7b Oil Burners

Design of Oil Burners

Oil BurnersFuel: Furnace Oil

Advantages:– Easy transportation– Simple and less expensive design makes it very

attractive.Design of Oil supply System:

Considerations in Design of oil supply system–Safety–Cost–Convenience–Emergency provision

Heating loop:To minimize the energy consumed in pumping,

viscosity is reduced by heating the fuel. •Single Unit heating loop•Centralized heating loop

-Needed where a common header (mother piece) is placed between heater and pump.

Oil supply system with two stages of pumps:The heater must bear the very high oil pressure at the pump

exit. The oil pressure at the pump exit should be higher than 2Mpa for 120 T/H. As the pressure increases the complication of design increases.So Two Stage systems are used.

Atomizer Oil is injected through oil gun. To increase the contact area of fuel with air, fuel is spitted into fine droplets. This process is called as Atomization of Fuel.

“Finer the atomized oil droplets, the faster they Burn.”Types of Atomizer:• Mechanical Atomizer

• Steam or Compressed air Atomizer

• Low pressure air Atomizer

• Rotary Cup Atomizer

Mechanical Atomizer:

Pressure of the fuel oil itself atomizes the Fuel, also called as pressure jet atomizer.

Main Components:•Atomizer•Swirler•Flow distributor arranged along the burner axis

Oil flows through 1.set of small holes of flow distributor.2.Tangential grooves of oil swirler, where oil gets swirled.3.oil swirls rapidly in swirl room and spurt out from atomizing nozzle.

Mechanical Atomizer:

Angle of AtomizationAtomization Angle:

Atomized oil spurts out of Nozzle forming a Cone Shape. Angle of this cone is Atomization Angle.

Exit Atomizing angle(α):

Draw a line at the exit of atomizer nozzle tanent to the edge of the spray. Angle between the two tangents is Exit Atomizing angle.

Conditioned Atomized angle:At certain distance “x”from atomizing nozzle a line is drawn

perpendicular to the axis of oil spray. If we draw two lines from atomizer inlet to the points of intersection of perpendicular line with the spray profile we get a smaller cone, angle of this cone is Conditioned Atomized angle

“Conditioned angle is less than the Exit atomizing angle. The difference can be 20o or greater”

Atomizer

Spray atomizing size, (Size of Droplets): Larger droplet size dm depends on the relative velocity, Wrel, between the oil droplet and the air as follows

143.03

P

FNdm

5.0

P

GFN

2rel

m W

Kd

Where, dm = largest droplet diameter,

K = coefficient depending on the oil viscosity and surface tension( about 600)

Sauter mean diameter,

FN is flow number ratio given by

AtomizerOil Flux density:

Volume of fuel flowing through unit area perpendicular to the dierection of oil spray in unit time.

•It should be uniform around the periphery otherwise result in poor Combustion

• Also very high flux density is not desirable at spray center a Recirculation Zone.

Design Methods

Calculation of Oil Flow Rate:

Assumptions:

•Oil is an ideal liquid with out any frictional resistance.

(A) Oil Enters tangentially and leaves in the shape of a conical jet

(B) High pressure produces a wide cone

(C) Low oil pressure produces a twisted narrow jet

(A) (B) (C)

• Centrifugal force acts on the liquid, By applying Bernoulli's Equation and law of conservation of mass, flow rate of

oil can be derived as

Design Methods

hKgPrG op 2.3600 2

Where µ is the flow coefficient given by

2

2 1

1

1

A

Where the ratio of oil flow area and the nozzle area,

2

2

1p

w

r

r

Design Methods

rp= nopzzle radius, m

rw= radius of oil film vortex inside the nozzle,m

PO= oil pressure at the inlet of nozzle above that at the exit,Pa

ρ = oil density, Kg/m3.

A = Characteristics flow constant given by

f

RrA p

f

0dAd

Total area of tangential grooves and R is swirl radius

For maximum flow rate We get,

Substituting we get,2

1

A

2

3

Design Methods

2.Calculation of Atomizing Angle:

11

81

2 w

utan

Where, u = tangential velocity & w = swirl velocity.

The actual cone angle is slightly different from that calculated

3.Emprical Method of Calculation of Flow Coefficient µ:

from tests carried out following relationship are established 88.0p

87.0p

Where,

μp= practical flow coefficient

αp= practical exit atomizing angle

µ, α = Theoretical values calculated from the above formula

Design Methods

Empirical formula method:

If A = 0.8 - 0.1.8; inlet oil pressure = 2.0 Mpa, oil viscosity = 3oE & Nozzle Diameter < 5 mm.

