Fans for gas flow: selection & operation

May 24, 2017

Guidelines for proper selection, operation and control

A mechanical fan is a machine used to create flow within a gas, and it consists of a rotating arrangement of vanes or blades that act on the gas. The rotating assembly of blades and hub is known as an impeller, a rotor or a runner. It is contained within a suitable form of housing or casing (case). The fan housing (or fan case) usually directs the gas flow and increases safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power including a steam turbine or an internal combustion engine may be used as well. Fans usually produce gas flows with high volume and low pressure, as opposed to compressors, which produce relatively high pressures. Guidelines for proper selection, operation and control of fans for gas flow are discussed in this article.

Axial fans

Axial-flow fans have blades that force gas to move parallel to the shaft around which the blades rotate. These fans blow gas along the axis of the fan, axially or linearly, and can be used in many applications. Standard axial-flow fans have diameters from 0.2 to 4 meters.

Variable-pitch fans are often used in industrial plants where precise control of static pressure and flow within supply ducts is required. The blades are arranged to rotate on a control-pitch hub, and its wheel usually spins at a constant speed. As the hub moves toward the rotor, the blades increase their angle of attack, which results in an increase in flow.

A well-known option, an axial fan’s power characteristic is usually nonoverloading, and it is usually used in applications with very low pressures and very high flows. The vane axial and tube axial can be selected for higher outlet velocities than the centrifugal fans, up to 10 meters per second or more. Axial fans should be connected to ducts by a tapered cone connection or similar connection.

Centrifugal fans

A centrifugal fan features a moving component called an impeller that consists of a central shaft around which a set of blades are positioned. These fans take gas to the impeller and spin the gas outward to the outlet by centrifugal force. As the impeller rotates, it causes gas to enter the impeller and move perpendicularly from the shaft. All in all, a centrifugal fan produces more pressure for a given gas volume compared to axial fans and is typically quieter than comparable axial fans.

Each of three types of centrifugal fan blades – radial, backward and forward – provides different characteristic performance types. The backward blade, the most commonly used type, is used extensively on clean and dirty gas streams and features nonoverloading power. As a rough indication, the usual operating efficiency range for the regular blade is approximately 65 to 80 percent, but for the streamlined type it is 80 to 92 percent.

While used in some services, radial blades are not popular. They can be used for handling gases with suspended solids or abrasive materials that are dirty or oily/greasy or have other difficult services. A fair running efficiency is approximately 50 to 73 percent for the straight radial blade and radial tip blade.

Like radial blades, forward blades are rarely used. They are usually shallow and operate at a slow speed for a given capacity, and they have low outlet velocity. The blades have poor operating characteristics for nearly all applications in which they are used because the power rises sharply with a decrease in pressure once the peak pressure for the fan has been reached. The operating efficiency range is approximately 55 to 65 percent.

Forced-draft fan impellers may be provided with backward inclined or backward curved gas-foil types. Induced-draft fan impellers may be radial, radial tipped, backward inclined or backward curved gas-foil type for operation in dirty gas environments.

Fan operation & gas flow

Different arrangements and configurations for fans include double-width, double-inlet and single-width, single-inlet. Overhung fans are usually used for small sizes and conventional applications. Between-bearing (BB) fans have been used for large or critical ones. In BB fans, the fan wheel is located between bearings where the bearings are mounted on independently supported pedestals to protect the bearings from the gas stream. As a rough indication, BB fans have been used when the impeller diameter is greater than 0.7 meters, power is greater than 100 kilowatts (kW) or speed is higher than 1,800 rpm. These fans are preferred for critical services, for example, with gas temperatures above 200°C; with toxic, flammable or hazardous gases; or in corrosive or erosive services that could be subject to fouling deposits that might cause rotor unbalance.

The selection and operation of fans requires great care. Generally, the following best practices should be considered:

  • Induced draft impellers should not exceed 0.8-meter diameter.
  • Reduced speed is desirable for critical services such as process gases, corrosive applications or erosive service.
  • Fouling deposits that result in unbalance should always be respected, and proper mitigations are required.

Fans should preferably have a stiff shaft. In other words, the operating speed is usually less than the critical speed. High-speed fans with flexible shafts exist, but their applications are often limited to small and noncritical fans. Most often, a wide operating range is required for an industrial fan. Fans should have turndown capability to 60 percent or less of the rated flow, and the performance curve (the pressure versus flow plot) of a fan should have a continuously rising characteristic, from the rated capacity to the surge. In many services, fans should allow the installation of fan blade cleaning systems.

