The image above illustrates the various losses and steps associated with the overall efficiency of an industrial boiler. Each arrow represents an energy loss that reduces the conversion of fuel into usable steam. Here is an analysis of the different elements in the diagram:
Fuel In (Energy Input)
This is the starting point of the process, representing all the energy contained in the fuel feeding the boiler. The goal is to convert this energy into usable steam with as little loss as possible.
Fuel-to-Steam Efficiency
This initial efficiency measure corresponds to the direct conversion of the fuel’s energy into steam. However, losses occur even at this initial stage, such as:
- Radiation Loss: Part of the energy is dissipated as heat emitted from the boiler’s surface.
- Exhaust Loss: The hot gases expelled through the flue carry away a significant amount of unrecovered heat.
See the technical bulletin on efficiency as a function of gas temperature for more information.
In-Service Efficiency
In-service efficiency incorporates not only the initial losses but also those related to the boiler’s daily operation. Among these losses:
- Start-Up Losses: Each start-up results in a period of inefficiency before the boiler reaches optimal operation.
- Pre- & Post-Purge Losses: The air blown before and after a cycle to secure the process results in heat loss.
- Blow-Down Losses: The discharge of water to remove impurities is accompanied by a loss of heat.
- Loss at High Turndown: When the boiler operates at a reduced load, efficiency decreases.
- Changing Loads: Rapid changes in steam demand disrupt efficiency.
- Radiation Loss at Idle/Stand-by: When the boiler is off, it continues to dissipate heat.
Steam Quality
In-service efficiency can also be diminished by poor steam quality when water is carried over in the steam. This water must be removed by purge devices (steam traps), resulting in additional losses.
Discrepancies between Fuel-to-Steam and In-Service Efficiencies
It is not uncommon to observe a fuel-to-steam efficiency between 80% and 85% (particularly for boilers equipped with high-performance economizers). However, this efficiency can be reduced to around 60% in service due to various factors:
- Poor burner selection.
- Oversizing of the boiler relative to actual needs.
- Lack of sophisticated controls or advanced regulation equipment.
Modern technologies are available to reduce this gap and optimize operating costs.
Understanding and reducing the losses associated with boiler efficiency is essential to improving performance and achieving significant savings. By integrating heat recovery technologies, optimizing purge cycles, and adapting control systems, it is possible to maximize efficiency while minimizing operating costs.