A fan inlet box is necessary for industrial application where space is tight and access to the centrifugal fan or blower is constrained. P-wheel type where blades are backward and straight. Various drive technologies cover a wide area of operating ranges. Table 4-16 presents a troubleshooting guide for centrifugal compressors and their lubrication systems. In the example shown below, the fan rotates in a clockwise direction. Typical Distribution of Unscheduled Downtime Events for Major Turbocompressore in Process Plants. Neise [298], and Neise and Barsikow [299, 300] used this separable form to generate separate similarity functions for fans of different sizes; examples are shown in Fig. (API Std. Therefore the function F is equivalent to a function F(m, ℜ, ϕ), where m=1, 2, … is the blade harmonic number. The gage static pressures are measured in millimetre of water. Fan Housing: Housing in a fan system improves efficiency as it reduces noise and acts as a protection for moving parts like impeller, hub etc. Epoxy coating is available upon request. The air enters the impeller in an axial direction and is discharged at the impeller outer periphery. PNEG-163-08 Centrifugal Fan 5 1. A fan usually consists of a single rotor with or without a stator. For more information on Delta electronically commutated fans, please Contact Us. The figure shows that a double exit fan can produce a higher volumetric flow rate than a single-outlet one when both channels are fully utilized. Designs: The following design models are available: ERAE/D: single inlet centrifugal fan . Self-cleaning type. 2. Variation of double-outlet fan efficiency versus flow rate in both channels. A schematic of the forward-centrifugal fan is given in Figure 1.1. If an elbow is required at a fan inlet due to space constraints, use an inlet box that has a predicted loss in lieu of the elbow. Double Inlet Forward Curved Centrifugal Fans The FDA series is DIDW centrifugal fans with forward curved impellers. Low noise and low energy consumption. Single, unspared centrifugal compressor trains support the entire operation of steam crackers producing in excess of 1,000,000 metric tons (approximately 2.2 billion lbs) of ethylene per year. Self-cleaning. Check belt drive for proper sheave selection and make sure they are not reversed. Center hub supported blade: This option is used for comparatively clean air as it has higher velocity and pressure (see Figure VI/3.2-1c). Important parts of a centrifugal blower. Marine fan with swing-out “Maxcess” casing. • Centrifugal fans are available in either Right Hand (RD) or Left Hand (LG) rotation. Centrifugal fans are capable of generating relatively high pressures. They can be small, with sub 0.125 kW motors, or large, with hundreds of kW motors driving fans. This would impact your inlet air density and affect your fan selection. (7.34b), determine. (6.151) represents a linearized model of the acoustic radiation. Finally, the sound pressure in proportional bands Δf ~ f may be written in the slightly modified form. Fan Parts-Impeller Design The centrifugal fans impeller have five basic blade shapes, and a number of impeller configurations (i.e) DWDI (Double width double inlet) or SWSI (Single width single inlet). . B-wheel type where blades are backward curved. Figure 4.25. A double-outlet centrifugal fan has a volute with two outlets. The function G(fL/c0,ℜ, ϕ) contains all the effects of acoustic resonances, but the representation (6.151) does not allow for aeroacoustic feedback. The fans are suitable for supply or extract applications in commercial, process and industrial HVAC systems. High pressure centrifugal fans, forward & backward curved blades, industrial centrifugal fans, external rotor motor Important types of fans for industrial and domestic applications for which sound power levels are given in Fig. RD-SB Rectangular Silencer Sheet RED-SB Elbow Silencer Sheet Selection and Pricing Request Form Single Inlet Centrifugal Fans - Backward wheels The BSB series is SISW centrifugal fans with high efficiency non-overloading backward curved impellers. Close; Rep Locator; Acoustical Consultant Locator; Licensee; Rep Login; Search. Figure 4.27 shows efficiency of the double-outlet fan versus flow rate at each channel. Eng., Ph.D., C.A. Forward curved centrifugal fans Single and double inlet Low noise level, relatively low air flow with high static pressure increase and a space-saving design. The scroll housing of single inlet centrifugal fans is open on one side. [300] and Yeager [301] of interest. 