Careful Fan Selection Can Bring Great Operating Benefits
Industrial Gas Engineering began manufacturing high temperature fan equipment in 1924. During the past 80 years, practical experience paired with engineering judgment has shaped our views regarding fans used in heat-treating furnaces today. Properly applied, forced convection can drastically improve heat transfer rates and furnace temperature uniformity in many applications. Examples are aluminum reheat and/or solution anneal, as well as temper and draw furnaces. In other applications, forced convection is used to circulate a controlled atmosphere to promote uniform interaction between the atmosphere and the workload. Examples are carburizing and nitriding. Improperly applied, fans can be an unending source of maintenance problems for little or no furnace performance gain and in the worst case, actually worsen furnace performance. Examples of improperly applied equipment include failure to consider the atmosphere when making alloy selections, and severe thermal shock on large and/or highly stressed (rotational) fans, especially with large sectional thickness variations. Design of the recirculation flow (path and rate of flow) must consider the desired results as well as the weight and geometry of the workload.
General considerations in fan selection
The trend in the market today is toward air-cooled equipment. This is done to minimize maintenance requirements and operating cost. With modern designs, even high-temperature (above 1750 F, or 954 C) atmosphere sealed fan equipment use standard bearings (operating below 200 F, or 95 C) in most applications. Long bearing life is ensured by keeping the bearing temperatures below 200 F, and the use of lithium grade two lubricant (factory fill on many bearings) is entirely adequate at these modest bearing temperatures.
A broad range of shaft-sealing arrangements-from lip seals to K/B sealed motors-address virtually any sealing requirement, from nitrogen to pure hydrogen and/or vacuum service. The trend is away from elaborate (and expensive) cartridge mechanical seals wherever possible, because many of these seals give little warning of failure and cause immediate shut down of equipment when failure occurs. Multichamber seals (which provide plenty of warning of failure) are often too long axially to allow use with high-speed fans. Cartridge seals routinely require water cooling, compounding the maintenance required.
Variable frequency ac motor controllers allow use of reduced size motors more closely matched to the actual operating conditions to increase flexibility while reducing cost.
Once the required flow volume is known (recirculation or exhaust), the easiest way to evaluate the system is at standard conditions (with the same acfm as at operating conditions). The error for basing calculations at standard conditions is slight, and one advantage is being able to use HVAC design guides readily to assist in evaluating system losses.
Maintaining proper application clearances (between the fan inlet and the nearest obstruction and/or the fan outlet and the nearest obstruction) are essential. Failing to do this will reduce fan performance, drastically in some cases. When abrasive particulates are present in the air stream, limiting the fan wheel tip speed can eliminate the need for specialized coatings.
If equipment is operated within its design limits, the main areas of concern are lubrication (type, amount, and frequency of relubrication) and water quality (where water cooling is still used).
Understanding fan designs
There are several different styles of fans, and each has certain benefits in particular applications, as well as some limitations. The following overview of selected fans provides a basis on which to make an educated choice for your application.
A multiblade fan, also referred to as a "squirrel cage" blower, offers a design that is suitable for use in low to medium static-pressure applications with moderate to high flow volumes (Fig. 1). The multiblade fan offers advantages of moderate physical size and cost, and it is the least sensitive to thermal shock among the fabricated fan wheel designs. It also can be applied to a wide variety of designs because they can be used as free-standing fans or cartridge assemblies using housings (i.e., single outlet scrolls, double outlet scrolls or diffusers).
Disadvantages of this fan design are that fan blades can fill with particulates (if the particulates adhere) deposited from the air stream, which drastically reduces performance. In extreme cases, the flow can be reduced to less than 50% of original. A properly designed housing (not a good place for creativity) is essential to achieve the necessary performance. Another disadvantage is that airflow can become unstable if the static pressure requirement is excessive. Unstable airflow is a surge in flow between two operating points of the fan performance curve, and may make the furnace perform so poorly it cannot be operated within process limits.
