/hr-ft2-°F
Tb = Bulk temperature of vapor, °R
Tw = Wall Temperature of vapor, °R
G = Mass flow of fluid, lb/hr-ft2
Cp = Heat capacity of fluid at bulk temperature, Btu/lb-°F
For two-phase flow,
htp = hlWl + hvWv
Where,
htp = Heat transfer coefficient, two-phase, Btu/hr-ft2-°F
Wl = Weight fraction of liquid
Wv = Weight fraction of vapor
7.4 IN TUBE PRESSURE DROP
The pressure loss in heater tubes and fittings is normally calculated by first converting the
fittings to an equivalent length of pipe. Then the average properties for a segment of piping and
fittings can be used to calculate a pressure drop per foot to apply to the overall equivalent length.
Friction Loss:
Dp = 0.00517/di*G2*Vlm*F*Lequiv
Where,
Dp = Pressure drop, psi
di = Inside diameter of tube, in
G = Mass velocity of fluid, lb/sec-ft2
Vlm = Log mean specific volume correction
F = Fanning friction factor
Lequiv = Equivalent length of pipe run, ft
And,
Vlm = (V2-V1)/ln(V2/V1)
For single phase flow,
V1 = Specific volume at start of run, ft3/lb
V2 = Specific volume at end of run, ft3/lb
For mixed phase flow,
Vi = 10.73*(Tf/(Pv*MWv)*Vfrac+(1-Vfrac)/rl
Where,
Vi = Specific volume at point, ft3/lb
Tf = Fluid temperature, °R
Pv = Press. of fluid at point, psia
MWv = Molecular weight of vapor
Vfrac = Weight fraction of vapor %/100
rl = Density of liquid, lb/ft3
Equivalent Length Of Return Bends:
The equivalent length of a return bend may be obtained from the following curves based on
Maxwell table and can be corrected using the Reynolds number correction factor.
Lequiv = FactNre*Lrb
Where,
FactNre = Reynolds number correction
Lrb = Equivalent length of return bend, ft
Return Bend Equivalent Length:
Graph 7 Return bend equivalent length curve
Reynolds Correction:
Graph 8 Return bend equivalent length Nre correction
Where,
G = Mass velocity, lb/sec-ft2
Di = Inside tube diameter, in
Visc = Viscosity, cp
7.5 GAS SIDE PRESSURE DROP ACROSS TUBE
Bare Tube Pressure Loss:
For bare tubes we can use the method presented here
Dp = Pv /2 * Nr
Where,
Dp = Pressure drop, inH2O
Pv = Velocity head of gas, inH2O
Nr = Number of tube rows
And the velocity head can be described as,
Pv = 0.0002307 * (Gn /1000)2 / rg
Where,
Gn = Mass velocity of gas, lb/hr-ft2
rg = Density of gas, lb/ft3
The Mass velocity is described as,
Gn = Wg / An
Where,
Wg = Mas gas flow, lb/hr
An = Net free area, ft2
And,
An = Ad - do/12 * Le * Nt
For staggered tubes without corbels,
Ad = ((Nt +0.5) * Pt/12) * Le
For staggered tubes with corbels or inline tubes,
Ad = (Nt * Pt/12) * Le
Where,
Ad = Convection box area, ft2
do = Outside tube diameter, in
Le = Tube length, ft
Pt = Transverse pitch of tubes, in
Nt = Number of tubes per row
Fin Tube Pressure Loss:
For the fin tube pressure drop, we will use following method.
