What’s the difference: Direct Vs Indirect Fired Heaters?


What is the technical difference between direct fired and indirect fired? In terms of process air heaters, what are the features, benefits and applications of direct fired vs indirect fired?

Direct Fired

“Direct Fired” means just that; the burner is fired directly into the process air stream to heat it. Air heaters that are direct fired are utilized in applications where the end product will not be affected by the by-products of combustion. Direct fired air heaters are typically less expensive and more efficient than indirect fired units, as direct-firing units convert nearly 100% of the combustion gases used to hot air. Below is a diagram of a direct fired air heater.

Indirect Fired

“Indirect Fired” means that the air is heated “indirectly”; the burner is fired into a heat exchanger. The air is heated when it passes over the heat exchanger and thus, the by-products of combustion remain inside the heat exchanger and do not come into direct contact with the process air. Indirect fired air heaters are used for chemically sensitive products, foods, chemicals, pharmaceuticals, ect.  Below is a diagram of an indirect fired air heater.

Which type of Air Heater is best for my application?

Do you need a hot air source, but are uncertain of which type of air heater will be best for your application?

Below are the industries that direct fired air heaters and indirect fired air heaters are commonly used in- look down the list and find your industry!

Direct Air Heaters are typically used in these industries:

Clay/ Sand


Laminate Curing



Cement Production

Composite Curing

Fertilizer Production

Indirect Air Heaters are generally used in industries where the end product cannot be contaminated by the by-products of combustion. These industries include:

Chemical Production


Food Processing

Hydrated Lime Drying

Grain/ Fiber Drying

Print Drying

Dairy Drying


Indirect Fired Air Heater – Stelter & Brinck

If you’re still uncertain of which air heater to choose, or have any questions, feel free to contact Stelter & Brinck; we would be happy to assist you.