Explanation of secondary (or auxiliary) ventilation

The three basic principle 

Ventilation studies should include three components that reflect the following basic principles:

  • Seek to eliminate the emission of polluting, disturbing and even dangerous substances in underground workings by deploying appropriate techniques of excavation, marining, etc…
  • Encourage the capture of all the products released (especially dust) as close as possible to the source, so that they do not spread in the atmosphere of the building site;
  • Dilute uncaptured residual pollutants in order to keep their concentration below the permissible thresholds.
Explanation of secondary (or auxiliary) ventilation

The SUB’ROCA engineers are at your disposal to assist and advise you in the dimensioning of your ventilation installations. Whether your project concerns a meshed network in an underground mine or a complex digging project in another environment, our teams have the technology to calculate the air requirements and performance necessary to ensure ventilation that complies with the health and safety of personnel.


This chapter specifies the main nuisances related to the execution of the works, as well as the limit values of pollutants and dust retained by the regulations.


Dust is defined as any solid particle having an aerodynamic diameter of not more than 100 micron-metres or a falling velocity, under normal temperature conditions, of not more than 0.25 m/s.

  • Total dust

Dust is deposited in the three levels of the respiratory tract: nose, pharynx, trachea, bronchial tree, alveoli, …

  • Alveolar dust

This dust gets into the pulmonary alveoli. They are considered dangerous.

  • Toxic dust

Silica: Crystalline silica is the most harmful form and three components have been identified in the mechanism of silicosis:

    • The quartz,
    • The tridymite,
    • The cristobalite.

Asbestos: The permissible limit value over 1 working hour is 0.1 fibre/cm3.


The occupational exposure limit value (OELV) to a chemical product (defined in article R. 4412-4 of the French Labour Code) represents the concentration in the air that a person can breathe during a given period of time. At this level, no organic or functional damage of an irreversible or prolonged nature is reasonably foreseeable. The value is expressed for gases in ppm (parts per million) or mg/m3.

Short-term limit values (STELV CT) are values measured over a reference period of 15 minutes. They are intended to avoid toxic effects due to peak exposures (short-term exposure). The CT TLVs replace the old ELVs measured over a period of up to 15 minutes.

The 8-hour exposure limit values (8-hour OELV) are measured over an 8-hour working time. They are intended to protect employees from the delayed effects of pollutants. The 8-hour TLVs replace the old average exposure limit values (ALVs).

The reference OELs are presented in the following tables:

Shooting fumes & diesel gas
Explanation of secondary (or auxiliary) ventilation
Natural substances
Explanation of secondary (or auxiliary) ventilation
Another substances
Explanation of secondary (or auxiliary) ventilation

The heat

The internal temperature of man must remain stable at 37°C+/- 0.5°C.

The main parameters that can cause a thermal imbalance are :

  • Physical work,
  • The average temperature of the radiation,
  • Air temperature,
  • Moisture in the air,
  • Air speed.

Working in a warm environment can generate depending on the nature and the exposure time:

  • Heatstroke or hyperthermia,
  • Syncopation,
  • Water deficiency or dehydration,
  • Sodium deficiency.

Having identified the nuisances in the site under consideration, it is now a matter of setting up a Ventilation Project.

General concepts of ventilation

Fresh air requirements

The determination of the fresh air requirements depends on the pollutants to be treated.

The recommended basic values for sizing the ventilation system are as follows:

  • For the dilution of gases emitted by combustion engines :
  • For the evacuation and collection of dust, gases from firin


Ventilation devices installed underground can be classified as follows :

  • Blower ventilation,
  • Suction ventilation,
  • Ventilation by air flow in the gallery system.

These different types of ventilation can be combined with each other, a combination of fan and blower, reversible fans, etc…

Blower ventilation

In blower ventilation, the ventilation duct brings the fresh air in front. The polluted air flows through the gallery itself, from the front to the outside

Explanation of secondary (or auxiliary) ventilation

Advantages and disadvantages of blower ventilation


  • The most active areas at the front are supplied with fresh air,
  • The speed of air ejection allows a good sweeping of the front,
  • The dilution of polluting gases is well ensured,
  • The fan, which is usually outside, remains fixed and independent of the front’s progress,
  • It is possible to control the temperature of the air supply.


  • Dusts and especially silica are dispersed,
  • The cork shoots that or pollutants generated by the front run through the entire gallery,
  • The shooting gases released from the sailor’s pile are not taken back to the source,
  • The pollution gradient increases from the front to the outside,
  • The workshops behind the front are located in the return flow of air polluted by the front,
  • Cold air sent to the front, if not warmed beforehand, can cause great discomfort in winter,
  • The polluted air coming out of the tunnel cannot be treated.

Suction ventilation

In suction ventilation, the ventilation duct extracts the polluted air close to the front and the fresh air arriving from outside through the gallery to the front.

Explanation of secondary (or auxiliary) ventilation

The advantages and disadvantages of exhaust ventilation


  • Allows the suction of pollutants at source, in particular dust and gases released during pickling, provided that the distance d = 5 log S is respected between the front and the suction,
  • Allows a quick evacuation of the firing cap, without polluting the tunnel behind the front,
  • Allows to treat the polluted air discharge at the exit of the gallery,
  • Provides a fresh air sweep from the outside to the front,
  • Can extract smoke from a fire.


  • The fan must be installed in a gallery (unless a rigid duct is used), which requires the fan to move forward regularly,
  • The pollution gradient increases from the outside to the front,
  • All the pollution is brought back to the most active area at the front,
  • If the suction point remains far from the front, the end of the dead-end gallery is no longer renewed and the level of pollution becomes uncontrollable (appearance of dead zones),
  • Effectiveness remains limited to a short distance from the front.

Ventilation solutions

Our ventilation systems are designed to ensure air renewal in quarries and underground mines. SUB’ROCA offers you a whole range of dedicated axial fans. SUB’ROCA assists you in the dimensioning of your installations.

SUB’ROCA offers ventilation solutions adapted to your work in the basement or « top-down », including the construction of station boxes or shafts.

SUB’ROCA supplies ventilation systems equipped with ducts for tunnels in excavation, up to JetFan (accelerator) for tunnels that are opening. Our equipment can be installed on TBMs.

SUB’ROCA has designed a compact range of high-performance fans in diameters from 250mm to 800mm for ventilating microtunneling excavations and underground networks.