Dp = ((f+a)*Gn2*Nr)/(rb*1.083E+109)
And,
For staggered layouts,
f = C2 * C4 * C6 * (df/do)0.5
For inline layouts,
f = C2 * C4 * C6 * (df/do)1.0
And,
a = ((1+B2)/(4*Nr))*rb*((1/rout)-(1/rin))
Where,
Dp = Pressure drop, inH2O
rb = Density of bulk gas, lb/ft3
rout = Density of outlet gas, lb/ft3
rin = Density of inlet gas, lb/ft3
Gn = Mass gas flow, lb/hr-ft2
Nr = Number of tube rows
do = Outside tube diameter, in
df = Outside fin diameter, in
And,
B = An / Ad
For staggered tubes without corbels,
Ad = ((Nt +0.5) * Pt/12) * Le
For staggered tubes with corbels or inlune tubes,
Ad = (Nt * Pt/12) * Le
Net Free Area, An:
An = Ad - Ac * Le * Nt
Where,
Ad = Cross sectional area of box, ft2
Ac = Fin tube cross sectional area/ft, ft2/ft
Le = Effective tube length, ft
Nt = Number tubes wide
And,
Ac = (do + 2 * lf * tf * nf) / 12
tf = fin thickness, in
nf = number of fins, fins/in
Reynolds correction factor, C2:
C2 = 0.07 + 8 * Re-0.45
And,
Re = Gn * do/(12*mb)
Where,
mb = Gas dynamic viscosity, lb/ft-hr
Geometry correction, C4:
For segmented fin tubes arranged in,
a staggered pattern,
C4 = 0.11*(0.0 5*Pt/do)(-0.7*(lf/sf)^0.23)
an inline pattern,
C4 = 0.08*(0. 15*Pt/do)(-1.1*(lf/sf)^0.20)
For solid fin tubes arranged in,
a staggered pattern,
C4 = 0.11*(0.0 5*Pt/do)(-0.7*(lf/sf)^0.20)
an inline pattern,
C4 = 0.08*(0. 15*Pt/do)(-1.1*(lf/sf)^0.15)
Where,
lf = Fin height, in
sf = Fin spacing, in
Non-equilateral & row correction, C6:
For fin tubes arranged in,
a staggered pattern,
C6 = 1.1+(1.8-2.1*e(-0.15*Nr^2))*e(-2.0*Pl/Pt) - (0.7*e(-0.15*Nr^2))*e(-0.6*Pl/Pt)
an inline pattern,
C6 = 1.6+(0.75-1.5*e(-0.70*Nr))*e(-2.0*(Pl/Pt)^2)
Where,
Nr = Number of tube rows
Pl = Longitudinal tube pitch, in
Pt = Transverse tube pitch, in
Stud Tube Pressure Loss:
For the stud tube pressure loss we will use the following method,
The general equation for staggered or inline tubes,
Dp = Nr*0.0514*ns((Cmin-d0-0.8*ls)/((ns*(Cmin-do-1.2*ls)2)0.555))1.8*G2*((Tg+460)/1460)
Where,
Dp = Pressure drop across tubes, inH2O
Nr = Number of tube rows
Cmin = Min. tube space, diagonal or transverse, in
do = Outside tube diameter, in
ls = Length of stud, in
G = Mass gass velocity, lb/sec-ft2
Tg = Average gas Temperature, °F
Correction for inline tubes,
Dp = Dp*((do/Cmin)0.333)2
And,
G = Wg/(An*3600)
An = Le*Nt*(Pt-do-(ls*ts*rs)/12)/12
Where,
Wg = Mass flow of gas, lb/hr
An = Net free area of tubes, ft2
Le = Length of tubes, ft
Nt = Number of tubes wide
Pt = Transverse tube pitch, in
ls = Length of stud, in
ts = Diameter of stud, in
rs = Rows of studs per foot
CHAPTER 8 HEATER STACK DRAFT ANALYSIS
Fired heaters come in numerous configurations and designs. These configurations include
systems that are forced draft, induced draft or a combination of both, but many are natural draft.
The natural draft designs will be discussed in this section.
Regardless of the mechanics involved, the stacks purpose is the same, to safely disperse the
products of combustion into the atmosphere. For environmental reasons, many stacks are
required to discharge at a particular height. But in most cases they are designed only to meet the
needs of the furnace or furnaces they are designed for. In the case of the natural draft furnace, the
stack serves another purpose, that of assuring that the furnace stays below atmospheric pressure
throughout the setting.
For most stack designs, a gas velocity at the exit of about 15 to 25 ft/sec is sufficient to discharge
the gasses into the atmosphere at a rate that will assure they disperse properly. Additionally,
most natural stack are designed for 125% of the design flue gas flow to assure that if the furnace
is operated above the design point that it will still operate safely.
