Difference between revisions of "Ventilation"

Revision as of 21:26, 1 March 2012

From Queen's University Mine Design Wiki

This article is an exploration of the topic of "Underground Mine Ventilation" in conjunction with the Queen's University Mine Design Wiki

Background

The main objective of an underground mine ventilation system is clear: to provide airflows in sufficient quantity and quality to dilute contaminants to safe amounts/concentrations where personnel are required to travel and work. This requirement is integrated into the mining law of those nations who possess that type of legislation. The degree of "quality' and "quantity" varies with regulations set from nation to nation, depending on a number of parameters: mining history, contaminants of greatest concern, the apparent dangers associated with those hazards and the political/social structure of the nation. The general requirement is for any personnel to work and travel in an environment that is safe and comfortable.

Ventilation Design Planning

General Considerations^[1]

Air Quantities

Air quantities where diesel machines are used are determined from manufacturers' specifications or by testing and certification done on individual units by the Mine Safety and Health Administration (MSHA). Generally, to provide 1.0m3/s•kW for each unit is satisfactory, otherwise air quantities are usually determined by practical considerations and experience.

Velocities should not exceed 7.6m/s for both comfort and for reducing head losses due to friction. They should also be kept abopve a minimum of 0.25m/s. In workplaces, air velocities should be between 1.0-2.0m/s in stopes and between 2.0-3.0m/s in drifts.

Overall Mine Layouts

In a shaft mine, intake air should be directed down the main personnel materials shaft(s) and exhausted by some other shaft(s), preferably sunk exclusively for exhuats. In a ramp-entry mine, the intake would normally be the adit(s) where personnel enter, sometimes with a separate adit for exhaust, connected to the workings by raises and winzes.

Main ventilation openings should be laid out for minimum interference with mining operations.

It is also important to consider the direction of flow in the opening and effect of air temperature and humidity on the opening. For example, downcast shafts have moisture and freezing problems in wet, cold climates, whereas upcast shafts have condesation and timber decay in hot humid climates. This is explored in more detail below.

Air-Moving Devices

Fans are the main air-moving devices in underground ventilation systems.

1. Booster fans are used to give additional air-moving power where needed or to divert airflow. They are often needed in situations where the mine workings are widely scattered.

2. Auxiliary Fans are used in directing air to indivual faces or small areas.

Surface vs. Underground Location of Main Fan

Neglecting the relevance of mining laws, the advantages of a surface (blower or exhauster) location are:

1. Better, simple and safer control in event of emergency

2. Easier access for repairs

3. Simpler and cheaper installation and power supply

4. Damage to fan by fire or explosion is less likely

5. Recirculation of leakage effects less pronounced

Advantages of an underground (booster)location are as follows:

1. Stronger and more positive ventilaiton of deeper and more remote workings

2. Intake and exhaust shafts are free of airlocks and fan installations, allowing their use for hoisting or haulage

3. Simplifies modification of ventilation system by addition of fans to new levels and/or remote areas

4. Handles less air than exhauster and (more than blower) for given quantities at face

Taking all factors into account the location of the main fan at the surface is usually preferable from the standpoint of its greater safety. However, the complexity of ventilation networks in multilevel mines weighs heavily against the exclusive use of surface fans. The topic of mine fans, their design and usage, among other related topics is explored more in detail below.

Multiple-Fan Ventilation

Modern metal mine ventilation practice is to ventilate with multiple fans, usually surface fans in combination with booster fans undeground. Boosters are typically installed near the downcast shaft to provide positive pressure to the workings. In addtion to intake boosters, exhaust boosters may also be installed underground near exhaust shafts or adits. Exhaust boosters are often needed in mines where the workings have been extended beyond the pressure-generating capabilties of the main exhaust fan.

Ventilation schematic for mature stope mine^[1]

Similar complex situations are often found in other older mines with extensive workings. However, the safety factor of the backup systems is inherent in multiple-fan installations, as the failure of one or even several fans will result in only a reduction, rather than a complete loss, of ventilation.

Reuse of Ventilation Air

Reuse of air is allowable in most metal mine situations and is preferred for economic reasons. Reuse reduces the air-quantity requirement of the mine and usually simplifies the ventilation network. However, the used air is mixed with fresh air before use. In cases where dust or heat is excessive, filtration or cooling will be needed. For example, in some South African mines fume filters are used to extract nitrous oxide fumes from development blasting when blasting is done on shift.

Leakage

Leakage is the most common cause of inefficient distribution of air in mines. In metal mines leakage in mine ventilation systems can average out to 25%. However, leakages not in excess of 15% are attainable with good ventilation practice.

Ventilation Planning For Various Mining Methods^[1]

The physical outline of the ventilation system depends largely on the mining methods in use to extract ore from the mineral deposits. Each type of metal and nonmetal mining method has its own characteristic ventilation system. For our purposes only the methods of overhand cut & fill and square-set stoping (transervse/longitudinal stoping) will be used as examples for the basis of ventilation design.

Overhand Cut & Fill, Square-set stoping Ventilation

Common level stoping ventilation layout^[1]

Common layout for square-set stoping^[1]

A common arrangement for square-set stoping is shown to the right.