1.137.0 ApreeAR deg57200 37.0

Nozzle characteristics may also be obtained to allow calculation of more practical values of the flow coefficient µ using the equation

2

2 1

1

1

A

AR

rA p

p

862.0

094.2

Where, Ap is a modified practical value of the characteristics coefficient

By using the practical flow coefficient and friction loss we can calculate oil flow rate as follows

hKgPPrG opp 23600 2

Design Methods

Frictional Loss of the Atomizer can be calculated as

Pa 22 vP

Where the velocity in the swirl chamber is

fg

Gv

3600

Wd

Re

Reynolds number is based on the groove dimension

Where, 5.02 OPW

bhhbd 2

Circle Mechanical( Return Flow) Atomizer

The Oil pipe arrangement for a return oil or circle oil atomizer

Cross sectional view of a return oil inner circle type atomizer

C. Circle Mechanical( Return Flow) AtomizerInlet Area Charging Atomizer:

Flow rate is adjusted by changing the inlet area of tangential groove

Circle Atomizer:

Oil Enters swirl chamber through tangential holes in swirl section. one part of oil swirls out from the atomizer nozzle: the other part returns from the swirl chamber through a number of circle holes on the baack of the swirl chamber. This oil returns to the pump. It is called inner circle atomizer.

There is another design known as outer circle atomizer, where oil enters through the central hole and returns through peripheral holes.

Multiple holes for return oil can be drilled in a circle on the back plate of the swirl chamber. When the diameter of the holes is 1.3 times the diameter of the atomizer nozzle, the maximum adjustment ratio nmax an be about 3.

min

maxmax G

Gn

Steam (or compressed Air) Atomizer

•Consumes High steam

•Atomization is assisted by either high –pressure air or high pressure steam. Accordingly they are of the steam-mechanical atomizer or pure steam atomizer type.

Types:-•Pure steam Atomizer•Steam-mechanical Atomizer type

Pure Steam Atomizer

Steam Mechanical atomizers are two types:

Inner mixing type– Steam and Oil meet in the mixing chamber at the tip of burner.

Outer mixing type– Oil meets the steam outside the atomizer, chances of high

pressure oil entering the steam to contaminater steam is minimum

Design of Y-Type Atomizer

1.Number of Holes, n:

Between 5 and 30 per atomizer

2.Oil hole diameter, d1:

For chosen number of holes, n, the hole diameter d2 can be found from the total cross sectional area of the oil holes

Design of Y-Type AtomizerRequired Oil flow rate G,

3

30

1

Kg/m oil, ofDensity

0.7 Usually t,Coefficien Flow

Kg/h Atomizer, of rate flow Oil

Where,

.23600

G

PPP

PFG

3.Steam or Air Hole Diameter,d2

The diameter d2 is based on the mass flow rate m2

air compressedfor 2.14 saturated,for steam,1.99 heatedsuper for 09.2

mholes,air of area onalcrosssecti TotalF

tcoefficien flow

where,

Kg/h)(3600

22

222

sPFm

Design of Y-Type Atomizer

23 PP

Graph represents:

Flow coefficient µ for flow of oil through oil holes plotted as a function of oil hole exit velocities and different ratios of areas of air/steam hole(F1),oil hole area (F2) and mixing hole area (F3) for Y-Type manometer.

4.Mixing Hole Diameter d3:

Ratio of mixing hole diameter and steam hole diameter (d3/d2) is of the range 1.4-1.8.

Design of Y-Type Atomizer5.Length of Mixing Hole: l3 :

The distance between the exit ioil holes and steam holes l2 is an important parameter, allowing good mixing of steam..

Following ratio’s may be used:

0.50.4

7.0

33

31

dl

dl

6.Angle between Oil and steam Hole:

Recommended range of angle is 50-65o

7.Atomizing Angle:

Preferred range: 70-900.Actual atomizing may be about 15-250 larger than the Value.