In this sketch, the area marked "A" is the height available, for the
differences in the density of the ambient air and the flue gas, to create
the draft required. Normally the draft required is that which will result
in a slightly negative pressure at point "B". It should be noted that, for
most heaters, the draft at point "C" required by the burners to induce
combustion air is not considered in setting the stack height. The
burners are normally sized to use only the draft in the firebox.
Fig 10 draft in heater
Pressure loss across stack entry:
This pressure loss can normally be considered as a sudden entry since the area of the outlet gas
plenum in the heater is usually much greater than the area of the inlet to the transition. A sudden
entry pressure loss can be approximated by the following equation.
Dp = 0.34 * Vh
Where,
Dp = Pressure drop, inH2O
Vh = Velocity head at inlet area, inH2O
Pressure loss across stack transition:
This pressure loss can normally be considered as a gradual contraction since the area of the inlet
and the outlet are usually close in area. A gradual contraction pressure loss can be approximated
by the following equation.
Dp = Ca * Vh
Where,
Dp = Pressure drop, inH2O
Vh = Velocity head at outlet area, inH2O
Ca = Coefficient based on included angle
And the coefficient can be described as,
Included Angle Ca
30
0.02
45
0.04
60
0.07
Pressure loss across stack damper:
This pressure loss is normally accounted for by rule of thumb. This may be 0.5 or 0.25 velocity
head. Assumed 0.25
Dp = 0.25 * Vh
Where,
Dp = Pressure drop, inH2O
Vh = Average velocity head of stack, inH2O
Stack friction loss:
For the stack friction loss, the following equation is used
Dp = (0.002989 * 0.018 * rg * Vg2) / Ds * Ls
Where,
Dp = Pressure drop, inH2O
Vg = Average velocity of stack, ft/sec
rg = Density of flue gas, lb/ft3
Ds = Stack diameter, ft
Ls = Stack length, ft
Stack draft gain:
The draft gain will be taken based on the height, "A" on above sketch.
Gd = (ra - rg)/5.2 * A
Where,
Gd = Draft Gain, inH2O
rg = Density of flue gas, lb/ft3
ra = Density of ambient air, lb/ft3
A = Height of gas path, ft
Pressure loss across stack exit:
This pressure loss, since it normally exits to atmosphere, can be considered as a sudden exit. A
sudden exit pressure loss can be approximated by the following equation.
Dp = 1.0 * Vh
Where,
Dp = Pressure drop, inH2O
Vh = Velocity head at inlet area, inH2O
Velocity head of gas:
Vh = Vg2 * rg / 2 / 32.2 / 144 * 27.67783
CHAPTER 9 DUCTING PRESSURE LOSS
Fired heater designers utilize ducting for many purposes in a fired heater design. They are used
for connecting flue gas plenums to stacks, distributing combustion air to burners, transferring
flue gas to and from air preheat systems, etc. The pressure losses through ducting pieces may be
individually analysed or the may be analysed as a system.
Straight duct run friction loss:
Dp = (0.002989 * Fr * rg * Vg2)*Le/De
Where,
Dp = Pressure drop, inH2O
Fr = Moody friction factor
rg = Average gas density, lb/ft3
Vg = Velocity of gas, ft/sec
Le = Equivalent length of piece, ft
De = Equivalent diameter of piece, ft
And for round duct,
De = Diameter
And for rectangular duct,
De = (2 * Width * Height)/(Width + Height)
90° Round section elbow loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
90° Rectangular section elbow loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Elbow of any degree turn loss:
This may be used for a rectangular or round duct elbow of N ° turn.
Dp = Vh * C90 * N/90
Where,
Vh = Velocity head of gas, inH2O
C90 = Loss coefficient from above for 90° turn
N = Number of degrees of turn
Sudden contraction loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Gradual contraction loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
No contraction change of axis loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Sudden enlargement loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Gradual enlargement loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Sudden exit loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
90° Rectangular miter elbow loss:
Dp = Vh * Cl
Where,
Vh = Velocity head of gas, inH2O
Cl = Loss Coefficient From Table
Draft gain or loss:
The draft gain or loss will be taken based on the height of the upward or downward flow of the
flue gas. If the flow is upward, the pressure loss is negative.
