What are the two distinct methods of constructing air heaters?

The correct choice identifies the two distinct methods of constructing air heaters as tubular or regenerative.

Tubular air heaters utilize a series of tubes through which air passes, allowing it to be heated by contact with the hot surfaces of the tubes, often heated by hot flue gases. This design is highly efficient as it maximizes the surface area for heat transfer and is effective in various industrial applications.

Regenerative air heaters, on the other hand, work by temporarily storing heat in a medium that is then used to preheat incoming air before it enters the heating system. This method is particularly beneficial in enhancing energy efficiency by recycling heat, which significantly reduces fuel consumption and emissions.

Both methods have their unique advantages and are chosen based on specific application requirements, such as efficiency, design, and available space. Understanding these construction methods is crucial for professionals in the field, as they directly impact the performance and efficiency of the heating systems.

The other options present different concepts or types of systems that do not accurately describe methods for constructing air heaters. This helps clarify why tubular and regenerative are the appropriate choices.