Power Plant Engineering

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


If we examine the arrangement of feedwater and hot gases within a Steam Boiler or steam generator, it can be classified into two main types: the Fire tube boiler and the water tube boiler respectively. Most of the boilers are classified into these categories. In our previous study, we covered the basic working principle of a boiler and its operational functions. In this article, we will discuss the operational principle, components, variations, benefits, and drawbacks of the Fire tube boiler. Let’s commence with the definition.

What is a Fire Tube Boiler?

In a fire tube boiler, hot flue gases typically flow through small tubes that are surrounded by or immersed in feedwater. The name “fire tube” originates from the fact that these small tubes contained fire or flue gases in them.

Typically, these kinds of boilers are unsuitable for large-scale power plants. Why? The reason is their operation at low pressure, which leads to the production of steam with moisture. Also, it is commonly referred to as wet steam or saturated steam, due to the presence of tubes immersed in water. As a result, these boilers are used widely in small-scale industries.

Taking into account the axis of the boiler shell, a fire tube boiler has the capability to function both as a vertical and a horizontal boiler. The combustion chamber is an integral component of the boiler, classifying it as an internally fired boiler. These boilers generate steam with a pressure typically around 10 bar (with a maximum range of up to 15 bar), thus categorizing them as low-pressure boilers.

Fire Tube Boiler Diagram

Fire Tube Boiler Diagram

Fire Tube Boiler Parts

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

Fire Tube Boiler Mountings

Fire Tube boiler mountings are preinstalled safety devices that work smoothly. Let us discuss

  • Safety Valve: The safety valve, an important component among fire-tube boiler fittings, serves as a mechanical safeguard in thermal power plants. Its role is to protect against potential explosions caused by excessive internal steam pressure within the boiler. It is crafted with automatic functionality, this mechanical device swiftly opens to release the excess internal pressure and automatically shuts when pressure subsides to the prescribed value.
  • Combustion Chamber: This is a closed chamber inside a fire-tube boiler where combustion takes place by burning the fuel. We can say that this chamber provides greater heat to the boiler.
  • Firetubes: Through this channel or pipe, the hot flue gas passes to the smokebox. As this is a fire-tube boiler, the regular flow of hot gases can damage these tubes. Generally, these tubes are made up of seamless steel e.g. carbon steel and alloy steel. The materials of these tubes are different for low-pressure, medium-pressure, and high-pressure boilers respectively.
  • Chimney: This is an open chamber attached to the firetubes of a fire-tube boiler. Generally, the hot flue gases are released by the burning of fuel in the combustion chamber and they pass through the small tubes (emerged in water). Then it exhausted out into the atmosphere through a chimney (placed above the boiler as shown in the above figure).
  • 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. 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.
  • Pressure Gauge: This indicates the pressure of steam in a boiler. It is mounted on the upper section of this boiler. The main purpose of this mechanical device is that it is used for monitoring the internal pressure of a fire-tube boiler. In most cases, the Bourdon-type pressure gauge is widely used in a steam boiler.
  • Steam stop valve: The main function of the steam stop valve is to shut off or regulate the flow of steam from the fire tube 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 level indicator: This is an important part of fire-tube boiler fittings that indicates the level of water in the boiler constantly to an observer. 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 a 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.

Fire Tube Boiler Accessories

Fire Tube boiler accessories are used to increase its efficiency. Let us discuss

  • Air Pre-heater: This mechanical device recovers the heat from the boiler’s hot flue gases which are exhausted out through the chimney. It means there is no heat loss. So we can say that this device plays an important role in increasing the efficiency of a fire-tube boiler.
  • Economizer: This is also a heat exchanger that is placed at the exhaust section of a fire-tube boiler to produce saturated steam (moist or wet steam). The temperature of the boiler feedwater is raised by an economizer. How? Simply by bringing down 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. The fire-tube boiler utilizes two types of heat (sensible heat and latent heat) to convert water into steam. A superheater utilizes only sensible heat to superheat the steam in order to increase its thermal efficiency and enthalpy of a boiler.

Types of Fire Tube Boiler

Generally, fire tube boilers are classified into various types that are given below. 

Application of Fire Tube Boiler

Generally, fire tube boilers are used in different sectors that are given below.

  • Most of the fire tube boilers are used in Railways.
  • Also, these boilers are used in the marine sector i.e. Ships.
  • A few boilers are also used in small-scale industries.

How does a Fire tube Boiler work?

In order to generate heat, first of all, the coal is heated inside the combustion chamber. Then the hot flue gases started moving through these small tubes to heat the water that surrounds these tubes inside the fire tube boiler. The hot flue gases transfer the heat energy from the surface of the flue tube to the water by the process of thermal convection. The combustion gases are directed through the small tubes, while the water is directed through the boiler shell respectively.

Schematic Diagram of a Fire Tube Boiler

When the water begins to boil inside the boiler, steam is formed. Now it is directed out from the boiler through the outlet pipe or you can say delivery pipe to the other parts of the boiler. In order to fulfill the fluid loss, make-up water is added to it. In this type of channel, the water level within the boiler shell must always be maintained by the operator, so that the tubes are covered in water.

Otherwise, these small tubes could overheat and become damaged soon. In order to extract the excess internal pressure, a mechanical device i.e. safety valve is placed on this boiler. If we calculate the overall efficiency of fire tube boilers then it is nearly about 75%

Advantages of Fire Tube Boiler

The advantages of a fire tube boiler are given below.

  • Due to the simple structure or design of a fire tube boiler, it can be easily installed.
  • If we consider the working function, then it is very simple.
  • If we consider the pricing value, then the fire tube boiler is cheaper than the water tube boiler.
  • It is widely used in small-scale industries (The reasons are low pressure, simple working function, and quality of steam).
  • Quite Compact in construction and economical.
  • The fluctuation of steam demand in this boiler can be met easily. It is light in weight.
  • Normally this boiler requires a low maintenance cost.
  • It can be operated with a less skilled person (this is also a factor for which small-scale industries are widely using this boiler)
  • The cleaning procedure is very simple and easy because of the large boiler shell.
  • Also, It provides a cost-effective heating solution.

Disadvantages of Fire Tube Boiler

The disadvantages of Fire tube boilers are given below.

  • The transportation of these boilers is not so easy because their parts cannot be detachable.
  • Due to the large quantity of water, the necessity of steam pressure rising time is quite high. (As you know too much water takes more time to convert into steam)
  • Here in this boiler, the output steam pressure cannot be very high since the water and steam are kept in the same vessel.
  • The steam produced in this boiler is not very dry (because of the moisture content).
  • These boiler systems are not considered where high-capacity steam is required. Generally, it happens for two reasons first of all this is a low-pressure boiler and secondly, the output steam is not very dry.
  • The steam generation rate in these boilers is very low ( nearly 9 tonnes per hour TPH ) as compared to others.
  • It would be very dangerous if there is bursting because a large amount of hot water and steam have been collected inside. And it happens when there is a leakage in the joint portions of this boiler.

This is all about the “Fire 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.

MechStudy Team

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