Many operations at process plants require propelling gas forward. Centrifugal fans commonly handle that duty. Unfortunately, engineering, operations and maintenance teams largely ignore fans, frequently forgetting or overlooking them. This lack of attention often results in design, manufacture and installation of fans without the quality and provisions necessary for reliable long-term performance.
A centrifugal fan consists of a wheel with blades on its circumference and a casing to direct the flow of gas into the center of the wheel and out toward the discharge. The configuration and details of these components affect the gas movement as well as the efficiency and reliability of the fan. Often, fans generate relatively low differential pressures — as a rough indication, below 1.1 Barg. Blowers usually can achieve higher pressures than fans, say, for instance, 1.1 Barg or above. (For information on centrifugal blowers, see: “Make Success with Centrifugal Blowers a Breeze.”) Compressors can achieve much higher pressures. Centrifugal fans usually operate against pressures of 0.1 to 0.8 Barg but some operate at 0.9 Barg or even a little more.
Commonly used centrifugal fans typically run at speeds between 800 rpm and 4,000 rpm, although high-speed fans exceed these limits. Some fans feature a gear system as an integral part of the machinery to increase speed. Other high-speed fans are directly driven, e.g., by high-speed electric motors. A high-speed impeller/wheel, whether gear- or direct-driven, can rotate very fast, say, up to 6,000 rpm or even more.
Some fans are multi-stage devices, accelerating gas as it passes through each stage. In single-stage fans, gas doesn’t take many turns; hence such units are considered more efficient. One usual characteristic of fans — which can be a disadvantage in some applications — is that gas flow tends to drop drastically as system pressure increases. Fans most often are used in services that aren’t prone to clogging. Blowers are a better choice when some degree of clogging is expected because they can produce enough pressure (say, more than 1.5 or 2 Barg) to blow clogged materials free.
Selection And Operation
Any fan selected should meet capacity and pressure stipulations, operational specifications, desired machinery size, weight and cost, as well as ruggedness and reliability requirements. Many small fans are overhung models but this design isn’t possible for medium- and large-size devices. Lots of fans, particularly medium and large units, are between-bearing (BB) models. In this type, the wheel or impeller assembly is located between bearings; the bearings usually are mounted on independently supported pedestals and protected from the gas stream. As a rough guideline, opt for a BB fan if impeller diameter exceeds 0.5 m or driver-rated power tops 90 kW. High-speed, say, over 2,000-rpm, fans in any size need special care and preferably should have a BB configuration. Usually, low temperature gases (say, below -10°C), high temperature services (above 160°C) and challenging gases (toxic, flammable, corrosive, erosive, hazardous, etc.) demand very careful selection, special materials, specific seal systems, bearing protection and a BB design. In addition, each particular service poses its own set of considerations and requirements. For instance, reduced speed is desirable for erosive service. Services subject to fouling deposits require special attention. Fouling can cause rotor unbalance that impairs fan performance; therefore, a smart rotor-dynamics configuration, insensitive to some degree of unbalance, makes the most sense for these services.
Wheels And Blades
A fan wheel consists of a hub to which a number of blades are attached. The blades can be arranged in different ways; we’ll look at the main ones: forward-curved, backward-curved and radial.
Forward-curved blades arch in the direction of the fan wheel’s rotation. Such blades are especially sensitive to particulates. They provide a comparatively low noise level and generally find use for relatively small gas flows with a relatively high increase in pressure.
Backward-curved blades arch against the direction of the fan wheel’s rotation. Smaller fans may have backward-inclined blades, which are straight, not curved. Larger machines use backward-curved blades with properly selected and shaped curvatures. These models provide good operating efficiency. Units with backward-curved blades of one form or another commonly serve in many applications. Such fans can handle gas streams with different characteristics and specifications such as those with low to moderate particulate loadings, etc. They easily can be fitted with wear protection but certain blade curvatures can be prone to solids’ build-up. Backward-curved wheels often are heavier than their forward-curved equivalents because they run at higher speeds and require stronger construction. Backward-curved fans can have a high range of specific speeds. They are the most common units for relatively high pressure, medium flow applications. However, they also find use in a wide range of size, flow and pressure services.
Radial fans have wheels whose blades extend straight out from the center of the hub. Radial-bladed wheels often are favored for particulate-laden gas streams because they are the least sensitive to solids’ build-up on the blades. However, they frequently generate greater noise and lower efficiencies. Radial blades appear in fans running at high speeds, low volumes and relatively high pressures, e.g., ones used in industrial vacuum cleaning, dust collecting, pneumatic conveying and similar systems.
Ideally, a fan should come with wheels or impellers that provide the highest efficiency consistent with the application. Backward-curved fans are more efficient than radial-blade ones and, therefore, are suitable alternatives for medium- and high-power ratings.
