Steam Boiler: February 2011

Role of Feedwater Heaters in a Steam Boiler

Role of feedwater heaters in a steam boiler is important to make efficiency of steam boiler higher. A feedwater heater is a heat exchanger that has function to heat feedwater before to be supplied to a steam boiler. The feedwater heater is used to bring feedwater closer to temperature of the steam boiler water. It aims to increase steam boiler efficiency 1% with each 10^oF rise in feedwater temperature. This increase efficiency can make saving in required fuel to heat steam boiler water.

There are two general types of feedwater heaters are used in a steam boiler plant:

1. Close feed water heater.

This type of heater is an indirect heater because the steam and water are separated by tubes and the water is heated by conduction. The closed feedwater heater contains two important parts: tubes and shell. Feedwater flow through the tubes and shell-side of the feedwater heater contains hot steam, generally use steam from exhaust turbine or waste heat generated in the steam boiler plant to improve overall plant efficiency. The feedwater is heated as a result of heat transfer from steam to feedwater.

The close type of heater can heat water with higher temperature than close type. The close feed water heater is located before the steam boiler feed pump with has low pressure and a high pressure feedwater heater is located at the downstream side of the steam boiler feed pump

2. Open feed water heater.

This type of heater is a direct contact heater as the water is mixed directly with steam. The open feedwater heater is located at the suction side above steam boiler feedwater pump. This is necessary so that the feedwater pump can be supplied with a head pressure to permit higher feedwater temperature.

Water Treatment Before Supplied to Steam Boiler

Water treatment is the process of making the raw water from any natural source, such as a river, well, or lake, suitable for feeding into a steam boiler. Water must be treated to prevent scale deposit and to control corrosion. If a steam boiler surface is covered with scale, heat normally absorbed by the water goes up the stack instead, the tube metal temperature of steam boiler rises to the point of failure because scale mostly calcium and magnesium, is an excellent insulator and slows the transfer of heat to the water.

The objective of water treatment, combining the external treatment and internal conditioning of a steam boiler, in one word is cleanliness—cleanliness of the wetted parts. This, in turn, facilitates the production of clean steam, which keeps the steam boiler, piping, and turbine protected. External water treatment is done before water is fed into the steam boiler and is differentiated for a better clarity from the internal water conditioning within the steam boiler island. It converts raw water to feedwater. Water conditioning is the dosing of appropriate chemicals at proper places to the treated water to prevent damage to the internal surfaces of a steam boiler and make the steam suitable for a turbine or process.

Water treatment before supplied to steam boiler consists of the following stages:

1. Clarification (sedimentation followed by filtration): to remove suspended solids

2. Softening or demineralization: to remove hardness and dissolved solids

3. Deaeration: Oxygen in the boiler causes corrosion in the form of deep pits. Deaerator work on the principle that hot water holds less dissolved gas than cold water. Heated water in a pot has bubbles that form under the surface long before the water boils. These are dissolved gases coming out of solution as the water warms up.

Deaerator performs three functions:

§ Removal of dissolved non condensable gases (air) such as oxygen, nitrogen, and carbon dioxide.

§ Heating of feedwater

§ Storage of feedwater

Heat Transfer within Steam Boiler

Fig. 1: Heat Transfer within Steam Boiler

A steam boiler is designed to absorb the maximum amount of heat released from the process of combustion. Heat transfer within steam boiler is accomplished by three methods: radiation, convection, and conduction. The heating surface in the furnace area receives heat primarily by radiation. The remaining heating surface in the steam boiler receives heat by convection from the hot flue gases. Heat received by the heating surface travels through the metal by conduction Heat is then transferred from the metal to the water by convection.

The relative percentage of each heat transfer within steam boiler is dependent on the type of steam boiler, the designed transfer surface, and fuels. Each of these methods is discussed in more detail below:

§ Radiation

Radiation is the process of continuous interchange of energy by means of electromagnetic waves without a change in the temperature of the medium between two bodies involved. Radiation is the most important method of heat transfer in the furnace of steam boiler. The amount of heat transfer depends on the area of the heating surfaces and hot surfaces in the furnace. The physical laws governing transmission of heat transfer by radiation is: Heat is transmitted in the straight line, heat can be reflected and refracted, and heat is radiated in all direction.

The steam boiler tubes absorb the radiant heat from flame and radiate a small portion of the heat back to the furnace. Radiation is present in all steam boilers.

§ Conduction

Conduction within steam boiler is the transfer of heat from one part of a body to the other or from one body to another in physical contact. It is the flow of energy that occurs in a substance without appreciable movement of molecules due to lattice vibrations and flow of free electrons. Heat flow from flue gases to water/steam across the metal wall in a steam boiler, loss of furnace heat to the surrounding atmosphere through insulation, and heat loss from hot steam/water pipes through insulation are all examples of conduction.