Radial fan (fabricated and cast)
A fabricated radial fan is suitable for use in medium to high static-pressure applications with low to moderate flow volumes (Fig. 2). Key advantages are its low cost and application flexibility including cartridge assemblies installed without using a housing at very low static pressure requirements where efficiency is not a concern. It also has excellent flow stability over a broad flow range of static pressures. In general, IGE limits fabricated radial fans to a maximum temperature of 1780 F (970 C), as many of the processes requiring radial fan wheels (especially above 1780 F) are corrosive to the weld material used to construct the fan wheel. This can lead to more excitement than is desirable. Specialized radial fans using pressure blower designs are also used in applications requiring high static pressures at low flows and/or applications requiring high static pressures at high flows where physical size is a constraint. These designs are usually limited to a maximum of 1400 F (760 C).
A disadvantage of a fabricated radial fan is that larger and/or higher alloy fans are sensitive to thermal shock. In addition, the fan has a larger physical size and lower mechanical efficiency compared with a multiblade or an axial fan of equivalent capacity.
While a cast radial fan is similar in style to a fabricated radial fan (Fig. 3), its cast construction offers the advantage of being less sensitive to thermal shock while increasing the potential maximum use temperature limits and types of corrosive atmospheres (such as carburizing) that can be used. Its disadvantages include higher cost and limited size range availability. The size limits are dictated by the sizes commonly used by original equipment manufacturers.
Backward incline fan
This style of fan is suitable for low to medium static-pressure applications with low to moderate flow volumes (Fig. 4). It has the advantages including a non-overloading characteristic, which is useful in applications where the static pressure varies during operation and low power consumption due to its high mechanical efficiency. These should not be used where thermal shock (above 15 F, or 8 C, per minute) is a concern.
Among its disadvantages are its large size and/or higher alloy requirement (due to higher tip speeds), which increase costs more than multiblade and axial fans of equivalent capacity. Tight inlet clearance between the inlet cone and fan wheel mean additional design and installation time to ensure alignment initially and in operation. Failure to maintain this clearance will cause severe vibration when the fan wheel and inlet come in contact with each other. While very large high capacity backward incline fans are used in a variety of low-temperature applications from boiler draft to aluminum aging (primarily due to high mechanical efficiency), they are rarely used at high temperature (above 1200 F, or 650 C), as any distortion of the fan wheel, inlet or the associated mounting can cause interference and severe vibration. Inlet cone and proper inlet conditions are critical to achieve chart rated performance.
Axial fan (fabricated and cast)
This style of fan wheel is suitable for use in low static pressure applications having moderate to very high flow volumes with flow parallel to the fan shaft at the fan wheel (Fig. 5). Fabricated axial fan wheels have been used as high at 1900 F (1040 C). These high temperature units are often used at higher speed at lower temperatures in furnaces either operating in temperature "steps," or for other processes, and then used at high temperature at limited speed to stir the atmosphere.
Advantages of a fabricated axial fan include good mechanical efficiency and high flow volume for the physical size compared with other designs, and it can be used for flow toward or away from the plug, so applications can be quite varied.
A disadvantage of the fabricated axial fan is that relatively high tip speeds are required to generate static pressure, which increases the alloy requirement, although not as drastically as the backward incline designs. Static pressure requirements above 1 in. water column (25.4 mm H2O) require the use of a shroud with nearly all designs, and some designs require shrouds below this pressure level. Another disadvantage is that larger sizes can be sensitive to thermal shock due to the large sectional thickness variations between components.
Cast axial fans are similar to the smaller fabricated axial fans, and are intended for applications involving continuous high-temperature exposure (typically with a corrosive atmosphere) or thermal shock (such as tip-up furnaces).
Advantages of the cast construction are that the fans are less sensitive to thermal shock and they can be used with a broad range of atmospheres and temperatures. These are not appropriate for applications requiring high static pressure.
On the other hand, the cast construction increases their cost and limits the size range capabilities. IGE currently supplies a limited range of cast axial fan wheels to 29 in. diameter. While larger fans are quite feasible, there has been insufficient demand to justify the high tooling cost to produce larger fans.
Reversing axial fan
A reversing axial fan is suitable for low static pressure applications with very high flow volumes requiring alternating flow directions. The key advantage of this fan design is its flexibility of application, since the airflow direction can be reversed frequently to achieve the desired results. The most common application for this equipment is aluminum sow homogenizing and/or reheat for rolling.
Disadvantages of this fan design are that higher tip speeds and/or larger fans are required than for single-direction flow and they have slightly lower mechanical efficiency. IH