Power Plant Engineering

What is a Water Tube Boiler? How Does It Work?


According to the position of water and hot gases, a boiler or steam generator is classified into two types i.e. Fire tube Boiler which we have discussed earlier and another one is the Water Tube Boiler. In order to generate high-temperature and superheated steam, these boilers are mainly used in large-scale industries. Let us start with the definition!

What is a water tube boiler?

In a Water tube Boiler generally, the fluid i.e. water flows through the small tubes surrounded by hot flue gases. Why is it named so? Due to the presence of water inside these small tubes, it is named so. Generally, these types of boilers are used in large-scale industries for high-capacity applications. Because this boiler has some different factors like the custom design, high efficiency, generating high-pressure superheated steam, and also the quality of steam. Because Hoop stress or Condensed tangential pressure originates in the tube, this boiler easily generates high-pressure and superheated steam.

The internal part of this boiler consists of a tiny steam drum, small diameter tubes, and other components like an economizer, superheater, Air preheater, etc. These components produce high-volume vapor for high-capacity applications. In our previous article, we studied Fire-tube boilers and their types, and here we will discuss the working principle of a water tube boiler, its types, parts, advantages, and disadvantages respectively.

Water Tube Boiler Diagram


Parts of Water Tube Boiler

Generally, Water tube Boiler parts and components are classified into two categories, which are given below.

  • Exhaust Stack or Chimney: This is an open chamber that is attached to the Induced Draft Fan (ID Fan). This ID Fan pulls out the flue gases from the boiler furnace via an Air Preheater and electrostatic precipitator (ESP) where the remaining ash is collected over a hopper and combust gases are extracted out to the atmosphere through this chimney.
  • Air Registers: They maintain the correct fuel-to-air ratio and also reduce the formation of hazardous gases like smoke, carbon monoxide, and nitrogen oxide.
  • Downcomers: These are the tubes present inside the boiler through which cold water flows from the steam drum to the mud drum. It picks up heat from the firebox and restores the water supply to the mud drum.
  • Riser Tubes: These allow hot water i.e. steam flow from the mud drum to the steam drum. Radiant Tubes:- This tube contains the boiler feedwater that is heated by the radiant heat from the burners and boiled to form steam that is returned to the steam drum.
  • Steam Drum: This reservoir plays a key role in a water tube boiler. This is the place where water or steam meets with each other. Generally, it stores the steam that is produced in the boiler and sends back water to the mud drum for reuse. So we can say that a steam drum in a boiler acts like a phase-separator.
  • Mud Drum: This is a chamber that is placed at the bottom part of the boiler. It stores earthy matter and unwanted particles like sand deposited from the water which can damage the interior part of a boiler.
  • Sight Glass: A glass tube is utilized on steam-type boilers for giving observable signs of the water level in boilers. Strainer:- This is a type of device as a filter to hold solid elements letting a fluid supply.
  • Safety Valve: The safety valve is a mechanical device and also the most important part of a boiler’s construction. It preserves human life as well as property in a thermal power plant. Normally it prevents the explosion when there is an excessive internal pressure of steam inside a boiler. This device is designed in such a way that it automatically opens and releases the excess internal pressure. Then it closes automatically when the internal pressure decreases to the prescribed value.
  • Pressure Gauge: This indicates the pressure of steam in a boiler. It is mounted on the upper part of this boiler. The main purpose of this mechanical device is that it is used for monitoring the internal pressure of a boiler. In most cases, Bourdon-type pressure gauges are widely used in steam boilers.
  • Feed Check Valve: This mechanical device is a non-return valve that is placed at the end of the delivery pipe of the feedwater pump in a boiler. Where is it located? It is mounted on the boiler shell and slightly below the normal water level. The primary function of this valve is that it allows a unidirectional flow of water from the feed pump to the boiler shell.
  • Water Level Indicator: This is an important part of boiler fittings that indicates the level of water in the boiler constantly to an observer or an operator. It is a safe mechanical device upon which the correct working of the boiler depends. Here we can see that the lower portion of the indicator is dipped in water and the upper portion is connected with the steam chamber in the boiler. These fittings may be seen in the front section of this boiler. In case of urgency, if the water level falls down then the water will be resupplied by the operator through the feed pump. It is mostly located in the steam boiler.
  • Steam Stop Valve: The main function of a steam stop valve is to shut off or regulate the flow of steam from the boiler to the steam pipe or from the steam pipe or delivery tube to the engine respectively. It is designed in such a way that the main body of the steam stop valve is made up of cast steel and the valve and nuts are made up of brass with excellent resistance and limited dimensional tolerance.

