Lecture 04 v2

PLANT DESIGN - A.Goldis, D. R. Lewin Furnace Design1 4/5-

054410 Plant Design

LECTURES 4/5: FURNACE DESIGN

Alon Goldis and Daniel R. Lewin Department of Chemical Engineering

Technion, Haifa, Israel


Lecture Objectives After this lecture, you should:

Be familiar with correct operating practice of industrial furnaces, with emphasis on:– Operation principles– Role of burners– Air supply and flue gas removal– Performance monitoring

Be familiar with the principal control configurations implemented for furnace regulation

Be able to design the radiant section of an industrial furnace


Furnaces

Distillation furnace (sig. C6 verso)Author: Brunschwig, Hieronymus, ca. 1450-ca. 1512

Title: Kleines Distillierbuch


Furnaces


Applications

1. Vacuum charge heater

2. Reformer Furnace

3. Crude furnace

4. Pyrolysis furnace

5. Visbreaker furnace

6. Hydrocracker furnace

7. Air heater

8. Oil heater

9. Others


Operation principles

Process Fluid:

Fluid characteristics of the process fluids should be considered before designing a heater. For example, very high viscosity fluids have tendency to attain very high film temperature, as the fluid in the film does not readily mix with the bulk fluid. This results in uneven distribution of heat in the fluid and develops hot spots, where vaporization and degradation occurs. Heat Duty:

Total furnace heat duty is the sum of heat transferred to all process streams, including auxiliary services such as steam super heaters. Amount of heat duty affects the selection of type and configuration of heater.


Operation principles – cont’dAverage Radiant Heat Flux:

By making the design radiant flux as large as possible, we lower the heat transfer surface required, thus reducing both the heater size and its cost. However, this will result in higher maintenance cost due to shortened life of components and coke deposition. Allowable average radiant heat flux rate is a function of various factors such as heater type, feedstock, service, coil outlet temperature etc. and, therefore, established by experience.

Mass Flow Velocity:

To minimize coking and fouling in coils, fired heaters should be designed with high enough mass velocities. However, too high a mass velocity will cause a high coil pressure drop, resulting in high pumping or compressor costs, increased design pressure of the coils and upstream equipment. Design mass velocity is usually kept in the range of 250-350 lb/sec-ft2. Under turndown conditions, mass velocity should be kept above 150 lb/sec-ft2 in order to prevent excessive coking and fouling of the coils.


Operation principles – cont’dVaporization:

It is desirable to avoid a situation of 100% vaporized stream. Foreign material or polymer formed in tankage, which does not vaporize, may deposit on the tube and cause coking. Therefore, limit the maximum vaporization in limited in practice to about 80%.

Tube size, number of passes and fluid pressure drop:

A combination of the tube size and number of passes is selected to satisfy the mass flow velocity, throughput and fluid pressure drop requirements.

Turndown:

Set by process considerations. Turndown rates of 60% can be used without falling below mass velocity rates needed to prevent excessive coking rates. Burner turndown is a function of burner design and the type of fuel. Burner turndown does not normally affect furnace turndown.


Operation principles – cont’dStack temperature and optimum heater efficiency:

The economic stack temperature or the optimum efficiency of the heater is a function of fuel value, inlet oil temperature, investment cost of the incremental convection section and the required rate of return from incremental investment. Stack temperature usually ranges from 350°F to 700°F. However, a temperature of 250°F can be achieved for low sulfur fuel using air preheater. Stack temperature must be high enough to prevent acid condensation on the convection section inlet tubes and air preheater.

Tube/coil materials:

Usually made from carbon steel, alloy steel or stainless steel pipes. Tubing material is selected based on service life, corrosion resistance and cost. Allowable stresses in the tube material decrease with increasing temperatures, therefore, higher tube temperatures require thicker tubewalls or higher alloy-content. Carbon steel is the most widely used material for heater tubing where corrosion resistance is relatively mild.


Burners

Burners start and maintain combustion in the firebox by mixing fuel and air due to fuel gas pressure and air draft.

The mixing of fuel and combustion air occurs in the gas phase, so, all liquid-fuel burners use atomizing devices to break up the liquid mass into micron-size droplets.

When steam is not available for atomizing and oil is the only fuel available for firing then air atomization or mechanical atomization can be used

The number of burners depends on the size of the heater and the heat duty to be supplied.


Burners

Atomizer

Oil Lance Assembly


Burners – Cont’dBurners should be selected to provide stable combustion with the following characteristics:

- Ability to handle wide range of fuels.