Forced-draft-fan wheels/impellers often come with backward-inclined or backward-curved blades. Induced-draft-fan impellers may be radial, backward-inclined or backward-curved, depending upon the gas environment. Induced-draft-fan blades usually are used in small fans, say, those below 0.7 m in diameter; induced-draft fans aren’t common for large applications.
Performance And Reliability
Operation at part load is very common for fans, so turndown and operating range are important. As an indication, plants often desire a turndown of 60% or less of the rated flow. This usually is provided using a variable speed drive (VSD), an inlet guide vane (IGV) system or a combination of the two. Some services have relied on other turndown systems such as a bypass, pressure reduction at suction, etc., but VSD, IGV or VSD+IGV commonly are specified because they are more efficient and more reliable. Surge or other instabilities are major factors limiting the minimum achievable capacity. The fan performance curve (pressure versus flow) preferably should show a continuously rising characteristic from the rated capacity to the surge. Note that many fan curves just plot theoretical values. Instead, strive to get performance curves corrected for the particular job gas at the specified conditions and based on performance tests of actual equipment at the shop or site performance tests at a very realistic configuration.
Shaft seals need great care because many fans have suffered seal problems or failures. In general, seal selection/configuration and seal maintenance/operation have been challenging. Many different seal systems for fans exist — e.g., labyrinth type, floating bushing, close-clearance annulus, honeycomb type, etc. Choose a type and model of seal that minimizes leakage from or into the fan over the range of specified operating conditions and during periods of idleness. Select a seal that can handle all foreseeable operating conditions, including those during startup and shutdown as well as any special operating modes expected. Many applications rely on double seals. For instance, fans in processing applications (except air services) with negative pressure at the shaft seals usually should have double seals. A seal for a critical service (such as toxic, flammable, etc.) generally requires provisions for a centralized buffer gas injection to minimize leakage.
Bearings are critical components in fans. Indeed, bearing problems have caused many trips. Hydrodynamic radial and thrust bearings have been popular because of their high reliability and theoretical indefinite life. They are common in large fans, say, above 150 kW. These bearings also are used in many fans in the medium range of size and power rating as well as those in critical applications. Insist on hydrodynamic radial and thrust bearings for challenging services such as high-speed ones, those for high temperatures (say, above 150°C), etc. Tilting-pad bearings have been used in high-speed fans. These bearings should be self-aligning and preferably supported on near-rigid pedestals independent of the fan housing to ensure that vibration, differential thermal movements or other forces from the fan housing don’t affect them.
However, many manufacturers still use rolling-element bearings for small and some medium fans. Rolling-element bearings have limited life and relatively low reliability. Manufacturers of many of these fans can’t use hydrodynamic bearings without changing their designs. Hydrodynamic bearings need more space and auxiliaries, an expensive oil lubrication system and numerous other provisions that make such a fan expensive and challenging. When you must opt for a fan with rolling-element bearings, take a few important steps. First, specify a high rated life, say, above 90,000 hours or 100,000 hours, for the bearings. (Some users have requested and gotten vendors to accept 140,000 hours!) Think about access and maintenance provisions for the bearings. Regardless of bearing type, getting to all bearings preferably shouldn’t require dismantling ductwork or the fan casing. Overhung models should have provisions for supporting the rotor during bearing maintenance.
Many fans need circulating lubrication oil for their bearings. Filtering and cooling this oil before it goes to the bearings is essential. Provide a full-flow dual filter set with replaceable elements and suitable filtration degree (preferably 10 microns or less). Ideally, locate the filter set downstream of the coolers. Ensure the flow and conditions of the lubrication oil suffice to properly lubricate and cool the bearings. As a rough indication, keep the rise in oil temperature through the bearing and housing to less than 30°C. For most oils, the bearing outlet oil temperature should be below 75°C. Very small fans or blowers may use grease instead of oil to lubricate bearings. Temperature limits also usually apply. For grease lubrication, restrict the maximum bearing housing temperature to less than 75°C, preferably under 70°C. Don’t allow oil or grease to leak outside the bearings; equip bearing housings with replaceable seals (usually, multistage labyrinth-type) and deflectors, both made of non-sparking materials.
All fans in even moderately high temperature services, say, handling gases above 150°C, preferably should have special provisions for cooling the bearings, i.e., properly configured oil, water or other cooling. For these hot services, the fan manufacturer should perform a thermal analysis for the bearings and seals taking into account the heat conducted by the shaft in these hot gas applications to optimize the cooling systems, lubrication oil system and overall thermal management.
Many fans use gear units or have integral gearing; these machines require a carefully selected lubrication oil and a proper lubrication oil system. Where a common system supplies oil to different pieces of equipment, such as a fan, a gear and an electric motor, select an oil whose characteristics are the optimum balance for all these applications.
AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at [email protected].