8. Diameter , Dair, of steam hole center line:

12

dnDair

Design of Y-Type Atomizer9.Annular Diameter, Dmix of the Mixing Hole:

For geometry it is given by

2

sin2 321a

airmix lllDD

10.Droplet sizes:

mmW

dq

mv

drsteam

oil

drop 3.0

2.01.03

5.0

1.05.0 11200

at density steam

dyne/cm oil, theof tension surface

cm hole, mixing ofdiameter d

oil gsteam/ grate,n consumptio Steam

g/s holr,flower oilm

(Cst) oil ofViscosity

where,

steam

3

oil

oil

oil

q

v

air compressedfor 38.3

steam saturatedfor 22.3

steam heatedsuper for 33.0

/5.0

k

k

k

smvPkw steamsteamcr

Low pressure Air Atomizer

Design Procedure:

1.Diameter Doil:

mmWG

mWGDoil

5.0

5.0

.8.18

36004

2.Cross sectional Area of Nozzle:

2196 mmP

GA

O

oil

Where,

µ= Oil Nozzle flow coefficient = 0.2-0.3,

Po= Oil Pressure before Nozzle in Pa, N/m2

ρ = Oil Density Kg/m3

3.The Diameter of air inlet tube is found to be,

Where, Da= Air Consumption rate Kg/h

ρa = Air Density Kg/m3

Wa = Air Velociy10-15 m/s

4.Exit Section Area, Aa of air nozzle

Where, Va = air flow rate, m3/h

Pa = Air pressure befoe the nozzle, Pa

µ = Air nozzle exit for flow rate coefficient = 0.6- 0.8


mmW

GD

aa

aa

8.18

2196 mmP

VA

a

aaa

5.Theorotuical size (radius) of the spread of atomized oil droplets, r

6.The approximate flame lengthis given by

Where, L = flame length in m,

Vair = air flow rate per unit of fuel, m3/Kg

dnoz = diameter of atomizer nozzle,m

Other Types,

1. K-type atomizer

2. R-type atomizer

3. RK-type low-pressure atomizer

4. TB-type low-pressure atomizer


mm

Pr

a

300200

mdV

L nozair

60422

Cross sectional view :

b) Operating principles and Design Calculation:1.Radial velocity um,

Where,

Q = Oil flow rate,m3/sDdrop = Inner diameter of the rotary cup, mN = Rotary speed of cup, rpmV = kinetic viscosity of oil, m2/sθ = Slope of inner surface of the cup

Rotary cup Atomizer

3

122 sin66.0

cupm vD

NQu

• Substituting the value of um

1.Oil Film Thickness

Where, DL= Diameter of the cup mouth

VT = tangential velocity of the oil

a = radial distance from the cup mouth from the axis of the cup

Rotary cup Atomizer

mcup uhDQ .

31

22 sin

0484.0

nD

Qvh

21

22

22

42

aaD

V

uD

V

QS L

mL

T T

363.0. 81.0

19.014.167.081.0

LD

vQN

Critical condition to form a continuous oil film is

Where, ρ = oil Density

Q = critical oil flow rate

σ = surface tension( 2.5 Kg/m when ρ = 900 kg/m3

N = Revolution

v = Viscosity2.The tapering angle, θ,of the rotary cup:

To avoid larger droplet size, θ is selected to be larger than 35o

3.Effect of air nozzle position and the impact on the quality, valid for a >5 mm

Where , σ = Surface tension g/cm

ρa = air density, g/cm2

Mr = Weight ratio of air and oil

Vr = Velocity ratio of air and oil

VT = tangential velocity of oil, cm/s

cmVVV

Q

MaaD

Vd

rrTrLa

rs

5.0

235.125.025.0

21.05.04

15.0

065.01

59.0106

Rotary cup Atomizer

• Swirl air register:

Classification:

Turbo air Registers

Tangential moveable vane air registers

Axial movable impeller air registers

Fixed Tangential impeller air registers

Advection-Flow Air Register:

• Simple Design

• Small resistance and low NOx formation

Air Register

Air Register

Venturi-type advection- flow air register Without root air primary air passing through flame holder around atomizer.

With Root Air where primary air is premixed with the oil in the gun

Air Register

Good atomization and good air arrangement are primary Objectives

•Root air is necessary

•Early mixing should be Intense

Effect of position of the oil nozzle with respect to air register exit on mixing of air with fuel

• Size and location of Recirculation Zone should be appropriate

•Later stage Mixing should be intense Too

A good practical design of air register should have

•Root air

•Flame holder in center air to help stabilize the flame and increasing early mixing.

•Improvement adjustment of the secondary air and reduction in its swirl intensity.