Homestake Mine ventilation layout^[1]

Case Study: Homestake Mine

The ventilation layout of the Homestake Mine is shown to the left. The mine is ventilated by three relatively independent circuits. For the upper part, the No. 2 shaft is teh intake adn there is an intake fan at the collar of the the shaft and exhaust is blown into an open-cut mine. The middepth part of the mine intakes by the No. 5 shaft with exhaust up the Ellison shaft with fans that are undeground boosters with an axhust fan at the collar of the Ellison. The deepest part of the mine is dsignated the 'Oro Hondo' circuit in which intake is via the Ross shaft and exhaust is up the Oro Hondo shaft, with the main air motivation supplied by an exhaust fan at the collar of the Oro Hondo.

In the cases present overhand cut and fill and transverse stoping are the main employed stoping methods of mining. In the cut adn fill system, air is directed into the the stope with auxiliary fans from the drift on the bottom level of the stope; air is often blown through a cooler before entering the stope. Exhaust is through ventilation boreholes that are sunk from the upper main level of the stope along tthe strike of the stope in its early days of development.

Transverse stoping is similar to the VCR mining method in which it is necessary to provide ventilation for the top sill of the stope and the bottom sill of the stope where muck is picked up. Both of these sills are driven by crosscutting from main level drifts. In the top sill, air is directed through an auxiliary fan to teh face through vent tubing. In the bottom sill, ventilation is flow-through since there are crosscuts in either end of the sill to the drift.

Design of Ventilation Systems^[1]

Major and Minor Features

In planning the layout of a ventilation system and determining the proper method of ventilation for a prospective mine or changes and extensions to an existing mine, attention must be given to many factors. These factors affect both the major and minor features of the ventilation system. Major features of the system are concerned with the fan, the main openings and airways and the primary circuits. Minor features involve the details of distributing air to individual working places. The desirable major features to be concerned with in ventilation system design are:

1. Utilize haulage or hoisting openings as intakes

2. Utilize all available openings and connections to the surface in transmitting air

3. Utilize ascensional ventilation, i.e. direct air to the lowest active level in themine, allowing it to course through workings as it ascends

4. Limit distance of air travel to a minimum

5. Maintain a balance of resistance between main intake and return airways

6. Reduce control devices in main operating openings to a minimum

7. Avoid leakage and recirculation

8. Circulate air from active zones to caved ground

9. Split air as often as needed and as close to the fan as possible (but limit in a hot mine, where velocity must be maintained for cooling)

10. Consider surface location and exhausting arrangement for at least some main fans

11. Avoid interventilation of adjacent mines, but utilize them for escape routes

12. Use centrally located shaft(s) as intake(s) and peripheral shafts as exhausts

13. Avoid transmitting air in opposite directions in adjoing airways.

The minor features of a ventilation system are reflected in the details of planning ventilation in conjunction with mine operation:

1. Degree of concentration of workplaces

2. Position of active workings

3. Ground condition and methods of support required

4. Position of local sources of heat, cold, gas or dust

5. Position of sealed zones or fire zones

6. Condition of abandoned workings

7. Precooling requirements of hot zones in deep mines

In general:

1. Attempt to coordinate mining and ventilation systems, rather tnan superimposing a ventilation system on an existing, rigid or predetermined mining method

2. Utilize the primary development openings as main airways in the ventilation system and other development openings as secondary airways

3. Restrict the number of workplaces on one split of air, in order to have a single section or stope ventilated by one split. Aside from providing fressh air in all workings, splitting ensures that disruption of ventilation one has no effect on others

Design Procedure

Design of a ventilation system consists of the following steps:

1. From the proposed mine plan, lay out a system of air courses in accordance with the features mentioned above

2. Determine air-quantity requirements. The limiting design criterion may be imposed by any of the following: Legal Quantity Requirements, Legal Quality Requirements, Empirical Guidelines

3. Tentatively select main and booster fan positions

4. Calculate resistances in the various points of the circuits and calculate head and power requirements for each fan. Computer analysis is the typical way of performing this, otherwise the techniques employed in analytical solutions are shown in detail here and here

5. Select number of options, revise requirements, run economic analyses of the various options and redesign the system if necessary **

6. Evaluate each option thoroughly from the viewpoint of safety **

7. Decide on a final plan **

**: for the course requirements it is assumed that these steps would not be necessary as they go too into depth of design

Ventilation Design Rules of Thumb

Some rules of thumb that are useful for determining air qualities/quantities:

1. Drift Velocities. Air velocity in main-level drifts should be 1.0-3.0m/s, with 2.0m/s a good average. Where drifts open up into repari galleries or other large openings 0.25m/s is adequate

2. Stope Velocities. Air in open stopes should move at about 2.0m/s, unless excessive cold or heat is a problem. Reduce the velocity to 0.5m/s for cold air or increase to 3.0m/s or above for hot air.

3. Development-Working Velocities. When dead-end ventilation is needed, vent tubing should be carried about 5m behind the face. Air moving from the tubing should of quantity such that the same speeds are felt as mentioned above. Air velocity is reduced when the air exits the tubing hence the velocity felt at the face should be felt faster than what quantity of air is actually moving through the tube.

General Layout of Systems

Many mines working vertical (or steeply dipping) veins continue to practice ascensional ventilation. This usually supplies adequate air and should be considerded if contamination from dust or gas in the workings is not a problem. This system does not work if there is a fire as the ascensional system may contaminate the entire mine with lethal gases.

