Fan characteristic curve

In practice, the fan characteristic curve—i.e., the total pressure difference dependency on the volume flow—is determined experimentally. The measured results are then for the impeller and casing together. Since the losses are greater outside the design point, the fan efficiency is high at the design point. 

The fan volume flow q, and its corresponding Ap,g, can be found when a Ap,o( - chart is drawn the duct parabola and experimental Ap, t both equal f(q ) (Eig. 9.47). The experimental curve Apj j = f q ) is called the fan characteristic curve, and the duct static pressure drop dependency on the duct volume flow is the characteristic curve. The characteristic curve intersection point is called fan operating point. 

Consider the fan characteristic curves for two different rotational velocities ft] and n-,. Select the operating point for the characteristic curve as The corresponding point is q,i) for characteristic curve n. ... 

FIGURE 9.53 Fan characteristic curve for two rotational velocities, n, and rij, and three fan affinity parabolas. 

A fen can operate only along its characteristic curve, but after that fan is placed in a fixed system, it can operate only at the one point where pressure-volume conditions match the pressure-volume system curve calculated based on the system resistance, see Figure 12-134. Thus, if the fan characteristic curve is superimposed on the plot of the system, the point of intersection will be the point of operation. To change this point requires changing at least one condition on the fan or the system. 

Selection should be made for minimum power input, which is also likely to be the quietest fan for the duty. Performance is usually quoted for a standard condition of 1.2 m /kg. Calculations of system resistance are best carried out at the same condition. The user may find only the static pressure quoted. If total or velocity pressure are also quoted or the outlet velocity can be calculated the designer can calculate how much pressure can be recovered after the exit. Any mismatch due to difficulty in calculating system resistance will cause the volume to rise or fall, to settle on the fan characteristic curve. 

Figure 5.6 Fan characteristic curves. From Ref. 7 with permission.

They are normally fitted with 10-12 blades, the blade tips inclined away from the angle of rotation. They have the advantage of being very efficient, with fan static efficiencies in excess of 80% for airfoil blades. This type of fan has a nonoverloading characteristic curve. 

Figure 9.42 shows the typical characteristic curve of a centrifugal fan, where the blades are backward curved. The figure also shows the theoretical characteristic curve when the slip factor is 1 and when it is smaller than 1. Characteristic curves for a real fan are closer to the isentropic one at the design point. At this point the efficiency is maximum. 

Solution. Draw into the iv diagram the characteristic curve of the fan and the duct-pressure-drop volume flow dependency. The latter is a parabola passing through the origin with the following equation ... 

In a Ap-q, chart, the characteristic curve of the ducts is drawn. In the same chart is drawn the characteristic curve of both fans. At each volume flow q, the total pressure of each fan is added. In this w ay, we get a new - q ... 

The speed of an electric motor can be changed by altering the frequency of the electric current. This is because the ratio is the same as 60 or 50 f/p (f = the frequency of the current, p = the number of poles in the stator). Frequency converters are built of electronic components, frequently combined with microprocessors. They provide good motor protection and are superior to the traditional bimetal protection. The characteristic curve for a pump and fan motor is also quadratic, making lower demands to the frequency converters When the frequency of the electrical current is changed in the frequency converter, the main AC supply is transformed into DC. The DC is then treated... 

Figure 12-130A. Characteristic curves for straight radial blade. (Used by permission The Howden Fan Company.)...

Note that this less dense air requires less horsepower, and also that the fan can produce only 1.66 in. static as compared to 2 in. with standard air. The system resistance must he adjusted to accommodate this lower static pressure otherwise, the fan will follow its characteristic curve hy reducing its flow until it discharges at the static pressure of 1.66 in. 

The working pressure limits of the propeller fan, depending on its diameter, are of the order of 150 Pa. The characteristic curve has a pronounced trough , which should be avoided in application if at all possible, since wide variations in air flow can occur for a small change in pressure. Performance varies with aperture shape, clearance and position. 

FIG. 10-87 Approximate characteristic curves of various types of fans. 

Centrifugal fans are classified according to their blade geometry - radial, forward curved, backward curved, and air foil. The radial fan s major characteristic is its ability to compress gases to a higher pressure but delivers lower flow rates than the other fan types. Its characteristic curve is shown in Figure 5.6. The... 

The forward-curved-blade fan is designed for low to medium flow rates at low pressures. Because of the cupped shaped blades, solids tend to be held in the fan, and thus this fan is also not suitable for pneumatic conveying of solids. In the characteristic curve for the fan, shown in Figure 5.6, there is a region of instability to the left of the pressure peak. Thus, the fan must be operated to the right of that region. The horsepower increases continuously with increasing flow rate. 

Method of Using Characteristic Curves.—To illustrate the use of these characteristic curves a problem may be taken, as follows What will be the size of a fan with corrugated blades of the type covered by Fig. 12 which will be required to deliver 50,000 cu. ft. per minute against 1.5 in. static pressure, the fan to operate at approximately maximum mechanical efficiency The diameter of the fan found by calculation will vary according to the point on the curves of Fig. 12 taken as a basis, and will... 

Positive-displacement fans, blowers, and eompressors are similar in action to positive-displacement pumps, and include reciprocating compressors, two- or three-lobe blowers, and screw compressors. However, with gases, the almost vertical characteristic curves, bend to the left more than for liquids because of the greater tendency for slip. 

The performance of a fan is characterized by its characteristic curve and represents the relation between the overall pressure rise and the inlet volume flow rate for a certain speed of the fan. It is known also as the pressure-volume curve. The appropriate characteristic chart showing these curves for various speeds, as well as the power consumption and the efficiency as functions of the flow rate, should be available with every fan. 

As the air pressure to be developed by the fans varies greatly with the specific load of the cooler, fans having not too flat a characteristic curve (pressure-volume curve) should be chosen. Simple damper control is uneconomical it is preferable to use inlet vane control. 

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