FAN TYPES In order to cover a wide range of applications, fans are manufactured in a variety of type. They can be classified under three general types : (a) Centrifugal, (b) Axial and (c) Mixed flow. Table 1 compares the typical characteristics of some of the most common fan types. CENTRIFUGAL FAN A fan in which the air enters the impeller axially and is discharge radially into a volute-type casing. Centrifugal fans are divided into four general classifications: Forward Curve (FC), Backward Inclined (BI), Radial Blade, and Tubular Centrifugal. The speed for a given type centrifugal fan impeller is determined by the tip speed necessary to produce the required absolute particle velocity (Fig.29). This absolute particle velocity vector relative to ground (S) has two components, one radial (r) and the other tangential (t) to the wheel. Fig. 29 Centrifugal Fan Wheels The velocity of the air relative to the blade is indicated by the blade vector (B) which is nearly tangential to the blade though some slip occurs. The length of the tip speed vector (R) as shown in the diagram, indicates the relative wheel RPM to produce a given capacity. By examining the relative length of the R vector, it can be seen that the FC fan requires the lowest tip speed for a given capacity, while the BI requires the highest tip speed. Forward Curve Centrifugal Fan The forward curve centrifugal fan travels at a relatively slow speed and generally is used for producing high volumes at low static pressure. 23
Fig. 30 Charestic Curve for FC fans Typical operating range of this type of fan is from 30% to 80% wide-open volume (Fig. 30). The maximum static efficiency of 60-68% generally occurs slightly to the right of peak static pressure. The horsepower curve has an increasing slope and is referred to as an overloading type. The FC fan will surge but the magnitude is typically less than for other types. Advantages of the FC fan are low cost, slow speed which minimizes shaft and bearing size and wide operating range. The disadvantages are the shape of its performance curve which allows the possibility of paralleling, and an overloading of the motor may occur if system static pressure decreases. Also, it is not suitable for material handling because of its blade configuration. It is inherently weak structurally. Therefore, forward curved fans are generally not capable of the high speeds necessary for developing higher static pressures. Backward Inclined Fan Backward inclined fans travel about twice the speed of the forward curved fan as previously indicated by the velocity vector diagram. The normal selection range of the backward inclined fan is approximately 40-85% of wide open CMH. (See Fig. 31). Maximum static efficiency of about 80% generally occurs close to the edge of its normal 24
Fig. 31 Characteristic curve for BI fan operating range. Generally, the larger the fan the more efficient for a given selection. The magnitude of surge for a BI fan is greater than for an FC fan. Advantages of the BI fan are higher efficiency and non-overloading power curve. The power curve generally reaches a maximum in the middle of the normal operating range, thus overloading is normally not a problem. Inherently stronger design makes it suitable for higher static pressure operation. The BI fan s disadvantages include first, the higher speed which requires larger shaft and bearing sizes and places more importance on proper balance, and secondly, unstable operation occurs as block-tight static pressure is approached. This fan is also unsuitable for material handling. A refinement of the flat-blade, backward inclined fan makes use of airfoil shaped blades. This improves the static efficiency to about 86% and reduces noise level slightly. The magnitude of surge also increases with the airfoil blades. Characteristic curves for airfoil fans are shown in Fig. 32. 25
Fig. 32 Charateristic Curve for Airofoil Fan Radial Blade Fans Radial blade fans (Fig. 33) are generally narrower than other types of centrifugal fans. Consequently, they require a larger diameter wheel for a given capacity. This increases the cost and is the main reason why they are not used for air conditioning duty. The radial blade fan is well suited for handling low volumes at relatively high static pressures and for material handling. Absence of surge and a nearly straight power curve with linear relationship with CMH are its other advantages. This proportional relationship allows capacity control to be actuated from motor power input. Disadvantages of this type of fan are higher cost and lower efficiency. Fig. 33 Characteristic Curve for Radial Blade Fan 26
Tubular Centrifugal Fans Tubular centrifugal fans, illustrated in Fig. 34, generally consist of a single width airfoil or BI wheel arranged in a cylinder to discharge air radially against the inside of the cylinder. Air is then deflected parallel with the fan shaft to provide straightthrough flow. Vanes are used to recover static pressure and straighten air flow. Characteristic curves are shown in Fig. 35. The selection range is generally about the same as the scroll type BI or airfoilblade wheel, 50-85% of wide open volume. However, because there is no housing of the turbulent air flow path through the fan, static efficiency is reduced to a maximum of about 72% and the noise level is increased. Fig. 34 Tubular Centrifugal Fan Frequently, the straight-through flow results in significant space saving. This is the main advantage of tubular centrifugal fans. Fig. 35 Characteristic Curve For Tabular Centrifugal Fan AXIAL FANS Axial fans are divided into three groups - propeller, tubeaxial, and vaneaxial. The propeller fan (Fig. 36) is well suited for high volumes of air at little or no static pressure Tubeaxial and vaneaxial fans (Fig. 37), are simply propeller fans mounted in a cylinder and differential. are similar except for vane type straighteners on the vaneaxial. These vanes remove much of the swirl from the air and improve efficiency. Thus, a vaneaxial fan is more efficient than a tubeaxial and can reach higher pressures. Note that with axial fans the BkW is maximum at block-tight static pressure. With centrifugal fans the BkW is minimum at block-tight static preaure. 27
Advantages of tube and vaneaxial flow fans are the reduced size and weight and straight-through air flow which frequently eliminates elbows in the ductwork. The maximum static efficiency of an industrial vaneaxial fan is approximately 85%. The operating range for axial fans is from about 65% to 90%. The disadvantages of axial fans are high noise level and lower efficiency than centrifugal fans. In recent years, more sophisticated design of vaneaxial fans has made it possible to use these fans at pressures comparable to those developed by the airfoil backward inclined fans, with equal overall efficiency. These fans have variable pitch blades which can be activated by an external operator. For large-size fans requiring motor power above 75 kw, it is comparatively simple to change the fan characteristics by using either a manual or pneumatic controller. The disadvantage of these fans is their high noise level; sound traps are generally required both upstream and downstream. Despite this added acoustical requirement, the initial cost of these fans compares favorably with the airfoil BI fans Fig. 36 characteristic curve - Propeller MIXED FLOW FANS This type of fan has an airpath through the impeller which is between that of the axial and centrifugal types. It is capable of being constructed to provide either axial or radial discharge and produces more head than a comparable flow fan. (Fig. 38) Fig. 37 characteristc curve - Vane Axial Fig. 38 Mixed Flow Fan 28