The general practice is to put underground booster fans on as many levels as necessary, drawing air directly off the intake shaft. Air is then directed through the workings with or without the use of auxiliary fans and passes into the exhaust shaft. Positive-pressure systems are recommended in all metal mines with either booster fans at various levels or a single intake fan on the surface where this is considered more practical. Exhaust fans can be used as a supplement to keep the air moving in the right direction and to supply the necessary additional head, but they should not be used as the only air movers because splitting is difficult to control efficiently.

Ventilation Systems and Network Analysis

Introduction

The quantified planning of distribution of airflows along with the locations and capacities of fans and other ventilation controls that create the appropriate conditions through the system are the most essential parameters in the design of a new underground mine. As shafts and airways are created the ventilation system must be adequate to supply sufficient ventilation to working extensions. ^[2]

Any operating mine must be considered a dynamic system causing the ventilation planning to be a continuous and routine process. Ventilation network analysis can be summed up by the quantitative determination of airflow for given locations and the duties of fans with the constant resistances of the branches of a ventilation network with set positions within the network. The desired airflow of a given network will be determined in function of airway resistances, fans and regulators. ^[3]

Fundamentals of Ventilation Network Analysis

The schematic diagram of an integrated ventilation system is composed of branches representing a set of single airways or a group of openings that connected in a certain manner is considered to behave effectively as a single airway. The sealed off areas of insignificant leakage, stagnant dead ends and headings that produce no induction effects on the main airflow are omitted from the network schematic. The points where the branches connect are called junctions or nodes. The surface atmosphere allows the top of the shafts or other openings to be connected to each other through a common pressure sink. ^[4]

Kirchhoff’s Laws

Gustav R. Kirchhoff (1824-87) was the first to state a law that describes the relationships explaining the behaviour of electrical current in a network of conductors. Kirchhoff’s Law can also be applied to fluid networks (i.e. closed ventilation systems at steady state). The basic relationship states that the mass flow entering a junction equals the mass flow leaving that junction. ^[3]The equation can be represented mathematically by:

where: M are the mass flows, positive and negative, entering junction j.

Let it be noted that mass flows is equal to the volume flow multiplied by the air density such that:

where:

Q = volume flow (m3/s)

ρ = air density (kg/m3)

Therefore:

It is assumed that, in subsurface ventilation systems, the difference in air density around any single junction is negligible. Adding the following equation to help check the accuracy of airflow measurements that are taken around a junction:

Kirchhoff’s second law can be used in ventilation by stating that the sum of all pressure drops around a closed path, or mesh, in the network must be zero after considering the effects of fans and ventilating pressures. The ensuing equation is called the steady flow energy equation, initially for a single airway. ^[3]

where:

u = air velocity (m/s)

Z = height above datum (m)

W = work input from fan (J/kg)

V = specific volume (m3/kg)

P = barometric pressure (Pa) and

F = work done against friction (J/kg)

Since we can consider a number of such branches will form a closed loop or mesh within the network, the algebraic sum of all the differences in height (delta Z) must be zero and the sum of the changes in kinetic energy can be set as negligible. The equation for the sum of each of the remaining terms around the mesh, m, is the following:

The summation of the terms corresponds to the natural ventilation energy, NVE, which originated from thermal additions to the air. The equation now becomes:

Multiplying all terms by the air density converts everything into pressure units:

However:

ρF =p (frictional pressure drop, equation)

ρW=pf (rise in total pressure across a fan) and

ρNVE=NVP (natural ventilating pressure, equation)

Hence, the equation can be turned into Kirchhoff’s second law:

Such a relationship is utilized as a quality assurance check on a pressure survey or as a method of determining a value for the natural ventilating pressure. ^[5]

Deviations from Square Law

The rational form of the square law,Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle p = RtρQ2} , and the traditional form, can both be used in the condition of dully developed turbulence. The coefficient of friction ,f, influences both the ational resistance, Rt, and the Atkinson resistance, R, for the duct or airway. Once the flow enters the transitional or laminar sections of the Moody chart then the value of f becomes a function of Reynolds Number and the corresponding values of resistance for that airway will vary with the airflow. Values of resistance must be calculated according to the appropriate values of Reynolds Number if the form of the square law is to be obeyed. ^[6] The relationship then becomes:

where values of the index, n, lie in the range of 1.8 to 2.05 for a variety of pipes, ducts and fluids.

When working with routes along a main ventilation system, it has been seen that n lies very close to 2 but may have a lower value for leakage flows through stoppings or old workings. The n value adapts since flows enter the transitional or even the laminar regimes that can cause it to go as low as 1.0 and the pressure drop flow relationship becomes:

where:

= laminar resistance

Mine Fans

Introduction

A fan utilizes mechanical energy of rotating impeller to produce air flow and increase total pressure. The grand majority of fans used in typical underground mining practices use electric motors.

