How a steam boiler works

A boiler is a device in which fuel is burned and the heat released is transferred to water to produce steam.  

Boilers are widely used in chemical, sugar, wine, beer, beverage and food-processing plants. Steam supplies heat for cooking, heating, drying, pasteurisation and concentration. Saturated steam is commonly used for process heat transfer. Industrial boilers vary in capacity and operating pressure according to the application.

Steam pressure and temperature must follow the boiler design and process requirements. Saturated-steam pressure and temperature have a defined relationship; 2.0 MPa and 250°C should not be presented as a universal saturated-steam operating pair.

Industrial boilers can be classified by their combustion arrangements and water-circulation systems, among other features.

A. Main components of a boiler system

• Boiler feedwater tank
• Boiler
• Steam-using equipment

B. Boiler operating principle

A boiler converts heat released by fuel into thermal energy in steam. Saturated steam is commonly supplied for air heating in drying, equipment cleaning and heating processes in textile, sugar, chemical, brewing and beverage plants. Steam turbines driving generators commonly use superheated steam.

C. Process description

Feedwater meeting the required quality is pumped from the tank to the boiler. The burner may use fuel oil or another suitable fuel. Combustion gases transfer heat as they pass through the boiler. The original three-pass illustration labels the steam at 150°C and the chimney outlet at 200°C; these are illustrative values, not operating limits for every boiler. After delivering heat in the process equipment, steam condenses and recoverable condensate returns to the feedwater tank.

D. Opportunities to improve boiler energy efficiency

Flue gas still carries heat when it leaves the boiler. Subject to the system design, heat recovery may be considered for the following purposes:

• Superheating steam where the process requires it.
• Preheating feedwater.
• Preheating combustion air and fuel where appropriate.

When is superheated steam needed?

Superheated steam is used where the process or turbine requires it. For saturated-steam distribution, suitable insulation, drain points and steam traps are needed to remove condensate. Adding a superheater is not a substitute for effective condensate drainage.

Spirax Sarco: steam mains and drainage