Strong and rugged fan housings with a plate thickness of 5 to 6 millimeters (mm) or thicker is preferred. For medium and large fans, bolted and gasketed access doors – with minimum 600 mm-by-600 mm, for example – are required in the scroll and inlet box to access the fan internals for inspection, cleaning, rotor balancing and any internal bolting necessary for rotor removal.

Fan housings should be able to accept some external forces and moments from the suction and discharge connections. The external loads that will be imposed on the fan housing from the ancillary equipment such as ducting, silencer and filters should be limited. These items and fan housing should be provided in a way that the distortions from imposed loads do not affect the fan’s performance and cause internal rubs.

The sizing of a fan and its driver is a subject of great debate. Often, proper safety margins have not been considered, and the fan or its driver is undersized. In the sizing of a fan driver, possible variations in operating temperature and gas density from startup through various possible scenarios of operation should be considered. The driver power rating should be at least 110 percent of the maximum power required, considering all specified operating conditions. Often a good recommendation is to increase the driver power rating to 112 to 115 percent.

The fan control parameters can be a combination of inlet condition, discharge condition and flow. The fan control and capacity control are usually accomplished by variable speed, the suction throttling (by a damper or variable inlet guide vanes) or discharge bypass or adjustment. Variable fan speeds are well-known in many applications.

Manufacturing, reliability & maintenance

Shafts should be one piece of forged steel heat-treated after rough machining. Shafts smaller than 150 mm in diameter can be fabricated from hot-rolled steels, and shaft bearings should be accessible without dismantling duct works or fan casing. Hydrodynamic radial and thrust bearings are preferred for fans.

Standard rolling-element bearings have failed in many fan applications. The rating life (L10) is the number of hours at the rated bearing load and speed that 90 percent of a group of identical bearings will complete or exceed before the first evidence of failure. In various fans, bearings with L10 rating life of more than 80,000 hours with continuous operation at normal conditions (for instance more than nine years) have been specified. However, with the actual loading conditions more than the specified normal conditions or other unforeseen problems and issues, even rolling-element bearings with calculated long lives have failed prematurely. Specifying expensive rolling-element bearings with high ratings is possible, but they do not usually offer better reliability than simple types of hydrodynamic radial and thrust bearings.

Hydrodynamic radial and thrust bearings are mandatory for critical services or medium and large fans. They should be specified and used for fans above 150 kW. They should always be used for critical applications such as toxic or flammable gases or when gas temperature exceeds 180°C. Hydrodynamic radial and thrust bearings are also preferred for some small fans and even so-called noncritical fans because they offer better reliability and better overall cost over any rolling-element bearing for 15 to 20 years of operation. However, for many small fans, hydrodynamic bearings are not feasible and rolling-element bearings are the only option.

Hydrodynamic radial bearings should be split for ease of assembly, precision-bored and sleeve type with steel-backed, babbitted replaceable liners (or shells). These bearings should be equipped with anti-rotation pins and positively secured in the axial direction.

Thrust bearings are critical components in any fan. Hydrodynamic thrust bearings should usually be of the babbitted multiple-segment type, provide equal thrust capacity in both directions and be arranged for lubrication to each side. Thrust bearings should be self-aligning. Ample loading factors are always recommended considering the reported cases of overloading of thrust bearings and a long list of problems associated with them. As an indication, hydrodynamic thrust bearings should preferably be selected at around 40 percent of the bearing manufacturer’s maximum allowable load rating for all specified operating conditions; In other words, a 2.5 safety margin is recommended.

While lubrication oil has also been used for the bearing cooling, some limits on this oil cooling capability exist. Generally, the rise in oil temperature through the bearing and housing should not exceed 30°C under the most adverse-specified operating conditions, and the bearing outlet oil temperature should not exceed 80°C. When the inlet oil temperature exceeds 45°C, special consideration should be given to bearing details, oil flow and allowable oil temperature rise. Operation above these limits might be possible using sophisticated synthetic oil, but such oils are expensive and special. Synthetic oils are routinely used in special fans such as those with high operating temperatures, but their applications should be limited to fans on which mineral lubrication oils cannot be used.

Damper and inlet variable guide vane (IGV) systems have been used in many fans, but the details of their design and operation should be carefully reviewed to ensure their long-term reliability because they have been known to introduce some operational issues and problems. IGV operating mechanisms should be located outside the flowing gas stream. The mechanism should be readily accessible for in-place inspection and maintenance, and it should have bolted attachment construction to permit removal if necessary. Proper and reliable provisions should be furnished for lubrication of these mechanisms during operation.

Amin Almasi is a senior rotating machinery consultant in Australia. He is a chartered professional engineer of Engineers Australia and IMechE and holds bachelor’s and master’s degrees in mechanical engineering and RPEQ. He is an active member of Engineers Australia, IMechE, ASME and SPE and has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea and reliability.

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