Boiler Design Basic Steps

Boiler design is the process of designing boilers used for various purposes. The main function of a boiler is to heat water for steam generation. Steam produced can be used for a variety of purposes including space heating , sterilization, drying, humidification and power generation. The temperature or condition of steam required for these applications is different, so how to design a boiler varies accordingly.

boiler design

Accessories and mountings are devices, which form an integral part of a boiler but are not mounted on it. They include economizers, super-heaters, feed pumps and air pre-heaters. Accessories help in controlling and running the boiler efficiently. Certain common mountings include:

  • Feed check valve – regulates the flow of water into the boiler and prevents the back flow of water in case of failure of the feed pump.
  • Steam stop valve – regulates the flow of steam that is produced in the boiler to the steam pipe, and 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 – removes water from the shell at regular intervals to remove the various impurities that may be settled at the bottom of the shell.
  • Safety valves – automatically prevent the steam pressure from exceeding safe levels
  • Water-level indicators – indicate the level of water in the shell

What are the basic steps in Boiler Design

Based on the abovementioned definition of a boiler and defining the accessories and mountings of a boiler system, we can now discuss the physical and mechanical principles that a boiler compiles with during its functioning process.
Normally a boiler is designed to suit user’s requirements. Therefore, the boiler design starts with the basic specifications stated by user or customer.
The customer’s preliminary specifications usually contain the following items:

  1. Steam penetration – Maximum Continuous Rating (MCR)
  2. Steam pressure
  3. Steam temperature
  4. Load range (Boiler turndown)
  5. Fuel

The basic boiler design often begins with just these four or five parameters, and continues to diversify into finer details along the way, as further details of operation; site-specific requirements get discussed and determined. For an optimum design of a boiler system, it is recommended to take the following steps to fulfill a reliable and durable design of a boiler.
Here are the basic steps in how to design a boiler:

    1. Calculation of boiler Heat Duty
      • Using desired steam parameters, calculate the enthalpy of steam to be generated
      • Multiplying enthalpy with the rated steam generation, obtain the total heat value of steam at rated steaming (boiler heat duty)
      • Boiler heat duty is expressed as MKCal/hr or MW or HP
    2. Calculation of Burner Heat Duty and fuel firing rate
      actually, Burner heat duty is defined by the rated steam enthalpy at MCR divided by boiler efficiency

      • With an assumed boiler efficiency number, calculate the heat duty of burner
      • Knowing the NCV of fuel, calculate MCR rate fuel firing at MCR
    3. Fixing furnace geometry
      • Select the burner(s) suitable for the calculated fuel firing rate
      • From burner size and flame clearance requirements, configure front wall geometry
    4. Calculation of furnace volume required
      • Using guideline/ limit for VHRR (Volumetric Heat Release Rate)
    5. Calculation of required FEGT
      • FEGT (Furnace Exit Gas Temperature) has a direct effect on heat available to the superheater
      • The flue gas temperature entering super heater is determined from the desired final steam temperature, i.e. the degree of superheat
    6. Calculation of required EPRS
      • EPRS (Effective Projected Radiant Surface) is the radiant surface available to pick up the heat from flue gas
      • This is calculated from the required value of FEGT
      • EPRS is the surface of furnace tubes directly exposed to radiation, plus the surface formed by membrane strips/ fins
      • This determines the number of tubes required in the furnace section
    7. Freezing furnace configuration
      • With EPRS, Burner flame clearance requirement and front wall dimensions. Calculate furnace depth and configure furnace.
    8. Calculation of convection bank surface
      • This is done by using FEGT and Heat absorption required for steam generation, then it is necessary to calculate the heat transfer surface required in boiler bank and subsequently, using this to calculate number of tubes required in generating bank
    9. Selecting Steam Drum
      Let me explain more about the Steam drum size, which is governed by the following factors:

      • Boiler steaming capacity at MCR
      • Circulation ratio
      • Expected load fluctuations
      • Steam-water separation equipment (in steam drum)
      • Number of generating bank tubes entering steam drum (for ligament efficiency)
    10. Calculating economizer surface
      • Calculate the minimum flue gas temperature required to avoid dew point corrosion
      • This is grossly affected sulfur content in fuel
      • Higher the sulfur content, higher is the required flue gas temperature at stack
      • From economizer inlet flue gas temperature and the desired stack temperature, calculate economizer surface
    11. Calculating FD/ ID fan duty
      • From MCR fuel firing rate and excess air requirement of burner, calculate air flow rate required
      • From burner data sheet and air duct configuration, calculate FD fan head
      • From flue gas side pressure drop, gas temperature and total gas weight, calculate ID fan duty and size the fan
    12. Calculation of stack height
      • Stack height is calculated based on the following criteria:
        • Weight SO2 emitted from stack at 6MCR operation
        • Natural draft considered as available while calculating ID fan duty
        • Regulation governing dispersal of pollutants (population around the boiler plant)
    13. Workout boiler mountings and fittings
      • Decide the numbers, location, and capacities of safety relief values on steam and water circuits (follow applicable regulation)
      • Work out the sizing of direct water level gauges (follow applicable regulation)
      • From MCR fuel firing rate and excess air requirement of burner calculate air flow rate required
      • Calculate expected thermal expansions at various locations and develop a thermal expansion drawing
    14. Carrying out stress analysis calculations for the configured boiler
      • Stress analysis calculation brings out the stress pattern for each pressure part
      • It is mandatory to prepare a complete stress analysis calculation under rated temperature and pressure and obtain approval from statutory authority, before commencing manufacturing/ assembly of the boiler
    15. Developing safety protection system (PLC) and control system (DCS)for the configured boiler
      • Follow NFPA-85 guidelines for BMS
      • Follow designer standard for control loops

Modern water tube boiler design benefits

What are the benefits of modern boiler design? To answer this important question we must know that modern boiler design offers several benefits. In the past, improper design of boilers has caused explosions, which led to the loss of life and property. Modern designs attempt to avoid such mishaps. Further, mathematical modeling can determine how much space a boiler will need and the type of materials to be used. When the design specifications of a boiler are determined, design engineers can estimate a cost and time schedule for the construction.

How to design a boiler? First, we must know that it may be based upon:

  • Production of a maximum quantity of steam with minimal fuel consumption
  • Economic feasibility of installation
  • Minimal operator attention required during operation
  • Capability for quick starting
  • Conformity to safety regulations
  • Quality of raw water: how hard or soft the water is will determine the material of the boiler.
  • Heat source – the fuel to be burned and its ash properties or the process material from which the heat is to be recovered.
  • Capacity/steam output required, usually measured in tonnes per hour or kg/s.
  • Steam condition – pressure, temperature, etc.
  • Safety considerations
  • Mechanical constraints
  • Cost restrictions
  • Monetary cost
  • The tensile strength of material must be considered while using any joining processes.
  • Simple Vertical Boiler Working , Fire Tube Boiler , Package Boiler ,
  • Boiler books, Water tube Boiler

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