Primary Ventilation Circuits

Primary ventilation circuits are designed to supply fresh air to workplaces and dilute and remove contaminants resulted from the mining processes. The time required to flush blasting contaminants from the mine can range from a few minutes up to many hour.

The effective and timely removal of contaminants particularly after blasting is high on the priority lists for mine managers, as time lost on production waiting for fumes to clear is very costly.

There are two basic circuits used 1. Parallel circuits, and 2. Series circuits.

Both have their advantages and disadvantages and many mines use a combination of both circuit types.

7.4.1 Parallel (One Pass) Circuits

Parallel circuits adopt a general design philosophy of ‘one pass, flow through’ air.

This concept ideally lends itself to large tonnage stopes or stoping blocks and may not be suitable for use in some narrow vein steeply dipping orebodies. In this situation, a circuit is established to direct air from the primary intake circuit over the areas of activity and exhaust directly to the return air circuit. It is usual for return air circuits to be areas requiring minimal access and therefore all contaminants are removed directly from the mine. This is particularly important in the event of a fire occurring in the work place as it minimises the areas (and personnel) affected by the smoke, gases and particulates.

With multiple level accesses, it is necessary to control the air entering the activity area (usually by regulation of the exhaust). The down side of this

To Primary Exhaust To Primary

Exhaust

TYPE A: End Access

TYPE B: Central Access

type of circuit is that short-circuiting occurs each time a stope is opened or a ventilation control is damaged.

Whilst providing improved conditions in work areas, these circuits also decrease re-entry times but only if the access does not form part of the return air circuit.

It is usual to provide airflow to all scheduled production locations, even when not in use. This provides flexibility to move from one location to another without the necessity to adjust ventilation circuits. Mine operators often plan to minimise total airflow by campaigning air from one location to another, however experience tells us that this seldom, if ever, occurs and all locations operate with less than desirable airflow rates.

For this type of circuit to be successful, ventilation controls must be well maintained.

7.4.2 Secondary Fans and Parallel Ventilation Circuits.

Because of the geometry of narrow vein orebodies, it may prove difficult to maintain the philosophy of flow through ventilation without the need for extensive development and therefore the use of secondary fans during production must be considered.

This type of circuit is normally adapted to narrow vein orebodies with long strike lengths. LHD tramming distances are usually kept below 250m. In these circuits, air intakes via the access (decline) and is returned to the primary exhaust at each orebody access.

7.4.3 Series Ventilation Circuits

A system most often used in narrow plunging orebodies, is a simple series ventilation circuit. These circuits rely on secondary fans to ventilate in orebody development and production with the return air from these activities being mixed with the primary intake airflow and re-used at the next activity.

This type of circuit has the advantage of simplicity of control and minimal development for ventilation.

Although the utilisation rate (total airflow to the working areas / total airflow through the mine) can be as high as 75%, it is typically 65% with 40% not uncommon.

These circuits have disadvantages including (but not limited to):

• The need for high pressure fans (hence higher power costs),

• Low fan power efficiency for the majority of the operating period (the fan only operates at maximum efficiency in a fully developed mine).

• Decreasing airflow with depth, and

• Contaminants from activities (and fires) affect all down stream personnel.

Although not encouraged, it is recognised that this is the system used in development areas and in reality will continue to be used in some production areas.

7.4.4 Use of Stope Voids as Airways

There is often a misconception that stope voids will upcast airflow causing much frustration when the air downcasts through the stope.

Because stope voids form connections between levels they create airways parallel to the ramp or intake shaft. Generally the pressure distribution is such that the air in the ramp (intake system) is down casting and it follows that the air in the stope will also downcast air. Obviously if the air in the ramp is up casting then it follows that the air in the stope void will also upcast.

BASIC MINE VENTILATION MANAGEMENT

If stope voids are required to up cast air, then it is necessary to design and install the appropriate ventilation controls to cause the pressure distribution required. In general, if stope voids are required to upcast, the primary return airway must not be lower than the top of the stope void.

The reliance on stope voids also has the high probability of production blasting covering the stope brow thus preventing flow through ventilation.

7.4.5 Recirculation

Recirculation occurs when air is kept within a closed circuit. It should not be confused with the situation when air is reused, as in series ventilation circuits.

Recirculation occurs when a fan is installed in an airway in which the natural flow of air along the airway is less than the operating capacity of the fan. In some Australian legislation there is a requirement to install secondary fans such that the air delivered to the fan is greater than 1.3 times its open circuit capacity. This legislation was introduced in times when small drives (2.0m x 2.0m) and low capacity fans (5 to 7 m3_{/s) were common. As a general Rule of thumb an}

airflow of at least 1.5 times the open circuit capacity of the fan is required in airways with large cross sections and higher capacity fans. Even then minor recirculation is possible, depending upon the siting of the fan in the airway.

Although not recommended, some minor recirculation may be acceptable provided the, work place temperature, contaminants in the airflow and the clearing time for blasting gasses remain within acceptable levels.