Definition of boiler output

The following specifications have been defined as result of the consumption analysis:

  • Maximum steam output
  • Minimum steam output
  • Documentation of safety margins
  • Concept for future operational changes to the steam output demand
  • Possibility of a chronological course of steam output

The individual boiler outputs can be defined with this data.

Single-boiler systems

When selecting the output size of the boiler, it should be certain that the boiler system subsequently operates in the range of 40 – 90% of the maximum output of the steam boiler as the efficiency is particularly high in this range.

Defining the minimum and maximum steam output also establishes the necessary control range of the system.

Required control range =
Minimum steam output
Maximum steam output

Use of a single boiler is a convenient solution if the control range during normal operation on weekdays is between the following values:

  • Single flame-tube boiler: 1 to 0.125 (control range 1:8)
  • Double flame-tube boiler: 1 to 0.061 (control range 1:16)

The following output sizes are available:

  • Single flame-tube boiler: 175 – 28,000kg/h
  • Double flame-tube boiler: 18,000 – 55,000kg/h

Multi-boiler systems

It may make sense to use a multi-boiler system for several reasons. The following description aims to cover the various reasons for distribution of the nominal steam output. However, as many distribution variations are possible, it is not possible to provide a full and comprehensive assessment encompassing all aspects. The decision to opt for a single system or to distribute among several boilers must always be made for each project individually and should be done by the operator and planner with the support of the plant engineer and boiler manufacturer.

Security of supply and redundancy

Distribution of the boiler output among several generation units is necessary, if the security of supply also needs to be maintained when a unit drops out. For example, this is necessary in hospitals or in the pharmaceutical industry.

In this case the reserve unit must provide the minimum output required to ensure continued operation.

Furthermore, in food companies, such as dairies or in sugar manufacturing, and industrial firms, such as the paper and printing industry, breakdown of the steam generator unit would often be disastrous from an economic standpoint.

Partial load operation and steam boiler system control ratio

Reasons for distributing the total output between several units are:

  • Difference between smallest and biggest heat consumer
  • Cyclically fluctuating steam demand, e.g. between day and night
  • Different steam demand on workdays as opposed to weekends

The smallest power requirement is normally far less than the smallest load of an individual boiler unit, and it therefore makes sense to adapt the output distribution to the light load. This avoids a costly and environmentally-polluting on/off switching operation of the combustion system and premature wear.

In large-scale systems the output limit of the heat source determines the number of units. The total output should ideally be distributed among units with identical construction. Reduction in the number of spare parts kept in stock and part replaceability alone are sufficient reasons for doing so. If efficient operation cannot be achieved with the smallest load using the smallest unit determined in this manner, only then should an adapted low-load unit be used.

Technical report: avoidable loads on steam shell boilers

Starting duration cold start/heat maintenance system

Fast availability of the maximum steam output is from time to time also a good reason to use a multi-boiler system. While a cold boiler needs roughly an hour until it reaches the operating state and is ready to deliver its full output, a boiler which has been kept warm and is in the standby state can respond to such a request in only 5 minutes. In this process, heat maintenance is more efficient and gentle using a steam-heated heating coil as opposed to a combustion system.

Info on Heat maintenance system

Optimised operating costs

The question as to how many boilers should be installed in a system and what their respective outputs must be investigated with a view to reducing the operating costs (to a minimum). Particularly in cases where the steam demand fluctuates cyclically, e. g. week/weekend load or if the heating load fluctuates depending on the season, it makes sense not to select the same boiler output for the individual units.

Use of sequential control

The boiler stresses and the operating costs can be optimised by defining each boiler as either a base-load boiler or peak-load boiler and by using a state-of-the-art sequential control.

Info on System Control SCO

Space requirements – installation requirements

Most boiler systems are set up in a separate boiler house, or at least in a separate area of the building, because special conditions relating to installation and operation must be observed due to the potential dangers associated with their operation.

In some countries, such as Germany, for smaller boilers with a lower safety relevance, there might be reduced requirements regarding the location of the boiler (please check your local country requirements). It is possible to build these in an existing boiler room or energy centre.


Maximum value

Maximum boiler steam output

2,000 kg/h

Maximum permissible operating pressure

32 bar

Maximum water content up to low water

10,000 l

Maximum product of water content and permissible operating pressure

20,000 l · bar

Steam boiler with facilitated installation conditions (example Germany)

In some systems, it may be necessary due to the installation conditions for the total steam output to be shared by several boilers that satisfy the above conditions. Eased installation conditions are frequently used in hospitals, small laundrettes or food production plants if a separate boiler house is not available and the boilers can be set up in the cellar for example.

Sensible distribution of the boiler output

A number of requirements in relation to failure safety and the required control range, and also sensible distribution of the boiler output between several boilers, are stated as examples in the following table in order to satisfy the requirements:


Distribution of the boiler output

Failure safety of 100% of steam output

100:100, between 2 boilers

Failure safety of 80% of steam output

80:80, between 2 boilers

Failure safety of 50% of steam output

50:50:50, between 3 boilers

Control range > 1:8

50:50, between 2 boilers or 1 double flame-tube boiler

Control range > 1:20

30:70, between 2 boilers

Control range ≤ 1:20 + failure rate 80%

40:40:40, between 3 boilers

Distribution of boiler output to satisfy requirement examples

Additional combinations for distributing the boiler output between several boilers are possible. The investment, operating and maintenance costs must be taken into consideration when deciding on the steam output distribution.

When the total output is distributed between several boilers, a sequential control must be used. It applies the switch-on/shutdown and heat maintenance logic of the individual steam boilers.

Info on System Control SCO