Air velocity CFM
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Here is a complete overview of air velocity and CFM:
Air Velocity & CFM (Cubic Feet per Minute)
What Is CFM?
CFM measures volumetric airflow - the volume of air moving past a point per minute. It combines both the speed of the air (velocity) and the size of the opening it passes through.
Core Formula
CFM = Velocity (FPM) × Area (sq ft)
Where:
- CFM = Cubic Feet per Minute (airflow volume)
- FPM = Feet Per Minute (air speed/velocity)
- Area = Cross-sectional area of the duct or opening (sq ft)
Rearranged forms:
- Velocity (FPM) = CFM ÷ Area
- Area (sq ft) = CFM ÷ Velocity
How to Find Duct Area
| Duct Type | Formula |
|---|---|
| Round duct (diameter d in inches) | Area = π × (d/24)² |
| Rectangular duct (W × H in inches) | Area = (W × H) / 144 |
Example - Round duct, 10" diameter, 600 FPM:
- Area = π × (5/12)² = 0.545 sq ft
- CFM = 600 × 0.545 = 327 CFM
Room Ventilation Formula
CFM = (Room Volume in ft³ × ACH) ÷ 60
Where ACH = Air Changes per Hour (recommended values by room type):
| Room Type | Recommended ACH |
|---|---|
| Bedroom | 5 - 6 |
| Living room | 6 - 8 |
| Kitchen | 7 - 8 |
| Bathroom | 7 - 8 |
| Laundry room | 8 - 9 |
| Attic (with A/C) | 12 - 15 |
Example: 12 × 14 ft room, 9 ft ceiling, 6 ACH
- Volume = 12 × 14 × 9 = 1,512 ft³
- CFM = (1,512 × 6) ÷ 60 = 151 CFM
Velocity from Velocity Pressure (Standard Air)
For standard air (70°F @ 29.92" Hg):
V (FPM) = 4005 × √VP
Where VP = velocity pressure in inches of water column (measured with a pitot tube or manometer).
For non-standard air (density correction):
V (FPM) = 1096.7 × √(VP / density)
ASHRAE Recommended Duct Velocities
| Application | Velocity (FPM) |
|---|---|
| Residential supply ducts | 600 - 900 |
| Commercial supply ducts | 1,000 - 1,500 |
| Main trunk ducts | 950 - 3,500 |
| Return air ducts | 600 - 1,200 |
- Too low → poor distribution, stagnant zones
- Too high → noise, pressure drop, energy waste
Fan Laws (Scaling CFM)
| Fan Law | Formula |
|---|---|
| CFM scales with RPM | CFM₂ = CFM₁ × (RPM₂ / RPM₁) |
| Static pressure scales with RPM² | SP₂ = SP₁ × (RPM₂ / RPM₁)² |
| BHP scales with RPM³ | BHP₂ = BHP₁ × (RPM₂ / RPM₁)³ |
Measurement Tools
- Anemometer - measures air velocity directly in FPM or m/s
- Pitot tube + manometer - measures velocity pressure to derive FPM
- Balometer/flow hood - measures CFM directly at a grille or diffuser
Quick Conversions
| From | To | Multiply by |
|---|---|---|
| CFM | m³/hour | × 1.699 |
| CFM | L/s | × 0.472 |
| FPM | m/s | × 0.00508 |
If you have a specific scenario (duct size, fan selection, room calculation, etc.), share the details and I can work through the numbers for you.
Air velocity Community and Family medicine
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I now have all the content needed from Park's Textbook. Here is the complete answer from a Community Medicine perspective:
Air Velocity - Community & Family Medicine (Park's Textbook)
Definition
Air velocity refers to the speed of air movement and is one of the four key physical factors - along with temperature, humidity, and heat radiation - that determine the "cooling power" of the air on the human body. As Professor Lee stated:
"The problems of ventilation are physical, not chemical; cutaneous not respiratory."
Measurement of Air Velocity
1. Anemometer (for outdoor/high velocities)
- Consists of four hemispherical cups attached to the ends of 2 crossed metal arms, with a vertical spindle connected to an "anemometer box"
- Wind velocity is recorded on a counter called the cyclometer
- Wind velocities are recorded in open flat country at a height of 10 m
- Velocities measured in metres per second (m/s)
| Wind Speed | Description |
|---|---|
| 0.5 m/s | Complete calm - smoke rises vertically |
| 3.3 m/s | Slight breeze - leaves rustling |
| 10 m/s | Strong wind - larger branches moving |
| 15-20 m/s | Storm |
| 25-30 m/s | Gale |
| >30-50 m/s | Hurricane |
2. Kata Thermometer (for low/indoor air velocities)
- The instrument of choice for measuring low air velocities (e.g., in rooms, workplaces)
- Can record air velocities as low as 10 feet per minute
- An alcohol thermometer with a glass bulb 4 cm long, 1.8 cm in diameter
- Readings marked from 100°F to 95°F on the stem
- Two types used: dry Kata and wet Kata (bulb covered with wet muslin)
Procedure:
- Immerse bulb in hot water (above 130°F) until alcohol rises to the reservoir at top
- Suspend in air at point of observation
- Record time (seconds) for spirit to fall from 100°F to 95°F using a stopwatch
- Repeat at least 4 times; discard the first reading; average the last three
- Divide the Kata Factor (marked on stem by manufacturer) by the average cooling time = rate of cooling in millicalories per cm² per second
Types by cooling range:
| Type | Cooling Range | Alcohol Colour |
|---|---|---|
| Standard Kata | 100°F - 95°F | Red |
| High Temperature Kata | 130°F - 125°F | Dark blue |
| Extra High Temperature Kata | 150°F - 145°F | Magenta |
Note: In tropical countries where atmospheric temperature may be above 100°F, the standard Kata cannot be used - the High or Extra High Temperature Kata must be used instead. Kata thermometers have silvered bulbs to reduce errors due to radiation.
Air Velocity in Thermal Comfort
Air velocity is one of the four factors of thermal comfort (alongside temperature, humidity, and radiant heat). It contributes to the "cooling power" of the air.
Indices of Thermal Comfort (in order of evolution):
| Index | Components | Notes |
|---|---|---|
| Air temperature alone | Temperature | Inadequate alone |
| Temp + humidity | Temperature + humidity | Still unsatisfactory |
| Cooling power (Kata) | Temp + humidity + air movement | Dry Kata ≥6, Wet Kata ≥20 = comfort |
| Effective Temperature (ET) | Temp + humidity + air movement | Developed 1923 by Houghton & Yaglou (Pittsburgh Lab, ASHVE) |
| Corrected Effective Temperature (CET) | Temp + velocity + humidity + mean radiant heat | Most complete index; uses Globe thermometer |
- Effective Temperature (ET): The temperature of still, saturated air that induces the same sensation of warmth/cold as the actual environment. Obtained from special charts using the three variables.
- Corrected Effective Temperature (CET): Improvement over ET; replaces dry-bulb temperature with the Globe thermometer reading to account for radiant heat. Preferred when a radiation source is present.
Wind Direction
Measured by a wind vane - an arrow that turns freely on a vertical axis, erected at 10 m above ground level. If motionless for 3 minutes, wind is described as "calm." Directions grouped into 4 main (N, E, S, W) and 8 or 16 sub-categories.
Source: Park's Textbook of Preventive and Social Medicine, p. 833 & 851
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