Background Information
(A) What is a boiler?
A boiler is a heat transfer device that produces steam or hot water for use in heating, industrial processes, or power generation. They are generally closed vessels that use a fuel source or electricity to heat up water or generate steam. The basic configuration of a boiler includes a furnace where fuel is burnt to produce heat and a heat exchanger made of pipes and tubes or a heat transfer medium, which allows water to be heated above its boiling point.
There are different types of industrial boilers based on variations in their designs.
(B) Classification of Boilers
There are many industrial boilers based on the variation in their configuration. Understanding the differences among the various designs can help a prospective end-user determine which one meets his/her specific needs. Choosing between the different variations of boilers in the market can be simplified by looking at four main aspects of a boiler. These are: End product of the boiler (2) Design and construction (3) Heat source (4) combustion technology.
End Product of the Boiler
A boiler can be configured to produce either hot water or steam.
Hot water boilers are used to produce hot water required for hydronic or hot water heating systems. Typical applications include domestic and commercial hot water applications, comfort heating, commercial dishwashers, car washers, etc.
A steam boiler on the other hand is used to heat water to boiling point in order to convert the water to steam for use in different applications. Typical application of steam includes: driving turbines used for electric power generation. Varied industrial process heating, and agricultural applications.
Classification Based on Design and Construction
Based on the Design and construction, boilers are classified hit at least eight groups: viz.
(a) According to the location of the boiler shall Axis. The metal casing that encloses the tubes and the furnace of a boiler is referred to as a shell. The Axis of the boiler shell can be (i) Horizontal (b) Vertical (iii) Inclined. When the axis of the boiler shell is horizontal the boiler is called horizontal boiler. If the axis is vertical, the boiler is called a vertical boiler. If the axis of the boiler is inclined, it is known as an inclined boiler.
(b) According to the flow of medium inside the tubes. Based on this classification we have (a) fire tube boilers and (b) water tube boilers:
The boiler in which hot flue gases from the fuel burning in the furnace pass through tubes surrounded by water in the shell is known as a fire tube boiler. When water is inside the tubes and hot gases are outside, the boiler is called a water tube boiler.
(C) According to Boiler Pressure:
According to the maximum pressure of the steam the boiler can raise, boilers are classified as:
(i) Low pressure (3.5 -10 bar)
(ii) Medium pressure (10-25 bar)
(iii) High-pressure boiler (>25 bars)
(D) According to the Draft used:
The draft is the method of circulation of air inside the boiler. The boiler needs a supply of air to support combustion. Based on the Draft used, boilers are classified into (i) Natural draft boiler
(ii) Artificial draft boiler.
If the circulation of air is provided with the help of a chimney, the boiler is known as a natural draft boiler. When either a forced draft fan an induced draft fan or both are used to provide the flow of air the boiler is called an artificial draft boiler.
(E) According to the method of water circulation: If water circulation takes place due to a difference in density caused by the temperature of water, the boiler is called a natural circulation boiler. When the circulation is done with the help of a pump the boiler is known as forced circulation boiler.
(F) According to the position of the furnace: When the furnace of the boiler is inside its drum or shell, the boiler is called an internally fired boiler. If the finance is outside the drum the boiler is called an externally fired boiler.
(G) According to a number of tubes: A boiler having only one fire tube or water tube is called a single tube boiler. The boiler having two or more, fire or water tubes is called a multi-tube boiler.
(H) According to Boiler Mobility: When the boiler is fixed at one location and cannot be transported easily, it is known as a stationary boiler. If the boiler can be moved from one location to another it is known as a portable or mobile boiler. The boiler which can work on the surface of the water is called a marine boiler.
COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS.
As stated earlier we can distinguish boilers into fire tubes and water tubes based on the flow of medium inside the tubes.
A fire tube boiler is comprised of a set of tubes that are fixed to opposite sides of the tank (shell). One side of the tube banks is exposed to the furnace where the combustion of fuel takes place. Hot gases from the furnace pass through the tubes in order to transfer heat to water inside the shell.
Fire tube boilers are sub-divided into three groups as follows:
- Horizontal Return Tubular (HRT) Boilers. HRT boilers typically have horizontal, self centered, fire tubes with a separate contribution chamber.
- Scotch, Scotch Marine, Or Shell Boilers: These designs have fire tubes and combustion chambers housed within the same shell.
- Firebox Boilers: Firebox designs have water-jacketed fireboxes and are configured to allow for (max) three passes of combustion gases.
Compared to water tube boilers, fire tube boilers are: more fuel efficient, easier to operate, simpler in design and construction, and have cheaper initial costs. However, they are limited to low-pressure and low/medium-capacity applications.
Water Tube Boilers
Unlike fire tube boilers, water tube boilers feed water (rather than flue gases) through the tubes, while hot gases surround the tubes in the shell.
Water tube boilers are used when steam demand is high and the pressure required is also high. They are, however, more complex to construct, making them more costly. Also, water tube boilers are less tolerant of poor water quality, thereby requiring water treatment and conditioning to operate efficiently.
CLASSIFICATION BASED ON HEAT SOURCE (FUEL TYPE USED)
Based on the nature of fuel used to produce heat in a boiler, they are broadly classified into:
(i) Solid
(ii) Liquid
(iii) gaseous
(iv) electrical
(v) Nuclear energy
In practical terms, the commonly used fuel for boilers is coal, Biomass, Natural gas, oil, wood, electricity, and nuclear fuel.
