What are the two general classes of forced circulation boilers?

The two general classes of forced circulation boilers are indeed controlled circulation and once-through boilers. Controlled circulation boilers operate by maintaining a specific flow rate of water through the boiler. This is typically achieved by utilizing pumps to create the necessary pressure and flow, allowing for efficient heat transfer. This design is effective in managing water temperatures and preventing localized overheating, making them suitable for various applications, including high-pressure systems.

On the other hand, once-through boilers operate in a different manner, where water is heated and converted to steam in a single pass through the boiler without returning to the drum. This design allows for high efficiency and quick response times to changes in load, producing steam on demand and minimizing the risk of water impurities or carryover into the steam line.

Both types are categorized under forced circulation as they rely on mechanical means (pumps) to facilitate fluid movement, distinguishing them from other designs that might rely on natural circulation processes. Understanding these classes aids engineers in selecting appropriate boiler systems based on operational requirements and efficiency standards.