Pitot-static tube6.2.9. Fixed point traverses

PURPOSE AND SCOPE OF SURVEYS

AIR QUANTITY SURVEYS

6.2.10. Hot body anemometers6.2.11. Tracer gases6.2.12. Measurement of cross-sectional area

PRESSURE SURVEYS

6.3.1. Gauge and tube surveys6.3.2. Barometer and altimeter surveys

ORGANIZATION OF PRESSURE-VOLUME SURVEYS

6.4.1. Initial planning6.4.2. Survey management6.4.3. Quality assurance

AIR QUALITY SURVEYS

Chapter 6. VENTILATION SURVEYS

6.1.

6.2.

6.3.

6.4.

6.5.

6.2.1. Rotating vane anemometer6.2.2. Moving traverses6.2.3. Fixed point measurement6.2.4. Density correction6.2.5. Swinging vane anemometer (velometer)6.2.6. Vortex-shedding anemometer6.2.7. Smoke tubes6.2.8.

rigour of the ensuingdata analysis depend upon the purpose of the survey. Perhaps themost elementary observation in underground ventilation is carried outby slapping one's clothing and watching the dust particles in orderto ascertain the direction of a sluggish airflow.

Measurements of airflow should be taken in all undergroundfacilities at times and places that may be prescribed by law. Evenin the absence of mandatory requirements, a prudent regard for safetyindicates that sufficient routine measurements of airflow be taken(a) to ensure that all working places in the mine receive theirrequired airflows in an efficient and effective manner, (b) thatventilation plans are kept up to date and (c) to verify that thedirections, quantities and separate identity of airflows throughoutthe ventilation infrastructure, including escapeways, are maintained.Similarly, routine measurements of pressure differentials may be madeacross doors, stoppings or bulkheads to ensure they are also main-tained within prescribed limits and in the correct direction. Thelatter is particularly important in underground repositories fortoxic or nuclear materials.

One of the main differences between a mine ventilation system anda network of ducts in a building is that the mine is a dynamicentity, changing continuously due to modifications to the structureof the network and resistances of individual branches. Regularmeasurements of airflow and pressure differentials underground arenecessary as a basis for incremental adjustment of ventilation con-trols.

During the working life of a mine or other underground facility,there will be occasions when major modifications are required to bemade to the ventilation system. These will include opening up newdistricts in the mine, closing off older ones, commissioning new fansor shafts, or interconnecting two main areas of the mine. Theprocedures of ventilation planning are detailed in Chapter 9. It isimportant that planning the future ventilation system of any facilityis based on reliable and verified data. Ventilation surveys that arecarried out in order to establish a data base for planning purposesmust necessarily be conducted with a higher degree of organization,detail and precision than those conducted for routine control. Thischapter is directed primarily towards those more accurate surveys.

A major objective of ventilation surveys is to obtain the fric-tional pressure drop, p, and the corresponding airflow, Q, for eachof the main branches of the ventilation network. From these data,the following parameters may be calculated for the purposes of bothplanning and control:

6-2

6.1. PURPOSE AND SCOPE OF VENTILATION SURVEYS

A ventilation survey is an organized procedure of acquiring datathat quantify the distributions of airflow, pressure and air qualitythroughout the main flowpaths of a ventilation system. The requireddetail and precision of measurement, and the

air" that have passed through the

6-3

-S S

Most of the techniques of observing airflow are, therefore, combina-tions of the methods available for measuring mean velocity and thecross-sectional area.

Prior to the invention of anemometers in the nineteenth century,the only practicable means of measuring rates of airflow in mines wasto observe the velocity of visible dust or smoke particles suspendedin the air. An even cruder old method was to walk steadily in thesame direction as the airflow, varying one's pace, until a candleflame appeared to remain vertical. Modern instruments for the mea-surement of airspeed in mines divide into three groups depending upon(i) mechanical effects, (ii) dynamic pressure and (iii) thermaleffects.

6.2.1. Rotating vane anemometer

The vast majority of airspeed measurements made manually under-ground are gained from a rotatinq vane (windmill type) anemometer.When held in a moving airstream, the air passing through the instru-ment exerts a force on the angled vanes, causing them to rotate withan angular velocity that is closely proportional to the airspeed. Agearing mechanism and clutch arrangement couple the vanes to a pointerwhich rotates against a circular dial calibrated in meters (or feet).The instrument is used in conjunction with a stopwatch and actuallyindicates the number of "meters of

= m2 -.= u x Am3

Q

qualitv ofthe air. These measurements may include wet and dry bulb tempera-tures, barometric pressures, dust levels or concentrations of gaseouspollutants.

