SYSTEM SURGE, FAN SURGE AND PARALLELING The three main reasons for unstable airflow in a fan systems are (1) System surge, (2) Fan Surge and (3) Paralleling. System Surge System surge occurs when the system resistance and fan performance curves do not intersect at a distinct point but rather over a range of volumes and pressures. This situation does not occur with backward inclined (BI), airfoil (AF), and radial fans. However, it can occur with a forward curve centrifugal fan when operating, as shown in Fig. 22. In this situation, because the fan curve and system curve are almost parallel, the operating point can be over a range of airflow and static pressures. This will result in unstable operation known as system surge, pulsation, or pumping.

Fig. 22 System Surge

System surge should not be confused with “paralleling,” which can only occur when two fans are installed in parallel.

Fig. 23 Fan Surge Explanation

Fan Surge Fan surge is different from system surge, they may or may not occur at the same time. (Fig.23) For any fan, the point of minimum pressure occurs at the center of rotation of the fan wheel and the maximum pressure occurs just at the discharge side of the wheel. If the wheel were not turning and this pressure differential existed, flow would be from the high pressure point to the low pressure point. This is opposite from the direction air normally flows 18

through the fan. The only thing that keeps the air moving in the proper direction is the whirling of the blades. Stall occurs unless there is sufficient air entering the fan wheel to completely fill the space between the blades. This shows up in Fig. 24 as fluctuation in air volume and pressure. This surge can both felt and heard and occurs in nearly all fan types, to varying degrees, as block-tight static pressure is approached. The radial blade is a notable exception. While the magnitude of surge varies for different type of fans, (being greatest for airfoil and least for forward curve),the pressure fluctuation close to block-tight may be on the order of 10%. For example, a fan in surge developing about 600 Pa of total static pressure might have pressure fluctuation of 600/10 of an Pa. This explains why a large fan in surge is in tolerable. Equipment room walls have been cracked from the vibration of ducts serving a fan in surge.

Fig. 24 Fan Surge

Selections should not be made to the left of the “surge point” on the fan curve. This point, which defines a system curve when all operating speeds of the fan are considered, varies for different fan installations. For instance, stable operation can be obtained much further to the left when the fan is installed in an ideal laboratory type situation. These conditions, of course are seldom encountered in field applications. Consequently, most manufacturers do not catalogue operating ranges all the way to the surge line. However, since the catalogue cut-off point is basically one of engineering judgment, conservative catalogue performance data will provide operating ranges, which will allow stable operation with any reasonable field ductwork design.

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PARALLELING The third cause for unstable operation is paralleling, (Fig. 25), which can occur only in a

multiple fan installation connected with either a common inlet or common discharge, or both in the same system, particularly when large volume of air must be removed. The combined air flow-pressure curve in this case is obtained by adding the airflow capacity of each fan at the same pressure. (Fig. 26)

Fig. 25 Unbalanced Parallel Operation

Fig. 26 Parallel Fan Operation 20

The total performance of the multiple fans will be less than the theoretical sum it inlet condition are restricted or the flow into the inlets is not straight. Some fans have a “positive” slope in the pressure-air volume curve to the left of the peak pressure point. If fans operating in parallel are selected in the region of this “positive” slope, unstable operation may result. The closed loop to the left of the peak pressure point is the result of plotting all the possible combinations of air volume at each pressure. If the system curve intersects the combined air volume pressure curve in the area enclosed by the loop, more than one point of operation is possible. This may cause one of the fans to handle more of the air and could cause a motor overload if the fans are individually driven. This unbalanced flow condition tends to reverse readily the result that the fans will intermittently load and unload. This “pulsing” often generates noise and vibration and may cause damage to the fans, ductwork or driving motors.

Fig. 27 Effect of Scroll Dampers

Fig. 28 Elimenation of Paralleling This requires the installation of scroll volume (outlet volume) dampers (Fig. 27). It serves to change the shape of the fan scroll and thus, for each position of the damper, there is a corresponding different performance curve. The fan curve resulting from various positions of the outlet volume dampers is shown in Fig. 28. The purpose is to change the fan curve sufficiently such that the sum of the 21

difference curve will intersect the single fan curve at A’ and provide stable operation. The performance may be reduced slightly and a corresponding increase in RPM should be made to achieve the specified conditions. However, this is rarely done since difference is typically negligible. To correct, the scroll volume damper is merely pushed down on both fans until the static pressure and noise level pulsation disappear. Generally, they are then left in this position permanently. The curve generated by the damper at this point is so shaped that the sum of the differences curve intersects at only one point. Fans operated in parallel should be of the same type, size and speed otherwise undesirable performance complications may result. It is strongly advised that the advice of the fan manufacturer be sought when considering the use of fans in parallel. The use of axial flow fans in parallel presents very real potential noise problems unless special measures are taken at the design stage; add-on noise control is not normally possible. A noise problem often encountered with fans operating in parallel is beating. This is caused by slight difference in speed of rotation of the two theoretically identical fans. The resulting low frequency beating noise can be very annoying and difficult to eliminate. The problem can be likened to the stroboscopic effect of a fluorescent light illuminating a rotating wheel with a slight difference between the frequencies of rotating of the wheel and the AC supply to the light.

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