Steam Boiler: Steam Boiler Basic

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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.

Water Tube vs Fire Tube Boiler

Water Tube vs Fire Tube Boiler is comparison between two types boiler which is viewed from several aspects. Water tube and fire tube boiler is steam boiler classification which is categorized based on tube type. Commonly water tube boiler is appropriate for significant gas streams going above enormous amounts of generated steam (lb/hr) and also can certainly manage high temperature and pressure of steam. Fire tube boiler is appropriate for low pressure of steam, commonly under 500psig.

Water Tube vs Fire Tube Boiler can be seen in several aspects below:

Water Tube Boiler’s aspects:

- Gas flow : higher than 50.000 pounds/hour

- Temperature of gas inlet : small to adiabatic firing

- Pressure of gas : commonly under 2 psig

- Water inventory : low

- Location of sootblower : everywhere within boiler

- Multiple pressure of steam : yes

- Multiple modules : yes

- Heat flux- vapor area : could be high when use fin tubes

- Heating surface form : fin tube and also bare tube

- Combustion firing : doable

- Superheater location : everywhere in the gas way utilizing screen area

Fire Tube Boiler’s aspects:

- Gas flow : generally less than 50.000 pounds/hour

- Temperature of gas inlet : small to adiabatic firing

- Pressure of gas : commonly high up to 2100 psig

- Water inventory : high

- Location of sootblower : at exit boiler or inlet boiler

- Multiple pressure of steam : no

- Multiple modules : no

- Heat flux- vapor area : commonly low

- Heating surface form : bare tube

- Combustion firing : doable

- Superheater location : at exit boiler or inlet boiler

Within water tube boiler, expanded areas could be employed to create it stream-lined if the gas flow is thoroughly clean. The pressure drop of flue gas may also be decrease when compared with an equal fire tube boiler due to design intensiveness. Water tube boiler could be scaled-down and also consider less, especially when the gas stream is significant, excess one hundred thousand pounds per hour.

Superheater could be applied in the two boiler types. Within water tube boiler superheater might be positioned in an most effective area of gas temperature. A protect screen area or a huge convection area prefaces the superheater. Within fire tube boiler, it should be positioned at possibly the exit gas or inlet gas, generating the design fewer adaptable and weak to deterioration or slagging.

When the discharged gas is concentrated naturally, the areas could be cleaned simply by employing retractable soot blowers. Generally, the kind of boiler to be utilized for a specific condition is established by the practical knowledge of the boiler maker. Occasionally a collaboration of water and fire tube boilers is applied to match exceptional demands. From explanation above comparison between Water Tube vs Fire Tube Boiler can be known

Hot Water Heating Boiler

Hot water heating boiler is one of boiler types which is classified based on use. This boiler is utilized in the heating system to heat water at certain temperature and the hot water is distributed to the expected location. Hot water heating boiler in a house will help convenience especially in the winter season. The heating process is made of piping, pump, and boiler.

Hot water heating boiler based on its capability to heat water at certain temperature can be categorized into three types:

Low temperature hot water heating boiler - this boiler can operates and heat water until temperature 95^o C.
Medium temperature hot water heating boiler - this type of boiler can perform and increase temperature of water range 95^o - 120^o C
High temperature hot water heating boiler - this boiler type is used to heat water until temperature above 120^o C.

For medium temperature type and high temperature type, because of its temperature, water lead to change its phase to be steam. It must be prevented by applying pressure to the boiler system. There are several methods to generate pressurized hot water heating boiler as follow:

Nitrogen pressurization method – this method uses nitrogen that is compressed in a vessel tank and distributed together with water as shown in Figure 1 below.

Figure 1: Nitrogen Pressurization Method

Air pressurization method – this method is same with nitrogen pressurization method, the difference is the medium, in this method uses air as the medium.

Diaphragm air pressurization method – this method uses compressed air as above method but it uses diaphragm of neoprene as shown in Figure 2 below among water and air.

Figure 2: Diaphragm Air Pressurization Method

Pressure pump method – this method uses pump to produce pressure in the hot water heating boiler system as shown in Figure 3 below.

