Air Preheat System

The air preheat system is used to preheat the combustion air going to the burners. Since it cools the flue gas further, while removing heat, it improves the efficiency of the furnace. Using an air preheat system will frequently result in overall efficiencies above 90%.

These systems utilize air heaters of various designs. Some designs of air heaters lend themselves to a smaller heater such as a tubular air heater mounted on the heater or a regenerative type heater which would more often be found on very large heaters. Some of these air heaters are discussed below.

The regenerative air heater is widely used in the boiler and power generation industry. For this reason, when the fired heater industry developed a need for air heaters, these designs were the first to be used. They consist of a setting enclosing a large rotor which holds baskets of heat exchange surface. This rotor causes the baskets to pass through the hot flue gas, where the material heats up, and then through the cold air, which it heats. Even though these were very good at exchanging large amounts of heat, they had a downside of leakage from the higher pressure side(air) to the lower pressure side(flue gas) and of course they have moving parts. The sketches below show the basics of this type air heater.

Rotor	Baskets

Recuperative, Tubular Air Heater The recuperative air heater comes in a variety of types. The one shown below is designed to set atop the fired heater(in place of the stack). The air is drawn in across the shell side of the exchanger, then directed to the burners. After combustion, the flue gas travels through the heater and then the air heater on the tube side, where it exists to the artmosphere.

This recuperative air heater utilizes cast tubes and has become the most popular air heater in use. It has very low, if any leakage across the heat transfer surfaces. It is durable with no moving parts. If corrosion is a problem, the cold end units are usually glass tube.

Heating Medium Air Heater This air heater comes Is part of a recirculating hot oil system which recovers heat from the flue gas exiting the fired heater and uses it to preheat the combustion air going to the burners.