6.56. Air is discharged at 90 degrees to the inlet airstream. Figure 4.25 depicts variation of total pressure in channel 1 versus flow rate at each channel, and Figure 4.26 shows the same for channel 2. https://airprofan.com/industrial-fan-applications/ - Air needs some runway on its way into a centrifugal fan. In fact, all control devices for axial fans are applicable for radial fans also, except the fan blade pitch control (meant for axial fans only). Highly efficient and air volume change is negligible with changing pressure. POLLRICH GmbHEisenhuettenstrasse 26 | 57074 Siegen GermanyPhone +49-271-66123-0 | info@pollrich.com, POLLRICH is a member of FLT, the research-network for ventilation technology, Designs and Characteristics of centrifugal fans, Impeller diameter from 280 up to 1.250 mm, Impeller diameter from 280 up to 2.240 mm, Volume flow up to 250.000 m³/h (single inlet design), up to 500.000 m³/h (double inlet design), Pressure-rise up to 15.000 Pa (single stage), up to 30.000 Pa (double stage), Shaft support with overhung arrangement or on both sides, Single inlet, single width (SISW) or double inlet, double width (DIDW), Mounting on bearing block (steel or concrete), Impeller diameter from 280 up to 4.000 mm, Volume flowup to 800.000 m³/h (single inlet design), up to 1.900.000 m³/h (double inlet design), Pressure-riseup to 33.000 Pa (single stage), up to 56.000 Pa (double stage), Bearings grease or oil lubricated, encapsulated or pedestal as well as plain bearings, Mounting on bearing block (steel or concrete) or on sprung- or strip foundation possible. Sizes Available: 180-400 mm / 7.09-15.75” Casings: The casings of the high efficiency centrifugal fans are made of galvanized sheet metal. centrifugal fans, of types and sizes required, and whose products have been in ... design for fans 30” and larger, include openings for tachometer. Delta manufactures centrifugal fan and EC motor assemblies for a range of industries and applications. Provide linkage for manual or automatic operation; 6.53 illustrates the primary components of the centrifugal fan, and Fig. Bath of these devices alter the direction of the flow into the impeller. These fans are composed of forward blade impeller, dynamically balanced motor-roller assembly, IP44 motor, Class B, with permanent ball bearings greased for life. Highly efficient. It is important to minimize this reversing flow. With over 2000 variations of diameter, width and length type, specifications are virtually tailor-made to individual needs. where G(x, k0y) is dimensionless and L is a dimension scale of the fan, say its rotor diameter. There is no boundary between fully axial or fully radial categories. Centrifugal Fan: Types The major types of centrifugal fan are: radial, forward curved and backward curved (see Figure 5.3). The double inlet pulls air over the motor, completely utilizing motor heat in the process; Downstream heaters for dual inlet fans are available for high-capacity in-bin drying; Engine-driven dual inlet fans are also available; Centrifugal In-line Fans. To generate these functions it must be noted that the spectral character of the G(fL/c0) function is determined by acoustic resonances in the casing, and these occur at the same frequency at all speeds. Sizes of this series are in accordance with AMCA standard 99-0098-76 and DIN 323 R20. Ad is listed under centrifugal fan, extraction and fans, Industrial Machinery. Large fans, such as those used in air-conditioning systems, have lower fundamental resonance frequencies, but higher-order acoustic modes of the casing volume continue to be important. For example, if you need a pressure blower to pull 60 inches of water through your system, that pressure is all negative at the fan inlet. Radial fans rotate slower but have larger diameters. Here the motor and impeller are mounted on a very large hinged door which can be swung out for access and maintenance, without disturbing any associated ducting. The combining element of our Heavy Duty Centrifugal Process Fans is the optimum in relation of efficiency and energy-consumption. Close; 1-800-565-8401; info@vibro-acoustics.com; Rep Login; Go to Website . A large range of stsndard Models covering most user requirements are generally available. In centrifugal fans, the air flows through the action of deflection and centrifugal force. Similitude considerations for centrifugal fans must therefore include both aerodynamic and acoustical resonance factors. It does not make any difference which of the two outlet channels has a constant throttle point when the other is throttled. The airstream is diverted either side of the motor compartment and then rejoins again downstream. It is a long time that people do not attempt to model forward-blade fan performance with the Euler energy equation. The volumetric flow rate and total pressure are measured in m3/s and millimetre of water, respectively. They represent the ideal solution for a wide range of cooling tasks. When the impeller rotates, air is allowed to enter the fan around the open space between the impeller and the shaft and forced to proceed at a right angle to the shaft at the outlet. An inlet bell for centrifugal fans having a fan wheel (l5,24) with a cover plate (l4, l9), the inlet bell opening into the cover plate with an intermediate gap (I) for recirculation air, the inlet bell (ll, 20) being provided with a conical inlet part (12,2l), which tapers towards the flywheel (15,24), and a … Never lift with slings or timbers passed through the fan inlets. 