Heat transfer through the furnace tubes is by conduction. It is natural that thick plate does not conduct heat as quickly as thin plate. Thicker plate is also able to withstand more force.
This is of particular importance in the furnace tubes where the flame temperature may be up to 1 800°C, and a balance must be struck between:
• A thicker plate, which has the structural strength to withstand the forces generated by pressure in the boiler.
• A thinner plate, which has the ability to transfer heat more quickly.
The equation that connects plate thickness to structural strength is Equation 3.2.1:
Equation 3.2.1 shows that as the plate thickness gets less, the stress increases for the same boiler pressure.
The equation that connects plate thickness to heat transfer is Equation 2.5.1:
Equation 2.5.1 shows that as the plate thickness gets less, the heat transfer increases. By transposing both equations to reflect the plate thickness.
For any one boiler, if the heat transfer rate (
) is increased, the maximum allowable boiler pressure is reduced.
A compromise is reached with a furnace tube wall thickness of between 18 mm and 20 mm. This translates to a practical pressure limit for shell boilers of around 27 bar.
Shell boilers are manufactured as packaged units with all the ancillary equipment fixed into position. After manufacture, the packaged boiler must be transported to site and the largest boiler which can be transported by road in the UK has an output of around 27 000 kg/h.
If more than 27 000 kg/h is required, then multi-boiler installations are used. However, this has the advantage of providing better security of supply and improved plant turndown.
Today’s highly efficient and responsive shell boiler is the result of more than 150 years of development in:
• Boiler and burner design.
• Material science.
• Boiler manufacturing techniques.
• Control systems.
To guarantee its successful and efficient operation, the user must:
• Know the conditions, environment, and demand characteristics of the plant, and accurately specify these conditions to the boiler manufacturer.
• Provide a boiler house layout and installation that promotes good operation and maintenance.
• Select the control systems that allow the boiler to operate safely and efficiently.
• Select the control systems that will support the boiler in supplying dry steam to the plant at the required pressure(s) and flowrate(s).
• Identify the fuel to be used and, if necessary, where and how the fuel reserve is to be safely stored.
Advantages of shell boilers:
• The entire plant may be purchased as a complete package, only needing securing to basic foundations, and connecting to water, electricity, fuel and steam systems before commissioning. This means that installation costs are minimised.
• This package arrangement also means that it is simple to relocate a packaged shell boiler.
• A shell boiler contains a substantial amount of water at saturation temperature, and hence has a substantial amount of stored energy which can be called upon to cope with short term, rapidly applied loads.
This can also be a disadvantage in that when the energy in the stored water is used, it may take some time before the reserve is built up again.
• The construction of a shell boiler is generally straight forward, which means that maintenance is simple.
• Shell boilers often have one furnace tube and burner. This means that control systems are fairly simple.
• Although shell boilers may be designed and built to operate up to 27 bar, the majority operate at 17 bar or less. This relatively low pressure means that the associated ancillary equipment is easily available at competitive prices.
Disadvantages of shell boilers:
• The package principle means that approximately 27 000 kg/h is the maximum output of a shell boiler. If more steam is required, then several boilers need to be connected together.
• The large diameter cylinders used in the construction of shell boilers effectively limit their operating pressure to approximately 27 bar. If higher pressures are needed, then a water-tube boiler is required.