fired heater - 2013 fw talk
DESCRIPTION
Fired HeaterTRANSCRIPT
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PRESENTED BY :
RUBY SAHU
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OVERVIEW
What is a Fired Heater
Main Components
How does Fired Heater Work
Selection of Fired Heaters
Design Parameters
Typical Design Parameters for different Fired Heaters
Special Design Consideration
Roles and Responsibility of Fired Heater Engineer
Fired Heater Design
Thumbs rule
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FIRED HEATER
A combustion equipment
Produces energy by combustion fuels
Provide the necessary heat to the process fluids (inside tubes) for different process:
Distillation
Cracking
Reforming
Hydrotreating
Isomerization
Process fluid IN
@ temperature T1
Process fluid OUT
@ temperature T2
T2 > T1
Fuel
Air
Heater body at
Negative Draft
FLUE GAS
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MAIN COMPONENTS VERTICAL CYLINDRICAL FIRED HEATER
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MAIN COMPONENTS BOX TYPE FIRED HEATER
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OPEN
BURNER
DAMPER
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ID Fan
CLOSE
BURNER
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50% 50%
FD Fan FD Fan
OPEN
BURNER
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HEAT TRANSFER
50% 50%
APH
ID Fan
FD Fan FD Fan
CLOSE
BURNER
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Selection of Fired Heater
Criteria Cylindrical Box Type
Space available Less More
Heat duty (MMBtu/hr) All radiant < 5 > with
convection > 120
Very long tube - Horizontal tubes in box
heater
High temp. service - Tubes at box centre with
burner on both the sides
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DESIGN PARAMETERS PROCESS DESIGN PARAMETERS
Process unit & heater type Number of passes Heat absorbed Fluid flow rates, temperature & pressure (inlet and outlet) Fluid Properties (viscosity, specific heat, thermal
conductivity) Fouling Factor Average Heat Flux Turndown and overdesign requirements
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DESIGN PARAMETERS OTHER SPECIAL DESIGN PARAMETERS
Thermal efficiency required Draft mechanism and Air pre heater requirement Forced draft fan & drivers Induced draft fan & drivers Soot blowers Fuel type
For burner selection Excess air determination
Stack emissions limits
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Typical Heater Design Parameters
(1) CRUDE HEATER
Thermal Cracking Tendency :LOW
Outlet Temperature, °F :625 to 700
Pressure drop, psi :150 to 250
Mass velocity, Lb/(sec-ft2) :250 to 350
Radiant heat flux, Btu/(hr-ft2) :10000 to 12000
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Typical Heater Design Parameters
(2) VACUUM HEATER
Thermal Cracking Tendency :LOW/ Slight
Outlet Temperature, °F :715 to 800
Pressure drop, psi :50 to 75
Mass velocity, Lb/(sec-ft2) :250 to 350 (Except outlet tube)
Radiant heat flux, Btu/(hr-ft2) :9000 to 12000
Size outlet tubes for less than sonic velocity
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Typical Heater Design Parameters
(3) VISBREAKER HEATER
Thermal Cracking Tendency :MEDIUM/HIGH
Outlet Temperature, °F :840 to 890
Pressure drop, psi :210 to 350
Mass velocity, Lb/(sec-ft2) :300 to 400
Radiant heat flux, Btu/(hr-ft2) :8000 to 12000
Minimum process fluid velocity, ft/sec :6
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Typical Heater Design Parameters
(4) DELAYED COKER HEATER
Thermal Cracking Tendency :HIGH
Outlet Temperature, °F :920 to 940
Pressure drop, psi :350 to 400
Mass velocity, Lb/(sec-ft2) :350 to 450
Radiant heat flux, Btu/(hr-ft2): • Single Fired Heaters 8000 to 10000
• Double Fired Heaters 12000 to 15000
Minimum process fluid velocity, ft/sec :6
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Special Design Consideration
(1) Chemical Fouling
(2) Average heat flux
(3) Inside film temperature
(4) Fluid velocity and Residence time
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(1) Chemical Fouling (Coking)
Decomposition/ Cracking of Process Fluid
Depends upon: Fluid Composition
Residence Time
Impacts: Pressure drop inside tube
Poor Heat transfer
Increase in tube metal temperature/ tube failure
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(2) Average heat flux
Crude heater Flux selection : Non-Fouling/Coking services : 12000 Btu/(hr-ft2)
Mildly Fouling/Coking services : 10000 Btu/(hr-ft2)
Highly Fouling/Coking services : <9000 Btu/(hr-ft2)
Effects of Lower Heat Flux : More radiant surface area required
More process fluid pressure drop
More passes to fit pressure drop
More expensive heater
Lower inside film temperature
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(3) Inside Film Temperature
Primary indicator for fouling potential for fouling/ coking services
Can be measured from: Tube metal temperature-TMT
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(4) Fluid Velocity and Residence Time
Fluid velocity affects:
Tube metal temperature
Potential return bend erosion
Residence time affects:
Inside film temperature
Coking
Tube metal temperature
