Calculation of air required

It is extremely important for the people who work underground to have fresh air to breathe. If the air is full of diesel gas, blasting gas or dust, there is a large risk that life-threatening accidents/injuries can arise.

Ventilation computations for underground work are normally performed using a computer program. This is based upon SIA 196. It is however quite simple for anyone who wishes to acquire a sense for the magnitude of the amount of air required to perform a simple computation on their own.

In order to efficiently dilute dangerous gasses and dust, a return speed for air in the tunnel of between 0.3 and 0.6 m/s is normally required (if a risk exists of the presence of methane gas, then 1 m/s should be used).

The objective is to determine how much air in m3/s is needed at the bottom of the tunnel in order for the return velocity to be sufficient and in order to be able to dilute gasses so that it is possible to breathe the air without it posing a health hazard.

In order to clear diesel gas out, a minimum of 4 m3 of air/kW/min. is required at the bottom of the tunnel (6 m3/kW/ min. for a motor without a particle filter).

An estimate is made of how many kW are being used simultaneously in the tunnel and then the computation is made to determine the total amount in m3 of air/second.

Each person who is in the tunnel needs 3 - 5 m3 of air/minute and a computation is made to determine how many people are normally in the tunnel at the same time, then the total amount is worked out in m3/minute.

The two results obtained are then added together and approx. 10% is added for blasting gas. The end result is what at a minimum is needed in terms of air at the bottom of the tunnel in m3/minute.

In order to compute how many m3/second have to come out of the fan, the leakage in the ventilation ducts have to be computed.

A well-mounted ventilation line of good quality has a leakage of less than 1%/ 100 metres. If the ventilation line is poor, a higher amount of leakage must be reckoned with. This value is added to the quantity of m3/minute that has been computed for the bottom of the tunnel.

The minimum value arrived at in m3/ minute should then be increased by a margin of safety.

Once you know how much air is needed in terms of m3/second, then you go to the Jensen Technical specification where you can read off the burst pressure point in the duct for different diameters.

Correct computational formulas look like the following:

a) Computational formulas:

Static pressure in the duct:

Δp = λ • Δx/D • ρ/2 Ux


Δx = length (m)

Δp = pressure loss over Δx

λ = average coefficient of friction

D = diameter of the duct (m)

ρ = specific gravity of 1 m3 of air (kg)

Ux = average velocity in the length concerned (m)

b) Dynamic pressure in the duct:

Δp = ρ/2 U2

c) Pressure loss at intake, bends, outlets and also changes of diameter, etc.

For each, we have to calculate:

Δp = ζ • ρ/2 U2

ζ = Zeta coefficient for singular loss

d) In order to arrive at the total pressure (pt) required, we must add:

pt = Δp + Δp dynamic + Δp2 + Δp2 .....

e) Computation of energy consumption for the fan is based upon the following formula:

N = (Q Pfan)/( ηv • ηm)[kW]

Q = airflow from the fan (m3/s)

Pfan = total pressure from the fan (Pa)

ηv = efficiency of the fan (0,4 0,85)

ηm = efficiency of the motor (approx. 0,95)

In order to avoid electricity costs which are too high, the ventilation line must have as large a diameter as possible.

A large diameter makes it easier for the air to flow and requires less power from the fan.

If the pressure in the ventilation line is too high (requires a lot of power from the fan), the diameter must be increased.

If this is not possible due to insufficient room, then two ventilation lines should be mounted instead of one. The same quantity of air in two ducts instead of one duct requires ½ the quantity of air, 1/4 of the pressure and 1/8 of the energy consumption (the power from the fan).

It is always economically sound to invest in high quality for the ventilation line and to treat it well. A good, properly installed and non-leaking ventilation line provides not only a good working environment for the employees, but it also provides such large energy savings that a 20 - 40 % difference in the price of the ventilation line is insignificant.

A true breath of fresh air

Jensen Ventilation Flexible Ducting System, we deliver fresh air to all underground operations around the world.