Fans are an integral part of atmosphere heat treating owing to the need to have consistent atmosphere composition at the part surface. In fact, because the atmosphere-surface interface is where we are trying to maintain – or prevent in the case of neutral hardening – an ongoing chemical reaction, the agitation of the furnace atmosphere is critical. But anyone who has been in heat treating for a significant time has heard many complaints about fan performance or failure. Poor performance or failure is often the result of a poor match between the process and the fan design. What are the best fan designs for a given process? In the end, it’s mostly about the wheel.
In fan-speak, the rotor of the fan that moves air or a mixture of gases is called the fan’s wheel. There are many different wheel designs, but five basic wheel types are found inside heat-treating furnaces. The names of the types describe the blades on the wheel. They are forward-curved, axial, backward-inclined, radial and radial-tipped. Airfoil blades and backward-curved blades, which are the most efficient type of fan blades, are not commonly used for this application due to the presence of impurities in the air stream.
Forward-curved wheels (Fig. 1) are used extensively in heat-treating applications because they are among the quietest designs and can be constructed of materials allowing deployment at temperatures up to 2000°F (1093°C). They are generally the smallest wheel size you can get for a given airflow requirement. They are suited to creating moderate pressure and flow volumes with relatively low efficiency, but they require a housing to create the aerodynamics for adequate gas movement. As such, furnaces deploying forward-curved wheels often integrate the housing for the wheel into the furnace wall or ceiling.
Forward-curved designs will accumulate dirt, however, which will degrade performance and potentially result in damage to the fan from resulting imbalance of the wheel. This limits forward-curved designs to clean processes such as tempering, annealing and homogenizing applications. Forward-curved wheels (Fig. 2) are not well-suited to carburizing.
Wheels using backward-inclined blades (Fig. 3) are among the most efficient designs for fans handling dirtier airstreams. They can deliver large volumes of gas with moderate pressure generation and low noise. While they can handle airstreams with particulates, they are not suited to sticky particulates. Their design is also not suited to temperatures in excess of 1000°F (538°C). The most common application for this wheel design (Fig. 4) in heat treating is for cooling or combustion fans.
Axial wheels look like propellers (Fig. 5). They can push moderate-to-high volumes of gas while creating moderate pressure but with lower efficiencies than centrifugal fans. They are tolerant of dry or sticky particulates, making them suitable for dirtier processes such as carburizing, and can be constructed for deployment at temperatures up to 2000°F (1093°C).
Many of these axial fans can be designed to be reversible in nature. By changing the direction of rotation of the wheel, the direction of the airflow can be changed. This feature (not available in centrifugal fans) makes the axial fan an attractive option in many furnaces to improve air circulation.
Axial fans are often deployed without any housing, though this dramatically reduces their efficiency and the pressure they can create. To eliminate this performance degradation, some designs use a static ring (Fig. 6) of material around the blade to create an effect similar to a radial housing. The gap between the tip of the blade and the static ring is critical to the performance of the fan and needs to be checked during maintenance.
The radial-blade wheel is often called a paddle-wheel design (Fig. 7). The paddle-wheel design results in the wheel shedding dirt – including sticky particulates – as it spins, making it a self-cleaning wheel. This makes it particularly well-suited to dirtier applications like carburizing.
The design also allows these wheels to spin faster than other designs, so they cover a wide range of volumes and pressures. Their efficiency is low, however, and they tend to be quite noisy, particularly when being used to create higher pressures. Radial-blade wheels (Fig. 8) can be fabricated for deployment at temperatures up to 2000°F and can accommodate blade liners for use with highly abrasive particulates such as ceramic particles. They are rugged enough that they are even used for moving media such as wood chips.
As its name suggests, the radial-tipped blade (Fig. 9) has features of a backward-curved blade but with the tip of the blade radial to the center of the wheel. This enables the wheel to be more efficient while still allowing it to handle small amounts of particulate matter.
Selecting the Best Fan Wheel for Your Process
It is a straightforward procedure for selecting the best fan wheel for your furnace. Essentially, one must characterize the environment within which the fan will operate and the gas stream that the fan will be moving. The critical parameters one will need to define are:
- Over what temperature range will this fan operate?
- Does the process have particulates? If yes, are the particulates sticky?
- What volume (CFM) of gas do I need to move and at what pressure?
- Are there any components of the gas stream that are corrosive or abrasive?
- Is it possible to integrate a housing for the fan wheel into the furnace ceiling or wall to create the correct aerodynamics, or is it necessary to use a wheel that does not require a housing?
- Are there concerns about efficiency or sound that must be addressed?
Armed with answers to these questions, one can identify the wheel type best suited to a given process. One can also provide answers to these questions to their fan supplier, where experienced application engineers can make recommendations and provide a drawing for integration into the overall furnace design.
Fans in heat-treating processes can be a point of frustration for many because they may be an afterthought in furnace design in some cases. It is not usual to see a furnace originally designed for one process repurposed for a different process. When this happens, it is possible that the original wheel is not suited to the new process, in which case performance may suffer until the wheel is updated to match the demands of the new application.
For example, if a furnace is changed from a dry-particulate application to a sticky-particulate application, the wrong wheel will accumulate the sticky particles, develop an imbalance and, ultimately, result in fan failure and potential load loss. Similarly, using a wheel that does not provide sufficient flow volume or pressure could result in poor circulation around the load and subsequent degradation in surface properties of the treated parts.
By optimizing fan selection to the process rather than the furnace, one can maximize both performance and lifespan. The intent of this article is to provide a guide for optimization, but, when in doubt, your fan provider will have experts ready to assist.