Dp = (ra - rg)/5.2 * A
Where,
Dp = Draft gain or loss, inH2O
rg = Density of flue gas, lb/ft3
ra = Density of ambient air, lb/ft3
A = Height of gas path, ft
CHAPTER 10 BURNER TYPE AND SELECTION
There are many aspects of burner design that need to be considered in conjunction with the
design of a fired heater. The major considerations that shall be accounted for in burner selection
and design are discussed below
10.1 DRAFT
Burners are broadly categorized into two types : natural and forced draft. Burners are sized based
on consideration of total air side pressure drop or draft loss across the burner.
Natural draft burner
The combustion air for natural draft burners is induced through the burner either by the negative
pressure inside the firebox or by the fuel gas pressure educting the air through a venturi. Natural
draft burners are the most found burner in general refinery service.
Forced draft burner
They operate with combustion air supplied at a positive pressure. The term “forced draft” is so
designated because the combustion air or other oxygen source is normally supplied by
mechanical means .They have higher available sir supply measured as higher air pressure
compared to a natural draft burner. This can allow the use of smaller or few burners for the same
equivalent heat release from a natural draft burner.
10.2 FLAME STABILITY
Above all the other consideration a burner shall operate safely and be stable within the burner
operating conditions. A stable flame is one where the root of the flame is firmly attached to the
designed flame stabilization point( the region within a burner that acts as a continuous ignition
zone for the flame), with no signs of the flame root jumping between other possible stability
zones.
The mixing of the combustion air with the fuel is critical to flame stability. Mixing energy can be
provided by the fuel discharge velocity and its direction of flow. Natural draft burners have to
rely more on fuel energy for mixing than do forced draft burner. They are more likely to have
poorer mixing.in case of forced draft burner turbulence is created within the burner improving
the mixing process and enhance flame stability.
10.3 DESIGN EXCESS AIR
Perfect combustion is achieved when all the fuel is burned using only the theoretical amount of
air. Perfect combustion cannot be achieved in a fired heater. Complete combustion is achieved
when all the fuel is burned using the minimal amount of air above the theoretical amount of air
needed to burn the fuel. With complete combustion, the fuel is burned at the highest combustion
efficiency. Incomplete combustion occurs when all the fuel is not burned, which results in the
formation of soot and smoke.
Excess air is air supplied to the burner that exceeds the theoretical amount needed to burn the
fuel. Combustion air requirements are based on the composition of the fuel used and the design
of the burner. Fuels commonly used contain nitrogen, ash, oxygen, sulfur, carbon and hydrogen.
When a fuel has a large volume of nitrogen that must be accepted along with the desired oxygen,
more excess air should be provided. That excess air has a chilling effect on the flame. Some fuel
particles fail to combine with oxygen and pass out of the stack unburned.
Water vapor is a by-product of burning hydrogen. It too subtracts heat from the flame and
becomes steam at flue gas temperature, passing out of the stack as vapor mixed with the
combustion products.
Natural gas contains more hydrogen and less carbon per unit of heat content than oil and
consequently its combustion produces a great deal more water vapor which withdraws a greater
amount of heat from the flame. Therefore gas efficiency is always slightly less than oil
efficiency.
Reducing excess air below design level will typically have following effects on emissions
pollutant
NOx
SOx
CO
Combustible
particulates
Effect on excess air
Decrease
No change in total Sox, however less SO2
converted to SO3
Increase
Increase
Increase
Table 2 effect of pollutant on excess air
10.4 COMBUSTION AIR PREHEAT
The addition of heat to the combustion air increases the efficiency of the combustion process.
Higher air preheat temperature will increase flame temperatures. This will increase the
concentration of NOx in the flue gas while the mass will reduce. Hence the extent of air preheat
at design conditions and consider this when specifying equipment for low emissions of NOx
10.5 TURBINE EXHAUST GAS
The oxygen for the combustion of fuel in fired heater can be supplied by the flue gas streams
such as the exhaust from a gas turbine . Gas turbine exhaust streams contain between 13 to 17
volume percent of oxygen at temperature between 454 ◦C and 565 ◦C .Burner can operate with
oxygen content down to approximately 15 volume percent in turbine exhaust streams, below this
level combustion can become unstable
10.6 COMBUSTION AIR ADJUSTMENT
Burners are normally provided with airside control devices to adjust the air rate into the burner.
