Control devices in a ventilation system

Three Elements

A Pressure Source or Fan

Connected mine airways

Control devices : stoppings, doors, airlocks, regulators and air crossings

Auxiliary fan: for coursing the air to the heading faces (with brattice cloth, ducts)

Booster fan: If a high resistance district is getting insufficient air

Typical elements of a mine ventilation system

Stoppings:

In developing a mine, connections made between intakes and returns.

When theses are no longer required for access or ventilation,

they should be blocked by stoppings to prevent short-circuiting of the airflow.

A stopping may be of permanent or temporary in nature

Temporary: made of brattice cloth, plastic cloth with wire netting reinforcement

can be hung as curtains or nailed onto a frame work

Permanent: For long term control,

between main intake and main return

Stoppings can be constructed from brick with lime or cement, concrete blocks or

fireproofed timber blocks.

Prefabricated steel stoppings may also be employed.

Stoppings should be well keyed into the roof, floor and sides, particularly if the strata are

weak or in coal mines liable to spontaneous combustion.

Every stopping between the main return air way shall be constructed of masonry or brickwork, Every stopping between the main return air way shall be constructed of masonry or brickwork, of at least 25cm in thickness .of at least 25cm in thickness .

Doors

Doors are employed when a gallery has to be kept available for haulage and/or traveling and at thesame time airflow through the gallery has to be prevented.

For example, intake and return roadways must be kept separated but at the same time, at intervals of say, 300 to 500m, access might be needed from one side to the other which is provided by having a door.

Doors are commonly made of mild steel sheets, strengthened with steel flats or angle irons, and may be of the single leaf double leaf type set in a steel frame with hinges.

In the past, doors made of wooden boards set in wooden door frames, (often with a regulator fitted at the centre) were commonly Used but these were prone to greater air leakage in normal operation and posed risk of damage at the time of a mine fire.

Doors should be of self-closing typecan be easily achieved by keeping the door frame slightly tilted against the vertical (sayby 5° to 10°) in opposite direction to its opening with top of the door frameoffset by about 3 to 5 cm. Some of the other methods of making a door self closing are:

to use a piece of old rope to act as a spring against which the door opens to have an arrangement of a weight which is lifted when the door is opened or to install a compressed air operated cylinder.

Where a door is required to be opened frequently for passage of men and material,an attendant appointedbut the cost goes up

The door open against the intake air so that the air pressure helps the door to keep it shut. It is also advisable to fit doors with latches to prevent their opening in cases of emergency when the direction of pressure differentials may be reversed. Contoured flexible strips attached along the bottom of the door assist in reducing leakage, particularly when the airway is fitted with rail track.

The space between the frame of a door and the roof and sides of the roadway should be builtup with masonry or concrete not less than 25 cm in thickness.

When a door is needed only for a short period, as near the working face, or needed urgentlyat some location, it may be built by simply suspending a few pieces of beltconveyors from the roof level. These are known as flap doors.

Airlock doors

Ventilation doors located between main intakes and returns are usually built as a set of two or more to form an airlock.

This prevents short-circuiting when one door is opened for passage of vehicles or personnel.

The distance between doors should be capable of accommodating the longest train of vehicles required to pass through the airlock.

For higher pressure differentials, multiple doors alsoallow the pressure break to be shared between doors.

With two doors in series, at least one door can always be kept shut when tubs or men are to pass through the other one.

In important locations, such as near the pit bottom, usually a set of three or four doorsare provided, one of which is arranged to open in the opposite direction to the others so that at least that door remains shut when air current is reversed in the mine in case of an emergency.

needed on the surface at every shaft or incline ordinarily used for lowering/ raising of persons or materialswhere a mechanical ventilator is installed. Otherwise, air will not be sucked from the mine in case of an exhausting fan but will come through the shaft opening.

Surface air lock

not necessary if the incline or shaft is used exclusively for ventilation purpose and the fan occupies the whole of the available cross section of the opening.

Steel box type airlock inside shaft (the so called German type airlock

Steel box type surface airlock in headgear

a door fitted with one or more adjustable orifices. Its purpose is to reduce the airflow to a desired value in a given airway or section of the mine.

The most elementary is a rectangular orifice cut in the door and partially closed by a sliding panel. The airflow may be modified by adjusting the position of the sliding panel manually.

Passive regulators may be actuated by motors, either to facilitate their manual adjustment or to react automatically to monitored changes in the quantity or quality of any given airflow.

Regulators

Where intake and return air have to cross each other, an air crossing is needed.

Usually made as an overcast, : Roof is ripped over one of the galleries, two chambers constructed and one of the aircurrents, usually the return is made to go via the top chamber.

Steps may be provided on both sides to help persons travelling in the return road to climb up to and down from the top chamber.

Air crossings

Poor construction of overcast

Excellent construction for overcast

Undercast: the floor-dug up to make the two chambers, and one of the air currents, usually t the return, is made to flow through the lower chamber.

In mines working thick or contiguous seams, crossing of intake and return air currents may be effected through natural air crossings provided by galleries in different horizons in the same seam or in contiguous seams.

Undercast- Not preferred as it may give rise to the problem of water accumulation in the lower gallery.

