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Water-tube boiler parts and Working Principle

In this article, we will learn about the High-Pressure water tube boiler definition, parts, and their functions, water tube boiler working principle and applications.

Water-tube boiler Definition:

A closed vessel in which water flowing inside the tubes and hot flue gas pass outside the tube is called a water tube boiler. Hot flue gases transfer their heat to water in the tubes. The Water after gaining from flue gas converts into steam. Steam further used for different industrial purposes. Generally, Water-tube boilers use for large pressure applications. Thermal power plants use a high-pressure water tube boiler to produce steam.

Water-tube boiler Parts and functions

Steam Drum:

The steam drum is a basic part of water tube boilers. In this drum, steam separated from heated water. Hot water droplets fall to the bottom of the steam drum. These hot droplets further send to the heat cycle. Steam leaves the drum from the top of the drum. Feed Water for heating cycle enters from the bottom of the steam drum.

Mud or Water Drum:

Lower Drum/ Mud drum is large in diameter and all the risers and downcomer generating tubes are remote from its bottom. Mud or water drum directly attached to the steam drum with large no of straight tubes bundles called boiler bank tubes. Solids, mud and other sediments settle down in this mud drum. Periodic blowdown valves used to remove these solids or mud depositions from the mud drum. Blowdown valves connect to TDS control to fully drain the mud dream when the boiler is out of service.


Downcomer is a piping arrangement that connects the steam drum to the distribution header. Feedwater from the steam drum moves to the bottom of the boiler through a downcomer. Further feed water from the bottom of the boiler enters into the distribution header.

Distribution headers:

Distribution header is a set of large piping arrangement which transfer feed water from downcomer to risers.

Water Tube Boiler Diagram
Water Tube Boiler


Tubes or piping arrangement in a combustion chamber enclosure called risers. Feedwater and steam run in riser tubes. The heat from the combustion chamber (flue gas) transfers to the feed water in the riser tubes. Hot water and steam mixture from riser enters the steam drum where steam separated from hot water and heating cycle repeats again. In a riser, water flows from bottom to top. Therefore, due to the direction of water flow, these heat transfer tubes called risers.

Combustion chamber:

The combustion chamber located at the bottom of the water tube boiler. The air and fuel mix and burn in the combustion chamber. It is lined with the risers.

Water walls:

Large water tube boilers have water-cooled furnaces. Basically, Water walls form to cool and protect the furnace lining. Combustion space of furnace partially or fully shielded with water walls. Water walls consist of vertical tubes in which water from the boiler circulates and connect with upper and lower headers. The water wall heating surface is almost 10% of the boiler heating surface, yet represents as much as 50% of the total heat absorption.

The water walls perform the following functions:

  • Protect the insulated walls of the furnace from high temperature
  • Absorb heat from the furnace to increase boiler evaporation capacity or unit generating capacity
  • Make the furnace airtight, avoid erosion of refractory materials


Economizer collects heat from exhaust gases and transfers this heat to the feed water. Thus, Economizer increases the feedwater temperature and increases boiler efficiency. It consists of small diameter tubes in which feed water flowing inside the tubes and exhaust gases circulate outside the tubes. Exhaust gases transfer their heat to the feedwater and leave the boiler through the chimney.

Air heaters:

Air heaters also boost boiler efficiency. It recovers heat from the flue gases to heat the combustion air. Thus, fuel saved that use to heat the air-fuel mixture to its ignition point. In air preheater, the temperature of flue gases must not decrease from dew point. Otherwise, moisture from flue gases condenses and combines with sulfur, carbon mono oxide, and carbon dioxide. These oxides and sulfur with condensate makes highly corrosive sulfurous and carbonic acids and reduces the air heater life.


Superheater use to produce super-heated heat. Saturated steam from the boiler passes from the superheater tubes and gain heat from furnace hot gases. In superheater heat transfer occurs due to radiation or convection or both.

Baffle plates:

Baffles defect the hot gases back and forth between the riser tubes. This baffle plate deflection produces greater heat absorption in the boiler tubes. It also permits a better temperature difference between boiler tubes and gases. Baffles maintain flue gas velocity and eliminate dead packets. It also prevents high draft losses and maintains deposit fly ash.


Force draft (FD) and induced draft (ID) fans used in the water tube boilers. FD fan installed before the boiler furnace and used to deliver air to the furnace and also helps to exhaust air through the chimney. On the other hand, the ID fan installed outside the economizer or preheater. These fans also help to exhaust the gases through the chimney.

Water Tube Boiler Working Principle:

To understand the water tube boiler working principle, we will discuss the simple above water tube boiler diagram. This boiler consists of a steam drum (Upper Drum) and a mud drum (Lower Drum). Feedwater with the help of a feedwater pump enters the steam drum. Water from the steam drum moves downward to the mud drum through downcomer and further moves upward through the distribution header and water tubes to the riser.

This upward movement of water from the mud drum to the riser is due to water density difference. The Air-fuel mixture burns in the combustion chamber and produces hot gases. These hot gases transfer their heat to the water in water tubes. Heated water has a low density as compared to cold water. Therefore, water in water tubes moves upward to riser/ circulating tubes. You can see clearly in the above water tube boiler diagram, hot gases move from the upper side of the water tubes and transfer their heat to the water and then hot gases move right to left in the water tubes. The mixture of steam and water droplets further moves toward the steam drum. In the steam drum water and steam separated via baffles or steam separators or anti priming pipe. The upper portion of the steam drum contains steam space and the lower portion of the steam drum contains boiler feed water. Thus, a heating cycle completed and steam moves towards the main steam system. In large units or system requirements, sometimes steam from steam drum moves toward superheater where superheat steam produces with the help of exhaust flue gas.

To maintain the water level in the steam drum, the three-element controller used in large industries. Three elements controller sense the water level, the steam flow, and feedwater flow to control the feedwater flow. Economizers also use to heat feed water through exhaust flue gas. Safety valve, pressure gauges and other boiler mountings and accessories used in the water tube boiler for safe operation.


  • Variety of process applications in industries as textile, paper, sugar mill, etc
  • Steam Power Plants
  • Chemical processing industry
  • Railways
  • Marine Applications
  • Pulp and Paper manufacturing plants
  • Refining units

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