Design of Burners

8/3/2019 Design of Burners

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Prepared By:-Ramnarayan Meena P11TD001

Krishnat Patil P11TD053

Sudesh Powar P11TD008

Hitesh Thakare P11TD040

Ashish Mogra P11TD041

Design of Burners

Submitted To :-

Dr. S.A. Channiwala


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CONTENTS

History of Burners

How does combustion occur?

What is a burner? Mixing of air and gaseous fuel

Characteristic features of jet

Behavior of free (unconfined) and confined jet

Role of primary air

Degree of recirculation

Selection of burner


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History of Burners

History of burners dates back to early shipping days, when fuel oil first started

replacing coal as the ships primary fuel source.

Since then, burner designs and construction has come a long way, but the

principles behind their operation has remained the same.


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Why do we want to use Burners .. ??

Boilers

Thermic fluid heaters

Industrial oxidizers

In drying applications


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Burners - Introduction

The burner is the device used to combust the fuel with an oxidizer to convertthe chemical energy in the fuel to thermal energy

Specifically, it is a device used to provide a controlled exothermic oxidation

reaction. However, the device itself is not consumed in the reaction. (e.g.

wooden torch is not a burner.)

A burner is designed to provide stable operation and an acceptable flame

pattern over a specific set of operating conditions.


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To provide acceptable operation, burner must be designed to provide

FiveMs:-

Meter the fuel and air into the flame zone.

Mix the fuel and air to efficiently utilize the fuel.

Maintain a continuous ignition zone for stable operation over the

range.

Mold the flame to provide the proper flame shape.

Minimize pollutant emissions.


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Burner Design - Introduction

Fig. 1. Schematic of an industrial Combustion Process.


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Components ofCombustion System

1.Burner:-

It combusts the fuel with an oxidizer to release heat

2. Load:-

Material that will be processed.

3. Combustor (Furnace, Heater or Dryer):-

Inside which burner and load are located.

4. Heat Recovery Device:-

To increase the thermal efficiency of the overall combustion system.

5. Flow Control System:-

Uused to meter the fuel and the oxidant to the burners.6. Air pollution control system:-

To minimize the pollutants emitted from the exhaust stack into the

atmosphere


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Need for Atomization

Even though fuel oil is classified as a flammable liquid, most fuels will not burn

easily in a liquid state.

If you were to drop a lit match in a container of fuel oil, it would PROBABLY go

out almost immediately (dont try this .. !!).

In order for fuel oil to burn, it must first be transformed from a liquid to a

vaporised state atomised.

Atomisation increases the exposure of the fuel to the oxygen in the air thispromotes combustion.

A nozzle rated at 0.60 US gallons/hr. can generate as many as 50 million

droplets of oil in an hour.


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EFFECTS OF BAD ATOMISING

If atomisation is incomplete, the droplet sizes are too large for complete

combustion.

The larger droplets will escape the flame only partially burnt. This can usually

be seen as fire flies when looking at the flame.

This will not only result in a poor flame, but also soot deposits being formed

inside the combustion chamber.

In addition the combustion plants efficiency will reduce causing excessive fuelusage for the required energy output.


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BURNER DESIGN FACTORS

1. Fuel :-

1. Whether liquid or gaseous fuel is used .. ??

2. Adv of gaseous fuels over liquid fuel.

3. Need of Dual fuel burners.

2. Oxidizer

1. Predominantly, air is used as oxidizer .2. However, problems associated with its use.

3. Preheated air and FlGR

3. Gas Recirculation

1. FuGR Technique used to induce furnace gases to be drawn into the burner

to dilute the flame.2. This dilution is accomplished to:-

a. minimize NOx emissions by reducing the peak temperatures in the flame.


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3. Gas Recirculation

1. FuGR Technique used to induce

furnace gases to be drawn into the

burner to dilute the flame.

1. This dilution is accomplished to:-

a. MinimizeNO

x emissions byreducing the peak temperatures in the

flame

b. To increase the convective heating

from the flame because of the addedgas volume and momentum.


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BURNER COMPONENTS

Ignition System

Plenums

Burner TileControls

Flame Safety System


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Ignition System

important component in the

burner system to ensure safe and

reliable operation.

Plenums:-


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Plenums

It is used to homogenize the incoming

gas flows to evenly distribute them to

the outlet of the burner.

This is important to ensure proper

operation of the burner over the entire

range of operating conditions, especially

at turndown.

If the plenum is too large, then the flows

may be unevenly distributed across the

burner nozzle outlet.

If the plenum is too small, then thepressure drop through the plenum may

be excessive.


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Burner tile/ block or quarl

it helps shape the flame and protects

the internal parts from overheating.

In the majority of designs, the burner

tile is made of some type of ceramic

that often contains alumina and silica,depending on the temperature

requirements.

There may be holes through the tile to

enhance mixing of furnace gases with

the gases fed into the burner.

The tile may have bluff body

components that enhance flame

stability.


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flame safety system

critical to the safe operation of

the combustion system.

include some type of flame

scanner or flame rod to ensure

that either the burner or the pilotis operating.

These are connected to the fuel

supply system so that the fuel

flow will be stopped if the flame

goes out to prevent a possible

explosion for unignited fuel gases

contacting a hot surface

somewhere in the combustor


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Calculatingthe Heat Releasefroma Burner

HR = mfxHV

where,

HR is the heat release of the burner,

mfis the mass flow rate of the fuel, and

HV is the heating value of the fuel.


