Ventilation calculator
Airflow and air changes per hour
Dilution ventilation
Hood coefficient of entry
Formulas and sources
- Q = V × A
- Air changes per hour = Q × 60 ÷ room volume
- qd = (387 × lb) ÷ (MW × min × d)
- Qd = (qd × Km × 10⁶) ÷ Ca
- Ce = √(VP ÷ |SPh|) = √(1 ÷ (1 + K))
The formula comes from
OSHA Technical Manual, Section III, Chapter 3 (Ventilation Investigation), IV.F.3.d and Appendix III:3-2(opens osha.gov in a new tab)“Velocity = Distance/Time, or V = D/T Squeeze off a quick burst of smoke. Time the smoke plume's travel over a two-foot distance. Calculate the velocity in feet per minute. For example, if it takes two seconds for the smoke to travel two feet, the velocity is 60 fpm. ... Slot velocity is calculated by dividing the total volume flow rate by the slot area”
The formula comes from
OSHA Technical Manual, Section III, Chapter 3, General Exhaust (Dilution) Ventilation Systems and Appendix III:3-2(opens osha.gov in a new tab)“The number of air changes per hour is the number of times one volume of air is replaced in the space per hour. In practice, replacement depends on mixing efficiency. ... ACH, AC/H (air changes per hour): The number of times air is replaced in an hour.”
The formula comes from
OSHA Technical Manual, Section III, Chapter 3 (Ventilation Investigation), General Exhaust (Dilution) Ventilation Systems(opens osha.gov in a new tab)“q d = [(387)(lbs)]/[(MW)(min)(d)] ... Q d = [(q d)(K m)(10 6)]/C a ... K m = mixing factor to account for poor or random mixing (NOTE: K m = 2 to 5; K m = 2 is optimum) C a = acceptable airborne concentration of the material (typically half of the PEL).”
The formula comes from
OSHA Technical Manual, Section III, Chapter 3 (Ventilation Investigation), V. Hoods(opens osha.gov in a new tab)“A hood's ability to convert static pressure to velocity pressure is given by the coefficient of entry (C e), as follows: C e = Q actual / Q ideal = √(VP / |SP h|) = √(1 / (1 + K)) where: K = loss factor VP = velocity pressure in duct SP h = absolute static pressure about 5 duct diameters down the duct from the hood.”
Source
OSHA Technical Manual, Section III, Chapter 3 (Ventilation Investigation), IV.G.1(opens osha.gov in a new tab)“Hood placement must be close to the emission source to be effective. Maximum distance from the emission source should not exceed 1.5 duct diameters.”
FAQ
How do you calculate airflow in cfm?
Multiply the average air velocity in feet per minute by the open area in square feet: Q = V × A. OSHA's Technical Manual uses the same relationship the other way round for slot velocity, the total volume flow rate divided by the slot area.
How do you calculate air changes per hour?
Air changes per hour = Q × 60 ÷ room volume, with Q in cfm and volume in cubic feet. OSHA's Technical Manual defines it as the number of times one volume of air is replaced in the space per hour, and notes that real replacement depends on mixing.
What is the dilution ventilation formula?
First find the evaporation rate, qd = (387 × lb evaporated) ÷ (MW × minutes × d). Then the dilution air is Qd = (qd × Km × 10⁶) ÷ Ca, where Km is a mixing factor of 2 to 5 and Ca is the acceptable concentration in ppm, typically half the PEL. OSHA's worked numbers: 2 lb of toluene over 60 minutes with Km 3 and Ca 50 ppm needs about 8,400 cfm.
What is a hood's coefficient of entry?
Ce is the ratio of actual to ideal flow into a hood: Ce = √(VP ÷ |SPh|) = √(1 ÷ (1 + K)), where VP is duct velocity pressure, SPh is hood static pressure measured about 5 duct diameters downstream and K is the hood loss factor. A higher Ce means a more efficient hood.
How far can a capture hood be from the source?
OSHA's Technical Manual says the maximum distance from the emission source should not exceed 1.5 duct diameters. Capture velocity falls off quickly with distance: about a tenth of duct velocity at one diameter and a hundredth at two.
Units follow the OSHA Technical Manual: cfm, fpm, square feet and inches of water gauge. Dilution ventilation suits low-toxicity vapors released evenly; the manual recommends local exhaust for more hazardous materials and point sources. This tool is for study and estimation, not a substitute for a ventilation engineer's design or a measured survey.