§ Convection

Convection is the process of transferring heat by movement energy through a material. When any fluid in a steam boiler is heated, its density is decreases and the fluid become lighter. Then the heavier or cooler fluid will flow and replace the heated portion of the fluid. In turn the cooler portion becomes heated. The flow of the cooler fluid to heated area is called convection. Heat transfer by convection normally occurs from a lower to higher elevation of steam boiler.

Steam Boiler Efficiency

Steam boiler efficiency is very important information to indicate among other factors the fuel usage, which in turn determines operating cost. There are several ways of defining steam boiler efficiency:

§ As-Found Efficiency: is the efficiency measured in the field for steam boilers existing in a state of repair or maintenance. It is used as the baseline for any subsequent efficiency improvements.

§ Tuned-Up Efficiency: is the efficiency after operating adjustments (low excess air) and minor repairs have been made.

§ Maximum Attainable Efficiency: is the result of adding currently available efficiency improvement equipment, regardless of the cost considerations.

§ Maximum Economically Achievable Efficiency: differs from that above in that it accounts for realistic cost considerations with efficiency improvement equipment added only if it is economically justifiable.

ASME Power Test Code PTC 4.1 has defined two methods for determination of efficiency:

§ The input & output method

The input-output method has defined the efficiency of steam boiler as the useful energy output divide by the energy input.

The energy input is measured in Btu/h. The following is an example of a simple way to calculate steam boiler efficiency

Question: A gas-steam boiler type fire tube has capacity of 12,300,000 Btu/h. The stack loss is 1,400,000 Btu/h. What is the efficiency of the steam boiler?

Answer:

Stack Loss = 1,400,000 Btu/h

Input capacity = 13,500,000 Btu/h

Output of the steam boiler = 12,300,000 - 1,400,000 = 10,900,000 Btu/h

§ The heat loss method

Calculation steam boiler efficiency based on the heat loss method is not simple; all parameter of heat loss should be considered. The total heat loss is the sum of heat losses due to steam leaks, radiation, blowdown, heating of combustion air, condensate, make up water, and stack.

Requirement of Water for Steam Boiler

Fig.1: Water Tank of Steam Boiler

Water is only medium for generation of steam. In a steam boiler system, water is continuously fed as feedwater that is converted into steam and delivered constantly to main steam header. The quality of water is obviously an important factor to make steam boiler has maximum combustion & heat transfer efficiency, safe operation, minimum maintenance, and long working life. Good quality of water is a necessity for economic and efficient operation of a steam boiler.

There are various types of water processed and used in the steam boiler system:

§ Raw Water : raw water is available from nature. An unlimited quantity of raw water is available form ponds, lakes, rivers, and oceans.

§ Make up water: Make water is the water supplied from municipal water system, wells, and other sources, for addition of new water to the boiler required to replace the water converted to steam.

§ Steam condensate: When steam has performed its work, it transfers heat and reverts to liquid state called steam condensate.

§ Feedwater: Feedwater is the treated water that is stored in a water tank after necessary treatment.

§ Steam Boiler water: This is the water that circulates within the steam boiler system. Chemicals are injected into the steam boiler water, making it suitable for use under pressure in the system

Requirement of feed water before be supplied to steam boiler should be follow the following criteria:

§ Freedom from scale

Raw water contains hardness like dissolved salt, mainly calcium and magnesium compound, silica scale, iron deposit that is formed as iron oxide and accumulate on steam boiler surface. If hardness is present in the feedwater and not controlled chemically, then scaling of the heat transfer surfaces occur, reducing heat transfer and efficiency making frequent cleaning necessary. In extreme cases, local hot spots can occur, leading to mechanical damage or even tube failure.

§ Freedom from oxygen

Oxygen make the water corrosive, causing localized pitting of the steam boiler metal.

§ Freedom from acid attack

If the PH is too low, the acidic solution will attack metal surfaces of the steam boiler.

§ Freedom from caustic embrittlement

If the PH value is too high and the water is contain high alkaline solution that work itself into the crack may cause caustic embrittlemnet.

Combustion System of Steam Boiler

Combustion is the name given to the chemical reaction between fuel and oxygen which results in the production of heat. The oxygen is usually derived from the air, which unites chemically with the carbon, hydrogen, and a few minor elements in the fuel to produce heat. A steam boiler designer and operator should understand the fundamental principle of combustion process to assure satisfactory service and high efficiency.

