Fired Heater - 2013 FW Talk

PRESENTED BY :

RUBY SAHU

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

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

MAIN COMPONENTS VERTICAL CYLINDRICAL FIRED HEATER

MAIN COMPONENTS BOX TYPE FIRED HEATER

OPEN

BURNER

DAMPER

ID Fan

CLOSE

BURNER

50% 50%

FD Fan FD Fan

OPEN

BURNER

HEAT TRANSFER

50% 50%

APH

ID Fan

FD Fan FD Fan

CLOSE

BURNER

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

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

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

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


(2) VACUUM HEATER

Thermal Cracking Tendency :LOW/ Slight



Mass velocity, Lb/(sec-ft2) :250 to 350 (Except outlet tube)


Size outlet tubes for less than sonic velocity


(3) VISBREAKER HEATER

Thermal Cracking Tendency :MEDIUM/HIGH





Minimum process fluid velocity, ft/sec :6


(4) DELAYED COKER HEATER

Thermal Cracking Tendency :HIGH




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

Special Design Consideration

(1) Chemical Fouling

(2) Average heat flux

(3) Inside film temperature

(4) Fluid velocity and Residence time

(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

(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

(3) Inside Film Temperature

Primary indicator for fouling potential for fouling/ coking services

Can be measured from: Tube metal temperature-TMT

(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

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

FIRED HEATER DESIGN

NUMERICAL EXAMPLE FOR SIZING

A FIRED HEATER WITH

CAPACITY OF 40MM Btu/hr

(SERVICE :HOT OIL HEATER)

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

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

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

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

TMT = Tout+ Est.50°F

= 518 + 50

= 568°F

From the graph,

BWT = 1440°F

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

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

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

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

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

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.

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

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

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

Thank You

Fired Heater - 2013 FW Talk

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