Figure 3: Pressure Pump Method of Hot Water Heating Boiler
(Source: Book-The Boiler Operator Handbook-NIFES-Graham & Trotman)

Basic Boiler System

A steam boiler is composed of 2 basic systems as shown in Figure 1. One basic boiler system is the water-steam system which is known as boiler waterside. In waterside of boiler, water is heated and released by movement via water tubes, then water is transformed to steam, and also it retains as steam in the system.

Figure 1: Basic Boiler System Diagram
(Source: Book-Boiler Control System Engineering-G.F Gilman)

Steam boiler need to manage chemical stability. The method where this is performed could interact to control system of feedwater. The quantity of blowdown should be regarded in the control design of feedwater, particularly when the blowdown is continually performed. Frequently, the flow of blowdown is separated by content level ratio of the flow of feedwater. The popular method pertaining to managing the chemical content level is continuous blowdown.

On big scale boiler this could possibly be performed automatically by way of measuring conductivity of boiler water to regulate blowdownn rate. Mixing ratio control of blowdown with conductivity and comparing feedwater flow to blowdown is carried out to get blowdown rate. Blowdown is usually obtained and conductivity is measured manually in utility plants. This is needed on a regular schedule or maybe any time the conductivity becomes very high.

The other basic boiler system is the fuel-flue gas-air system. This system is as well-known as the boiler fireside. This specific system supplies heat which is transferred into boiler fireside system. Fuel is the input for this system and to perform combustion or firing needs air and ignition system. The combustion air and fuel is as well-known as wind box. Ash and gas will be the output of this system.

Simple Vertical Boiler

Simple vertical boiler is a boiler which is constructed vertically and produces steam at low pressure and in small quantities. Therefore simple vertical boiler is used in low power generation or in places where space is limited. Construction of this boiler type is shown in Figure 1.

Figure 1: Simple Vertical Boiler

Simple vertical boiler consists of wall cylinder which surrounding the cylindrical fire box. Cylindrical fire box is tapped on it where steam flow to the surface. At the bottom of fire box there is grate. The fire box is equipped with two or more slope transverse pipe F, F. The slope aims to raise heating surface in addition also to improve water circulation.

Hand hole is made cleaning deposit purpose. A man hole is created above to so people can enter the boiler for cleaning. A ash hole is made on the bottom of boiler for disposal of ash buildup. The space between boiler wall and fire box is filled with water to be heated.

Waste Heat Boiler

Waste heat boiler is one of type boiler based on method of combustion. When waste heat at medium or high temperature is available, so waste heat boiler can be installed economically. If waste heat boiler needs more steam from the steam which is generated using the hot exhaust gases, can be used an additional burner that uses fuel. If the steam does not directly be used, steam can be used to produce electric power using steam turbine generators. It is widely used in heat recovery from flue gases of gas turbines and diesel engines.

Figure 1: Waste Heat Boiler

Thermic Fluid Heater Boiler

Currently, thermic fluid heater boiler has been widely used in various applications for indirect heating process. By using petroleum fluids as heat transfer medium, these heaters provide constant temperature. Combustion system consists of fixed grate with mechanical draft arrangements.

Thermic fluid heater boiler modern with oil-fired consists of double coil, construction of three passes and is fitted with pressure jet system. Thermic fluid, which acts as a heat carrier, heated in the heater and circulated through the user's equipment. Fluid transfer heat through heat exchanger to the process, then the fluid is returned to the heater.

Thermic fluid flow at the user end is controlled by a control valve that is pneumatically operated, based on the operating temperature. Heater operates at high fire or low fire depending on the return oil temperature which varies depending on system load.

The advantages of thermic fluid heater are:

- Closed operating system with minimum losses as compared to steam boilers.

- Operating system is not pressurized even for temperatures around 250 C compared to the needs of the steam pressure of 40 kg/cm2 in a similar steam system.

- Automatic control settings, which provide operational flexibility.

- Thermal efficiency is good because there is no heat loss caused by blowdown, discharge condensate and flash steam.

The overall economic of thermic fluid heater boiler depend upon the specific application and the reference basis. Thermic fluid heater boiler with coal-fired which has range thermal efficiency 55-65%. Thermic fluid heater boiler is most comfortable to use than the most common boiler. Incorporation of heat recovery devices in the exhaust gas will further enhance the thermal efficiency.

