Boiler design

Boiler design is the process of designing of boilers used for various purposes. The main function of boilers is to transfer heat produced by the combustion of fuel to water and ultimately to generate steam. Steam produced in a boiler is used for a variety of purposes including space heating, sterilisation, drying, humidification and power generation. The temperature or condition of steam required for these applications is different, therefore boiler designs vary per types of requirements.

Contents 1 Boiler Design 1.1 Considerations 1.2 Prerequisites 1.3 General design procedures 2 Material selection 3 Manufacturing techniques 4 Types of boiler accessories and mountings 5 Advantages and disadvantages of contemporary boilers 6 Testing of boilers 7 References 8 Bibliography 9 External links

Boiler Design

Considerations

• Safety: In the past, improper design of boilers has caused accidents. This led to loss in life and property. By designing an appropriate boiler, we can avoid mishaps and also save resources.
• Mathematical Modeling can be done. This allows the knowledge of how much space a boiler will need and the type of materials to use.
• Predicting response of system to input and surrounding.
• Save cost of trial and error.

When the design specification of a boiler is determined, then the conceptual design and engineering details such as the type of boiler, the area of heat exchanging surfaces, and other variables can be selected, and consequently design engineers can estimate a cost and time schedule for the whole construction.

Prerequisites

Essentials of designing a steam boiler include:

1. Production of a maximum quantity of steam with minimal fuel consumption
2. Economic feasibility to install
3. Minimal operator attention required during operation
4. Capability for quick starting
5. Conformity to safety regulations

• Input needed to design a boiler
  • Quality of raw water : how hard or soft the water is will determine the material of the boiler. Hard water with a lot of impurities may need to be neutralised before being fed into the boiler. It can also decide the type of boiler e.g. water tube, smoke tube/fire tube or coil type boilers
  • Heat source — the fuel to be burnt and its ash properties^[1] or the process material from which the heat is to be recovered.
• Output desired
  • Capacity / steam output required: It is usually measured in tonnes per hour or kg/sec. Depending on its rate a convenient unit is chosen.
  • Steam condition - pressure, temperature, super heat if any, dryness fraction.
  • Type of load/ steam demand (fluctuating or steady)
• Safety considerations
  • Ability to respond to changing conditions/ steadiness of steam condition / operating parameters - this may decide the type of instrumentation (level switches or level transmitter (single element control or three element control)) To control hazards in a boiler, following methods are used: Process and/ or material substitution, Engineering Controls, Administrative Controls, Personal Protective Equipment.
• Mechanical constraints
• Cost Restrictions

The monetary cost of boiler production is also an important factor. Iron quality is a consideration, which is costlier, but makes more durable boilers. In many structures iron like eureka, pine, etc., having good mechanical and thermal properties may be used. They are similar to the homogeneous steel but have less plasticity.

General design procedures

• Type of boiler to be designed (fire tube, water tube etc.) according to available resources and desired output.
• Calculations associated with boiler design. The thermal calculations, involve net heat transfer, heat loss, work done, and efficiency of the system. While mechanical calculations include load bearing capacity (limit of pressure it can withstand before bursting) of drum of boiler, thermal expansion of metal piping and streses produced due to heating of pipies, etc.
• Software available to calculate the necessary parameters.
• AutoCADPro-V or other designing software for boilers.
• Types of grates and furnaces used according to fuel used and cost.

Material selection

Many factors have to be considered such as strength, safety, cost, efficiency, etc. Selection of the material is a very important step in making boilers. The economic and the environmental pressures also need to be taken into consideration while choosing the materials. A boiler shell (the main part where water is heated) is made up of wrought iron and steel. Tubes may be made of brass or copper. There are many criteria for selection of materials used in boilers. However tensile strength is one of the important ones. Cast iron, ductile iron, carbon steel and aluminium are generally used for the body or valves.