- Predictable flame patterns for all fuels and firing rates.

- Good turndown ratio between maximum and minimum firing rates.

- Low noise and NOx levels.

- Provision for safe ignition, easy maintenance. - Low excess air operation.


Burners – Take a look on the inside

The operator checksthe “quality” of the fire

Burners in action


Air Supply & Flue gas removal

The firebox has slightly negative pressure due to density difference

of flue gases and outside cold air. To overcome the frictional losses

and to maintain the negative pressure in the firebox sufficient stack

height is provided for exiting the flue gases to atmosphere.

Air supply is carried out using one of the following methods: Natural Draft (NF)- air is drawn by the draft created by the

stack Forced Draft (FD) - air is supplied by a centrifugal fan (blower) Induced Draft (ID)- flue gases are driven out by a centrifugal

fan Balanced Draft - When both FD and ID fans are used


Air Supply & Flue gas removal – cont’d Cold Flue gas

Hot Flue gas

Cold Air

ID Fan

FD Fan

Preheater

Damper


Excess Air

Excess air is expressed as a percentage of the theoretical quantity of air required for complete combustion of the fuel. Typically, oil fired forced draft heaters and gas fired natural draft heaters need about 10-15% excess air where as oil/combination fire natural draft heaters need 15-20% excess air. Lowering the excess air helps in reducing NOx emissions from the fired heaters and also minimizes heat losses with the flue gases.

On the other hand, low values of excess air cause heavy smoke


Heat flow in furnace

Radiation section

Tubes are installed along the walls and roof of the combustion chamber

Convection section

The tubes in this section usually have extended surfaces, such as fins or studs to improve heat transfer increasing the overall efficiency of the heater

Atmosphere

Shield (bridge-wall) section

The first 2-3 bared tubes rows in the convection section which "shield" the remaining tubes from the direct radiation


Performance Monitoring

Some of these key parameters are: 1) Process stream flow and Temperature; 2) Temperature measurement; 3) Fuel firing; 4) Tube skin temperature; 5) Flue gas temperature; 6) Flue gas draft profile and analysis.

Most fired heater operations can be optimized to improve efficiency and save money. Some of the common problems observed with fired heater operations are uneven flow distribution in various passes, high excess air operation, high stack temperature, fouled convection section, flame impingement and over firing. Optimizing the performance of a fired heater requires close monitoring of key parameters on both process side as well as combustion side.


Performance Monitoring- cont’d

Process stream flow and temperature:Poor flow distribution to the coils in multi-pass fired heaters leads to low flow to one or more passes causing overheating, coking and tube burnout in those passes. It is important to use flow controllers on each pass of heaters processing liquid hydrocarbons, where low flow in one pass can lead to excessive vaporization, increased pressure drop and further flow reduction. Also, keep a watch on the flow regime and the

coil velocities at the outlet of heater coils, in vaporizing services.

Slug flow and higher velocities could cause vibrations in tube and failure due to erosion.


Performance Monitoring- cont’d Temperature measurement:

Temperature measurement at the outlet of each pass is used as a guide for adjusting the flow rates of each pass as well as for calculating the process heat duty. It is also recommended to measure the temperature of process fluid at the outlet of each pass in the radiant and convection sections which helps in calculating the process heat duty split between the radiant and convection sections.

Tube skin temperature:

Tube skin temperature provides guidance in setting the maximum firing rates. Tube skin thermocouples are recommended for each pass.


Performance Monitoring- cont’d Fuel firing: Fuel firing rate is used to calculate the heat release in firebox. In a simple fuel firing control scheme, process fluid outlet temperature controller provides the set point for the pressure controller on burner fuel supply.

Flue gas temperature: Monitoring of the temperature of flue gases at outlet of radiant section, inlet of convection section and outlet of convection section is useful in establishing the maximum firing rate, heater efficiency and convection section fouling.


Performance Monitoring- cont’d Flue gas draft profile:Measure the draft at various places such as burner level, radiant section outlet, convection section outlet and downstream of stack damper. An insufficient draft may lead to positive pressure inside the firebox, causing flue gas leakage from the openings. On the other hand, a high draft draws more combustion air into the firebox reducing the efficiency of the system.

Flue Gas Analysis: Flue gas analysis helps in maximizing the combustion efficiency. Using oxygen analyzer in the flue gases, excess air can be controlled by varying furnace draft.


FYI - NOxSince 1970, EPA (U.S. Environmental Protection Agency)

hasbeen tracking six principal pollutants: Carbon monoxide (CO) Lead Nitrogen oxides, Particulate matter Sulfur oxides Volatile organic compounds.

