October 1996, Volume 6

If you have ever worked in an industrial plant, you have probably encountered a situation involving some difficulty in starting a motor driven centrifugal fan. This problem is especially prevalent with fans developing high pressure, since high pressure is created by increasing tip speed and subsequently increasing impeller diameter. A larger diameter impeller is more difficult to start with a fixed speed electric motor than a smaller diameter impeller.

An electric motor which is direct coupled to a centrifugal fan must overcome the fan's inertial resistance and aerodynamic resistance. This latter load can generally be minimized or eliminated if an adequately sealing control damper is furnished as an integral part of the fan (see Figure 1). However, little can be done to minimize the inertial start-up load which is commonly referred to as the "WR²" load and which is determined by the wheel design.

Figure 1

Higher WR² values are associated with impellers having mass concentrated near the periphery of the wheel such as the airfoil fan impeller. A radial blade fan impeller has considerable mass concentrated near the center of rotation and for a given condition can commonly have a lower WR² value (see Figure 2). Of course, such factors as wear pad design and material of construction play a significant role in ultimately determining the final WR² value.

Figure 2

A fan should always be started with the inlet or outlet dampers closed. If start-up difficulty occurs, the aerodynamic load factor should be investigated because it is sometimes difficult to perceive that so much leakage can occur with the damper in a closed position. With a damper closed, the fan is developing low amounts of flow and high suction downstream of the inlet damper. The pressure at the inlet of the fan can sometimes exceed twice the rated pressure of the fan with the dampers in the closed position. Because of this high pressure differential across the inlet damper, a small of gap at the damper blade edges will result in excessive leakage and high start-up aerodynamic load. Not much leakage is required to elevate motor start-up load.

Also, during motor start-up and immediately after the fan is at synchronous speed, the motor amps will remain high even with the damper closed. This phenomenon is a result of lagging power factor and occurs when the motor is lightly loaded. In other words, during the transient time after start-up and until the fan dampers are opened, the motor amp reading is not a true indication of motor horsepower.

Figure 3 is a typical fan performance curve for a pressure fan providing air to an industrial process. This fan is designed to develop 70,000 ACFM at 30 in. w.g. of static pressure. However, if the dampers are leaking during startup, the aerodynamic load can be significant, because in this case the leakage rate through the closed damper can easily be 5000 to 10,000 ACFM. In conclusion, a fan developing high pressure should be furnished with a tight sealing inlet damper if start-up problems are to be prevented.

Figure 3

The second factor affecting start-up is the inertial start-up load which is determined by the fan designer. This is always specified in the fan catalog or in the fan general arrangement drawing. This WR² start-up requirement of the fan rotor must be matched with the start-up capability of the electric motor. The electric motor manufacturer must be consulted to determine motor start-up capability. If these two values are matched, a fan rotor should be brought up to rotating speed within five to ten seconds, which is commonly considered acceptable. During this transient start-up condition, the motor amps to the windings of the motor can sometimes exceed 3 to 4 times the nameplate value. Obviously, considerable heating occurs during the start-up condition, so it is for that reason that the start-up period is kept to a minimum. Type motor windings, motor construction, and the type of insulation are all variables that the motor designer utilizes to increase WR² starting capability.

Figure 4 and 5 are typical motor torque curves which can be combined with the fan torque curves (Figure 6) to ultimately determine start-up time. This numerical calculation is not included in the scope of this newsletter since it is generally performed by the motor manufacturer.

Figure 4Figure 5

Figure 6

In conclusion, if you experience start-up problems with a fan, you should immediately check to make sure that a minimum air flow occurs during start-up. Also, the WR² starting capability of the motor should be evaluated and compared to the WR² starting requirement of the fan rotor. Both of these values are established by the fan and motor manufacturer respectively.

For additional information on articles appearing in this newsletter, contact Process Equipment Company at (205) 663-5330 or E-mail to: information@process-equip.com