Steam Generator & Fire Tube Boiler

Steam Generator or Boiler

It is defined as, "a closed vessel in which steam is produced from water by the combustion of fuel". Fuel may be solid (wood, coal), Liquid (diesel, petrol), Gas (pulverized coal).

• Complex integration of furnace, superheaters, evaporators, economizer, burners, fans, duct, collector.

Application of Boilers

• Generation of Electric Power
• Process requirement in Pharmaceutical for mixing liquid (because steam is good heating medium)
• For heating biuldings in cold weather for hot water supply
• Sterilization of surgical equipment's in hospitals

Ineffectiveness of Early Design of Boiler

• Since, heat transfer area is very small, so it is ineffective.
• Initially water is closed.

Primary Requirements of a Boiler

• Large heating surface to absorb maximum amount of heat.
• Water must be contained safely.
• Steam must be of required pressure, temperature, quality, flow rate.

Factors while Selecting a Boiler (keep balance between Initial cost and Maintenance cost)

• Quality and Quantity of required steam and its pressure and temperature.
• Floor Area.
• Type of fuel (can be dual fuel) and water available.
• Accessibility for repair/inspection,
• Comparative initial cost.
• Operating and maintenance costs.

 Classification of Boilers

A. Fire Tube Boiler

It is defined as, "If fire is inside the tube and water is flowing outside for heat transfer".

• We do not use fire tube boiler for power generation.
• Advantage  ↣  Low cost, compact design and size.
• Boiler Mountings  ↣  Fittings and devices which are necessary for the operation and safety of a boiler.

Types of Fire Tube Boiler

1. Typical Externally Fired Fire Tube Boiler 
2. Oil Fired Pressurized Package type Fire Tube Boiler

1. Typical Externally Fired Fire Tube Boiler

Combustion is taking place outside the shell of boiler, so called External Fired Fire. Parts of Typical Externally Fired Fire Tube Boiler are:

• Damper  ↣  controls the flow of air (like flapper).
• Coal Bed  ↣  where coal is placed in combustion chamber for combustion.
• Fire Door  ↣  to put coal inside and for inspection purposes.
• Grate  ↣  where fuel is combusted.
• Manhole  ↣  used for inspection and cleaning (shape is elliptical).
• Waterfeed Point  ↣  where water enters into shell.
• Cast Iron Supports  ↣  main structural unit.
• Mudholes  ↣  where all the mud is settled and collected.
• Water Level Indicator  ↣  show level of water and pressure gage is connected to steam space.
• Blow Down Valve  ↣  Help in regulating salt concentration, used to remove impurities (sediments)
• Taking samples to calculate ppm, TDS, alkalinity, pH and for review treatment process.
• Fusible Plug  ↣  protect boiler from overheating by melting it which creates empty space between combustion chamber and water space.
• Safety Valve (or Ramsbottom)  ↣  used to keep pressure under design pressure. If pressure increases, excess steam will like out from boiler.
• Butterfly Valve  ↣  regulating flue gases and prevent something coming into chimney if boiler is not in operating conditions.

2. Oil Fired Pressurized Package Type Fire Tube Boiler

• Fuel used is Oil (which is used to heat the water).
• As the name suggest Internally fired fire  ↣  furnace is inside the boiler.
• Calibration of of components is valid for 1 year.

Working:

Oil from Oil Tank  ↣  Strainer (for filtering or removing sediments)  ↣  pass through valve (which regulate the oil)  ↣  Oil pump (increase pressure)  ↣  passed into Oil Heater which heats oil by Electricity or Bleed Steam (to decrease Viscosity, increase fluidity and combustion)  ↣  oil is burn in air in Oil Burner  ↣  Damper is used to control airflow  ↣  Corrugated flue tube is used (which provides strength, help in thermal expansion and compression)  ↣  Tubes (in which gas to be heated pass through)  ↣  Exchange heat.

Some Additional Components 

• Steam Stop Valve  ↣  Two valves are used. One for normal operation (before superheater tubes) and other for emergency or mal-function.
• Superheater  ↣  Used to convert saturated steam into superheated steam.

Mountings and it's Types

Boiler Mountings are defined as, "Fittings and devices which are necessary for the operation and safety of a boiler". All these mountings should be calibrated to check against the standards. 

• Safety Valve  ↣  it works on the principle of pressure applied (spring force and fluid pressure force).
• If FS < FP  ↣  valve opens, steam went out.
• We can adjust spring force by a screw on top of Safety Valve.

• Water Level Indicator  ↣  used to maintain water level.
• Steam or Water Cock  ↣  open in normal position (during maintenance, we close it).
• Drain Cock  ↣  used to clean water level indicator.
• Glass Tube  ↣  has markings (showing water level) surrounded by shield (Guard glass cover).

• Pressure Gage  ↣  used to calculate the boiler pressure.
• During Hydrotest of boiler  ↣  Boiler pressure should be 1.5 times the design pressure.

• Steam Stop Valve (or Globe or Butterfly Valve)  ↣  it controls the flow of steam as per the requirements.
• Indicator  ↣  it shows by how much percent valve is open?

• Feed Check Valve (or Non-return Valve)  ↣  Installed on the suction side of boiler after pump discharge (which pump water to boiler).
• It prevent the scaping of water from boiler when pump is stopped.

• Fusible Plug  ↣  used to prevent overheating of boiler.
• Melting point is very important factor.
• It always dipped in water.
• When water level drops ↣ plug is overheated ↣ fusible alloy melts ↣ steam enters in combustion chamber ↣ get condensed ↣ combustion stops.

• Blow off Valve (or Cock)  ↣  used to remove sediments from boiler.

Circulation

It is defined as, "The flow of water and steam within the boiler circuit". There are two types of circulations:

1. Natural Circulation  ↣  If circulation occurs on the principle of density difference.
2. Forced or Controlled Circulation  ↣  If circulation occurs due to pump. Used when pressure is in between 120 - 180 bars.

Steam Drum

• To store water and steam to meet load requirement.
• Aid in circulation
• Provides space which separates water out of steam before goes to superheater.
• Remove impurities in Water using Down line.
• Provide place for water treatment (using chemicals) into the boiler.

Drum Internals

These are devices installed within the boiler steam drum includes types of steam separators, chemical feed lines, boiler feed water lines, continuous blow off lines.

Separation of Steam and Water take place at:

• At low pressure (up to 20 bar)  ↣  gravity separation is used.
• At high pressure  ↣  drum internals (like baffles, scrubber, cyclones) are used.

Boiler Water Wall

Brick Work is eliminated in the modern water tube boiler by using the tubes to form the furnace walls also called Water Cooled or Water Wall.

1. Boiler Casing 
2. Metal Casing 
3. Metal
4. Boiler Sheet
5. Insulation
6. Cladding

Points to be Noted

• In industries  ↣  1 inch = 25 mm
• Insulation  ↣  refractory (40% Aluminum trioxide, 55% SiO2, 5% Ether Oxide).  
• Welded Fins  ↣  to provide strength to the tubes and to carry heat transfer.
• Space between Water tubes  ↣  to counter heat expansion.

Note:

• How a Fire Tube boiler works?

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).