Figure 1: Thermic Fluid Heater Boiler

Boiler Type O and Type A

Boiler Type O

Boiler type O has 2 drums, they are steam drum and mud drum. The composition of the convection tubes and water wall tubes forming the letter O. Combustion chamber is placed in the middle between convection tubes and water wall tubes. Boiler type O can be shown in the Figure 1 below:

Figure 1: Boiler Type O

Legend:

1 : Furnace (combustion chamber)

2 : Steam drum

3 : Mud drum

Boiler Type A

Boiler type A has 3 drums, they are steam drum and two mud drums. Boiler type A can be shown in the Figure 2 below:

Figure 2: Boiler Type A

Legend:

1 : Furnace (combustion chamber)

2 : Steam drum

3 : Mud drum

Water Tube Boiler

In water tube boilers, boiler feed water flowing through the tubes into steam drum, water drum and header. Water is heated by gas burners or other fuel combustion to form steam and to be distributed into steam drum. Water tube boiler is selected if its steam and the steam pressure are very high as in the case of boilers for power generation.

Water tube boiler which is very modern designed with steam capacity of 4500-12000 kg / hour, with a very high pressure. Lots of water tube boilers are constructed in a package if used fuel oil and gas. For water tube that uses solid fuel, is not commonly designed package.

Characteristics of water tube boilers are the following below:

- Forced, induced and balanced draft help to improve combustion efficiency

- Less tolerant of water quality resulting from water treatment plant.

- Allows for the higher thermal efficiency.

Figure 1: Water Tube Boiler
(Source: United Nation Environment Program, 2008)

Bubbly Flow in Evaporation Process

Evaporation is the process to convert water into steam. Evaporation process occurs in steam boiler especially in water wall tubes area. One of evaporation process is bubbly flow. When temperature of water in water wall tubes is equal to saturation temperature of water, the little bubbles will arise especially near metal wall tubes. Saturation temperature of water will be reached faster in area near metal wall tubes because the heat is transferred first from metal wall tubes to water inside. Flow area of little bubbles can be called as Bubbly Flow.

Figure 1: Bubbly Flow in Evaporation Process

In the bubbly flow, bubbles of steam on one side can be small and round and on other side it could be great with a round and flat shape. In this condition the size of the bubbles is not always exactly same, but it is expected have uniform size.

Combustion of Natural Gas

Steam boiler can perform combustion with some fuels classification. Each fuel has advantage and disadvantage for use. One of fuel that usually used for combustion is natural gas. Combustion of natural gas can give higher efficiency than combustion of liquid or solid. But in the process, use natural gas for combustion is easy for explode and more dangerous, so the construction of steam boiler should consider safety and impact to high cost construction.

Hydrocarbons ethane and methane is the mainly content of natural gas. They can be danger because ethane and methane can burn easily. In the combustion system of steam boiler, to perform combustion need some requirement as follow; air supply, mixing of air and fuel, proper temperature and time. Natural gas can be burned with a blue flame or without a visible flame.

If one of requirement of combustion is not provided such as does not supply enough air and mixing composition of air and gas will lead to produce carbon monoxide, it is very poisonous and danger for human. So control of fuel and air is important to avoid incomplete combustion. Control range composition by volume of gas (based on hydrocarbon content) and air is the actual step to avoid burn explosively and violently. The danger range of gas is between 8% – 13 % and the safe range of gas is between 0 – 7 %.

A lot of concentrations of vapor exist, at certain range vapor will be diffused in the air and can lead to explosion. An explosion can occur if the mixture is contacted with open flame or spark. Therefore to prevent that concentration, it will be important to install safety shut of valve and perform purging at after and before combustion.

Fire Tube Boiler – Working Principle

Based on use classification, fire tube boiler is categorized as for process or heating boiler. It has range pressure not exceed 450 psi (small scale or medium scale). Design fire tube is regulated in ASME BPV Section I subpart PFT. Fire tube consists of tube and tube sheet.

Figure 1. Fire Tube Boiler

Fire tube design use tubes to flow hot gases and distribute it to the safe point of discharge through stack. Water at the outside of tubes is heated by hot tubes, so there is heat transfer between hot gases inside tubes and water outside tubes. Water is converted to steam for process purpose.