6.56. Self-cleaning type. 6.19), yielding a cross-spectral density that may be expressed in the general form, where the frequency has been made dimensionless on L/UT. S.L. In centrifugal fan, the air from the inlet enters the impeller axially through fan housing, where it picks up the acceleration and is radially sent out in 90° direction. Access to the motor and impeller in all these cases is then easy. The fan is gener­ ally operated at a constant speed. Blade inlet and outlet angles are about 90° and 175°, respectively, and the optimum number of blades is about 40. The variable inlet vane is an energy saver when compared with conventional dampers. Safety Safety Guidelines This manual contains information that is important for you, the owner/operator, to know and understand. Inlet Box PositionsFor Centrifugal Fans AMCA Standard 99-2405-83 NOTES: 1. For CFD simulation, it is needed to have a closed watertight model (sometimes called waterproof, or model negative, or wet surface) of the fan … ABOUT US Industrial Air Technology Corp. services the industrial fan market with a complete line of centrifugal industrial fans and industrial blowers. Representation of different types of turbomachines on the Cordier diagram. A centrifugal type exhaust/supply fan specifically designed for moderate size ventilation applications. Sizes Available: 180-400 mm / 7.09-15.75” Casings: The casings of the high efficiency centrifugal fans are made of galvanized sheet metal. Restricted or unstable flow at the fan inlet can cause pre-rotation of incoming air or uneven loading of the fan wheel yielding large system loss and increase sound levels. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Generally it rises the pressure upto a maximum of 0.07 bar (70 cm WG). Over the last 20 years centrifugal fans with backward curved blades have become more common within air handling. Long casings are normally used on fans contained within a ducting system which has elements on both the fan inlet and outlet (Installation Category D). Thus the motor is open to the cooler or cleaner ambient atmosphere. Bath of these devices alter the direction of the flow into the impeller. The name, ‘Centrifugal Fan’ is derived from the direction of flow and how the air exits the fan impeller radially from the outer circumference of the fan. Fig 2shows the pressures through a fan, each of which is described below: Inlet Pressure; is the static pressure on the inlet side of the fan. Figure 8.26. Impellers. (2.153) of Volume 1 suggests that the sound pressure at the exit plane, say, could be represented as the general functional form of Eq. If we presume that the temporal and spatial qualities may be separated, i.e., that, then the radiated sound spectrum can be written, where Λ represents a spatial correlation scale and the product k0 Λ=ωΛ/c0 represents the aeroacoustic coupling between the dipole field and the casing acoustics. OBJECTIVE: This study aims to design and develop a model of a centrifugal pump and analyze the various parameters such as the velocity, pressure at the outlet and inlet… See Figure 8.24. When a vane is in place, maximum performance is decreased. Centrifugal fans consist of an impeller in a casing having a spirally shaped contour. they can be stated as: bearing, impeller locking device, fan discharge casing, shaft seal plates etc. True bifurcated axial flow fan. Whether it’s high or low volume, high or low pressure, high or low temperature, clean or dirty gas streams, erosive or corrosive environments, Northern Blower has thousands of available centrifugal fan … P-wheel type: Backward and straight blades. The centrifugal fan is more complicated aeroacoustically than the axial flow fan, and the acoustic resonances in the scroll casing influence the sound spectrum. Experiments have been conducted to isolate the importance of acoustic resonances in the housing, and Moreland [296] has shown that small fans (fans less than 10 in. V-A Design; Resources; Careers; Contact. The fan is gener­ ally operated at a constant speed. B-wheel type: Backward curved blades. Pressure in the fixed throttled channel remains constant when the other channel is throttled. System Design, 29 General Ventilation Guidelines .....29 Process Ventilation ... Centrifugal fans are the ‘workhorse’ of the fan industry, operating in a wide variety of configurations, duties and ... the fan inlet. CalQlata has tried to keep the operation of this calculation option as simple as possible, given that it is recommended for general purpose calculations only and not for actual purchase specifications (see Fan Calculator – Technical Helpbelow). In addition, the fans are also available with different drives. Standard No. The air in the fan is pushed against the outside of the scroll by the movement of the fan wheel. Instead of the extension duct detailed above, a more simple “inspection” duct can be substituted. in diameter) can have a Helmholtz resonance in the frequency range of 100–1000 Hz. When the impeller rotates, air is allowed to enter the fan around the open space