The different types of underground mine fans are classified by their location:

• Main fans handle all the air passing through the system

• Booster fans assit the through-flow of air in discrete areas

• Auxiliary fans overcome resistance of ducts in blind headings

Furthermore fans are separated into two distinct categories based on their major mechanical design classifications:

• Centrifugal fan: air enters near centre of the wheel, turns through a right angle and moves radially outward by centrifugal action between the blades of a rotating impeller

• Axial fan: air passes through the fan along flowpaths that are aligned with the axis of rotation of the impeller without changing their macro-direction

The axial fans’ impeller rotates at a higher blade tip speed then a centrifugal fan of similar performance therefore producing more sound. Axial fans also have a stall characteristic at high resistance yet are more compact and can easily combined into series and parallel configurations, therefore making them more favourable for underground locations.

Illustrations of fan pressures^[3]

Fan Pressures

Fan Total Pressure, FTP, is the increase in total pressure, pt across the fan:

FTP = pt2 – pt1

Fan velocity pressure, FVP, is the average velocity pressure at the fan outlet:

FVP = pt2 - ps2

Fan static pressure, FSP, is the difference between the fan total pressure and fan velocity pressure:

FSP = FTP − FVP

Fan static pressure is regarded as the unit of the useful mechanical energy applied to the system. Manufacturers publish characteristic curves in terms of fan static pressure rather than fan total pressure. Typical fan pressures are illustrated to the right.

Impeller Theory & Fan Characteristic Curves

An important aspect of subsurface ventilation planning is the specification of pressure-volume duties required of proposed main or booster fans.

Idealized flow through a backward bladed centrifugal impeller^[3]

Centrifugal Impeller

The airflow enters at the centre of the wheel, turns through a right angle and, as it moves outwards radially, is subjected to centrifugal force resulting in an increase in its static pressure. The corresponding outlet velocity pressure may then be partially converted into static pressure within the surrounding fan casing. A rotating backward bladed centrifugal impeller is shown is illustrated to the right.

Idealized flow through a backward bladed centrifugal impeller^[3]

Actual Characteristic Curves for Centrifugal Impellers^[3]

In an actual fan, there are losses which result in the real pressure-volume curves lying below what is expected in theory. In all cases, friction and shock losses produce pressure-volume curves that trend toward zero pressure when there is no external resistance.

Frictional losses occur due to the viscous drag of the airflow on the faces of the vanes. A diffuser effect occurs in the diverging area available for flow as the airflow moves through the impeller which results in a further loss of available energy. The transmission of power is therefore not uniform along the length of the blade.

The shock (or separation) losses occur primarily at the inlet and reflect the sudden turn of near 90° as the airflow enters the eye of the impeller. As the airflow approaches the inlet, a vortex is applied by the wall effects of the fan. Shock losses can be reduced by installing either an inlet cone at the eye of the impeller or fixed inlet and outlet guide vanes.

The characteristic curves shown to the left are a result of the combined effect of these losses on the three types of centrifugal impeller. The non-overloading power characteristic, together with the steepness of the pressure curve at the higher flows, are major factors in preferring the backward impeller for large installations.

Actual Characteristic Curves for an Axial Fan^[3]

The losses in an axial fan may be divided into recoverable and non-recoverable groups. The recoverable losses include the rotational components of velocity that exist in the airflow leaving the fan. These losses can be recovered when operating at the design point by the use of guide vanes. Buildup of the swirling outlet air will occur however, as the fan digresses from the design point.

The non-recoverable losses include friction at the bearings and drag on the fan casing, the hub of the impeller, supporting beams and the fan blades themselves. These losses result in a transfer from mechanical energy to heat which is lost in its capacity for doing useful work.

The design point coincides with the maximum efficiency because at this point the losses are at a minimum. Operating at low resistance would not draw excessive power from the motor as the shaft power curve shows a non-overloading characteristic. However, the efficiency decreases rapidly in this region.

The disadvantage of operating at too high a resistance is a decreasing efficiency but, more importantly, the danger of approaching the stall point. Boundary layer breakaway takes place on the blades and centrifugal action occurs producing recirculation around the blades.

A fixed bladed axial fan of constant speed has a rather limited useful range and will maintain good efficiency only when the system resistance remains sensibly constant. This can seldom be guaranteed over the full life of a main mine fan. Fortunately, there are a number of ways in which the range of an axial fan can be extended:^[3]

Typical characteristic curves for an axial fan^[3]

1. The angle of the blades may be varied. Many modern axial fans allow blade angles to be changed, either when the rotor is stationary or while in motion. The latter is useful if the fan is to be incorporated into an automatic ventilation control system. The versatility of such fans gives them considerable advantage over centrifugal fans.

2. The angle of the inlet and/or outlet guide vanes may also be varied, with or without modification to the impeller blade angle.

3. The pitch of the impeller may be changed by adding or removing blades. The impeller must, of course, remain dynamically balanced. This technique can result in substantial savings in power during time periods of relatively light load.

4. The speed of the impeller may be changed either by employing a variable speed motor or by changing the gearing between the motor and the fan shaft. The majority of fans are driven by A.C. induction motors at a fixed speed. Variable speed motors are more expensive although they may produce substantial savings in operating costs. Axial fans may be connected to the motor via flexible couplings which allow a limited degree of angular or linear misalignment. Speed control may be achieved by hydraulic couplings, or, in the case of smaller fans, by V-belt drives with a range of pulley sizes

An example of an actual axial fan characteristic curve is shown to the right.