Air registers or dampers are provided for this purpose.
CHAPTER 11 AUXILIARY EQUIPMENT
11.1 AIR PREHEAT SYSTEM
The air preheat system is used to preheat the combustion air
going to the burners. Since it cools the flue gas further, while
removing heat, it improves the efficiency of the furnace. Using
an air preheat system will frequently result in overall
efficiencies above 90%.
Fig 11 Air preheat system
Regenerative Air Heater
The regenerative air heater is widely used in the boiler and power generation industry. For this
reason, when the fired heater industry developed a need for air heaters, these designs were the
first to be used. They consist of a setting enclosing a large rotor which holds baskets of heat
exchange surface. This rotor causes the baskets to pass through the hot flue gas, where the
material heats up, and then through the cold air, which it heats. Even though these were very
good at exchanging large amounts of heat, they had a downside of leakage from the higher
pressure side(air) to the lower pressure side(flue gas) and of course they have moving parts. The
sketches below show the basics of this type air heater.
Fig 12A regenerative air heater
Fig 12C basket
Fig 12B rotor
Recuperative, Tubular Air Heater
The recuperative air heater comes in a variety of types.
The one shown below is designed to set atop the fired
heater(in place of the stack). The air is drawn in across
the shell side of the exchanger, then directed to the
burners. After combustion, the flue gas travels through
the heater and then the air heater on the tube side, where
it exists to the artmosphere.
Recuperative, Cast Tube Air Heater
Fig 13 Recuperative, Tubular Air heater
This recuperative air heater utilizes cast tubes and has become the most popular air heater in use.
It has very low, if any leakage across the heat transfer surfaces. It is durable with no moving
parts. If corrosion is a problem, the cold end units are usually glass tube.
Fig 14 Recuperative, Cast Tube Air heater
Heating Medium Air Heater
This air heater comes Is part of a recirculating hot oil system
which recovers heat from the flue gas exiting the fired heater
and uses it to preheat the combustion air going to the burners.
Fig 15 Heating medium air heater
11.2 SOOT BLOWER
Soot blowers are becoming less and less important in the Petrochemical and Refinery industry
during modern times. The reason for this is that generally, any component of the flue gases that
can foul the tubes can also foul the air. So, as clean air requirements continue to get more
stringent, the resulting cleaner fuels are less likely to foul the tubes.
Retractable Soot Blower:
The retractable blower gets it's name from the fact
that the element that does the cleaning is retracted out
of the hot gas stream when it is not in use. Since it
has less nozzles than a fixed rotary blower, it can
produce a better cleaning velocity for the same steam
pressure. This type blower is used for the higher
temperature and dirtier fuel applications.
Fig 16 Retractable soot blower
Fixed Rotary Soot Blower:
Fig 17 Fixed rotary soot blower
The rotary blower is in a fixed position
and since the element is supported by the
hangers at different points, the element
requires less material than the retractable
blower. This blower costs less than the
retractable, but requires a greater volume
of steam. This type blower is used for the
lower temperature and cleaner fuel
applications.
Sootblower Recommendations:
Under normal operating conditions, sootblower selection can be done as follows:
A. Below 100 ppm vanadium or 25 ppm sodium or 2% sulphur in the fuel oil:
Rotary sootblower to 1800°F gas temperature.
Retractable sootblower above 1800°F.
B. 100 to 200 ppm vanadium or 25 ppm sodium or 2% sulphur in the fuel oil:
Rotary sootblower to 1300°F gas temperature.
Retractable sootblower above 1300°F.
Above 200 ppm vanadium or contents in excess of 25 ppm sodium or 2% sulphur in the fuel
C.
oil:
Rotary sootblower to 1050°F gas temperature.
Retractable sootblower above 1050°F.
Under control conditions other factors may be used to modify sootblower selection.