Combustion of steam boiler will not start in air or be maintained below a certain temperature, known as the ignition temperature. Once the fuel is ignited, it must supply sufficient heat to raise the temperature and establish continuous combustion. Thus the three requirements for combustion to be established are fuel, temperature/heat and air.

The combustion process of steam boiler should consider as follow:

1. Control of air supply

The amount of air required depends upon the fuel, the equipment used for combustion. It is important that the best proportion of air to fuel be determined and maintained in order to obtain the highest efficiency possible. Too much air results in excessive release of hot gases from the stack with a correspondingly high heat loss and reduction in efficiency.

If therefore exactly the right amount of air is provided to burn the fuel completely, the waste gases will only contain CO₂, steam, SO₂, and N₂. The CO₂ content will be less than 21% for burning pure carbon completely.

2. Mixing of air and fuel

Air and fuel must be thoroughly mixed since each combustible particle must come into intimate contact with oxygen contained in the air before combustion can take place. Combustion steam boiler equipment is designed to obtain the best possible mixing of fuel and air.

3. Temperature required for combustion

Combustion will start in certain temperature to accelerate oxidation. Operator steam boiler should maintain the air and fuel mixture at a temperature sufficiently high to promote combustion

4. Time required for combustion

An appreciable amount of time is required to complete the process of combustion of steam boiler. Fuel and air should have enough time to permit complete combustion, if equipment of steam boiler is operated at excessively high ratings, the time may be insufficient to permit complete combustion.

Reference:

Books:

1. Mohammad A. Malek: Power Boiler, Design, Inspection, and Repair ASME Code Simplified, MacGraw Hill 2004

2. Carl D. Shield: BOILERS Type, Characteristic, and Fuctions, MacGraw Hill 1982

3. Everet BW, Herbert BL: Steam Plant Operation Sixth Edition, MacGraw Hill 1992

4. NIFES (National Industrial Fuel Efficiency Service): The Boiler Operators Handbook Second Edition, Graham & Trotman 1991

Website:

1. www.triadboiler.com

Simple Flow Diagram of Steam Boiler

Fig.1: Simple Fowl Diagram of Steam Boiler

The simple flow diagram of steam boiler in Figure 1 shows that steam boiler comprises various system, such us a feedwater system, a fuel system, a steam system, product of combustion system, a stack system, and a blowdown system. The inputs for the boiler are fuel and feedwater, whereas the output of steam boiler are combustion product and steam. Like the product of combustion, the blowdown of water from boiler is also waste.

Feedwater is water that is supplied to steam boiler. Water is supplied to the boiler either by gravity return (in small installation) or by a boiler feed pump. In steam boilers, the water absorbs heat until it reaches the boiling point. Converted into steam, it then collects in the highest part of the boiler because of the difference in density between steam and water.

Steam is generated within the boiler as a result of heat transfer from fuel to feedwater, and pressure in the steam boiler will force the steam to discharge through the main steam pipe. The gas at the burner is burned in the furnace within the boiler and becomes products of combustion, transfer heat to feedwater, moves out of the steam boiler under furnace pressure, and ultimately to atmosphere through a stack.

When feedwater is converted into steam, sediment is left behind ath the bottom of the biler, and this sediment is removed by blowdown either at scheduled intervals or continuously.

In terms of a real situation, the boiler flow diagram is complex. For example, the simple gas burner shown in the diagram is a complete combustion equipment system comprising many valve, controls, safety features, burning equipment, and piping. The combustion equipment system is designed to make the combustion efficiency as high as possible. The same principle is applicable to other system, such as steam systems, blowdown systems, and product of combustion system.

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)

Steam Boiler Definition

Figure 1. Steam Boiler 2x7 MW

A steam boiler is defined as a boiler use heat to convert water into steam for a variety of application. Though useful to heat one's home or business, the steam can still be used to power steam engines or other, similar tasks. Steam is a key resource because of its availability, advantageous properties and nontoxic nature.

In technical terms, a steam boiler is defined as a closed vessel where water or some other liquid is heated, steam or vapor is generated, and steam is superheated. Or any combination of these functions is accomplished under pressure or vacuum for use external to itself by the direct application of energy from combustion of fuels, electricity, or solar energy.

A steam boiler consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. A steam boiler is driven by steam generated under pressure in a boiler. The amount of steam that can be generated per hour depends upon the rate of combustion of the fuel in the furnace and upon the efficiency of heat transfer to the boiler proper.

A steam boiler should be designed to absorb the maximum amount heat released in the process of combustion. This heat is transmitted to the boiler by radiation, conduction, and convection, the percentage of each depending upon steam boiler design.

Each form oh heat transmission gives special consideration in design steam boiler. All three forms of heat transmission occur simultaneously and cannot readily be distinguished from each other in the operation of a steam boiler.