Water Tube Boiler Accessories

The accessories of this boiler are used to increase its efficiency and are listed below.

  • Air Preheater:- In simple words, we can say that this mechanical device recovers the heat from the boiler’s hot flue gas which is exhausted out through the chimney. It means there is no heat loss. In this way, the efficiency of a boiler is increased.
  • Economizer:- This is a heat exchanger that is placed in the interior part of a boiler to produce saturated steam or moist/wet steam. Also, it raises the temperature of the boiler feedwater, by lowering the energy input and reducing the firing rates needed for the boiler output.
  • Superheater:- The main purpose of the superheater is to increase the temperature of saturated steam without raising its pressure. This boiler utilizes both sensible heat as well as latent heat to convert water into steam while the superheater utilizes only sensible heat to superheat the steam in order to increase its enthalpy and thermal efficiency of the boiler.

Types of Water Tube Boiler

  • Stirling Boiler
  • Benson Boiler
  • Simple Vertical Boiler
  • Babcock And Wilcox Boiler
  • Lamont Boiler
  • Loeffler Boiler
  • Yarrow Boiler

Application of Water Tube Boiler

  • Paper & Pulp Manufacturing Plants
  • Refining Industries
  • Chemical Processing Units
  • The large steam turbines of utility companies like Thermal Power Plant

Water Tube Boiler Specification

  • A high steam output (up to 500 kg/s)
  • High-pressure steam (up to 160 bar)
  • Superheated steam (up to 550°C)

Working of Water Tube Boiler

The heat energy is generated in the boiler by the combustion of the fuel i.e. coal, natural gas, or oil. Now the fuel is injected into the boiler through a burner. As water flows through the tubes, the combustion or flue gases heat the water and produce steam. How? When the flue gas comes in contact with the surface of the water tubes, heat transfer takes place. Then the steam is collected in the upper drum or steam drum while flue gases are extracted out into the atmosphere through a chimney.


Here several types of tubes are present inside the boiler. The steam-generating tubes are attached to the upper and lower drums respectively. Water flows through the tubes from the upper steam drum to the lower mud drum. The cold water flows from the steam drum to the mud drum through the downcomer’s tube. Similarly the hot water i.e. steam flows from the mud drums to the steam drum through the riser tube. Now the saturated steam entered into superheater tubes from the steam drum for reheating. This mechanical device removes the moisture contents without an increase in pressure and produces high-temperature superheated steam respectively.

The water level inside the steam drum of a Water Tube Boiler can be controlled by an operator. To maintain the water level inside the boiler, a water level indicator is used. Because the loss of water level inside the boiler can damage its part due to overheating. Also, excessively high water levels can cause the steam to become saturated with moisture. This can damage the steam turbine and reduce the reliability of various machines.

Advantages of Water Tube Boiler

  • The transportation of this boiler is easy.
  • For a given output it requires less floor area.
  • The operation of this boiler is easy and safe.
  • It shows a fast reaction to steam demand.
  • It shows highly efficient performance, up to 90% with an economizer.
  • It has a large evaporative capacity.
  • For cleaning, inspection, and repairing the process is very simple.
  • It shows rapid heat transmission.
  • This boiler can tolerate high steam pressures.

Disadvantages of Water Tube Boiler

  • The installation procedure for this boiler is time-consuming and complicated.
  • It cannot be manufactured in packaged form.
  • This boiler is uneconomical for small industries.
  • The maintenance cost of this boiler is higher than a Firetube Boiler.

This is all about the “Water tube Boiler”. Thanks for giving your valuable time. If you have any doubts related to this boiler please don’t hesitate, to ask in the comments section below. If you have any prior experience related to this boiler and know more about it, please share it with us. We promise that we will solve your doubts as soon as possible. And if you really like this article, don’t forget to share it with your colleagues and friends. Good Luck!

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