11.3 FANS AND BLOWER
Fans and blowers, as used in fired heater service, usually fall into one of the following
categories.
Forced Draft Fans
Induced Draft Fans
Purge Fans
Each of these purposes are usually served by a different type fan.
Forced Draft Fans:
Forced draft (FD) fans are used in heater service to supply combustion air to the burner(s). They
draw in oxygen rich ambient air and force it through the burner system for the combustion of
fuel. They normally are not employed to move the flue gasses through the furnace setting, but in
special cases, such as a positive pressure, single fan air preheat system, they may also provide
the energy for this purpose.
The FD fan is normally handling clean, ambient air. One exception is when the air preheater is
located upstream of the FD fan. In this case it is handling hot clean air. When handling clean
ambient air, the basic parameter of fan selection is the fan's efficiency
Induced Draft Fans:
Induced draft (ID) fans are generally used to pull the flue gas from the heater and discharge it to
atmosphere directly or through an air preheater. Applications where this discharge is direct are
unusual. An example might be where a fired heater was also being used as a waste gas flare. The
normal use of the ID fan is on a two fan air preheat system. A single ID fan system might work
but many factors would favor the FD fan single fan system over the ID fan.
On two fan air preheat systems the fan selection is usually dictated by system pressure drop and
type of fuel being burned. Centrifugal fans are almost always used since nothing but the wheel
and shaft are actually exposed to the hot gas stream
Purge Fans:
The purpose of the purge fan is to discharge from a heater setting any potentially explosive gas
mixtures prior to lighting of the burners. Purge fans are especially desirable on heaters that are
shut down and relighted on a regular basis. Heaters employing FD or ID fans do not normally
require additional purge fans. Running the FD or ID fan usually will purge the system.
Natural draft heaters, however, are often fitted with purge blowers. When a cold heater
experiences a light off failure, it can build up a potentially explosive gas mixture. As the heater
setting is cold, there is no "natural draft" available to remove these gases. The purge fan is
normally a small low pressure drop axial fan.
It is important to protect these fans when they are not being used, since the radiant heat they may
be exposed to would destroy the fan. A butterfly valve is often used to shut the fan off from the
hot gases in the firebox. Some purge fan applications require the injection of purge air at
multiple entry points.
11.4 DAMPERS, LOUVERS, & DIVERTERS
Dampers and louvers can generally be broken down by the following types.
Stack Damper:
This is a basic device found on most heaters and
furnaces. The stack damper is necessary to adjust the
draft in a natural draft furnace. Even if the designer
could calculate accurately enough to exactly predict
the draft in the furnace, he would still over design
the stack to take care of future modifications. This
damper is normally not of a tight seal type, in fact it
generally is manufactured with two or three inches
of clearance around the perimeter. This damper
would not normally be used to control combustion
air, but rather to assure that too much draft is not
available in the furnace. Too much draft would
result in air leakages into the furnace which would
result in reduced efficiency. Manual operation is
normally used, but actuators can be used for
automated operations.
Fig 18 Stack Damper
Butterfly Damper:
Butterfly dampers are single blade, low leakage or minimum leakage dampers
utilizing a round, square or rectangular blade. Butterfly dampers are well suited
for on/off service or flow control balancing. Electric, pneumatic, hydraulic and
manual gear actuators with pneumatic or electrical positioners can be used.
Parallel Blade Louver:
The parallel design is used primarily for isolation. The use of blade edge and
jamb seals achieves minimal leakage past the closed damper. These dampers
are used mostly in air preheat and heat recovery systems. They would be
used where a tight shut off is required, such as isolating one unit from
another in a multiple unit system. Electric, pneumatic, hydraulic and manual
gear actuators with pneumatic or electrical positioners can be used.
Opposed Blade Louver:
The opposed design exhibits the best flow control characteristics with
moderate leakage past the closed damper. This type damper would be
necessary if the accuracy of the flow amount is required or if the flow pattern
must be maintained, such as when entering a burner zone. Electric,
pneumatic, hydraulic and manual gear actuators with pneumatic or electrical
positioners can be used.
