Gas Fitter Practice Test

If you had a commercial air handler and needed to know the speed of the blower where the pulley on a five-horse power 3,450-rpm motor is 6-inch and the pulley on a two-inch-fifteen-foot drive shaft is 10-inch, what would be the rpm of the blower?

1. 3450 rpm

2. 2070 rpm

3. 1725 rpm

4. 863 rpm

The correct answer is: 2070 rpm

To determine the speed of the blower in this scenario, it's important to understand how pulley sizes affect the rotation speed (RPM) of the driven equipment—in this case, the blower. The principle underlying this concept is that the ratio of the diameters of the pulleys will inversely affect the speed of the rotation. Here, you have a motor with a 6-inch pulley that turns a 10-inch pulley on a drive shaft. The RPM of the motor is 3,450. The formula to calculate the RPM of the blower (or the driven pulley) based on the diameters of the pulleys is as follows: \[ \text{RPM}_{\text{driven}} = \text{RPM}_{\text{driver}} \times \frac{\text{Diameter}_{\text{driver}}}{\text{Diameter}_{\text{driven}}} \] Inserting the known values from the question: 1. RPM of the motor (driver) = 3,450 RPM 2. Diameter of the motor pulley (driver) = 6 inches 3. Diameter of the blower pulley (driven) = 10 inches Now, calculate the blower RPM: \[ \text{RPM}_{\text{