Two of the most common oxides of nitrogen are: NO and NO2.

In stationary source, combustion approximately 90% of NOx

formed is NO. After NO leaves a stack, in the presence ofsunlight, ozone, and volatile organic compounds , it

becomesNO2, which (in extreme cases) appears as a reddish-brownplume.

http://www.epa.gov/


FYI - NOxHow is NOx formed? Thermal fixation of atmospheric nitrogen and oxygen in

the combustion process. The formation rate of thermal NOx is dependent on the

reaction temperature, the local stoichiometric, and the residence time.

Thermal NOx is most readily influenced by the combustion system.

Breakdown of CH portions of methane and other hydrocarbons in the fuel and their subsequent combination with nitrogen in the air.

The rate of formation of NOx is dominated by combustion conditions and can be suppressed by modifying the combustion process. Both thermal and fuel NOx are promoted by rapid mixing of oxygen with the fuel. Thermal NOx is greatly increased by long residence time at high temperature.


Furnace control and operation

The PFD on the right shows the proposed control system for a furnace as seen from the perspective of the process.

What has not been taken care of?

Ref: W. Driedger,“Controlling Fired Heaters, Hydrocarbon Processing, April 1997


FUEL GAS FLOW CONTROL-OVERVIEW


FURNACE SAFETY SYSTEMS


SAFETY SYSTEM FOR PILOT GAS TRAIN

Isolation valve (FC)


Vent valve (FO)

BSLL

PI

Flame detector

i/o of Burner Management System (BMS)




Vent valve (FO)

Fuel FC valve

SAFETY SYSTEM FOR MAIN GAS TRAIN

PSLL

PSHH

TSHH

BSLL

PI


PROCESS-RELATED SAFETY SYSTEMS

It is not obvious that we should automatically shut off the process feed in the event of a failure.

TSHH

Isolation valve (FL)

Depressurization valve (FL)

FSHH

Why?

Why?


Setpoint to fuel FC is lowest of (required duty, ratio air flow) Setpoint to air

FC is highest of (required duty, ratio fuel flow)

Required duty

LEAD/LAG COMBUSTION CONTROL

This control system ensures that the fuel/air ratio in the furnace is always below the explosion limit


Radiation Section DesignRef.: Lobo & Evans, Heat Transfer, AICHE, Vol. 35, 1939

)T(TAασ Q (1) 4w

4gcpr F

Direct radiation in the radiant section of a direct fired heater can be described by:

Tw = Average tube wall temperature, °R

Tg = Effective gas temperature in firebox, °R

F = Exchange factor

Acp = Cold plane area of the tube bank, ft2

a = Relative effectiveness factor of the tube bank

s = Stefan-Boltzman constant, 0.173x10-8 Btu/ft2-hr-R4

Qr = Radiant heat transfer, Btu/hr


Relative Effectiveness Factor, a


Calculation of cold plane area-Acp

tubetubetubes LSN cpA (2)

To simplify the calculations with respect to coils arrangement inside the heater, it is convenient to express the tube area as an equivalent plane area for heat transfer by radiation.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 single sided firing:

For double sided firing:

Where,Ntube = Number of tubes Stube = Tube spacing, ftLtube = Effective tube length, ft

tubetubetubes LSN 2A (3) cp


Calculation of 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 back 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. This is a value considered typical for oxidized metal surfaces.

(Ref.: Mekler & Fairall in Petroleum Refiner, June 1952.)



Aw - Effective refractory area, ft2

aAcp - Equivalent cold plane area, ft2



Calculation of Aw/aAcp :

The total refractory area, AR, is simply the total of the refractory area exposed to the radiant section of the heater.

cpRw AAA (4)

Aw - Effective refractory area, ft2

AR - Total refractory area, ft2

aAcp - Equivalent cold plane area, ft2



Flue Gas Emissivity calculation : The gas emissivity can be described by the curve presented on the next slide. The tube wall temperature has only a minor effect. Therefore, the emissivity can be correlated as a function of LP and the gas temperature, Tg. Variations in tube wall temperatures between 600 and 1200°F cause less than 1% deviation from these curves.


Calculation of exchange factor - FLP – product of the

partial pressure (in atm) of the flue gas times the mean beam length, in ft. Thus LP has units atm-ft.

Mean Beam Length is computed using a formula that depends on the furnace configuration.