6.2. AIR QUANTITY SURVEYS

The volume of air passing any fixed point in an airway or ductevery second, Q, is normally determined as the product of the meanvelocity of the air, u, and the cross-sectional area of the airway orduct, A

m

. friction factors

While observations of airflow and pressure differentials areconcerned with the distribution and magnitudes of air volume flow,other measurements may be taken either separately or as an integralpart of a pressure/volume survey in order to indicate the

. natural ventilating effectsp/Q2). branch resistances (R =

. volumetric efficiency of the system

airpower (p x Q) losses and, hence, distribution ofventilation operating costs throughout the network

.

. distribution of airflows, pressure drops and leakage

I.5m upstream from his body. To commence the traverse, the instru-ment should be held either in an upper or lower corner of the airway,with the pointer reset to zero, until the vanes have accelerated toa constant velocity. This seldom takes more than a few seconds. Theobserver should reach forward to touch the clutch control lever whilea second observer with a stopwatch counts backwards from five tozero. On zero, the anemometer clutch is activated releasing thepointer and, simultaneously, the stopwatch is started.

The path of the traverse across the airway should be similar tothat shown on Figure 6.1. The aim should be to traverse the anemo-meter at a constant rate not greater than about 15 per cent of theairspeed. Ideally, equal fractions of the airway cross sectionalarea should be covered in equal times. This is facilitated by the

6-4

30, the anemometer should be clamped in afixed position relative to the rod and manipulated by turning the rodduring the traverse such that the instrument remains aligned with theaxis of the airway.

The traverse

The observer should face into the airflow holding the anemometerrod in front of him so that the instrument is facing him and at least

30 from the direction of the airstream. Hence,for most underground airways, allowing the anemometer to hang freelyat the end of the rod will give acceptable results. For airways ofinclination greater than

+ 5 per cent for anglesdeviating by up to

1.5m inlength, or greater for high airways. The attachment mechanismshould permit the options of allowing the anemometer to hang verti-cally or to be fixed at a constant angle with respect to the rod. Arotating vane anemometer is fairly insensitive to yaw and will giveresults that do not vary by more than

jewelled bearingsand give repeatable readings for velocities in the range 0.25 to 15m/s. High range instruments may have four vanes, low-friction rolleror ball bearings and can be capable of measuring air velocities ashigh as 50 m/s. Digital vane anemometers are available that indicatedirectly on an odometer dial or an illuminated screen. Modern hand-held instruments may also be fitted with a microprocessor to memorizereadings, dampen out rapid variations in velocity or into which canbe entered the cross-sectional area for the calculation of volumeflow. Two types of vane anemometer are included in the selection ofventilation survey instruments shown on Plate 1.

In order to obtain a reliable measure of the mean air velocity inan underground airway, it is important that a recommended techniqueof using the anemometer is employed. The following procedure hasevolved from a combination of experiment and practical experience.

6.2.2. Moving Traverses

The anemometer should be attached to a rod of at least

anemometer during a given time period. The clutch device is employedto stop and start the pointer while the vanes continue to rotate. Azero reset lever is also incorporated into the instrument. Low rangevane anemometers will typically have eight vanes,

anenlometer traverseFigure 6.1 Path of an

-S S

6-5

= - m2 = u x Am3

Q

u.

The cross sectional area, A, is determined using one of themethods discussed in Section 6.2.12. The calculation of airflow isthen completed as

m

+ 5 per cent tolerance, gives the observed mean velocity.In most cases, the time of each traverse at a station is the same,allowing the anemometer readings to be averaged before calculating themean velocity. The observed mean velocity must then be correctedaccording to the calibration chart or curve for the instrument (Sec-tion 6.4) to give the actual mean velocity,

(i) names of observers(ii) the location of the measuring station, time, date,(iii) anemometer readings and corrections(iv) dimensioned sketch of cross-section(v) calculation of area(vi) calculation of air volume flow.

The bookings are normally made by the stopwatch person.For eachtraverse, the anemometer reading is divided by the corresponding timeto give the air velocity. The mean of these, ignoring any valuesoutside the

Setsof traverses should be taken at different locations within each air-way. Where cross-cuts or other leakage paths affect the airflow thena sufficient number of additional measurement points should be trav-ersed in order to quantify the rate and direction of leakage.

Bookinq

The anemometer field book should be waterproof and laid out suchthat each double page has segments for

+ 2 per cent. Larger discrepancies may be expectedin airways where there is a highly asymmetric variation in velocityacross the airway, where the floor conditions are unstable, or whenobstructions exist in the cross section. Obviously, measuring sta-tions should be chosen to avoid such difficulties wherever possible.Another highly annoying cause of discrepancy is the opening of aventilation door during the period of measurement. At least two

f 5 per cent. In favourable steadystate conditions, experienced observers will often achieve repeata-bility to within

stopwatch observer calling out the elapsed time at ten second inter-vals. The complete traverse should take not less than 60 seconds andmay be considerably more for large or low velocity airways. Thefinal five seconds should be counted down by the stopwatch observerduring which time the traverse person stretches forward to disconnectthe clutch at the end of the time period. The length indicated bythe anemometer is immediately read and booked, and the instrumentreset to zero.

