Steam Boiler: Steam Boiler Classification

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Stoker Boiler Types

Stoker boiler types can be classified based on its type and based on the method of feeding fuel to the furnace. Stoker boiler types are generally classified as chain-gate or traveling-gate stoker boiler and spreader stoker boiler as explained below:

Chain-grate or traveling-grate stoker boiler

On the classification of this stoker boiler type, coal is fed to the end of steel grate that moves. When the grate moves along the furnace, coal is expected to be burned before it falls on the other end as ash. In this stoker boiler type, to adjust the baffles, air damper and grate need certain skill level; therefore, personnel in charge must be capable and qualified. In addition, the combustion of chain grate or traveling grate stoker boiler must be clean so as to produce a minimum amount of unburned carbon in ash.

Coal feed hopper is installed lengthwise along the bottom furnace where possibility feed coal can fall down along stoker grate. A coal grate is used to control the speed of fed coal into the furnace by controlling the thickness of fuel bed. The size of coal should be uniform because a large chunk will not burn out completely at the time of reaching the end of grate.

Figure 1: Chain Grate or Travelling Grate Stoker Boiler
(Source: www.cf.missouri.edu/energy/)

Spreader stoker boiler

Another type of stoker boiler is the spreader stoker boiler; this type utilizes combination of suspension combustion and also grates combustion. Spreader stoker feed coal continuously into the furnace above the combustion bed of coal. Larger particles of coal will fall into the grate while soft coal is burned in suspension, where it will be burned in the thin bed combustion of coal and the rapid combustion.

Combustion method in the spreader stoker type provides good flexibility related to the fluctuations of load, because of ignition will takes place rapidly if combustion rate increases. Furthermore, the spreader stoker boiler is favored when compared with other stoker boiler types in many industrial and power plant applications.

Figure 2: Spreader Stoker
(Source: Coal-Improved Techniques for Efficiency- Department of Coal India)

Package Boiler

Package boiler is a boiler that had been fabricated and available as a complete package. The entire pressure parts have been assembled in the workshop and ready to be sent to the field or site where power plant is located. On the field/site requires only connection and integration of course work in electrical connections, water pipes, steam pipes and fuel piping system to operate. Package boiler is one of classification of boiler based on erection. Design package boilers in general are the type of Fire Tube Boiler and have made up shell and tubes. This type has high heat transfer both radiation and convection.

Package boiler has the following characteristics:

Package boiler has higher level of thermic efficiency than other types of steam boilers.
A number of passes generate better heat transfer overall.
Induced or forced draft system generates proper combustion efficiency.
A large number of small diameter tubes can make package boiler has good convection heat transfer.
The small dimension of combustion chamber/furnace and the high heat which is released will result in more rapid evaporation.

Figure 1: Package Boiler
(Source: Spirax Sarco)

Package boilers can be classified based on the number of passes which mean the number of times the combustion gases through the boiler. The combustion chamber is placed as the first pass after the later one, two, or three sets of fire tubes. The most common type of boiler in this class is unit of three passes with two sets of fire tubes and exhaust gases exiting from the back boiler.

Classification of Furnace Steam Boiler

Furnace of steam boiler is the space in which combustion of fuel take place. Furnace can be classified in many types. The general classifications of furnace steam boiler are as follow:

§ Based on wall construction:

1. Air cooled refractory setting

2. Solid refractory setting

3. Water wall / water cooled

Fig. 1: Furnace Room of Steam Boiler

§ Based on pressure maintained:

1. Balance draft furnace

2. Pressurized furnace

3. Supercharged furnace

§ Based on fuel:

1. Coal furnace

2. Oil furnace

3. Gas furnace

4. Baggase furnace, etc

§ Based on firing method:

1. Hand fired furnace

2. Stoker fired furnace

3. Pulverized coal fired furnace

§ Based on heat transfer:

1. Radiation furnace

2. Convection furnace

§ Based on waste heat recovery:

1. Recuperative furnace

2. Regenerative furnace

Classification Steam Boiler Based on Tube Type

A steam boiler is closed vessel in which water, is transformed into steam by application of heat. Classification of steam boiler generally can be seen here and the heating transfer through tubes can be classified as:

§ Fire Tube Steam Boiler

Fig.1 : Fire Tube Steam Boiler
(source: reprinted from ref. 6)

Fire Tube Steam Boilers consist of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As hot gases flow through the tubes, they heat the water that surrounds the tubes. The water is confined by the outer shell of the steam boiler. To avoid the need for a thick outer shell, Fire Tube Steam Boilers are used for lower-pressure applications. Steam pressure is limited to 450 psig.

Advantages of Fire Tube Steam Boiler:

Þ Easy to clean

Þ Relatively inexpensive

Þ Compact in size

Þ Well suited for industrial applications

Disadvantages of Fire Tube Steam Boiler:

Þ Not suitable for pressure higher than 450 psig

Þ Limitation for high-capacity steam generation

§ Water Tube Steam Boiler

Fig.2 : Water Tube Steam Boiler
(source: reprinted from ref. 6)

Water Tube Steam Boilers are designed to circulate hot combustion gases around the outside of a large number of water-filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In older designs, the tubes are either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, Water Tube Steam Boilers can be fabricated in larger sizes and used for higher-pressure applications.

Water Tubes Boilers have the following advantages:

Þ Available in larger capacities

Þ Able to handle higher pressures

Þ Faster response to changing loads

Þ Able to reach very high temperatures

Þ Provide an adequate furnace to ensure complete combustion

Disadvantages of Water Tube Steam Boiler:

Þ High initial capital cost

Þ Cleaning is difficult due to design structure

Þ No commonality between tubes

Þ Bigger in physical size

Classification Steam Boiler Based on Fuel

A steam boiler is closed vessel in which water, is transformed into steam by application of heat. Heating is caused by combustion of mixture of fuel, oxygen, and heat source. Classsification of steam boiler generally can be seen here and the common fuels fall into four categories shown below:

§ Gaseous Fuel

A steam boiler is fired by gas. The gas used for the boiler may be natural gas, propane gas, or any other liquid petroleum gas.

§ Liquid Fuel

A steam boiler is fired by liquid fuel. The liquid used for the boiler may be heavy oil known as Bunker C is used because of low cost fuel.

§ Solid Fuel

A steam boiler is fired by solid fuel. The solid used for the boiler may be coal, coke, peat, wood.

§ Electricity Fuel

A steam boiler is fired by electric. The electricity is generated by solid, liquid, gas or nuclear.

The first three are used most commonly to fire steam boiler and are composed mainly of carbon and hydrogen with small amount of sulphur and incombustible material. Electricity is a second generation fuel and is used to heat small steam and hot water boilers for specialist application.

Advantages and disadvantage steam boiler based on fuel:

1. Gaseous Fuel

Advantages:

Good efficiency of combustion

Low cost fuel

Disadvantages:

High cost for the construction

Difficult to be found

2. Liquid Fuel

Advantages:

Residue of combustions are not numerous and easily cleaned

Easily to be found

Disadvantages:

High cost fuel

High cost for the construction

3. Solid Fuel

Advantages:

Easily to be found

Low cost for the construction

Disadvantages:

Residue of combustions are hardly cleaned

Difficult to find good material

4. Electricity Fuel

Advantages:

Most easy to maintain

Easily construction and fuel easily to be found

Disadvantages:

Bad efficiency of combustion

Low temperature of combustion

Classification Steam Boiler Based on Pressure

A steam boiler is closed vessel in which water, is transformed into steam by application of heat in under pressure condition. Classification of steam boiler generally can be seen here and based on pressure can be classified as:

§ High Pressure Steam Boiler

Fig 1: HP Steam Boiler

A high pressure steam boiler is boiler in which steam is generated at pressure exceeding 15 psig and operates hot water boiler above 160 psig or 250^o F. Application this type usually is used for utility plants, big industrial plants and process application. A high pressure steam boiler can be named as Power Boiler, in which design and construction rule following to rules of ASME Boiler and Pressure Vessel Code Section I – Power Boiler.