Dimension Ratio in furnace is the ratio of -

Wide : Height : Tube eff. Length


Calculation of exchange factor - FEffective 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. For most applications, this is an acceptable assumption. Still there are exceptions to be aware of.

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. In purpose to simplify the calculations it is convenient to set this temperature ( as a constant ) prior to the calculations.


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:

)T - (TAh Q (5) wgtcc

Where,Qc - Convection heat transfer, Btu/hrhc - Film heat transfer coefficient, Btu/hr-ft2- °RAt - Area of the tubes in bank, ft2Tg - Effective gas temperature in firebox, °RTw - Average tube wall temperature, °R


Film heat transfer coefficient, hc

This value cannot be calculated precisely, and is usually selected by experience or rule of thumb. The arrangement of the tubes as well as the firebox design contributes to this factor.

a. For horizontal tube heater: For small heaters , hc = 1.5

For multiple tube cells , hc = 2.8.

b. Vertical heaters: For L/D < 2 , hc = 2

For L/D > 2, hc = 3


Radiant heat transfer to shield ( bridge-wall) tubes

Another heat loss from the radiant section is a heat transferred to the shield tubes (if any), Qs. For the examples in this section, we will assume no shield tubes are present.


Heat balance in the radiant sectionThere are three primary sources of heat input to the

radiantsection of the furnace:

Summarizing:

outlossSRotherairrls Q Q Q Q Q Q Q (6)

Q rls - heat release by burners

Q air - heat of the combustion air

Q other - heat of the fuel and any atomizing medium

On the other hand, heat is also leaves the radiant section by:

QR - heat absorbed by radiant tubes

Qs - heat absorbed by shield tubes

Qloss – heat losses through the casing

Qout - the sensible heat of the exiting flue gas


Heat balance in the radiant section – cont’d

Qrls - Heat release by burners, Btu/hr

The heat released by burners can be calculated for defined

fuel composition and the heating values of it’s variouscomponents. For liquid fuels, the heating values are

obtainedby a calorimeter test. From these values and by using thecombustion equation, we can determine the composition

of theflue gas. For example - the combustion of methane could

bestated as:

O2HCO2OCH (7) 2224

But… What happens when a fuel gases contain many morecomponents and burning is carried out in air rather than in pure oxygen ?



Qair - Heat in combustion air, Btu/hr

The heat available in the combustion air, such as frompreheated air, or using Gas Turbine Exhaust, etc., is taken

asthe heat content above 60 °F, since that is the design

datumtemperature for fired heaters. For the purpose of thisdiscussion, radiant heat transfer (to air) can be

neglected,i.e., consider the air at 60 °F. 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.

These must be taken into account in heater design, however,

for the purposes of discussions, those can be neglected .



Qloss - Heat loss through setting, Btu/hr

These losses, referred to as Setting Loss or Radiation Lossare 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/hrFrom the flue gas composition, the overall enthalpy of the fluegas at a specific temperature can be calculated. Theseenthalpies can be obtained from the curves.


Class Exercise No.1

Design a radiant fired heater following the data below:

(1) Process Conditions (specifications) : - Heat Absorbed - 9,500 KBtu/hr - Tube Wall Temperature- 600oF - Fuel- Gas - Excess Air - 15 % (2) Mechanical Conditions (proposed initial estimates): - Tube Diameter – 4.5 in - Tube Spacing - 8 in - Tube Effective Length – 26.0 ft - Number Of Tubes - 30 - Area Of Flue Gas Exit - 42 ft2

- Radiant Arrangement - Box


Class Exercise No.1


Class Exercise No.1 - Solution

1.77784.5"8"

diameter TubeCenter to Center

Step 1: Find a

0.915



252026

ftinch

12

8inch30 ftft

tubetubetubes LSN cpA Step 2: Find Acp

For single sided firing:

Therefore: 2

sp 475.8ft 5200.915 A



R

2

2

A 2 2 2

2 8 26 2 8 (10 4") 2 (10 4") 26 42

1076.64

areaWL WH HL Exit

f t f t f t f t f t f t f t

f t

cpRw AAA

Step 3: Calculation of radiant exchange factor - F

Recalling eq. (4):

Where: AR is total refractory area, i.e. the total of the refractory area exposed to the radiant section of the heater.

2w R cp

2w

2cp

A A A 1076.6 475.8 600.84

A 600.841.2628

A 475.8

f t

f tf t



Step 4a: Beam Length Calculation.