As long as combustion process there are some passes usually used. At the first pass, the hot gases in distributed in fire tube boiler through furnace. In Figure 1, the rear head of lower section had sealed the hot gas from first pass and direct it to the tubes at the second pass to front head. The hot gases from front head is redirected to the third pass then directed again to the fourth pass and the end the hot gas is discharged through stack.

Steam Blow before Start up Boiler

Steam blow is performed before starting up boiler. It is necessary to be executed to eliminate foreign material, scale, and debris remaining in the pressure parts especially for superheater, reheater and piping steam line after construction step. The objective of the elimination is to avoid blockages of the steam filter at the steam turbine inlet and damage to the steam turbine. Steam blow requires the use of extensive temporary piping to allow the discharging steam to be blown safely to atmosphere.

Steam blow process should consider about requirement not only for safety, but also consider other requirement, so procedure of steam blow should be designed well. The following below are requirement of steam blow for safety operation:

§ Remove all of combustible materials in steam piping line boiler area such us paper, rags, wood, etc

§ Install silencer at the end of steam piping line to avoid nuisance disturbance.

§ All of personnel include in steam blow process should be equipped with good communication system to make proper coordination each other.

§ Isolate steam supply with closed stop valve when install target plate.

§ Design good temporary piping and support to resist enough the loading during steam blow.

Classification of Boiler Based on Erection

Steam boiler has some classifications. One of them is classification based on erection of steam boiler. The following below are the type of steam boiler regarding to erection methods:

1. Package Boiler

Package boiler is a boiler which all of pressure parts and non pressure parts are fabricated and assembled in workshop. The assembly is performed in workshop completely include equipment of burner, control, mechanical draft and its accessories (see figure 1).

Figure 1: Package Boiler

2. Field Erected Boiler

Field erected boiler is a boiler where some pressure parts and non pressure parts such as steam drum, water drum, are constructed in site area where steam boiler will be placed (see figure 2).

Figure 2: Field Erected Boiler

Thermodynamics Term

Steam boiler has main task to convert water to high quality steam. When water is heated at atmospheric pressure, water will be converted to steam at temperature 212 F. When water is heated at higher pressure than atmospheric pressure, the boiling temperature will increase.

Enthalpy is the heat energy which is required to heat water from temperature 0 C (3 F) to the boiling temperature. Enthalpy is expressed as Btu/lb (British thermal unit per pound). When temperature from 32 F is increased gradually until 212 F, water will go to the boiling point temperature. If heat temperature is added more than 212 F, the water will converted to steam and the temperature can called as steam temperature.

Saturation temperature is reached when temperature of water is same with temperature of steam. Saturation temperature shows that at the boiling process, temperature is constant. Enthalpy of evaporation which means the heat required to convert water liquid phase to steam phase, will occur when heat temperature is increased from the boiling temperature.

The total enthalpy of saturated steam is the sum of enthalpy of saturated liquid and enthalpy of evaporation. Another means is total heat which is required to convert water and produce steam totally (see figure 1).

From figure 1, can be known that phase 1-2 is water phase, 2-3 is the boiling phase where there are mixture between water and steam at constant temperature. Phase 3-4 is superheated phase where water is converted 100% to steam.

In steam boiler, phases 1-2 occurs in economizer, water drum, downcomer and lower drum, while phase 2-3 occurs on top 1/3 furnace tubes and steam drum. Finally phase 3-4 occurs in superheater and reheater.

Gaseous Fuel in Steam Boiler

Steam boiler can be classified based on fuel; one of the fuels is gaseous. Gas can be generally divided as three categories; natural gas, butane, and propane. Natural gas is gas which be found in offshore area, while butane and propane are gases which be found in crude oil and had been separated from it. Natural gas is usually used for fuel combustion in steam boiler or for industry such as manufacturing of steel. Butane and propane are generated as LPG (Liquefied Petroleum gases)

Gaseous fuel generally has unit to express calorific value at pressure and temperature in normal condition. The unit is Mega Joules per Cubic Meter or can be symbolized with MJ/m3. Butane and propane are kept in designed place which has pressure and the unit can be expressed as Giga Joules per metric Ton (GJ/tonne). The details explanation of natural gas, butane and propane as follow:

Natural Gas:

Specific gravity compared to air : 0.59
Specific gravity liquid compared to water : -
Calorific value (MJ/m3) : 38 – 39
Calorific value (GJ/tonne) : -
Flammability limits air/gas ratio % by volume : 5.3 – 14
Volume gas m3 perkg of liquid at normal pressure & temperature : -

Butane:

Specific gravity compared to air : 1.49 – 2.1
Specific gravity liquid compared to water : 0.57 – 0.58
Calorific value (MJ/m3) : 121.8
Calorific value (GJ/tonne) : 49.3
Flammability limits air/gas ratio % by volume : 1.8 - 9
Volume gas m3 perkg of liquid at normal pressure & temperature : 0.42

Propane:

Specific gravity compared to air : 1.4 – 1.55
Specific gravity liquid compared to water : 0.5 – 0.51
Calorific value (MJ/m3) : 93.1
Calorific value (GJ/tonne) : 50
Flammability limits air/gas ratio % by volume : 2.2 - 10
Volume gas m3 perkg of liquid at normal pressure & temperature : 0.54

Steam Separator in Boiler

Steam separator has main function to separate water and steam and this equipment is usually located in steam drum. Water surface in steam drum is turbulent, so make it easy to mix between steam and water. The principle of steam separator is make steam flow is changed in any direction. Because of density of steam is lighter than water make steam can be distributed easier than water. The water droplet which has higher density will be separated and dropped from steam. Moisture will be removed by steam separator to eliminate damage and erosion if water or wet steam is distributed to the steam line.

There are some types of steam separator. For small scale steam boiler, steam separator consists of dry pipe which has a lot of holes at the top and two holes at the bottom half. The mixture steam-water is directed through the top half holes dry pipe, turbulent moving force the mixture to separate between water and steam. Steam will flow to steam line and water will drop through bottom holes (see figure 1).

Fig. 1: Dry Pipe Steam Separator
Source: Book - Boiler Operator's Exam Preparation Guide - Theodore B. Sauselein

For big scale boiler which has complexity equipment, steam separator method use centrifugal force for better result. The mixture steam-water is forced to move around the cyclone and make the rotation (see figure 2). The more turbulent moving force the mixture separate easily.

Fig. 2: Centrifugal Steam Separator
Source: Book - Boiler Operator's Exam Preparation Guide - Theodore B. Sauselein

Classification of Coal as Fuel in Steam Boiler

Steam boiler can be classified by fuel. One of them is solid fuel which contains coal as fuel in steam boiler. Coal can be classified based on percentage of volatile matter, ash, fixed carbon and moisture (see table 1). Based on these matter coal can be named as peat coal, bituminous coal (1), bituminous coal (2) and anthracite.

Table 1: Classification Coal in Steam Boiler
Source: The Boiler Operators Handbook - graham & Trotman

Percentage of volatile matter can be defined as percentage of coal component exclude moisture, which is heated until temperature 905 C, then measure the weight loss. Percentage of moisture is percentage of wetness content or condensed liquid in coal which can be dried on temperature 105 C. Percentage of fixed carbon is percentage of carbon content when volatile matter is driven off. Percentage of ash is a residue from firing coal which can be burned.

High percentage of volatile matter will make coal can be burned quickly and more generate smoke and low percentage of volatile matter coal can be burned slower and produce less smoke.

Classification of Boiler Based on Use

Boiler can be classified based on several types. One of classification of boiler is based on use, they are:

1. Power Boiler

The use of boiler in power plants and big industrial boiler can be called power boiler.

2. Process Boiler

The use of boiler in industrial process can be called process boiler.

3. Steam Heating Boiler

Steam heating boiler is boiler which use firing directly from fuels (oil, electricity, gas, and solid fuel) and has operating pressure less than 15 psi.

4. Hot Water Heating Boiler

Hot water heating boiler is used in industrial process to produce hot water and heating building. Boiler has temperature boiler under 250 F and operating pressure boiler less than 160 psi.

5. Hot Water Supply Boiler

Hot water supply boiler is used in industrial process and building to supply hot water. Operating temperature and operating design are same with hot water heating boiler.