What are Boilers and Their Types?

What Is a Boiler?

A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.

Heat sources

In a fossil fuel power plant using a steam cycle for power generation, the primary heat source will be the combustion of coal, oil, or natural gas. In some cases, byproduct fuel such as the carbon-monoxide-rich off-gasses of a coke battery can be burned to heat a boiler, biofuels such as bagasse, where economically available, can also be used.

In a nuclear power plant, boilers called steam generators are heated by the heat produced by nuclear fission.

Where a large volume of hot gas is available from some process, a heat recovery steam generator or recovery boiler can use the heat to produce steam, with little or no extra fuel consumed, such a configuration is common in a combined cycle power plant where a gas turbine and a steam boiler are used.

In all cases the combustion product waste gases are separate from the working fluid of the steam cycle, making these systems examples of external combustion engines.

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How to Measure Boiler efficiency?

There are two methods to measure the boiler efficiency in the ASME performance test code (PTC) for boilers ASME PTC 4 and for HRSG ASME PTC 4.4 and EN 12952-15 for water tube boilers:

  • Input-output method (direct method)
  • Heat-loss method (indirect method)

Input-output method (or, direct method)

The direct method of boiler efficiency test is more usable or more common.

Boiler efficiency = power out / power in = Q × (Hg − Hf) / (q × GCV) × 100%

were

  • Q, rate of steam flow in kg/h
  • Hg, enthalpy of saturated steam in kcal/kg
  • Hf, enthalpy of feed water in kcal/kg
  • q, rate of fuel use in kg/h
  • GCV, gross calorific value in kcal/kg (e.g., pet coke 8200 kcal/kg)

Heat-loss method (or, indirect method)

To measure the boiler efficiency in indirect method, parameter like these is needed:

  • Ultimate analysis of fuel (H2, S2, S, C, moisture constraint, ash constraint)
  • Percentage of O2 or CO2 at flue gas
  • Flue gas temperature at outlet
  • Ambient temperature in °C and humidity of air in kg/kg
  • GCV of fuel in kcal/kg
  • Ash percentage in combustible fuel
  • GCV of ash in kcal/kg

How does a Boiler work?

Well, for a start, industrial boilers can cope with pressures much higher than a pressure cooker.

Industrial boiler systems are often made by welding together thick steel plates, allowing extremely high pressures to be made.

It has to be made incredibly strong to cope with the high pressure, as failure to do so will result in forces close to an exploding bomb!

The function of a boiler is to either produce hot water or steam.

Hot water boilers heat water for the purpose of domestic or commercial heating and hot water supply.

Steam boilers generate steam in order to power turbines for power generation and various other industrial heating applications.

To visualize the effects of steam generation using a boiler, think of the steam powering a turbine. When the steam passes through the blades of a turbine, the force turns the blades and accelerates the turbine.

Steam contains an enormous amount of energy, so it makes the turbine quite efficient and depending on the fuel used to boil the water, quite energy-efficient too.

Boiler

Types of Boilers

Different types of boilers are available in the market, which is manufactured by different manufacturers. The different types of boilers find their use in different applications. As time has passed, more efficient types of boilers have replaced the old and inefficient ones. 

Boilers can be broadly classified as shell tubes and water tube boilers.

Shell and tube boilers

Shell and tube boilers are also referred to as fire tube or smoke tube boilers. Fire-tube boilers; contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates.

Fire-tube boilers, typically have a lower initial cost, are more fuel-efficient, and are easier to operate. Their capacities are up to 25tons/hr and 17.5 kg/ cm2.

Different types of fire tube boilers:

1. Cornish boiler

These are the earliest form of high-pressure fire tube boiler. These consist of long horizontal cylinders with a single large flue containing the fire. Fuel is added to the grate area where it burns to produce hot gases.  The hot gases transfer the heat to the water.

Water takes heat and after some time it starts boiling to produce steam. Hot gases upon reaching the end of the fire tube, are divided into two sections, and each move into one of two-side flue which takes them once again to the front section of the boiler where they are moving into the bottom flue and bottom flue take them toward the chimney.

The chimney throws these gases out of the boiler into the atmosphere. Maximum heat transfer is taken place at the fire tube and shell section then taken place at the side flue and at last at the bottom flue.

For efficiency, the boiler was commonly encased beneath by a brick-built chamber.

2. Lancashire boiler

The Lancashire boiler is similar to the Cornish but has two large flues containing the fires. The pressure range of the boiler is about 0.7 MPa to 2 MPa and efficiency is 65 to 70%.  Fuel in these boilers is added into the grate which heats the gases.

Hot gases enter the front section of the boiler and leave the boiler from the back and then enter the bottom flue and start moving to the front section of the boiler.

In the front section, hot gases leave the bottom flue and enter the inside flue and move again towards the back of the boiler and enter the main outlet. 85% of heat is transferred when hot gases are in the fire tube while 15% is transferred when they are in the bottom and side flue.

3. Locomotive boiler

A locomotive boiler has three main components:

  • Double-walled firebox;
  • Horizontal, cylindrical “boiler barrel” containing a large number of small flue-tubes; and
  • Smokebox with chimney, for the exhaust gases.

Fuel is burned to produce hot gases. Fuel is feed through a fire hole. Hot gases are diverted to the fire tube with the help of a fire brick arch. Steam is collected in the steam drum which is placed at the top of the shell.

The wet steam goes through the inlet headers of the superheater and after passing through tubes, it returns to the outlet header of the superheater and is taken out for the steam engine.

Locomotive-type boilers are also used in traction engines, steam rollers, portable engines, etc.

