The overall thermal efficiency of a furnace is improved if the air that enters the radiant section around the burners is preheated. Economic studies are performed early in a project to determine the feasibility of such a system.
The two most common preheating systems are re-generative system and recuperative systems.
A typical regenerative system
- Hot flue gas exiting the convection section is diverted through a duct to a heat exchanger called an air preheater.
- Incoming fresh air is heated and sent by the forced draft fan to the burner plenum, where it enters the burner through the air register for combustion.
- An induced draft fan draws the flue gas through the preheater and sends the cooled gas back into the atmosphere through the stack.
Conventional Recuperative (Closed Loop) System
- Oil is circulated through the convection section tubes, heated and sent through the inlet air duct, where it releases its heat.
- The hot air then enters the burner for combustion and the hot oil is recirculated to a storage tank for recycling.
- From a layout standpoint, the hot oil system with a storage tank and pump is far less complex than the regenerative system.