Step 4: Calculation of flue gas emissivity

Furnace dimension ratio -

W:H:L = 8:10.333:26= 1:1.33:3.3 ~ 1:1:3,

So, the Mean Beam Length is: 2/3(Furnace volume)1/3

= 8.6ft

Step 4b: Partial pressures of flue gases

Partial pressures of a gas (in atm) depends of their mole fractions in the flue gas mixture. It can be assumed to be the sum of the partial pressures of CO2 ( 0.085 atm ) and H20 (0.1722atm ), that is, 0.2572 atm



Summarizing: PL = 8.6ft x 0.2572 atm = 2.212 atm-ft

Step 4: Calculation of flue gas emissivity

Assumption: the flue gas temperature ( Tg) is 1400 oF

0.47

PL



Step 5: Implementation of gas emissivity and the value

of Aw/aAcp, calculate the Exchange factor F

0.73



KBtu/hr 6128.9

))460600()4601400((73.0520915.00.173x10

)T(TAασ Q (1)448-

4w

4gcpr

F

/hr1102.7KBtu

)6001400(3026/ft12"

4.5"1.5)T - (TAh Q (5) wgtcc

ft

Step 6: Heat calculations

Assuming a small heater with one tube cell, hc=1.5, heat absorbed by radiant tubes by convection can be calculated:

Therefore, the total heat absorbed by radiant tubes is:

KBtu/hr 6.73211102.7 6128.9 QQQ crR

Recalling eq. 1 for calculating the heat absorbed in tubes by radiation:


Class Exercise No.1 - SolutionStep 7: DiscussionThe total heat absorbed in radiant tubes is far short of the 9,500 KBtu/hr heat transfer to satisfy process requirement.

2Rad , ;Flux (8) ftsurfacetubesradianttotalisA

AQ

RadRad

R

Possible reasons for this are:

1. The surface area of 30 tubes is to small

2. A bridgewall temperature of 1400 °F is to low – additional iteration need to be done !

3. Introducing a new parameter- the Flux Rate, which is a measurement of how hard heat is being absorbed by the tubes using the following equation:



Step 7: Discussion – cont’d

Note: for direct fired heaters, the average flux rate should be in the range 6-18 KBtu/hr-ft2.

2Rad 34.103026)

125.4

(

r9500KBtu/hFlux (8)

fthrKBtu

NLDQ

AQ

tubeseff

R

Rad

R



outlossSRotherairrls Q Q Q Q Q Q Q (6)

Step 8: The total heat balance

Recalling the total heat balance equation:

For the given example, the combustion air is at 60 °F, so Qair drops out of the equation. The fuel is gas at 60 °F, so there is no atomization and Qother also drops out . The furnace has no shield tubes, so Qs drops out also, reducing eq. 6 to:

outlossRrls Q Q Q Q (9)



Q0.01 Q (10) rlsloss

Step 8: The total heat balance, cont’d

Using the rule of thumb regarding the Heat loss and setting Qloss as 1% of heat released by the burners:

The heat released by the burners, Qrls, can be calculated via:

lbBtufueltheofvalueheatingLower

hrlbflowrateFuel

Where

/, fuelLV

/, fuelW

:

LVW Q (11) fuelfuelrls


Class Exercise No.1 - SolutionStep 8: The total heat balance, cont’dThe heat lost with the flue gases, Qout, can be calculated

from:

lbBtugasflueofEnthalpy

ratiofueltoAirfuelair

R

Where

R fuelair

/, gas flue

E

/

:

E)W(W Q (12) gas fluefuel/fuelout



outlossRrls Q Q Q Q (9)

Step 8: The total heat balance, cont’d

Rearranging eq. 9 in terms developed previously:

hrlb

EERQ

ERQ

fluefluefuelair

R

fluefuelairR

/ 56.752))379.88379.8879.180345201.0(20345

9500000W

))LV01.0(LVW

)WW(LVW01.0LVW

fuel

gas gas /fuelfuelfuel

gas fuel/fuelfuelfuelfuelfuel



%05.62%10075.15310

9500%100 x

hrKBTUhr

KBTU

xQQ

Effrls

R

Step 9: Furnace efficiency calculationFinding the fuel rate consumption provides the heat flow

released

from burners :

The efficiency of the heater is:

hrKBTU

lbBTU

hrlb

VLfuelWfuelQrls 75.153102034556.752


Lecture Summary This lecture has covered:

Correct operating practice of industrial furnaces, with emphasis on:– Operation principles– Role of burners– Air supply and removal– Performance monitoring

Principal control configurations implemented for furnace regulation

Design of the radiant section of an industrial furnace

Lecture 04 v2

Documents

Transcript of Lecture 04 v2