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Concept of Sonic & Subsonic Flow


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Equations for Calculating Fuel Flow Rate

Step I:-

D

etermine if orifice is operating above or below the critical pressure.Critical Pressure ratio is given as:

where Pc = critical pressure ratio

k = ratio of specific heats of the fuel.

If Pc > (Pb / Pt), then the fuel exits the orifice at Sonicconditions.

If Pc < (Pb / Pt), then the fuel exits the orifice at Subsonicconditions.

where, Pb = atmospheric pressure

Pt = fuel pressure in absolute, respectively.

)1/(

1

2

-

!kk

c

kP


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Continued .. Step II:- Determine the mass flow rate of the fuel through the orifice.

If the fuel exits the orifice at sonic conditions:-

If the fuel exits the orifice at subsonic conditions:-

)1(21

)21(

1

2

)/(

-

vv!

kk

cut

ctdf

kk

MWgRT

AgPCm

eeedf cAMcm V!

-

!

11

2

1

k

k

b

t

eP

P

kM


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where

The subscript e denotes the orifice exit,

Me = Mach number of the fuel,

Te = Temperature of the fuel,Ce = Speed of sound in the fuel, and

e = Density of the fuel.

MW

RT

P

ue

b

e!V

2

2

11

e

t

e

Mk

TT

!

2

1

-

!

MW

RukTC ee


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Atmospheric Gas Burners

Based on Bunsen Burners.

Components:-

Fixed Orifice, Spud Controllable shutter for air supply,

Venturi shaped mixing tube

Burner head with ports drilled in it.


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Fig. Atmospheric Gas Burner


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Design Data for Simple Aerated Burners

Gas flow rate through the orifice

where, V = gas flow rate,Nm3/h

Cd = Coefficient of Discharge,

= 0.8 to 0.9 for fixed tubular orifice or spud

A = Orifice Area, m2

P = initial gas pressure, cm Hg abs.

T = gas temperature, KS = specific Gravity of Gas, (for air S=1)

)1..(TS

P.pACd240,83V

(vvv!

OHcm,orificetheatdroppressurep 2!(


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Empirical Relation for Aeration Capacity of Burner

)2..(T

300

Q

AA

105C

PSP

4pm6

N

4

A vvvv!

Where, PA = primary air, per cent of theoretical air

P = gas pressure at orifice inlet, cm wg

CN

= netCV of gas, kcal/Nm3

Am = average area of mixing tube, m2 = (A1+A2)/2

A1 = throat area, m2, A2 = area of mixing tube outlet, m

2.

Ap = total port area, m2, Q = Heat input rate, kcal /h.

Primaryair requirement:-For water heaters, furnaces:-

A. if long flame is permitted 35-40% withNG and manufactured gases.

55% with LPG.

B. For Radiant Heaters 65%

C. Cooking Ranges 55-60%


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Throat to total port area ratio = 0.2 to 1.0

Air shutter area = 1.25 to 2.25 times total port area

Distance from gas orifice to throat = 0.5 to 2 times throat diameter

Length of mixing tube = 6 times throat diameter

Slope of mixing tube ~ 3

o

Area of burner head = 1.5 to 2 times total port area.

Burner output rate is given by,

Other Design Data

efficiencycombustionoftcoefficien,where

VCQ N

!

!

L

L


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Continued

Combining eq. (1) & (3), we get,

Also, from eq. (2), we have for primary air supply,

where, the ratio is known as Wobbe index.

It is a useful parameter in assessing the interchangeability of gases with

respect to the aerated burners.

S

CN

Q w

5.0

N

A

SC

1P

w

S

CN


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Oil burners

Fig.Main Components of Typical Furnace Oil Burner


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An oil burner is a heating device which burns fuel oil. The oil

is atomized in to a fine spray usually by forcing it under pressure

through a nozzle.

This spray is usually ignited by an electric spark with the air being

forced through by an electric fan.


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Components

Fuel injection

Oil pump

Electromagnetic valve Fan

Ignitors

Photocell Capacitor start motor

Order for starting an oil burner


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Oil burners consist of

an air register to control the combustion

air flow and a means of stabilising the flame and

the atomiser,

Figure 5.19 . An igniter may also be provided

to assist start-up.


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Pressure jetatomisers

The drop size produced by a pressure jet atomiser is strongly dependent

on the fuel viscosity and the surface tension. Radcliffe developed an

empirical expression for estimating the SMD ( Lefebvre 1989 ).

Normally oil supply pressures of 3.5 MPa (500 psig) and above are needed

with large burners utilising heavy fuel oil. The inner atomiser, usually known as the pilot, has a low capacity, typically

25 to 30% of full load while the outer, or main, atomiser carries 7075% of

full load.


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Schematic and exploded view of duplex wide turndown pressure jet

atomiser


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The drop size produced by a pressure jet atomiser is strongly dependent on the

fuel viscosity and the surface tension.

Radcliffe developed an empirical expression for estimating the SMD

( Lefebvre 1989 )

where = surface tension (N/m)

= viscosity (m2 /s)

m = mass flow (kg/s)

p = differential pressure drop across atomiser (Pa) .

Viscosities should be in the range 1520 cSt for satisfactory atomisation.

micronpm3.7SMD )4.0(25.02.06.0 (! RW


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THANK YOU

Any Quarries............?

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Design of Burners

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