Design of other types of Swirl Air Register

• Turbo Air Register

• Tangential Vane Air Register

• Axial Vane Air Register

Tangential Vane Air Register Axial movable Swirl type Air Register

Design of other types of Swirl Air Register

Design for Advection-Flow Air Register:

In venturi type air register, flow through the air register q,

Where, Q = Air flow rate through the register, m3/h

ξ = resistance coeffficient

Fin= Annular setion area at the inlet, m2

Fh = Section area of the throat, m2

P1-P2 = Pressure difference between inlet and throat

222

22211 hhh VVPVP

2

1

1

236003600

in

h

hhhh

F

F

PPFFVQ

11

12

fh

dw

w

d

DQ

Q


Where, Qw =outlet near tuyre

Q = Total flow

dfh = diameter of flame holder, m

Dd = diameter of duct where flame holder is located, m

22

111

1

D

d

w

r

Over all resistance coefficient,

Where, ξr = Resistance Coefficient at inlet of Advection flow air register

ξw = Resistance coefficient of the flame-holder


Resistance coefficient corresponding to the tuyere outlet is

Where, F = Calculating Section area, m2

F0 = Outlat area of tuyere, m2

2

22

2

0

111

1

F

F

D

dF

F O

w

rO

Design Principles of oil fired Boilers:

a)Combustion Volume:

To quantify residence time of fuel air in the furnace, Furnace volume heat release rate is used represented by

Where, B = fuel flow rate,kg/s

LHV = heating value, KJ/Kg wet

Vf = Furnace volume, m3

αx= excess air coefficient

Volume of gas,

Where, Tf = Average temperature of gas in furnace ,o k

Pf ,Patm = pressure of gas in furnace and in standard atmosphere.

)KW/m(. 3

fv V

LHVBq

KgNmLHV

V xfg /4186

08.01.1 3

)/(273

3 KgmP

PTVV

f

atnffgg

Effective Utilization volume,

Where, φeff = Volume effectiveness

Retention period of the fuel in the furnace,

Average temperature of the gas in the furnace,

Where, Tth = Theoritical Adiabatic flame temperature, o K

Tou= gas temperature at the furnace exit, o K

Where Tpre= preheat temperature of combustion, oC

Correlation of Pressure drop with volumetric Heat release:

feffeff VV

)(4186273

08.01.1s

PqT

V

BV

V

atmvxf

feff

g

feff

KTTT outhf 925.0

KTT prex

th

2737.03.0

2730

m

vqKP


Correlation of Pressure drop with volumetric Heat release:

ΔP = pressurte loss of the burner

K’ = Coefficient; value depends on arrangement of burner

m = empirical constant: value lies sin the range 0.9- 1.1

Furnace Cross sectional area:

Cross sectionbal area depends on the characteristics parameter known as Furnace grate heat release , qf

Where, B = Fuel flow rate, Kg/s

LHV = Lower heating value of fuel, KJ/Kg

Fs = Cross sectional area of furnace, m2

m

vqKP

2

sec

/.

mKWF

LHVBq f



Ratio of grate and the volumetric heat release rate gives the furnace height, h

Average Heat release rate for oil fired Boilers

Burner zone heat flux is also used at times for design. This index is defined by

furf

v

f HF

V

VLHVBFLHVB

q

q

sec

sec

.

.

2/.

mKWS

LHVBq

rb

Steam Capacity(t/h)

130 220 400 670-950 1000-1600

Qf( MW/m2) <4.0 4.07 - 4.77 4.19 - 5.23 5.23 - 6.16 <9.0

Arrangement of the burners

There are different burner arrangements:

Front wall, opposite wall, Tangential and Bottom Arrangements

•To avoid flame interference in swirl type burners a minimum distance is maintained between the burners.

•Clearance is provided to avoid impingement on the side and bottom of furnace

The minimum distance are

•The distance between center lines of adjacent Burners(2.5-3.0)d0

•The distance between the burner and the side walls (30-3.5) d0

•The distance between the burner and the furnace bottom 3.0 d0

where do is diameter of the mouth of the Burner.


Arrangement of Burners on the furnace wall of a 500MW Oil fired boiler, the direction of swirl in individual burner is shown by the arrow


A plot of total resistance coefficient of a mechanical atomizer against log(ARe), where A is the Characteristic nozzle co-efficient and Re is the Reynolds number on the Basis of the Groove:

1. Tangential groove atomizer

2. Tangential hole cylinder Atomizer

Thank you

Documents

7b Oil Burners