Furnace

FURNACE Mohamed Mosaad Ahmed Mohamed Ragab

What is a furnace? 

Furnace is the heart of industry .

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.  1. 2. 3.  1. 2.

Charging of material furnaces Intermittent or Batch type furnace or Periodical furnace and Continuous furnace. Waste heat recovery recuperative and regenerative furnaces.

MAIN COMPONENT OF THE FURNACE It is one of the most important pieces of equipment in the petroleum industry

Stages of the combustion process: 

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Mixing Ignition Chemical Reaction Dispersal of Products

To stack Convection tubes

Breeching

shock bank Firebox

Radiation tubes Firebox

Radiating cone

Radiating wall

Premixed and diffusion flame 

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Premixed flames are short, blue, noisy and the reactions are virtually complete. Diffusion flames are long, yellow, quieter and the reactions are incomplete.

Burners and firebox 

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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 releases heat .

Premixed burners

Burners and firebox

Tubes and walls 

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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 absorbed .

Shock bank 

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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 heat .

Radiant / convection tubes 

Radiant section  The

radiant tubes absorbed the radiant heat from burners and refractory .

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Convection section  Convection

flue gases .

bank mops up some of the heat left in the

Breeching 

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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 and out of the stack .

In / out of the heated fluid 

The charge is normally fed into the convection tubes and drown off at the radiant tubes .

To stack Convection tubes

Breeching

shock bank Firebox

Radiation tubes Firebox

Radiating cone

Radiating wall

Draft 

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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 flow of gases through the furnace .

Draft and stack 

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In Natural draft 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 

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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 furnace .

Forced draft 

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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 much or too little air of draft .

Performance Evaluation of a Typical Furnace

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

GENERAL FUEL ECONOMY MEASURES IN FURNACES 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 9) Use of Ceramic Coatings

Fuel loss 

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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 

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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 heat out of the heater as an exhaust ) .

Excess air

Radiation loss 

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heat may lost via radiation through the furnace wall. According to ambient temperature and wind velocity on expose surface

HEATER CONTROL

Heater control 

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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 in successive time periods.

Control Using CO and Draft Actuator

CO Analyzer Draft

O2 Analyzer

BSI Control addition to DCS

Heater Limits Registers

Low Fire

High Fire

Heater control