Fan discharge casing


Fan inlet


Fan Impeller discharge Figure 2: Centrifugal fan in scroll casing, with a backward inclined impeller (Source: CIBSE TM42:2006)


Backward curved blades Backward curved blades


Backward inclined blades Backward inclined blades


Backward inclined aerofoil blades Backward


inclined aerofoil blades


Figure 3: Illustration of centrifugal fan impellers


from the fan (the so-called ‘blast area’). This type of fan has been traditionally


driven by a motor with a belt and pulley arrangement. However, with the improvement in electronic speed controls and the increased availability of electronically commutated (‘EC’ or brushless) motors, direct drives are becoming more frequently used. This not only removes the inefficiencies inherent in a belt drive (that may be anything from 2% to more than 10%, depending on maintenance2


) but is also likely to lessen


vibration, reduce maintenance (fewer bearings and cleaning requirements) and make the assembly more compact.


Backward curved centrifugal fans Backward curved (or ‘inclined’) fans are characterised by blades that tilt away from the direction of rotation. They can reach efficiencies of towards 90% when using aerofoil blades, as shown in Figure 3, or with plain blades shaped in three dimensions, and slightly less when using plain curved blades, and less again when using simple flat plate backward inclined blades. The air leaves the tips of the impeller at relatively low velocity, so the friction losses within the casing are low and air-generated noise is also low. They may stall at the extremes of the operating curve. Relatively wider impellers will provide greatest efficiencies,


52 CIBSE Journal December 2011


and can readily employ the more substantial aerofoil profiled blades. Slim impellers will show little benefit from using aerofoils so tend to use flat plate blades. Backward curved fans are particularly noted for their capacity to produce high pressures combined with low noise, and have a non- overloading power characteristic – this means that as the resistance reduces in a system and the flowrate increases the power drawn by the electrical motor will reduce. The construction of backward curved fans is likely to be more robust and rather heavier than the less efficient forward curved fan. The relatively slow air velocity of the air across the blades can allow the accumulation of contaminants (such as dust and grease).


Radial blades Radial blades


Forward curved blades


Forward curved blades


Forward curved centrifugal fans Forward curved fans are characterised by a large number of forward curved blades. As they typically produce lower pressures, they are smaller, lighter and cheaper than the equivalent powered backward curved fan. As shown in Figure 3 and Figure 4, this type of fan impeller will include 20-plus blades that can be as simple as being formed from a single metal sheet. Improved efficiencies are obtained in larger sizes with individual formed blades. The air leaves the blade tips with a high tangential velocity, and this kinetic energy must be converted to static pressure in the casing – this detracts from the efficiency. They are typically used for low to medium air volumes at low pressure (normally <1.5kPa), and have a relatively low efficiency of below 70%. The scroll casing is particularly important to achieve the best efficiency, as the air leaves the tip of the blades at high velocity and is used to effectively convert the kinetic energy into static pressure. They run at low rotational speeds and, hence, mechanical generated noise levels tend to be less than higher- speed backward curved fans. The fan has an overloading power characteristic when operating against low system resistances. These fans are not suitable where, for


example, the air is heavily contaminated with dust or carries entrained grease droplets.


Radial bladed centrifugal fans The radial bladed centrifugal fan has the benefit of being able to move contaminated air particles and at high pressures (in the order of 10kPa) but, running at high speeds, it is very noisy and inefficient (<60%) and so should not be used for general purpose HVAC. It also suffers from an overloading power characteristic – as the system resistance is reduced (maybe by volume control dampers opening), the motor power will rise and, depending on the motor size, may possibly ‘overload’.


Figure 4: Forward curved centrifugal fan with integral motor (Source: EBMpapst)


Plug fans Instead of being mounted in a scroll casing, these purpose-designed centrifugal impellers can be used directly in the casing of the air-handling unit (or, indeed, in any duct or plenum), and their initial cost is likely to be lower than housed centrifugal fans. Known as ‘plenum’, ‘plug’ or simply ‘unhoused’ centrifugal fans, these can provide some space advantages but at the price of lost operating efficiency (with the best efficiencies being similar to that for housed forward curved centrifugal fans).


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