The axial-flow fan is usually direct-driven by mounting the fan blades on the motor shaft. The radial-flow fan is normally belt-driven but can also be direct-driven.

BALANCING THE SYSTEM

Balancing a system basically means sizing the ducts and adjusting the dampers to ensure each room receives the correct amount of air. To balance a system, follow these steps:

1. Inspect the complete system; locate all ducts, openings, and dampers.

2. Open all dampers in the ducts and at the grilles.

3. Check the velocities at each outlet.

4. Measure the "free" grille area.

5. Calculate the volume at each outlet. Velocity x Area = Volume

6. Area in square inches divided by 144 multiplied by feet per minute equals cubic feet/minute.

7. Total the cubic feet/minute.

8. Determine the floor areas of each room. Add to determine total area.

9. Determine the cfm for each room. The area of the room divided by the total floor area multiplied by the total cfm equals cfm for the room.

10. Adjust duct dampers and grille dampers to obtain these values.

11. Recheck all outlet grilles.

In some cases, it may be necessary to overcome excess duct resistance by installing an air duct booster. These are fans used to increase airflow when a duct is too small, too long, or has too many elbows.

Q46. What are the three common types of ducts?

Q47. What are the three types of return air systems?

Q48. Sheet metal ducts expand and contract as they heat and cool. True /False

Q49. What are the three types of dampers?

Q50. Once you have checked the velocities at each outlet, what is the next step when balancing the duct system?