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Roles and Responsibility of Fired Heater engineer
Sizing of the Fired Heater (Thermal calculation) Tube Layout Tube Diameter Number of Tubes Tube Metallurgy Software used: FRNC
Burner selection Type
Type of Fuel Draft mechanism
Special type of Burners such as Low NOx burner Number of Burners
Minimum clearance required Heat Duty requirement
Refractory Refractory material Refractory Thickness Design code: As per ASME C 680 code
Structural calculation Thickness of Plate/ Structural member sizes Wind/ Seismic Calculation Software used: STAAD Pro
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FIRED HEATER DESIGN
NUMERICAL EXAMPLE FOR SIZING
A FIRED HEATER WITH
CAPACITY OF 40MM Btu/hr
(SERVICE :HOT OIL HEATER)
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Design duty (Q) 40 MM Btu/h
Type Vertical Cylindrical/ Horizontal Convection Box, Gas Fired
Process Stream Hot Oil
Mass flow rate (m) 1931062 lb/h
Inlet temperature (Tin) 482 ˚F
Outlet temperature (Tout) 518 ˚F
Design pressure (Pdesign) 150 psig
Inlet Pressure (Pin) 60 psig
Outlet Pressure (Pout) 30 psig
Allowable pressure drop (ΔP) 30 psi
Efficiency (E) , minimum 85%
Specific Heat Capacity (Cp) @ inlet 0.567 Btu/ lb.˚F
Specific Heat Capacity (Cp) @ outlet 0.584 Btu/ lb.˚F
Average radiant Heat Flux Rate 10,000 Btu/ft2.h
Combustion Excess Air 20%
INPUT PARAMETERS
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Step1: Total duty calculation
Q = m x Cp x ΔT
m = 1931062 lb/ hr, the mass flow rate of the process fluid
Cp = 0.576 Btu/lb-°F, the average specific heat of the process fluid from the inlet to outlet
ΔT = 36°F temperature increase of the process fluid from inlet to outlet
Q = 1931062x0.576x36
= 40042501.63 Btu/hr
= 40MM Btu/hr
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Step2: Radiant and convection duty split calculation
Q= QR+QC
QR= the radiant heat
transfer absorbed by radiant heater coils from fuel combustion
QC = the convection heat transfer absorbed by convection heat transfer coils from fuel combustion
QR
QC
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QC= Q x (%stack + %Excess air)
%Stack = Percent convection
duty based on stack temperature and bridge wall temperature (BWT)
%Excess air = Percent convection duty based on excess air
QC
BWT
Stack
Temperature
20% excess air
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TMT = Tout+ Est.50°F
= 518 + 50
= 568°F
From the graph,
BWT = 1440°F
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Stack temperature is a function of excess air and overall heater efficiency.
η = calculated efficiency
+ Radiation loss
= 85 + 1.5
= 86.5%
From the graph,
Stack temperature = 470°F
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From the graph shown on
the right for 470°F and 1440 BWT, we obtain the convection duty split as 32.5%.
%Stack = 0.325
% Excess Air = 0
QC = 40 X (0.325+0)
= 13 MMBtu/hr
QR = 27 MMBtu/hr
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Step3: Radiant heating surface area calculation
RAD Surf = QR/Flux Avg
= 27 000 000/10 000
= 2 700 ft2
Total Effective tube length = (RAD Surf) / (Tube DO x π/12)
Assumption : 6 inch (in) pipe is selected having a OD of 6.625 in
(Tube DO x π)/12 = (6.625 x π)/12
= 1.734 ft
Radiant total effective length = 2700/1.734
= 1557 ft
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Radiant effective length = (Radiant total effective length
– Shield tube length)
Shield tube length = (TCD-G) x
Tubes per row TCD = (Qty of tubes x Radiant
spacing)/ π Assumption : Let us assume that the
Qty of tubes in the radiant coil is 48 and that tube spacing is 1 ft.
TCD = (48 x 1)/π = 15.28 ft
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Shield tube length = (15.28-1) x 8
= 114.24 ft
Radiant effective length = 1557- 114
= 1443 ft
Radiant effective tube length
= (Radiant effective length)/(Radiant tube Qty)
= 1443/48
= 30.6 ft
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To confirm this design the L/D ratio must be less than 2.75, in accordance with API 560:
= 30.6/15.28
= 2.0 (Approx) < 2.75
The design therefore complies with API 560.
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Thumb rules for design of Fired Heater
Radiant Section
Volumetric heat release maximum limit:
For oil fired : 12000 Btu/hr/ft3
For gas fired : 16000 Btu/hr/ft3
For vertical cylindrical heaters L/D ratio: <2.75
For vertical tube box heaters H/W ratio: <2.75
Maximum length for vertical tubes: 18.3m
Maximum unsupported length for horizontal tubes is lesser of 35 X OD
or 6m
Minimum distance between refractory and tube centre is 1.5 X Nominal
diameter
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Thumb rules for design of Fired Heater
Convection Section
Flue gas temperature should be below dew point temperature
Flue gas mass velocity (Kg/s-m2) Natural Draft: 1.5~3.0
Forced Draft: 3.0~4.5
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REFERENCES
ANSI/API STANDARD 560
FOURTH EDITION, AUGUST 2007
ANSI/API STANDARD 530
SIXTH EDITION, SEPTEMBER 2008
Direct Fired Heaters- A Practical Guide to their Design and Operation, by Roger Newnham
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Thank You