The Exhaust Velocity Calculator is a technical tool that determines the speed at which air or gas exits a duct, pipe, or ventilation system. It is essential in HVAC design, industrial exhaust systems, and safety ventilation planning. Knowing the velocity of exhaust gases helps engineers optimize fan selection, reduce pressure loss, and ensure compliance with ventilation standards.
This calculator is particularly useful for professionals working on fume hoods, dust collection systems, laboratory exhausts, and commercial kitchen ventilation. Proper exhaust velocity is vital to prevent backflow, maintain system efficiency, and protect indoor air quality.
Formula of Exhaust Velocity Calculator
Exhaust Velocity = Exhaust Flow Rate / Duct Cross-Sectional Area
Where:
- Exhaust Velocity is expressed in meters per second (m/s) or feet per second (ft/s)
- Exhaust Flow Rate is the volumetric flow (e.g., cubic meters per second or cubic feet per minute)
- Duct Cross-Sectional Area is in square meters (m²) or square feet (ft²)
Sub-Formulas:
For circular ducts:
Duct Area = π × (Diameter / 2)²
For rectangular ducts:
Duct Area = Width × Height
When flow rate is in CFM (cubic feet per minute):
Exhaust Velocity (ft/s) = CFM / (Area in ft² × 60)
This converts flow rate from per minute to per second, matching the velocity unit of ft/s.
Exhaust Velocity Reference Table
Below is a helpful table showing approximate velocities for common duct sizes and flow rates. This can guide engineers during preliminary design without full calculation.
| Duct Type | Size (in) | Flow Rate (CFM) | Duct Area (ft²) | Velocity (ft/s) |
|---|---|---|---|---|
| Circular | 8" diameter | 500 | 0.349 | 23.9 |
| Circular | 10" diameter | 750 | 0.545 | 22.9 |
| Circular | 12" diameter | 1000 | 0.785 | 21.2 |
| Rectangular | 12" × 6" | 800 | 0.5 | 26.7 |
| Rectangular | 24" × 8" | 1600 | 1.333 | 20.0 |
These values help verify that air is moving at proper speeds to prevent accumulation of hazardous fumes or excessive noise.
Example of Exhaust Velocity Calculator
Let’s say you are designing an exhaust system for a laboratory hood. The exhaust duct is 10 inches in diameter and handles 700 CFM of air.
Step 1:
Convert the diameter to feet:
10 in = 0.833 ft
Area = π × (0.833 / 2)² ≈ 0.545 ft²
Step 2:
Use the velocity formula:
Exhaust Velocity = 700 / (0.545 × 60) ≈ 700 / 32.7 ≈ 21.4 ft/s
So, the exhaust velocity is approximately 21.4 feet per second. This value can now be checked against design requirements or safety guidelines for proper ventilation.
Most Common FAQs
It belongs to the HVAC, mechanical design, and industrial ventilation calculators category. It is widely used in professional engineering and system design.
It ensures gases, smoke, or vapors are effectively carried through and out of the ductwork without stagnation or backdraft. It also affects fan performance, energy use, and system safety.
Yes. This tool is ideal for any exhaust system where maintaining a minimum airflow speed is crucial, including fume hoods, range hoods, and cleanroom exhausts. Always check the required velocity thresholds for your application.