Manufacturing techniques

• Materials used according to safety and cost constrains
• Types of Joining (welding, riveting etc.) used for different applications (high pressure areas, areas exposed to steam etc.)
• Insulation used for piping and storage drum.
• Tensile strength of material is to be considered while using any of the joining processes.^[2]

Apart from this, the material must exhibit ductility and strength qualities to the highest degree in order to withstand the immense strain that is put on them. Iron is mainly used for making the rivets at present but use of steel to make the rivets is also increasing. As the mild sheet used in boiler material is thickar,welding processes which can be employed are arc welding, oxy acetylene welding, submerged arc welding and seam welding.

In arc welding, consumable electrode is used.In downhand flat welding electrodes must be held less than 20 degree from the vertical and 3 mm far from the workpiece. Some precautions that should be taken are heavy gloves,hand shield or a helmet with window of coulered glass should be used to protect the face.

The main arc welding process is submerged arc welding. This sometimes also called as hidden arc welding.This is automated process.

Types of boiler accessories and mountings

accessories:The devices which form integral part of boiler but are not mounted on it. They include economiser,superheater,feed pump,air pre-heater etc. Accessories are not necessary but help in controlling and running the boiler efficiency. Mounting:These are fittings which are mounted for proper functioning of a boiler. They include pressure gauge,safety valve,water level indicator etc. Boiler can not work safely without the mountings.The following are some of the mountings which must be present on a boiler according to the Indian Boiler Act:

• Feed check valve: Placed on the head of the shell, the main functions of the feed check valve are i)To regulate the flow of water that is fed into the boiler ii)To prevent the back flow of water in case of failure of the feed pump.
• Steam stop valve: The main function of the steam stop valve is to regulate the flow of steam that is produced in the boiler to the steam pipe. It may also be used to stop the supply of steam from the boiler
• Fusible plug: Placed at the lowest level of water and above the combustion chamber, its function is to extinguish the fire as soon as the water level in the shell of the boiler falls below a certain marked level.
• Blow-off cock: The function of the blow off cock is to remove the water from the shell at regular intervals of time in order to remove the various impurities that may be settled at the bottom of the shell.
• Safety valves: Prevent the steam pressure to exceed its maximum value. They operate automatically. There are three main types of safety valves: 1)Dead weight safety valve 2) Lever safety valve 3) Spring-loaded safety valve.
• Water-level indicators: Placed in front of the boiler, used to indicate the level of water in the shell.

Advantages and disadvantages of contemporary boilers

Some advantages include:^[3]

• Steam produced can be supplied to external combustion engine such as steam engines and turbines.
• Steam produced can also be supplied at low pressures for industry processes.
• Steam can be utilized for producing hot water,which is used for heating installations at very low pressures.

Testing of boilers

Testing of each and every component used in a boiler is as important as the designing. Testing is done at two stages before the boiler is actually put to use. It takes place at two stages:

1. The testing of each and every manufactured component and
2. The testing of the final boiler.

Apart from testing the boiler components, it's important to test the water which is used for the generation of steam in the boilers as well. The main reasons for this is:

1. To obtain a continuity in heat exchange.
2. To prevent corrosion.
3. To obtain a high quality of steam.

A number water treatment processes are carried out before it is used in the boilers. They can be classified into external and internal treatment of water:

• External Treatment: In this case,the impurities are removed before the water is passed into the boiler.Some examples of external treatment processes are softening, deaeration,etc.
• Internal Treatment:this treatment is done within the boiler itself.It may be done alone or in combination with the external treatment processes.It is done to prevent the foaming of water in the boiler,react with the feed water which may be hard,etc.

References

Bibliography

• Buecker, Brad (2002.) "Basics of boiler and HRSG design." PennWell Corporation. ISBN 0-87814-795-0
• Malek, Mohammad A. (2005.) "Power boiler design, inspection, and repair: ASME code simplified." McGraw-Hill. ISBN
• Stromeyer, C.E. (1893.) "Marine boiler management and construction." Longmans, Green, and Co.

External links

• "Boiler Design and Efficiency." — from Forest Products Society