Steam Generator & Water Tube Boiler

Water Tube Boiler

It is defined as, "Boiler in which water flows into the tubes and are surrounded by flue gases (or flames)".

• High pressure Boiler
• Fast Steam Production (due to small ratio of steam and water content)
• Combustion efficiency is higher
• Require good water treatment (if sediments present can cause choking)

Working

Pulverized Coal (powdered form) burned in Combustion chamber  ↣  water is preheated in economizer  ↣  gas temperature decreased which from green house gas emissions  ↣  water from down-comer pass to the water wall (heated by gases)  ↣  heat transferred from flue gases to water (in Counterflow manner)  ↣  saturated steam is produced which is converted into superheated steam (600 - 750℃) by superheater.

• Drum  ↣  used to separate moisture content from steam (coming from water wall).

Boiler Accessories 

It is defined as, "Fittings and equipments required to increase the boiler efficiency".

1. Economizer

Device in which the waste heat of flue gases is utilized for heating the feed water (crossflow manner).

• Scrapers  ↣  when combustion gases passes, soot deposited on it which decrease heat transfer, collected in soot chamber.
• Blow off Valve  ↣  remove sediments.
• Butterfly Valve  ↣  if boiler is under maintenance, we close valve and hot gases went off to atmosphere.
• Brickwork  ↣  support structure and provide insulation.
• 100 % stainless steel wetted parts are used. 

2. Air Preheater

It is used to increase the temperature of air before it enters the furnace with the help of leaving hot flue gases before combustion.

3. Superheater 

It is used to increase the temperature of steam above its saturation point.

• Increase efficiency of plant by utilizing waste heat.
• It removes last traces of moisture from saturated steam.

Types of Water Tube Boiler

Following are the types of Water Tube Boiler:

a. Babcock & Wilcox Boiler 

• It is a type of Straight water tube boiler with Longitudinal Drum.
• Not used in Power Generation.
• Number of tubes depends on the diameter of drum.
• Baffles plates  ↣  which guides flue gases and ensure proper heating of water tubes. 
• It creates hindrance which convert flow to turbulent which enhances heat transfer.
• Anti-Priming Pipe  ↣  used to separate water from steam.
• It contains perforation sheets ↣ moisture content settle down ↣ steam enters to superheated tube heated by flue gases ↣ flows to steam stop valve.
• Ash Pit  ↣  Ash collected produced by combustion and removed.
• Damper (or Valve)  ↣  installed before chimney to regulate the air supply to fire box.
• Steel Girders  ↣  On which boiler assembly is suspended which allow contraction or expansion during boiler operation.
• Mud Box  ↣  where sediments or mud from water settle down.

b. Straight Water Tube Boiler with Cross Drum

• Number of tubes can be installed depends on the length of Drum.
• More number of tubes ↣ more steam generation.

c. Four Drum Bent Water Tube Boiler OR Stirling Boiler

It is defined as, "Bent tube, externally fired, water tube boiler, stationary, with natural circulation boiler".

• Pressure Range  ↣  maximum 60 Bar.
• Steam Production Rate  ↣  50 Ton/hr = 50,000 Kg/hr.
• Efficiency  ↣  (60 - 80) %
• Boiler Operation  ↣  Diameter up to 2m and length (5 - 8) m.

Construction & Working

• 3 Horizontal steam drum arranged upper part of boiler settings.
• Mud Drum arranged in Lower part.
• Drums are connected with each other by Equalizing (maintains steam level) and Circulating tubes (maintains water level). 

Water is feed into Drum 3 ↣ moves to mud drum (separate mud and removed from blow off valve) ↣ pure water pass to 1 and 2 drum. Pulverized Coal is burned on Grate (fire box) releasing hot flue gases ↣ deflected by Brick arch and baffles plates. Water in bent tubes connected into steam collected over the water level in drum ↣ steam is collected and pass to super heated tubes and collected from Steam stop valve.

Advantages 

• High steam production rate due to number of steam drum.
• High pressure operation and less pressure drop.
• Bent tubes allow free expansion and contraction (undue thermal stress).
• Permits great flexibility in Design.

Disadvantages

• More difficult to clean and inspect the bent tubes.
• Larger in size.
• Super heated temperature is limited.

d. Lamont Boiler OR Pump Assistance Circulating Unit

It is defined as, "a forced circulation, internally fired and water tube boiler".

• Works on Sub-critical pressure and temperature.

Working

• Combustion chamber is vertical where fuel burns.
• Water enters feed pump to provide water to economizer (preheat water) ↣ goes to drum ↣ water circulating pump ↣ increases pressure greater than boiler pressure ↣ Distributor header (distribute water to pump) ↣ Radiant evaporator (tubes) ↣ Convective evaporator ↣ goes to steam drum (where moisture content separated by baffles) ↣ pure steam goes to superheater ↣ temperature increases ↣ sent to turbine (primover).

e. Benson Boiler OR Once Through Water Tube Boiler

It is defined as, "a high pressure, drumless, forced circulation, internally fired water tube boiler".

• If boiler is operated above supercritical point  ↣  no need of drum.
• Require extremely clean water about 0.1 ppm/tds.
• Operating at 250 bar.
• Capacity is 135 ton/hr of steam
• Light in weight (because of absence of drum).
• Thermal efficiency is around 90%.
• Same as Lamont Boiler ↣ but no circulating pump and boiler drum.

Note:

• Entropie Steam Boiler Design and Manufacturing Plant

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).