between the impeller and the shaft and is forced to proceed at a right angle to the shaft toward the opening of the fan casing; see Figure II/2.1-1(b). Note that, as defined in Fig. Its special design of the motor-roller assembly provides great compactness, especially in single inlet models, and to avoid noise and vibrations the impeller of these centrifugal fans are dynamically balanced. The low pressure centrifugal fans of the CBM SMALL range are made of galvanised sheet steel protected with anticorrosive polyester paint and can be configured as single or double inlet. When the vanes are fully open it offers extremely low resistance. Variations in the total pressure of both channels are fairly similar. AWP DOUBLE INLET CENTRIFUGAL FAN Systemair’s range of centrifugal fans offer the engineers the flexibility to choose the most suitable sizes and configurations to suit any site condition. The side parts are produced with nut sets to fix … of single inlet centrifugal fans which are universally usable for many ventilation purposes either in HVAC or in machinery design industry and heavy duty installation business. The range presented in this catalogue consists of single inlet centrifugal impellers (TRE), single inlet centrifugal fans in steel scroll (GRE - GRF) and double inlet centrifugal fans (GDS - GDF). It is most important when you have a great vacuum. T-wheel type where blades are radial and straight. Highly efficient and air volume change is negligible with changing pressure. Avoid free discharges for inline and centrifugal fans. When plants in this size range experience emergency shutdowns of a few hours' duration, flare losses alone can amount to $400,000 or more. An industrial inlet box allows for a more efficient 90-degree turn close to the inlet to pull the air from any direction. Drive shaft. Sometimes centrifugal fans are called radial flow type fans. This type of fan produces, comparatively (with respect to axial fans), a higher pressure and less gas flow; they are noisier than axial fans. 6.54 illustrates the various types of fans. The maximum e ciency was 93.7%. The volumetric flow rate and total pressure are measured in m3/s and millimetre of water, respectively. It is most important when you have a great vacuum. Figure 4.28 shows a comparison between a double-outlet fan and a double-inlet single-outlet fan of similar construction. A double-outlet squirrel-cage fan. Centrifugal inline fans feature durable galvanized steel construction (aluminum is optional for selected belt drive models), which works in conjunction with a patented wheel design and deeply spun inlets to provide smoother flow through the ventilator. It is worth noting that the variable inlet vane control is extremely sensitive at lower load, meaning that with a slight change in angle there will be larger flow change, whereas the same characteristic is reversed at a higher load. 673 "Special Purpose Centrifugal Fans for Refinery Service" 1st. Despite the cost disadvantage, they have become more popular in air handling units because their efficiency is higher than forward curved centrifugal fans. The side parts are produced with nut sets to fix the mounting brackets, which can be fixed in steps of 90°. 4. outlet duct. Short casings are normally used on fans at the entry (Installation Category B) or at the exit (Installation Category C) of the ducting system. Table 2.1. This is not a drawback for this fan. Flow-field studies in this book and from other researchers have clarified that the flow is not uniform around the impeller or across its width. William K. Blake, in Mechanics of Flow-Induced Sound and Vibration, Volume 2 (Second Edition), 2017. This is shown in Figure 8.28. Hall Ph.D., in, Fluid Mechanics and Thermodynamics of Turbomachinery (Sixth Edition), International Journal of Heat and Mass Transfer, Only suitable for low–medium airflow rates. Since Table 4-16 cannot possibly be all-encompassing, refer to special sections in this book addressing such related topics as vibration-causing syndromes and bearing, gear, and coupling failure analysis and troubleshooting. For more than 100 years we are a reliable partner of any kind of machinery and engineering company. CBM-RE, low pressure centrifugal fans . Position on inlet box is designated in degrees clockwise from … Inlet diameter design is intertwined with practical consideration for impeller maintenance removal. The designations 1 and 2 only show which channel is throttled and where the pressure measurements are made. In fact, all the control devices, as stated for axial fans, are applicable for radial fans except the fan blade pitch control; this is a unique control device associated only with axial fans. There are no conclusive differences in the measured data for either channel. The design of the evasé depends upon the fan design and the fan operating conditions. Further, we will classify the various types of blades of an impeller and finally we will find out the velocity triangle for a centrifugal fan and blower. The latter may be used where the