Understanding Air Preheaters in Furnaces

An air pre-heater is a crucial component found in many large combustion systems, such as those in power plants or industrial furnaces. Its main purpose is to recover heat from the hot flue gases leaving the furnace and use this heat to warm up the incoming combustion air.

How an Air Pre-heater Works

Before air enters the furnace to mix with fuel and burn, it passes through the air pre-heater. Inside the air pre-heater, the hot flue gases, which would otherwise be exhausted, flow past surfaces that transfer heat to the cooler incoming air. This process significantly raises the temperature of the air.

Benefits of Heating Combustion Air

• Heating the combustion air improves the efficiency of the burning process (combustion).
• Higher air temperature helps the fuel burn more completely.
• This leads to less fuel consumption for the same amount of heat output.
• It also helps to recover heat from the waste flue gases, improving overall thermal efficiency of the system.

Analyzing the Options

Let's look at the given options:

• fuel: While fuel might be heated in some systems (e.g., fuel oil heaters), the primary function of an air pre-heater is not to heat the fuel itself.
• flue gases: Flue gases are the hot gases produced by combustion that leave the furnace. The air pre-heater takes heat from the flue gases to heat something else, it doesn't heat the flue gases themselves.
• air: This is exactly what an air pre-heater does – it increases the temperature of the combustion air before it enters the furnace.
• feedwater: Feedwater is the water that is turned into steam in a boiler. Feedwater is typically heated in a component called an economizer, using the heat from flue gases, but it's separate from the air pre-heater's function.

Therefore, the air pre-heater specifically increases the temperature of the air before it enters the furnace for combustion.