Process fans like FD fans and ID fans are used extensively in ‘Thermal systems’. FD fans are used as primary and secondary fans to support the manufacturing process. This terminology is used in the manufacturing industries to refer to systems that deal with generation, transfer, /use, or removal of heat energy. This often involves a combustion and heat exchange process, and the system is required for controlling temperature, heat flow, and energy conversion.

There are several examples of thermal systems, including boilers, furnaces, kilns and ovens, thermal power plants, and heat exchangers in chemical plants. The thermal systems use fuel, electricity, or chemical reaction for heat generation – these systems involve air/gas movements to support combustion or heat transfer, which requires process fans.

Draft fans are kinds of industrial fans, also called blowers – they are used in several manufacturing plants and factories. Draft fans are further subdivided into two types – FD (Forced Draft) fans and ID (Induced Draft) fans. The scope of this article is going to be focused on FD fans and the way they work as both primary air (PA) fans and secondary air (SA) fans.

What are forced draft fans?

Firstly, it will help to have a basic understanding of forced draft fans. FD fans are equipment used in thermal systems in certain manufacturing plants. These fans, which are installed at the furnace or boiler system’s inlet, take in fresh air from the outside atmosphere and push it inside the combustion chamber with increased pressure. In the chamber, the fuel gets mixed with this air, and this leads to the creation of positive pressure.

As a direct result of that, the process of combustion goes on in an unhindered way. The FD fans are suitable for use as primary as well as secondary air(SA) fans.

FD fans – PA and SA fans

These are some salient points related to how the FD fans play the dual role of both PA and SA fans.

A furnace or boiler system has two inlets to enable the flow of air into the chamber where combustion takes place. The FD fan that serves as the primary air fan is installed near the first inlet. And, the second inlet is the site where the blower acting as the secondary air fan is deployed.

Primary Air Fan: Normally, the air provided by the primary air fan is sufficient to make sure that the process of combustion goes on appropriately. This blower separates particles of fuel so that each of those is touched by the air coming in at high pressure. This FD fan is helpful in constantly maintaining the temperature inside the furnace at the needed level, too.

Secondary Air Fan: At times, there are situations where the air supplied by the primary fan is inadequate for the completion of the combustion process. The system then immediately requires an additional quantity of air. It is here that the FD blower operating as the secondary fan comes to the rescue. These secondary fans send the added air into the furnace/boiler, ensuring that the combustion process is fully completed without any wastage of fuel.

In the above circumstances, the presence of secondary air fans is imperative to make certain that there is no fuel wastage. Unwanted costs can be avoided as fuel wastage leads to economic waste, which is not good for a business. The usage of secondary fan leads to improvement in process efficiency, which is a value addition to the business.

Forced draft fans – Impellers used by Industry
The impeller used in the forced draft fan depends on factors like the application, air volume, pressure requirement, and impurities or dust content in the air.

• The most common one used in most industries is the backward curved or backward inclined impeller used in power plants, cement plants, steel mills, and boilers. This type of impeller enables high efficiency, stable airflow, lower noise, and is less prone to overloading.

• Radial or straight blade impellers are used where air is dusty and contains particles, like in cement, mining, and steel. Rugged and durable, these impellers are easy to clean.