Fans in Combination

There are situations in which it is advantageous to combine fans either in series or in parallel at a single location. Such combinations enable a wide spectrum of pressure-volume duties to be attained with only a limited range of fan sizes. In general, fans may be connected in series in order to pass a given airflow against an increased resistance, while a parallel combination allows the flow to be increased for any given resistance. Although ventilation network programs can allow each fan to be entered separately, it is sometimes more convenient to produce a pressure-volume characteristic curve that represents the combined unit.

Typical characteristic curves for an axial fan^[3]

Fans in Series ^[3]

For two fans in series, both pass the same airflow, but develop their own respective pressures. For three or more fans, the process of adding fan pressure remains the same. In individual fans need not have identical characteristic curves. However, if one fan is considerably more powerful than the other, or if the system resistance falls to a low level, then the impeller of the weaker unit may be driven in turbine fashion by its stronger companion. The weaker fan then becomes an additional resistance on the system. It is usual to employ identical fans in combination. A characteristic curve for fans combined in series is shown to the right. This shows two fans, a and b, located in series within a single duct or airway. The corresponding pressure-volume characteristics and the effective resistance curve are also shown. The characteristic curve for the combination is obtained simply by adding the individual fan pressures for each value of airflow. The effective operating point is located at C, where the resistance curve intersects the combined characteristic. Fans a and b both pass the same airflow, Q, but develop pressures pa and pb respectively The individual operating points are shown as A and B. ^[3]

Typical characteristic curves for an axial fan^[3]

Fans in Parallel ^[3]

For fans that are combined in parallel, the airflows are added for any given fan pressure in order to obtain the combined characteristic curve. For two fans in parallel, both pass their own respective airflows, but at the same common pressure. Three or more fans may be combined in parallel, adding airflows to obtain the combined characteristic curve. It is mandatory to employ identical fans when connected in parallel. This will reduce the tendency for one of them to approach stall conditions before the other. However, variations in the immediate surroundings of ductwork or airway geometry often results in the fans operating against slightly different effective resistances. A characteristic curve for fans combined in parallel is shown to the right. Fans a and b pass airflows Qa and Qb, respectively, but at the same common pressure, p. The operating point for the complete unit occurs at C with the individual operating points for fans a and b at A and B respectively.

Even when identical fans are employed, it is usual for measurements to indicate that they are producing slightly different pressure-volume duties. In the case of fans located in separate ducts or airways that are connected in parallel, the resistance of those ducts or airways may be taken into account by subtracting the frictional pressure losses in each branch from the corresponding fan pressures. In these circumstances, a better approach is to consider the fans as separate units for the purposes of network analysis.

An advantage of employing fans in parallel is that if one of them fails then the remaining fan(s) continue to supply a significant proportion of the original flow. The amount of flow depends upon the number of fans employed, the shape of their pressure-volume characteristic curves and the provision of non-return baffles at the fan outlets.

Fans may be connected in any series/parallel configuration, adding pressures and airflows respectively to obtain the combined characteristic curve. This is particularly useful for booster fan locations. A mine may maintain an inventory of standard fans, combining them in series/parallel combinations to achieve any desired operating characteristic.

Fan Performance

Power is delivered to the drive shaft of a fan impeller from a motor (usually electric) and via a transmission assembly. Losses occur in both the motor and transmission. For a properly maintained electrical motor and transmission, some 95 per cent of the input electrical power may be expected to appear as mechanical energy in the impeller drive shaft. The impeller converts most of that energy into useful airpower to produce both movement of the air and an increase in pressure. The remainder is consumed by irreversible losses across the impeller and in the fan casing producing heat energy.

Impeller efficiency may be defined as:

While the overall efficiency of the complete motor/transmission/impeller unit is given as:

Booster Fans

The employment of booster fans provides an enticing alternative to the capital penalties of driving new airways, enlarging existing ones, or providing additional surface connections. Unlike the main fans which, in combination, handle all of the mine air, a booster fan installation deals with the airflow for a localized area of the mine only. The primary objectives of a booster fan are:

• to enhance or maintain adequate airflow in areas of the mine that are difficult or uneconomic to ventilate by main fans

• to redistribute the pressure pattern such that air leakage is minimized.

The installation of underground booster fans can improve ventilation of a mine while producing significant reductions in total fan operating costs. However, these benefits depend upon skilled system design and planning. An inappropriate use of booster fans can raise operating costs if fans act in partial opposition to each other. Furthermore, if booster fans are improperly located or sized then they may result in undesired recirculation. ^[3]

Airflow Fundamentals

General Airflow Information

The airflow mechanics of a ventilation system occur mainly because of the differences in pressure between the intake and exhaust accesses. This means that in order for air to properly flow throughout an underground mine there must be differences in pressure between these accesses. This difference in pressure is known as the ventilating pressure, and the air flows in a direction from high pressure to low pressure. ^[7]

Pressure differences are created throughout the mine by imposing a form of pressure (i.e. a fan) at points throughout the mine. In order for air to flow properly the pressures must be able to overcome resistances that are created from certain aspects of the mine. These resistances can be either frictional losses or shock losses (otherwise known as mine static head). The greater the resistance that is present between points the less air flow there will be due to the reduction in ventilating pressure overcoming these resistances. If the pressure difference between two points remains the same, and the resistances are increased, the airflow between these two points will decrease, and vice-versa. The relationship between pressure, resistance, and airflow is shown in the expression below. ^[7]

With regards to the expression above, P represents the pressure, R represents the resistance, and Q represents the air flow.