§ Low Pressure Steam Boiler

Fig.2 : LP Steam Boiler
(source: triadboiler.com)

A low pressure steam boiler is defined as a boiler in which steam or other vapor is generated at pressure less than 15 psig or in which hot water is generated at pressure not exceeding 160 psig or temperature not exceeding 250^o F. A low pressure steam boiler can be named as Heating Boiler, in which design and construction rule following to rules of ASME Boiler and Pressure Vessel Code Section IV – Heating Boiler. Low pressure steam boiler are used for generating heat for industrial use or for heating systems found in healthcare facilities, apartments, universities, office buildings and historic landmark building.

Steam Boiler Classification

Steam boilers are built in the world from 200 BC until today, many developments have taken place that allow us to classify steam boiler in different ways. Hence steam boiler may be classified based on the following characteristics:

1. Classification of boiler based on pressure

§ Low Pressure

A steam boiler in which steam is generated at pressure less than 15 psi or a boiler in which hot water is generated at pressure not exceeding 160 psi and/or temperature not exceeding 250^oF

§ High Pressure

A steam boiler in which steam is generated at pressure exceeding 15 psi or a boiler in which hot water is generated at pressure exceeding 160 psi and/or temperature exceeding 250^oF

2. Classification of boiler based on fuel

§ Gaseous Fuel

A steam boiler is fired by gas. The gas used may be natural gas, propane gas, etc.

§ Liquid Fuel

A steam boiler is fired by oil. The oil used may be heavy oil known as Bunker C because of low fuel cost.

§ Solid Fuel

This type of steam boiler is fired by coal, wood, rejected product, etc

§ Electric Fuel

This type of steam boiler is fired by electricity. Usually used for research laboratory, food industries, and others where smoke pollution is prohibited.

3. Classification of boiler based on material

§ Steel

§ Cast Iron

4. Classification of boiler based on tube type

§ Fire Tube Boiler

A steam boiler in which the products of combustion pass through the tubes, which are surrounded by water.

§ Water Tube Boiler

A steam boiler in which the water passes through the tubes and products of combustion surround the tubes.

5. Classification based on circulation

§ Natural Circulation

§ Force Circulation

6. Classification of boiler based on method of combustion

§ Stoker Boiler

§ Fluidized Bed

§ Pulverized coal

§ Burner-Fired Boiler

§ Waste Heat Steam Boiler

7. Classification of boiler based on type of support

§ Top Supported Steam Boiler

§ Bottom Supported Steam Boiler

§ Middle and Girdle Steam Boiler

8. Classification of boiler based on furnace construction

§ Two Pass Steam Boiler

§ One and a Half Pass Steam Boiler

§ Single or Tower Type Steam Boiler

§ Down-Shot Steam Boiler

9. Classification based on use

§ Power Boiler

§ Process Boiler

§ Hot Water Heating Boiler

§ Hot Water Supply Boiler

§ Industrial Boiler

§ Commercial Boiler

§ Heat Recovery Boiler

10. Classification of boiler based on erection

§ Package Boiler

§ Field Erected Boiler

11. Classification of boiler based on mobility

§ Stationary Boiler

Stationary boiler is a boiler that is placed on permanent foundation such as boiler for power generation, industries and the others.

§ Mobile Boiler

Mobile boiler is a boiler that is placed on mobile foundation such as locomotive boiler and cab car.

12. Classification of boiler based on furnace position

§ Internally Fired Steam Boiler

§ Externally Fired Steam Boiler

13. Classification of boiler based on ASME Code

§ Power Boiler - ASME Boiler and Pressure Vessel Section I

§ Heating Boiler - ASME Boiler and Pressure Vessel Section IV

14. Classification of boiler based on heat source

§ Conventional Boiler

§ Heat Recovery Steam Generator (HRSG)