fan is at the lowest point of the system. Broadband noise is generated by the airflow through the impeller, on the walls of the casing, and over the ribs and stiffeners of the casing. Shi et al. A double-outlet fan has two cut-offs. True bifurcated fans can be installed vertically at high level in chimneys where the wind can blow through the motor compartment to give excellent cooling. In a variable vane-controlled system, the fan selection is generally done with full pressure and flow with no vane in place. Centrifugal fans in heating and ventilation industries usually supply more than one outlet port. On centrifugal fans the most commonly used are the preswirl inlet dampers located at the inlet box terminal flange and the variable inlet vane located in the inlet cone. The impeller design will depend on the aerodynamic duty and the operating conditions. Forward-curved squirrel-cage fans are the prime candidates for such applications. 6. Because Λ/L is a function of both ϕ and ℜ, the aeroacoustic coupling function can be expressed G(ωL/c0, ϕ, ℜ). Centrifugal compressors, blowers, and fans form the basic make-up of unspared machinery equipment in petrochemical process plants. Many of the features described for centrifugal fans in Sections 8.1 to 8.6 inclusive, are also applicable to axial and mixed flow fans. On double inlet fans with a single driver, the side with the driver is considered Fig 3 shows the velocity diagram for the air flowing into the fan (inlet) and out of it (outlet). In corrosive application, they are used with an inlet cone, and the vanes throw the air to the eye of the impeller for accurate and better control. The blockage effect is less but requires a diversion plate to be fitted to encourage a cooling air path if a TEFV motor, as discussed in Chapter 13, is fitted. The number of blades in the impeller may range from 6 to more than 60, depending on the configuration of the impeller. This wiring is normally contained within rigid piping or a flexible conduit. Fan static pressure and air volume for different centrifugal fans. Depending on the type of motor used, dual-inlet centrifugal blowers with modern EC technology can cover virtually any conceivable area of application. The impeller design will depend on the aerodynamic duty and the operating conditions. Inspect impeller for correct rotation for the fan design. Discharge orientation can be any of the standard Eurovent & ISO angles, along with any angle in between as a special design. Vane axial fans with downstream guide vanes and mixed flow fans are invariably provided with long casings. Centrifugal fan with scroll enclosure, In Practical Machinery Management for Process Plants, 1999. Flow direction in the meridional plane locates different fans in the Cordier diagram. With respect to axial fans, gas comes out here at higher pressure with less flow. Dixon B. 673, Section 1.4). All Products; Axial Fan – Centrifugal Fan – Industrial … Compared to the degree of study and measurement conducted on axial flow fans, centrifugal fans have received little attention. The fan can be mounted at any angle in any point along the duct work and straight-through air flow design allows easy installation. Fig. Drive mechanism. In other words, the best efficiency conditions are for symmetric operation. A Backward curved centrifugal fan is characterised by its cylindrical shape, several large curved blades and a conical inlet nozzle. Due to their low maintenance requirements, POLLRICH fans provide a continuous production process. 3. Energy-saving EC external rotor motors, conventional AC external rotor motors as well as IEC standard motors are used. Double Inlet Forward Curved Centrifugal Fans The FDA series is DIDW centrifugal fans with forward curved impellers. A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Just ask us – we’d like to help and advise you. Such instabilities are not still surveyed or documented but moving the rotor away from the inlet could initiate them even at high flow rates. Radial fans are industrial workhorses because of their high static pressures (upto 1400 mm WC) and ability to handle heavily contaminated airstreams. Information material in many languages available: click here. This is fitted with an access door of ample size for inspection, lubrication or cleaning. The Green’s function includes all the acoustic modality of the enclosure and may be dimensionally represented as. The aeroacoustic coupling function G(f D/c0, ℜ, ϕ) has been found to be a weak function of both the Reynolds number and flow coefficient. Whilst the moving air cools the motor, if there is high temperature or corrosive elements present, then it is desirable for the motor to be outside. A review paper, by Neise [7,58] outlines the primary noise-control measures for centrifugal fans, while design considerations have been discussed by Harris [295]. By continuing to use our site, you consent to the use of cookies. Tube axial fans may be provided with so-called “short” or “long” casings. Centrifugal fans The three types of centrifugal