Total Pressures Losses

The mine total head is the sum of all energy losses that occur within the ventilation circuit. This total head is comprised of the mine static head loss and the mine velocity head loss. The mine static head is the sum of the frictional losses and the shock losses. The mine velocity head is a loss that is accounted for at the end of a system, and it is a loss to the system because it is a discharge of kinetic energy into the atmosphere. The following expression gives the calculation of the total head loss due to both velocity head and static head losses. ^[7]

For design purposes, the aim should be to minimize these parameters as much as possible, such that equipment costs on ventilation systems can be decreased.

Velocity Head Loss

The velocity head (HV) is the loss in pressure that occurs due to the discharge of kinetic energy into the atmosphere that is wasted at the end of the system. This loss can be found using the following expression:

From this expression, v represents the velocity of the air running through the excavation, and can also be expressed through the volume air flow (Q) and the area (A) of the drift.

Therefore, for design purposes, these expressions should be taken into account; this is because the larger the volume air flow is the higher the velocity head loss will be, and the larger the area of the drift is, the lesser the velocity head loss will be ^[7]

Friction Head Loss

Friction pressure loss occurs when the air confined in an airway exerts itself perpendicularly to the wall of the airway and the friction on the walls of the airway consumes energy present in the airflow. Therefore friction losses depend on the roughness of the wall surfaces and velocity of the air. The friction head loss can be found using the Darcy-Weisbach equation shown next:

File:Equaition9.jpg

This expression shows the friction loss due to the walls of the airway in a specific length of duct. L represents the length of airway, D represents the diameter of the airway, and f represents the friction factor for the type of material used for the airway as well as the roughness of this airway. Furthermore, friction losses in airflow through mine airways contribute to 70-90% of the total head loss. Therefore, from a design perspective, size and material of airways should be analyzed completely before being implemented as they can greatly affect the flow of air throughout the mine.

Shock Loss

Shock losses happen from an abrupt change in the velocity of air in an airway. This abrupt change is a result of either a change in air direction or in airway area. A direct calculation of shock loss can be found using the following expression:

File:Equation10.jpg

From this expression, HX represents the shock loss, X represents the shock factor, and HV represents the velocity head loss (as shown above). Shock losses only contribute to approximately 10-30% of the total head loss; however, there are many different situations that can occur in the ventilation circuit that can cause a shock loss. Each shock factor varies depending on what the shock loss occurring actually is. The potential shock losses are changes in area in the mine system, fan losses, bends, and splits and junctions. ^[7]

Therefore, from a design perspective, changes in areas of the airway and locations of bends and junctions should be engineered such that the losses occur are not too high. Higher losses can lead to the need of higher changes in pressure between two points, and therefore will place a higher cost on the mine operation.

Gases and Pyschrometry

Gases in Underground Mines

As air travels through a ventilation circuit in an underground mine, it may become contaminated with one or more potentially harmful gases. These gases come from a variety of sources and must be carefully monitored to prevent mine workers from being exposed. Because ventilation systems must be designed to eliminate the hazards created by these gases, it is important for ventilation engineers to be familiar with the gases that may be present, their sources, and the hazards associated with them.

All of the hazardous gases discussed in this section can cause serious health issues and fatalities if they are present in significant quantities. Fortunately, modern ventilation systems in underground mines have made great steps in limiting worker exposure and creating an overall safer working environment.

Fresh Air

'Normal' atmospheric air is composed primarily of nitrogen (78%) and oxygen (21%). Nitrogen is quite inert, and does not pose any direct threat to worker safety aside from the possibility of oxygen displacement leading to asphyxiation.

Presence of oxygen is critical for human respiration. While normal fresh air contains approximately 21% oxygen, this number can be changed significantly when workers and equipment operate in confined areas. The amount of oxygen that a worker needs for respiration varies with changing levels of muscular exertion. A miner working vigorously will consume more oxygen and produce more carbon dioxide than one who is at rest. In most mining situations, however, the rate of oxygen consumption and carbon dioxide production by the workers is negligible when compared to the gas exchanges occurring in equipment with internal combustion engines, and the effects of oxygen displacement from other gas sources. A well-designed ventilation system should ensure that workers always have an adequate supply of fresh air to breathe.

Carbon Dioxide

Normal fresh air contains approximately 0.04% carbon dioxide. The gas itself is not toxic, but it can become dangerous at high concentrations primarily due to its displacement of oxygen. Because of its high solubility, carbon dioxide also acts as a respiratory stimulant. This combination of effects can lead to rapidly increased breathing rates and a lower availability of oxygen for respiration. Workers exposed to air containing 10 to 15 percent carbon dioxide will experience intolerable panting, severe headaches, rapid exhaustion and eventual collapse. While extended exposure (and deprivation of oxygen) can be extremely dangerous or fatal, administration of oxygen and rest can usually reverse any symptoms fairly rapidly. ^[3]

Carbon Monoxide

Carbon monoxide is a colourless, odourless, and highly toxic gas. Even very low levels of carbon monoxide (35 ppm) can cause headaches and dizziness with prolonged exposure over 6-8 hours. Symptoms progress more quickly and become more severe with increasing concentrations. Death can occur in less than 3 minutes at levels 12,800 ppm. ^[8]

Carbon monoxide is absorbed very readily by the haemoglobin in human red blood cells. When this occurs, a stable compound known as carboxyhaemoglobin is formed in the blood stream and does not decompose quickly. Because of its stability, carboxyhaemoglobin can gradually build up in the blood stream even when only low levels of carbon monoxide are present. As this compound builds up, the number of red blood cells available to transport oxygen within the body decreases, and vital organs become deprived of oxygen. ^[3]

In mines, carbon monoxide can be generated in large quantities during explosions or fires. On a more regular basis, the gas is also formed in smaller quantities by internal combustion engines and by blasting activities.