On the basis of construction, these can be classified as wet back boilers and dry back boilers.

4. Reversal Chamber

This is the posterior portion of the combustion chamber through which the flue gases travel from the first pass (furnace) to the second-pass tubes

Wet Back Boilers

In wet back boilers as the name suggests the reversal chamber is completely surrounded by water. The combustion reversal chamber is surrounded by water and therefore the heat in the flue gases is optimally utilized.

Radiation losses are reduced as none of the parts of the combustion chamber are open to the atmosphere instead, they are surrounded by water. That means fewer losses and lesser fuel bills. The most efficient modern boilers supplied are wetback type.

Dry Back Boilers

The reversal chamber is dry back boilers is not completely surrounded by water. The posterior part is exposed to the atmosphere. This leads to increased radiation losses, as the radiant heat is lost to the atmosphere instead of going to the water as in wet back boilers. Earlier generation boilers used to be dry back.

Thus, wet back boilers ensure lesser radiation losses and hence save fuel.

The layout of the tubes involves the number of passes the tube will make to pass the heat from the boiler furnace before being discharged. These can be two-pass and three-pass boilers.

Depending upon the layout of tubes boilers can be two-pass or three-pass boilers.

1. Two-pass boilers:

In two pass the combustion gases travel two times in the boiler.

Combustion gases should be cooled before entering the reversal chamber. Excess temperature causes overheating and cracking of the tube. The heat transfer rate is maximum at the first pass, this rate decreases with the increasing passes.

2. Three pass boilers:

A three-pass design provides three opportunities for heat transfer. The stack temperature of 3 passes will be lower than that of 2 pass boilers, of the same design and operating pressure. Efficiency is more than two pass boilers.

Each pass in the boiler should be designed with a cross-sectional area to achieve optimal flue gas velocity, which in turn maximizes heat transfer while also minimizing performance-robbing sooth build-up within the tubes.

Water-tube boilers

In water tube boilers, water and steam flow inside the tubes and the hot gases flow over the outside surface. Modern high-capacity boilers are of water tube type. The boiler circulation system is constructed of tubes, headers, and drums joined in an arrangement that provides water flow to generate steam.

Water tubes have high pressures and capacity than shell tube boilers. These boilers can be of single- or multiple-drum type. These have higher efficiencies than fire tube boilers.

Depending on layout boilers can also be classified as:

1. Longitudinal drum boiler.

The feedwater is feed in the drum.  The drum is placed above the heat source. The cooler water goes to the inclined tubes and the water is heating eventually in the hot tubes. As the water boils its density decreases and there is the circulation of hot water and steam.

Steam is separated from water in the steam drum and taken out. Longitudinal drum boilers range from 2250 kg/h to 3600 kg/h.

2. Cross drum boiler.

The drum in this type is placed in the cross to the heat source. The temperature obtained in this type of arrangement is more uniform. When the steam loads are high the upper tubes can become dry which causes them to fail.  The layout of tubes is made in such a way that large numbers of tubes are made available. The capacity of cross drum ranges from 700kh/h to 240000 kg/h

3. Stirling boiler:

A Stirling boiler has near-vertical, almost straight water tubes that zig-zag between a number of steam and water drum. Usually, there are three banks of tubes in a four drum layout.

The feedwater enters the left upper drum, from where it falls to the lower water drum. Water in pipes and two drums is heated, the steam produced rises in the upper drum from where steam is separated and taken off.

Package Boiler

These boilers come as a complete package. It requires only the steam, water pipework, fuel supply, and electrical connections to be made for it to become operational.

Package boilers are generally shell type with fire tube design so as to achieve high heat transfer rates

The packaged boiler is so-called because it comes as a complete package. Once delivered to the site, it requires only the steam, water pipework, fuel supply, and electrical connections to be made for it to become operational. Package boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection

The features of package boilers are:

  • Small combustion space and high heat release rate resulting in faster evaporation.
  • Large number of small diameter tubes leading to good convective heat transfer.
  • Forced or induced draft systems resulting in good combustion efficiency.
  • Number of passes resulting in better overall heat transfer.
  • Higher thermal efficiency levels compared with other boilers.

Which Type of Fuels do Boilers Use?

Combustion is the process of burning a fuel source. To create a reaction, there must be a fuel source, heat, and an oxidizing agent.

Boilers can be designed to burn a specific fuel, using any number of different technologies, but the main component to consider here is the heat source, or otherwise known as the fuel.

The fuel is one of the most important aspects of a boiler and is what burns inside the boiler to generate the heat.

There are many different sources that can be used.

  • Coal is a standard fuel source. In industrial boiler applications, the coal tends to be ground to a fine powder as it burns more completely than traditional bricks.
  • Electric can be used as a heat source, either by resistance heating coils or electrode units. Electric would normally only be used for smaller commercial or domestic use.
  • Electrode type applications require very high-water quality and conductivity to work effectively. Maintenance is key to electrode type applications too, as cleaning the insulators is required to prevent arcing between the electrodes.
  • Gas-fired boilers work by using either propane or natural gas, whereas oil-fired boilers work using gasoline or petroleum-based fluid.

An Example of Industrial Boilers Application

There are so many different applications that boilers are used for.

They are used in the food industry. Food, at various stages of production, needs to be heated or boiled as it is processed. The interesting use of boilers is in the brewing of beer!

During the beer brewing process, the malt needs to be ground and mixed with water, a process called mashing. This ‘mash’ is then heated, using steam, for several hours before the yeast is introduced to trigger the fermentation.