fan blades (radial, backward and forward) give three characteristic performances. Eleven sizes are available with a drive power of between 85 W and 3 kW. Sizes of this series are in accordance with AMCA standard 99-0098 R20. The fans are equipped with IEC flanged (B5) standard motors, protection IP55, class F, and they are suitable for frequency inverter controlled operation. The Cordier diagram compares performance and size of different turbomachines (Figure 1.2). Fan Parts-Impeller Design The centrifugal fans impeller have five basic blade shapes, and a number of impeller configurations (i.e) DWDI (Double width double inlet) or SWSI (Single width single inlet). Inlet duct. 3. Results are available for modifications in rotor geometry such as aerodynamic blade profiles or a removal of the shroud but since the rotor is the most expensive part of the fan, it is very important that modifications should not result in added fan price. This figure indicates that non-symmetric use of a double-outlet fan can reduce efficiency. For small fan, impeller is removed and installed axially from its inlet. Design of a Centrifugal Blower 4. Sizes of this series are in accordance with AMCA standard 99-0098-76 and DIN 323 R20. Automobile ventilation is an example where such fans serve both right and left sides of the cabin. Our centrifugal fans are, as a principle, energy-efficiently designed. Ed., Jan. 1982 DEFINITIONS AND TERMINOLOGY Evase - Is a diffuser or a diverging discharge transition piece. Single and double vane designs are available. The fans are available with forward curved impellers. Table 4-15. So, let us recall here first the fundamentals of a fan and blower and further we will see a very important type of fan and blower i.e. Fan Guards (1) Both inlet and outlet guards are available whenever the unit is Thus Eq. The radial/centrifugal fans have an impeller as the moving component; it is nothing but a set of blades or ribs suitably placed around a central shaft. Centrifugal fans have a finite acceptable control range, typically about 15 per cent below and 15 per cent above its design point. Centrifugal fan and blower. Self-cleaning. These are applicable for control devices that are provided with radial fans. They are normally larger for the same volume and pressure characteristics and are a little more complicated to construct and so more expensive. Centrifugal fans with forward curved blades always require a scroll housing (see Figure 2.16). performance of a centrifugal fan, which were inlet angle, outlet angle, and blade number, and obtained that the best combination of the three blade structure parameters using the response surface model (RSM) optimization method. A cheap radial-vaned centrifugal fan is required to provide a supply of pressurised air to a furnace. Figure 4.27. In addition, for radial/centrifugal fans, in general, there are two basic types of drives—that is, fixed speed and variable speed. In exceptional cases, higher performances are possible within this design. A central unit in such industries supplies conditioned air to various enclosures through different ports. Total pressure variation in channel 1 versus volumetric flow rate in both channels (C1 and C2 denotes channels 1 and 2, respectively). Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook, 2015. The encasement of the impeller has been identified as an important amplifier of the sound generated by the fan [296], and, as shown by measurements of Chanaud [297], unenclosed impellers generate fundamentally dipole sound, although with a slightly different dependence on rotation speed than for cased impellers. Readers interested in the flow field and acoustics of centrifugal blowers may find Denger et al. Our centrifugal fans are designed and built according to your specific requirements. Thus the fan case can alter the sound radiation, analogous to the way a half-plane alters the speed dependence of dipole sound from U6 to U5. Squirrel-cage fans rotate faster for similar load as radial fans but have a similar size as axial machines. Fan impeller - Is the assembly of the fan wheel and the hub(s). The specification requires that the fan produce a total pressure rise equivalent to 7.5 cm of water at a volume flow rate of 0.2 m3/s. However in situations are needed, our design & development will produce individual units designed for the specific requirements. Fan performance is generally tested to standards by considering ideal inlet and discharge airflow conditions. The simplest arrangement is a small number of radial blades, but the impeller commonly used in air-conditioning systems and in small ventilation units has a large number of smaller blades curved in the direction of rotation. Cooling discs may be used with bifurcated fans (see Section 8.9.4) where the air is above 75 °C and heat transmitted along the shaft could otherwise damage the motor. Figure 6.54. Chapter 4 of this book shows extensive variation in slip factor over the impeller exit surface that means the flow does not follow the blade passages.