Air Moving Equipment Fundamentals
Reversomatic, Quality fans since 1972
Fan Laws
The simplified form of the most commonly used fan laws include
- CFM varies directly with RPM
CFM1/CFM2 = RPM1/RPM2
- SP varies with the square of the RPM
SP1/SP2 = (RPM1/RPM2)²
- HP varies with the cube of the RPM
HP1/HP2 = (RPM1/RPM2)³
Fan selection criteria
Before selecting a fan the following information is needed:
- Air volume required - CFM
- System resistance -SP
- Air density (Altitude and Temperature)
- Type of service
- environment type
- materials/vapors to be exhausted
- operation temperature
- Space limitations
- Fan type
- Drive type (direct or belt)
- Noise criteria
- Number of fans
- Discharge
- Rotation
- Motor position
- Expected fan life in years
Fan performance tables and curves.
Performance tables provide a simple method of fan selection. However, it is critical to evaluate fan performance curves in the fan selection process as the margin for error is very slim when selecting a fan near the limits of tabular data. The performance curve also is a valuable tool when evaluating fan performance in the field.
Fan testing - laboratory, field
Fans are tested and performance certified under ideal laboratory conditions. When fan performance is measured in field conditions, the difference between the ideal laboratory condition and the actual field installation must be considered. Consideration must be also be given to fan inlet and discharge connections as they will dramatically affect fan performance in the field. If possible, readings must be taken in straight runs of ductwork in order to ensure validity. If this can't be accomplished, motor amperage and fan RPM should be used along with performance curves to estimate fan performance.
Fan troubleshooting guide.
Low Capacity or Pressure
- Incorrect direction of rotation - Make sure the fan rotates in same direction as the arrows on the motor or belt drive assembly.
- Poor fan inlet condition - There should be a straight clear duct at the inlet.
- Improper wheel alignment
Excessive vibration and noise
- Damaged or unbalanced wheel
- Belts too loose; worn or oily belts
- Speed too high
- Incorrect direction of rotation. Make sure the fan rotates in same direction as the arrows on the motor or belt drive assembly.
- Bearings need lubrication or replacement
- Fan surge
Overheated motor
- Motor improperly wired
- Incorrect direction of rotation. Make sure the fan rotates in same direction as the arrows on the motor or belt drive assembly.
- Cooling air diverted or blocked
- Improper inlet clearance
- Incorrect fan RPM
- Incorrect voltage
Overheated bearings
- Improper bearing lubrication
- Excessive belt tension
Ventilation rates for acceptable indoor air quality
| Space | Outdoor air required (CFM/person) | Occupancy (People/1000ft²) |
| Auditoriums |
15 |
150 |
| Ballrooms/Discos | 25 |
100 |
| Bars | 30 |
100 |
| Beauty shops | 25 |
25 |
| Classrooms | 15 |
50 |
| Conference rooms | 20 |
50 |
| Correctional facility cells | 20 |
20 |
| Dormitory sleeping rooms | 15 |
20 |
| Dry cleaners | 30 |
30 |
| Gambling casinos | 30 |
120 |
| Game rooms | 25 |
70 |
| Hardware Stores | 15 |
8 |
| Hospital Operating Rooms | 30 |
20 |
| Hospital Patient Rooms | 25 |
10 |
| Laboratories | 20 |
30 |
| Libraries | 15 |
20 |
| Medical Procedure rooms | 15 |
20 |
| Office Spaces | 20 |
7 |
| Pharmacies | 15 |
20 |
| Photo Studios | 15 |
10 |
| Physical Therapy | 15 |
20 |
| Restaurant Dining Areas | 20 |
70 |
| Retail Facilities | 15 |
20 |
| Smoking Lounges | 60 |
70 |
| Sporting Spectator Areas | 15 |
150 |
| Supermarkets | 15 |
8 |
| Theaters | 15 |
150 |
Copyright (c) 2009 Reversomatic.