Furnace

FURNACE

What is a furnace?

Classification of furnace

The classification of furnaces according to :  Heat transfer :-

1. combustion type (using fuels such oil fired, coal fired or gas fired.) 2. electric type.

 Charging of material furnaces

1. Intermittent or Batch type furnace or 2. Periodical furnace and

3. Continuous furnace.

 Waste heat recovery

1. recuperative and

MAIN COMPONENT OF

THE FURNACE

It is one of the most important pieces of equipment in the petroleum industry

Stages of the combustion process:



Mixing

Ignition



Chemical Reaction

Dispersal of Products

Radiation tubes Firebox Firebox shock bank Convection tubes Radiating cone Radiating wall To stack Breeching

Premixed

and

diffusion

flame



Premixed flames are short, blue, noisy and the

reactions are virtually complete.



Diffusion flames are long, yellow, quieter and the

Burners

and

firebox



Heat is produced by

burning fuel

at the burners in

the

fire box

.



The firebox is the

open area

above the burners .

Fuel

mixed

with air ignites at the burners and

Tubes

and

walls

 The tubes along the

walls of the fire box are radiant tubes .

 The tubes which can see

the burners are the radiant tubes and the shock bank .

 The walls and roof are lined with a material that reduces heat loss and radiates heat back to the tubes to increase the amount of heat

Shock

bank



The shock bank is located in the direct light of the

burners and in the stream of the hot flue gases .



The shock bank receives the convection and radiant

Radiant

/

convection

tubes



Radiant section

 The radiant tubes absorbed the radiant heat from burners and refractory .



Convection

section

 Convection bank mops up some of the heat left in the flue gases .

Breeching



The breeching is the duct which

gathers

the flue

gases and discharges them to the

stack

.



Flow gases flow from the

fire box

into the

breeching

In

/

out

of

the

heated

fluid



The charge is normally fed

into the convection tubes and

drown off at the radiant

Radiation tubes Firebox Firebox shock bank Convection tubes Radiating cone Radiating wall To stack Breeching

Draft

 The entire furnace structure must be air tight for

efficient furnace operation .

 All air should enter at the burners

 An air leak in any part of the furnace reduces

efficiency and increases the cost of operation

 Draft is the slight pressure deference that produce the

Draft

and

stack

furnace , draft is maintained by the stack .

 Using air doors and

stack damper

controlled the pressure inside the furnace slight lower the atm. Pressure

 The increase of the

stack height the increase of draft .

Forced

draft



A

short stack

can be used if forced draft fan is

present .



Forced draft permits

steady control

of the air at the

burners and efficient burning of the fuel .



Forced draft can make a good

purging

for the

Forced

draft

 The draft and burner pressure draw the air into the

furnace with enough energy to mix the fuel and air and maintain burning .

 The flow of flue gases loses energy as it passes

around the tube banks and the flow slow down due to friction .

 Draft must supply enough energy to overcome this

friction and maintain the flow of flue gases .

 Fuel and maintenance costs are increased by too

Performance Evaluation of a Typical Furnace

:

These furnace losses include

• Heat storage in the furnace structure

• Losses from the furnace outside walls or structure • Heat transported out of the furnace by

the load conveyors, fixtures, trays, etc.

• Radiation losses from openings, hot exposed parts, etc. • Heat carried by the cold air infiltration into the furnace • Heat carried by the excess air used in the burners.

EFFICIENCY AND FUEL

CONSERVATION

G

ENERAL

F

UEL

E

CONOMY

M

EASURES

IN

F

URNACES

Typical energy efficiency measures for an industry with furnace are:

1) Complete combustion with minimum excess air 2) Correct heat distribution

3) Operating at the desired temperature

4) Reducing heat losses from furnace openings

5) Maintaining correct amount of furnace draught 6) Optimum capacity utilization

7) Waste heat recovery from the flue gases 8) Minimum refractory losses

Fuel

loss

 About 5% of the fuel burned in the

world are lost or wasted .

 The main reason for this losses is the

misunderstanding of what is

required to burn fuel to the best advantage .

Excess

air

 The amount of air used to compensate the humidity of air

.(also calculated at max fuel flow rate ) .

 In industrial fired heaters, power plant steam generators,

and large gas-fired turbines, the more common term is

percent excess combustion air.

 For example, excess combustion air of 15 percent means

that 15 percent more than the required air is being used.

 Excess air reduction may save heat losses (it will carry the

Radiation loss



heat may lost via radiation through

the furnace

wall.



According to ambient temperature and wind velocity

Heater control

 Heater control is critical to good performance.  Firing must be coordinated with flow of the fluid

being heated to-prevent hot spots, too much thermal stresses, coking, scale formation,etc.

 The use of a natural draft, direct fired heater for

heating regeneration gas in a dry desiccant

dehydration system is a good example of heater control.

 It is a cyclic process with the heater being on and off

Control Using CO and Draft