Double Louvers:
The double louver design utilizes two banks of blades. Zero flue gas leakage
is achieved by pressuring the area between two closed banks of blades with
seal air. The parallel/opposed configuration provides zero leakage yet retains
good flow control characteristics. These dampers can be found in applications
where fast bypass capability is required during an upset condition. Double
louvers can also be found where overhead space is not available to install a
slide gate damper.
Slide Gate(Guillotine):
Slide gate isolation dampers are available in low leakage and zero leakage
designs utilizing either machine screws or chain drives. Slide gate dampers
can be furnished with electrical actuators, air actuators or hydraulic actuators.
Flow Diverter Dampers:
Flow diverter dampers are utilized on many process and heat recovery
systems to direct the flow in one or two directions. In a heat recovery
system, the diverter damper would normally direct the gas to a waste heat
recovery system or in a bypass mode through a silencer and stack. The
diverter damper can also be furnished with a single end pivoted blade to
direct the gas flow into one of the two outlets. The metallurgy of the
damper is based on the design temperature and pressure of the system.
CHAPTER 12 HEATER DESIGN AND ENGINEERING
The engineering used in heater design can be mainly classified into following type
12.1 PROCESS/THERMAL ENGINEERING
It mainly consists of burner design (type, numbers, burner circle diameter, etc) , radiant coil design
(number of passes, coil diameter and length , pitch circle diameter, etc), convection coil design (number
of passes, coil diameter and length ,number of rows, etc), refractory material and thickness , Anchoring
system and stack sizing
12.2 MECHANICAL ENGINEERING
It mainly consists of design of tube, design of fitting, design of tube supports, radiant , convection and
header box sizing, hydro test calculations.
12.3 STRUCTURAL / CIVIL ENGINEERING
It consists of load for radiant section, convection section and stack , support structure for pipping near
heater area as per the project requirement, design of pipe support anchors , civil load data
12.4 ENVIRONMENTAL ENGINEERING
It is mainly concerned with limitations for emission of NOx ,SOx , CO, particulate matter and unburnt
hydrocarbon
12.5 INTERFACE ENGINEERING
As the name suggest it involves interface within various departments involved in designing of fired
heater. The departments are process, instrumentation, piping , civil , electrical and machinery.
CHAPTER 13 LOAD DATA SHEET
Having understood components, construction and working of fired heater I moved ahead to
develop load data sheet which would help the company in preparing the project proposal at the
initial stage of the project in cost estimation and visualizing the 2-D view of the heater. It would
also help to find the potential problems that would occur at the time if erection of fired heater at
sight and hence finding the solution so forth. I developed load data sheet for vertical cylinder
heater as well as box heater. However , before starting the load data sheet I learnt how to
interpret general arrangement diagram (GAD) and data sheet.
Fig 19 tab 1 input sheet
Fig 20 Tab 2 fired heater 2D diagram
Fig 21 Tab 3 load calculation
Fig 22 total weight calculation
TIME LINE OF THE PROJECT
8 January– 12
February
Basics of fired
heater,components,
standard followed ,
engineering involved
, aid in live project
12 February– 12
March
Understanding fired
heater in detail, API
560 standard , John
Zink Hamworthy
handbook volume
1,2 and 3
12 March– 12 April
Learn how to
interpret GAD, data
sheet an developing
load data sheet for
vertical cylindrical
heater
12 April– 31 May
Developing load data
sheet for box heater,
understanding heat
transfer mechanism
in fired heater,
analysis of draft in
stack, report
preparation
REFERENCES
API 560 standard
Book name :
The John Zink Hamworthy Combustion Handbook, Second edition, Volume 1 – Fundamentals
The John Zink Hamworthy Combustion Handbook, Second edition, Volume 2 – Design and Operation
The John Zink Hamworthy Combustion Handbook, Second edition, Volume 3 – Applications
Lobo & Evans, Heat Transfer in the Radiant Section of Petroleum Heaters, AICHE, Vol. 35,
1939
Last page
Project Group Personal Details
Name of the Student: Shah Vrund Bhavinkumar
Permanent Address: D/2 Ganesh Park society , Nr. Kedar dham, Manjalpur, Vadodara , 390011
Email: vrund.smc17@sot.pdpu.ac.in
Mobile no:8238677755