Methane

Methane is a non-toxic gas that has no odour. It is commonly found retained in rock fractures, pores, or adsorbed on mineral surfaces. Methane gas is particularly common in underground coal mines because it is formed gradually by decomposition of organic material. Methane is very dangerous in mines because of its extreme flammability and explosive qualities. ^[3]

Methane burns cleanly to produce carbon dioxide and water vapour in open-air environments with sufficient oxygen presence. In most mine fires or explosions, however, this is not the case. The combustion is starved of oxygen, which leads to the production carbon monoxide. Indeed, many of the fatalities associated with methane fires and explosions in mines are caused by carbon monoxide poisoning. ^[3]

Hydrogen

Hydrogen is also a non-toxic gas, but it is even more explosive than methane. Hydrogen can occasionally be found in surrounding strata, but not usually in dangerous concentrations. Battery charging areas likely create the most risk for dangerous accumulations of hydrogen, especially if they are poorly ventilated. ^[3]

Sulphur Dioxide

Sulphur dioxide is a highly toxic gas that presents an immediate risk of death at concentrations as low as 400 ppm. Unlike carbon monoxide, the presence of sulphur dioxide (even at concentrations as low as 1-3 ppm) can be easily detected by its acidic taste and odor. In underground mines, sulphur dioxide is generated by oxidation of sulphide minerals and by internal combustion engines. Workers exposed to sulphur dioxide should be immediately administered oxygen, immobilized, and kept warm.^[3]

Nitrogen Dioxide

Of the three common oxides of nitrogen (nitric oxide, nitrous oxide, and nitrogen dioxide), nitrogen dioxide is the most toxic. Unfortunately, it is also the variation that is most commonly found in underground mines. Nitrogen dioxide can be recognized by its characteristic brown fumes. ^[3]

Nitrogen dioxide is very soluble in water, where it forms acids that can have irritating and corrosive effects. Concentrations of nitrogen dioxide greater than 200 ppm can be fatal. ^[3]

The oxides of nitrogen are most commonly generated by blasting and internal combustion engines.

Hydrogen Sulphide

Hydrogen sulphide is a very toxic gas that can be fatal to exposed workers at concentrations of 600 ppm. Workers who recover from hydrogen sulphide exposure are often afflicted with long-term health issues such as bronchitis and conjunctivitis. ^[3]

Hydrogen sulphide gas can often easily be recognized by its characteristic 'rotten eggs' smell, but relatively high concentrations of the gas can actually cause temporary paralysis of an exposed worker’s sense of smell. Scent detection alone should not be relied on to detect dangerous levels of hydrogen sulphide.

In mines, hydrogen sulphide is often generated by acidic breakdown or heating of sulphide ores. Areas with stagnant water pose especially high risks, especially when poorly ventilated.

Radon Gas

Radon gas is a by-product of the radioactive decay of uranium. Uranium is commonly found throughout the upper surface of the earth at an estimated average grade of 4 grams per tonne in crustal rocks. Uranium gradually decays through a series of steps which emit ionizing subatomic particles. While most of the products of these steps are solid elements, radon is not. The gaseous properties of radon allow it to more easily 'escape' from the rock masses it forms in, and also make it significantly more dangerous to underground mine workers than other products of radioactive decay. ^[3]

Radon gas can freely flow into the airways of underground mines, where it will then decay into microscopic radioactive particles. These particles can be breathed in by mine workers and retained in their lungs where they will further decay and continue to emit radiation. Over the long term, exposure to radon can cause lung cancer. ^[3]

Pyschrometry

Introduction

Psychrometry is the study of gas-vapour mixtures. In underground mining, we are most interested in investigating the properties and behaviours of water vapour and air mixtures in environments with variable temperatures and pressures.

All around the world, the composition of “dry air” is remarkably constant. “Dry air” refers to the mixture of gases we have all around us, assuming that no water vapour is present. With only slight local variation, the dry air of our lower atmosphere is comprised of approximately 78% nitrogen and 21% oxygen, with small amounts of other gases. Although the gas proportions remain fairly constant, the amount of moisture in the air (from water vapour) is highly variable.^[3]

Underground mines are typically wet environments. Water is commonly used for dust suppression, and it can often been seen seeping out of wall rocks. Water is also used by certain types of underground drilling equipment. Under varying temperatures, pressures, and humidity levels, some of this liquid water can evaporate and be carried along by the ventilation systems in the mine. When conditions change, the water can condense back into liquid form. It is important to understand these thermodynamic processes in order to accurately calculate and predict their effects on the local underground 'climate'.

