Air Flow Calculator

The Air Flow Pressure Calculator developed by us can help the user understand operation’s airflow issues, in pipelines, velocity tunnels, and other related instruments. The user can also find effective solutions to reduce maintenance in pipelines, velocity tunnels, eliminate hose malfunctions, avoid costly downtime, and improve operational efficiency in the overall system combined.

What is meant by Air Flow?

The Airflow as the name suggests helps to calculate the amount and pressure of air through a small opening such as in pipelines, velocity tunnels. This system works on majorly the following factors, which are:

  • Atmospheric Pressure also referred to as barometric pressure, is the pressure within the atmosphere of Earth within a certain range of kilometers. A standard atmosphere is a unit of pressure defined as 101,325 Pa, which is equivalent to 760 mm Hg. And Atmospheric pressure is the pressure caused by the mass of our gaseous atmosphere per unit area. It can be measured using mercury in the equation which is atmospheric pressure = density of mercury X acceleration due to gravity X height of a column of mercury.
  • Air Temperature is a measure of how hot or cold the air is. It is the most commonly measured weather parameter for calculating the temperature. More specifically, temperature describes the kinetic energy, or energy of motion, of the gasses that make up the air by their continuous motion and colliding nature. As gas molecules move more quickly, air temperature increases.
  • Air Velocity (distance traveled per unit of time) is most often expressed in feet per minute (FPM). Air velocity is measured by sensing the pressure that is produced through the movement of the air moving in one particular direction. Velocity is also related to air density with assumed constants of 70° F and 29.92 in Hg.

The values of p and ρ obtained by assuming g = 0, g are at an altitude slightly different from the geometrical altitude (hG) in the motion of the current of the air. This altitude is called geopotential altitude, which for convenience is denoted by ‘h’. The geopotential altitude can be defined as the height above the earth’s surface in units, proportional to the potential energy of unit mass (geopotential), relative to sea level. And this is relative to the flow of air in the atmosphere It can be shown that the geopotential altitude (h) is given, in terms of geometric altitude (hG).

Solved Examples on Air Flow

Question 1: Calculate the atmospheric temperature (T), pressure (p), density (ρ ), pressure ratio (δ ) , density ratio (σ ), speed of sound (a), coefficient of viscosity (μ ) and kinematic viscosity ( ) in ISA at altitudes of 8 km, 16 km, and 24 km.


It may be noted that the three altitudes specified in this example, viz. 8 km, 16 km, and 24 km, lie in the troposphere, lower stratosphere, and middle stratosphere regions of ISA respectively, hence airflow at that height is:

H(height) = 8 km 

Let the quantities be at 8 km

In troposphere, according to adiabatic law of expansion.

T=T - 0 λh


T0 = 288.15 K, λ = 0.0065 K /m 

Hence, T = 288.15 - 0.0065 8000 = 236.15K 

Directly using eqn of troposphere pressure:

p = δ = T/T

Substituting the values

236.15/288.15 = 0.35134 p       

Or 2 p = 0.35134 × 101325 = 35599.5 N/m 

ρ /ρ = 0.52516/1.225 = 0.42870 

Using eqn of adiabatic expansion:

P= (γ RT) 

where γ stands for standard cofficient,

Subsituting the values

=236.15 μ 

=1.5268×10 kg m s 

= 2.9072×10 m /s

By using a scientific calculator to further study the approximations:

In troposphere (h = 0 to 11000 m):

T= 288.15 - 0.0065 h. 

ρ = 1.225 [1-0.000022588h]

In lower stratosphere (h = 11000 to 20000 km): 

T=216.65 K. 

p = 22632 exp {-0.000157688 (h-11000)} 

ρ = 0.36391 exp {-0.000157688 (h-11000)}

In middle stratosphere (h = 20000 to 32000 km): 

T = 216.65 + 0.001h 

p = 5474.9 [1+0.000004616(h-20000)]-34.1632 

ρ = 0.08803 [1+0.000004616(h-20000)]-35.16

Air Flow Calculator

FAQs on Air Flow Calculator

1. How do you calculate the airflow rate?

Air velocity (distance traveled per unit of time) is usually expressed in Linear Feet per Minute (LFM). By multiplying air velocity by the cross-section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in Cubic Feet per Minute (CFM).

2. How do you calculate the CFM of airflow?

CFM = (fpm * area), where fpm is the feet per minute. To find the cubic feet per minute, substitute the FPM value with the area after the area is squared.

3. What is the normal airflow rate?

Normal flow rate is 1 atmosphere (101.3 kPa) or 14.696 psia at 32 0F (0 0C). The actual flow rate is the actual volume of fluid that passes a given point based on the given pressure and temperature of the process.

4. How do you calculate air flow through an opening?

Calculate airflow in a duct by measuring the airflow velocity in feet per minute (FPM) and multiplying by the duct cross-sectional area in square feet (ft2).

5. How do you calculate airflow in HVAC?

The calculation for CFM involves dividing the total volume of the space by the air exchange interval.