An air crossing should be strong, leak proof and have a large cross section with small aerodynamic resistance.

Inexpensive but efficient overcastIf an air crossing is weak, it may give way during an emergency such as an explosion and may cause short-circuit of air.

The partition and walls of the every air-crossing shall be at least The partition and walls of the every air-crossing shall be at least 25cm thick25cm thick if if constructed of constructed of masonary or of concrete not properly reinforcedmasonary or of concrete not properly reinforced, , and at least and at least 15 cm in thickness15 cm in thickness if constructed of properly if constructed of properly rein forced concreterein forced concrete..

Used for ventilation of blind headings

Line Brattice, commonly made out of treated jute cloth is used to course the air current from the nearest junction to the working face in a heading.

(treated to make it fire and rot resistant, cheapest and most favoured material)

Brattice cloth is nailed on to timber supports placed at 1 to 1.5 m intervals along the heading at ¼ to 1/3 width distance from one of the sides.

The heading is thus longitudinally partitioned into two airways of dissimilar c.s. permitting easy travel of man and machinery thru the large sized ones.

As brattice cloth made of jute though cheap, tends to be leaky.

Some countries have replaced it with plastic sheeting, plastic reinforced with nylon or polyster and combinations of plastic coatings/laminations on jute.

In India, corrugated or plain iron sheets are used in place of jute cloth to course the air current.

Line Brattice or Brattice partition

Auxiliary Ventilation

Auxiliary Ventilation

Line brattices used for auxiliary ventilation.

Brattice cloth, in general, leak heavily at the gaps in the roof and sometimes in the floor.

Hence, brattices are restricted to only short lengths of headings and should be kept extended right up to the face to give adequate quantity of air at the face.

Brattices increase the effective resistance of the heading by doubling the length of the airway and reducing the cross sectional area.

The alternative to the use of brattice cloth is auxiliary ventilation through ducts.

Auxiliary Ventilation

The choice between forcing and exhausting arrangements depends mainly upon the pollutants of greatest concern, dust, gases or heat.

shall be installed, located and worked in such a manner that-(i) a sufficient quantity of air shall, at all times, reach it so as to ensure that it does not re-circulate air; and(ii) there is no risk of the air which it circulates being

contaminated by any substantial quantity of inflammable or noxious gases or dust; and

ensure an adequate supply of air to within 4.5 metres of the face or blind end.

Forcing system with exhaust overlap Exhausting system with force overlap

Prepare Basic NetworkPrepare Basic Network

Ventilation survey of existing mineVentilation survey of existing mine Decide on method of work and rate of prodn of new mine at selected time phase of mine life

Decide on method of work and rate of prodn of new mine at selected time phase of mine life

Define acceptable ventilation standardsDefine acceptable ventilation standards

Calculate air quantity requirements based on methaneEmission, manpower, tonnage and air velocity

Calculate air quantity requirements based on methaneEmission, manpower, tonnage and air velocity

For hot and deep mines needing a/c calculate the extra qnty of air required

For hot and deep mines needing a/c calculate the extra qnty of air required

Estimate R values for airway branches in the networkEstimate R values for airway branches in the network

Conduct VNET simulation to get airflowdistribution and fan capacity.

Analyse results of Vnet simulation for airflowdistribution, air velocities,pressure loss and

Operating cost of the ventilation system

Modify Ventilation Parameters for generatingAlternatives for optimisation purposes if needed

Minor modification in network Major modification in vent system/standards

Select the optimum system for a given time phaseSelect the optimum system for a given time phase

Repeat the operation for various time phases in turn

Repeat the operation for various time phases in turn

Select the Ventilation System

Ascensional ventilation is preferable became:

Firedamp being lighter than air, is readily carried to higher levels.

Natural ventilation pressure assists the fan ventilation because the air gets hot during its travel in the mine and bas a tendency togo to higher levels.

If the fan stops, the air will continue to flow in the same direction by the natural ventilation.Descensional ventilation hassome advantages in hot deepThe main advantage isthat air has not to pass over water drains of the dip side and it reaches the working faceIn drier and cooler condition.

Section showing the principle of through-flow ventilation applied across the levels of a metal mine.

the actual geometry varies widely. Air moves in a through-flow mannerfrom a downcast shaft or ramp, across the levels, sublevels and stopes towards return raises, rampsor upcast shaft. Airflow across each of the levels is controlled by regulators or booster fans.Movement of air from level to level, whether through stopes or by leakage through ore passes or old workings tends to be ascentional in order to utilize natural ventilating effects and to avoid thermally induced and uncontrolled recirculation.

ventilation strategy of many metal mines

In the majority of cases, wherecontrolled vertical movement of the air is required, stope airflow systems employ ascentionalthrough-flow ventilation. Although auxiliary fans and ducts may be necessary at individualdrawpoints, every effort should be made to utilize the mine ventilation system to maintain continuous

airflow through the main infrastructure of the stope. Series ventilation between stopes should beminimized in order that blasting fumes may be cleared quickly and efficiently.

Simple ventilation system for shrinkage or cut-and-fill stopes.

Ventilation system for sub-level open stopes

Typical ventilation system for a block caving operation.