Basics of Pyschrometric Analysis

Psychrometric calculations are performed on a basis of mass, rather than volume, because the volume of a gas is subject to variation under changing temperatures and pressures. In a dry air system, the mass flow (of air) in an independent airway remains constant at all points. This assumption is not always true in wet systems such as mines, because water is present which can evaporate and add to the mass flow, or condense and take away from it. Because of this complication, most calculations are performed using a basis of a fixed mass flow of dry air, and a variable concentration of water vapor. The concentration of water vapour in the air mixture (usually expressed as kg water per kg dry air) is known as the moisture content or specific humidity of the air. ^[3]

As more and more water vapour is added to a system by evaporation, the air (or more correctly, the system) is said to become increasingly saturated. At a certain point, the partial pressure exerted by the water vapour reaches a limit and no more vapour can be added to the system. This limit changes with temperature and is known as the saturation vapour pressure. When this level is reached, the system is said to be fully saturated.

Related to this idea is the concept of the “dew point.” If an unsaturated air/vapour system is cooled under constant pressure, its saturation vapour pressure will gradually decrease without any change in the vapour pressure from the water present. Eventually the system will be cooled enough that it will become fully saturated. The temperature at which this occurs is known as the dew point. Cooling the system beyond this point would cause condensation to begin to occur.

Humidity

In psychrometric analysis, humidity is calculated in two ways: relative humidity and percentage humidity. Relative humidity (%) is calculated based on the vapour pressure of the system compared to the saturation vapour pressure of the system at the same temperature. Percentage humidity is very similar, but is calculated from the moisture content of the system compared to same system’s moisture content at the point of saturation. Over the normal atmospheric range, these two calculation methods give similar results. ^[3]

Pyschrometric Measurements

In underground mining, the most common tool used for measuring the presence of water vapour in air is known as a wet and dry bulb hygrometer (or psychrometer). The device consists of two thermometers, one of which has its bulb covered in a thin, water-soaked cloth. The dry bulb thermometer registers the air temperature as any regular thermometer would, while the wet bulb thermometer is cooled slightly due to the evaporation of the water. All relevant psychrometric parameters can be calculated based on knowledge of these two temperatures and the barometric pressure.

In order to measure accurately, these devices must have a constant flow of air over the wet bulb to prevent the establishment of a pocket of more saturated air. Static hygrometers simply rely on the air velocity at the site of their location to provide the required flow. “Whirling” psychrometers are commonly used in mine surveys, and are spun around a handle to provide sufficient airflow. Aspirated psychrometers provide the best accuracy and most consistent results by using small fans to draw air into the shielded container which contains the thermometers. ^[3]

Modern pyschrometer

An example image of a pyschrometer is shown to the right:

Modern Pyschrometric Chart^[9]

Pyschrometric Charts

Knowing the barometric pressure dry and wet bulb temperatures, it is possible calculate many relevant parameters (including moisture content, relative humidity, vapour pressure, enthalpy and more) using a set of equations known as the psychrometric equations. Without access to programmable calculators or spreadsheet software, these equations can be very cumbersome and intimidating to use. For convenience, visual representations of the psychrometric relationships were created for various barometric pressure. These plots are known as psychrometric charts (see image to left).

These charts can be used to quickly determine psychrometric parameters based on field measurements, and they provide an opportunity to better understand the relationships between the parameters as an air/vapour mixture undergoes thermodynamic change.

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5 1.6} John Wiley and Sons. "Metal Mine Ventilation Systems." John Wiley and Sons, 1997. p. 524 - 548. Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content Cite error: Invalid <ref> tag; name "Wiley" defined multiple times with different content
2. ↑ G.W McElroy. "A Network Analyzer for Solving Mine Ventilation Distribution Problems" U.S. Bureau of Mines Inf. Circ. 7704, 1954. p. 13.
3. ↑ ^{3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30} M.J McPherson. "Mine Ventilation Planning in 1980s." International Journal of Mining Engineering Vol 2, 1984. p. 185 - 227. Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content Cite error: Invalid <ref> tag; name "McPherson" defined multiple times with different content
4. ↑ J.J Atkinson. "Theory of Ventilation of Mines." North of England Institute of Mining Engineers No 3, 1854. p. 118.
5. ↑ H.I Hartman and Wang. "Computer Solutions of Three Dimensional Mine Ventilation Networks with Multiple Fans and Natural Ventilation". Int. J. Rock Mech. Sc.Vol.4, 1967.
6. ↑ H. Cross. "Analysis of Flow in Networks of Conduits or Conductors." Bull. Illinois University Eng. Exp. Station. No. 286, 1936.
7. ↑ ^{7.0 7.1 7.2 7.3 7.4} E.M De Souza. Fundamentals of Airflow. In E. De Souza, Mine Ventilation, n.d.
8. ↑ M. Goldstein. "Carbon Monoxide Poisoning" Journal of Emergency Nursing, Volume 34, Issue 6, 2008.
9. ↑ R.L Earle. "Unit Operations in Food Processing." Chapter 7 Figure 7.3, 1966.

External Sources

http://en.wikipedia.org/wiki/Wikipedia:Citing_sources

https://www.minedesignwiki.org/index.php/McPherson_-_Subsurface_Ventilation_Chapters

http://library.queensu.ca/

http://www.bacharach-inc.com/sling-psychrometer.htm