Boilers

Cast Iron Boiler

The main structure of this type of boiler consists of a number of cast iron sections known as boiler sections or fins. The burner flame is generated in the central space between the sections, and the hot combustion gases pass through specially designed passages to enhance boiler efficiency. To prevent direct flame contact with the section walls, the interior of the boiler can be lined with fireclay bricks and refractory materials. The thermal capacity of cast iron boilers is generally designed to be low to moderate. In high‑capacity boilers, each section may be composed of two pieces. For central heating systems with high thermal demands, cast iron boilers can be arranged in parallel. Cast iron boilers are primarily designed for hot water production and are less commonly used for steam generation. These boilers are generally suitable for low‑pressure applications; for high‑pressure systems, steel boilers are more appropriate. The sections of cast iron boilers are connected to each other via push nipples. Since the sections are separable, transportation and relocation are relatively straightforward. By adding or removing a limited number of sections, the thermal capacity of a cast iron boiler can be adjusted.

Cast iron boilers offer excellent resistance to rust and corrosion. However, their main drawback is the formation of cracks in the section walls—commonly referred to as boiler cracking. Since welding cast iron is difficult, a cracked section typically requires replacement. This process involves disassembling the boiler connections, replacing the damaged section, reassembling the unit, and addressing sealing and flue‑gas tightness issues. Cracking in cast iron boiler sections can result from the following causes:

• Direct flame impingement on the section walls, causing thermal stress
• Scale buildup on the internal surfaces of the sections, leading to longer burner run times and increased flame intensity in the combustion chamber
• Excessive water pressure inside the boiler
• Sudden heating or cooling (thermal shock) of the boiler

Steel Boiler

These boilers are mostly used for hot water and steam generation, often in high‑thermal‑capacity applications. They are manufactured as an integrated unit on a single base frame, making transportation significantly more difficult compared to cast iron boilers. The construction of steel boilers involves a large number of specially designed steel tubes. In some designs, water flows inside the tubes while hot combustion gases pass around them; in others, the combustion gases flow through the tubes with water surrounding them. The former configuration (fire‑tube) is more commonly used for steam production.

A major drawback of steel boilers is their susceptibility to corrosion, particularly at the tube‑to‑tubesheet joints at both ends of the boiler. Corrosion of tubes and steel plates reduces the boiler's service life. Another challenge is the potential for scale formation on surfaces in contact with water. To prevent this, it is necessary to use water treatment systems with precise hardness control. In HVAC applications, the capacity of steam boilers is often specified in terms of steam production rate (kg/h or lb/h).