What Does the Internal Structure of a Small Steam Boiler Look Like?

Steam boilers serve as indispensable key equipment in various industrial production processes, playing vital roles in drying, sterilization, disinfection, distillation and other process applications across numerous industries including food processing, paper manufacturing, chemical production, textiles, electronics, tobacco and pharmaceuticals. Different types of steam boilers exist, and their internal structures vary accordingly. Here, ZOZEN Boiler will provide an in-depth explanation of small steam boiler internal structures using two common examples: horizontal shell boilers and single longitudinal drum water-tube boilers.

Structure of Horizontal Shell Steam Boilers
The horizontal internal combustion boiler features a large-diameter shell containing a furnace chamber, with the burner positioned inside the furnace to form the combustion space. Both ends of the furnace chamber connect to front and rear tube sheets, with smoke tubes arranged between them. This design eliminates the need for external brick walls. As shown in the structural diagram below, the main pressure-bearing components include the shell, furnace chamber, front/rear tube sheets, and smoke tubes. These boilers typically have evaporation capacities below 20 t/h and operating pressures under 1.25 MPa.

FAQ: Small Steam Boiler Internal Structure 01
Internal Structure Diagram of Horizontal Shell Small Steam Boiler

Flue Gas Flow Path in Small Steam Boiler:
High-temperature flue gas generated by natural gas combustion in the furnace chamber flows from front to rear. At the rear, it turns into smoke tubes on both sides of the furnace, flowing forward to the front smoke box before turning upward into upper smoke tubes and finally flowing rearward to exit through the chimney.

Water Circulation in Small Steam Boiler:
Water near the furnace chamber and smoke tubes receives stronger heating, becoming less dense and rising, while water near the shell receives weaker heating, remaining denser and sinking - creating natural water circulation within the boiler.

Advantages of Horizontal Internal Combustion Boilers:

Complete factory assembly enables quick and simple installation

No brick wall construction required, saving space

Fast startup with quick steam generation

Disadvantages:

Smoke tubes prone to ash accumulation requiring frequent cleaning

Scale formation between tubes difficult to remove

Potential leakage at tube-to-sheet expanded joints

Flue Gas Flow Path Alternative Configuration:
High-temperature flue gas exits through the left rear outlet into the left convection flue, flows forward to the front transverse flue, turns into the right convection flue to flow rearward through the economizer before being exhausted by the induced draft fan and chimney.

Water Circulation Alternative Configuration:
Water from the upper drum descends through left/right convection bank tubes (less heated) to the lower header, which distributes it to left/right water wall tubes and more intensely heated convection tubes before returning to the upper drum.