Coal Fired Boilers
Coal is a standard fuel source for industrial boilers. It is used more in the pulverized form because pulverized coal burns more completely than larger clumps. The drawback to the use of cool fuel is that it is associated with environmental pollution. Hence, costly flue gas cleaning equipment like Scrubber and electrostatic precipitator have to be installed along with the boiler to remove harmful substances.
Biomass fired Boilers
All types of burnable organic materials including wood chips, rice husks, palm kernel shells, and a list of other agricultural waste products are collectively called Biomass and they are a cheap source of energy for use by the boiler. Biomass has proven to be economical in facilities where they are readily available. However, depending on the type of Biomass, environmental law may require the installation of scrubbing and filtering equipment for the stack gas.
Natural Gas fired boiler
This class of boilers uses propane or natural gas as its primary fuel. They are easily switched to use gasoline or other petroleum-based fluids as fuel sources with little modification.
Gas is a cleaner fuel than other sources of energy. They are highly combustible and have a high energy-to-heat ratio. Notably, it has distinct environmental advantages.
Oil-fired boilers:
This class of boilers uses gasoline or other petroleum-based fluid as their fuel source. Included in this group is a thermic fluid heater that uses oil as a heat transfer medium.
Electric Boiler
Electricity is converted to heat by the use of either resistance heating coils or electronic-type heating units to provide energy for boilers. A very important point to note is that Electric boilers require high-quality water to function effectively. Also, they are mainly used at the present time for low-capacity applications and for producing hot water.
Nuclear fired boilers
This class of boilers uses the energy generated by nuclear fission in a reactor to generate steam/hot water. They are mostly used in power plants.
Classification based on Contribution technology
The fuel combustion system of each boiler is configured to burn a particular type of fuel efficiently. In other words, the combustion technology of each boiler is designed for a specific type of fuel. The different systems of fuel combustion are: (1) fluidized bed (2) stoker and (3) thermic fluid heating.
Fluidized Bed System
This technology system bubbles air through a mixture of sand and solid fuel in the furnace to obtain a uniform heating temperature. Some notable advantages of this technology are: (i) Compact boiler design (ii) fuel flexibility (iii) high combustion efficiency (iv) reduced emission of noxious pollutants.
Fluidizes bed combustion is the new standard for boilers that uses Biomass as fuel.
Stoker system
Coal-fired boilers are adapted to stoker combustion technology. The two common types in use are (1) spread stoker and (2) traveling gate or chain gate stoker.
How to Specify Your Boiler
Selecting a boiler to meet your application requirements begins with drawing up the process/service conditions that the boiler must meet; under the following categories:
1) Capacity (Size)
The size of a boiler is depicted by the amount of steam or hot water it can produce in a unit of time usually one hour typically, this is measured in either of
(1) Pounds of steam per hour (Ib/hr)
(2) Gallons of water per minute (gpm)
(3) Kilogram of steam per hour (kg/hr)
(4) Tons of steam per hour (tons /hr)
The first step is to determine the capacity of the boiler that you need by accurately estimating the processing requirements of your industry.
2) Critical Process Condition
Critical process conditions of an industry involve (i) maximum temperature & (ii) maximum pressure requirements of the process. These conditions become the average conditions that the boiler must meet in service.
3) Desired Efficiency:
Also referred to as thermal efficiency, it defines what percentage of heat generated by the boiler is usefully engaged. It is calculated by subtracting the energy lost with the stack gas from the energy input and dividing it by the total energy input to the boiler. The efficiency you specify will determine the type of boiler to procure for your industry.
4) Type of Fuel
This is about the most important decision that, will determine your choice of boiler specification of the preferred type of fuel for your boiler should take into account, availability and cost of fuel. The different types of fuel to consider are Natural gas, oil, wood, coal, biomass, and electricity.
For instance, an agro-processing factory such as a rice mill with a lot of Biomass generated as a product will do well to consider using a biomass fuel-fired boiler.
5) Emission Norms & Environment Quality Requirements:
The other critical factor to consider regarding boiler selection apart from the choice of fuel has to do with the Emission norms of the environment where the boiler is to be sited. Areas with stringent pollution control regulations allow the use of fossil fuel only if pollution control equipment is installed along with the boiler to control noxious emissions.
6) Budget
The initial cost of the boiler as well as the running cost will also be considered in arriving at the type of boiler to install.
7) Added Features:
Industrial boilers come with a number of important features and accessories that an end user may wish to consider before making the final decision on the type to buy. Such features include (1) the presence of de-aerators for removing dissolved air in boiler feed water (2) safety valves to prevent excessive pressure build-up within the system (3) a superheater to raise the temperature thereby reducing condensation (4) water treatment and conditioning equipment to prevent fouling to boiler tubes. It may also be necessary to ascertain whether the boiler will be delivered as a packaged unit, requiring only minimum installation work.
HOW TO AVOID COSTLY ERRORS:
A wrong choice of boiler for an industrial plant comes with dire consequences. As such, it is always better to speak with a boiler engineer first before procuring a boiler. Some benefits derivable from such an exercise include:
(1) Selecting the right boiler to meet your process needs.
(2) Selecting the right size of steam tubes to convey the steam/ water from the boiler to the point of use.
(3) Installing proper steam trapping system.
At N.C. Gilbert Dev. Co. Ltd. We supply boilers that are guaranteed to meet your process needs because:
(1) We are boiler Engineers with years of experience.
(2) We are Agents/Representative to a reputable boiler manufacturing company – Kaifeng Xinli Boiler Equipment Co; Limited of China.
We will not only guide you in choosing the right boiler for your industry; but we will also procure, install, commission, and provide after-sales service to keep your boiler running optimally.