The procedure is repeated, traversing in the opposite directionacross the airway. Traverses should be repeated until three readingsare obtained that agree to within

inter-

6-6

a

= actual air density at time of measurement

Equation (6.1) shows that the density adjustment is effectivelyapplied only to the calibration correction and is ignored in mostcases.

6.2.5. Swinging vane anemometer (velometer)

In its most fundamental form, the swinqinq vane anemometer(velometer) is simply a hinged vane which is displaced against aspring from its null position by a moving airstream. A connectedpointer gives a direct reading of the air velocity. The air entersport at the side of the instrument. This can be fitted with

$4&KPc = air density at time of calibration

and Pm

cc = correction from instrument calibration curve or chartobtained from a value of u

= indicated velocityUj_

Pmwhere u = corrected velocity

(6-I)-c

PCu = Ui + Cc

pointtt correction factor for that station.This is typically 0.75 to 0.8 for the fixed point located some onehalf to two thirds the height of the airway. Subsequent routinereadings may be obtained simply by taking an anemometer reading atthe fixed point and applying the appropriate calibration and fixedpoint corrections. Provided that the measuring station is welldownstream of any bends or major obstructions and the airflow remainsfully turbulent then the fixed point correction factor will remainnear constant as the airflow varies.

6.2.4. Density correction

For precise work, anemometer readings may be further correctedfor variations in air density:

Anemometer traverses may also be employed at the ends of ducts.However, it is recommended that the technique should not be used forduct diameters less than six times that of the diameter of the ane-mometer.

6.2.3. Fixed point measurement

An estimate of duct airflow may be obtained by holding the ane-mometer at the centre of the duct and multiplying the correctedreading by a further correction factor of 0.8. A similar techniquemay be employed for routine check readings taken at well establishedmeasuring stations in airways. The reading obtained from a station-ary anemometer at a known location within the cross-section should,initially, be compared with that given from a series of traverses inorder to obtain a "fixed

Reynold's numbers. The length of airway should be chosen suchthat at least one minute elapses during the progression of the smokebetween the two marks. The dispersion of the smoke cloud oftencauses the downstream observer some difficulty in deciding when tostop the stopwatch. Due to the uncertainties inherent in the tech-nique, smoke tubes are normally employed as a last resort in slowmoving airstreams.

6-7

caplamp is located at each mark. Thetime taken for the cloud of smoke to travel the length of airwaybetween the marks gives an indication of the centre-line velocity ofthe air. This must then be adjusted by a centre-line correctionfactor to give the mean velocity. The correction factor is usuallytaken to be 0.8 although a more accurate value can be calculated forknown

phialcontaining a granulated and porous medium soaked in titaniumtetrachloride or anhydrous tin will produce a dense whitesmoke. This is released upstream of two fixed marks in the airway.An observer with a spot-beam

Vortexproduction depends upon the fluid velocity. In the vortex-sheddinganemometer, the vortices may be sensed by the pulsations of pressureor variations in density that they produce. One apparentdisadvantage noticed in practice is that when sited in a fixed loca-tion underground for monitoring purposes they require calibration forthat specific location. They may also require electronic damping toeliminate large but short lived variations in signals caused by thepassage of vehicles.

6.2.7. Smoke tubes

Smoke tubes are perhaps the simplest of the mechanical techniquesemployed for measuring airflows and are used for very low velocities.A pulse of air forced by a rubber bulb through a glass

changeable orifices or probes to give a range of measurable veloci-ties. Oscillations of the vane are reduced by the eddy current dam-ping produced when an aluminum strip connected to the vane movesbetween two strong permanent magnets. The delicacy of the velometertogether with its pronounced directional bias have limited its use inunderground surveys. However, it can serve a useful purpose in giv-ing spot readings as low as 0.15 m/s in gassy mines where hot wireprobes are prohibited.

6.2.6. Vortex-shedding anemometer

For continuous monitoring systems, both'rotating vane and swing-ing vane instruments with electrical outputs have been employed. How-ever, they both require relatively frequent calibration checks whenused in mine atmospheres. For this type of application, thevortex-sheddinq anemometer is preferred as it has no moving parts.

When any bluff object is placed in a stream of fluid, a series ofoscillating vortices are formed downstream by boundary layer breaka-way, first from one side of the body then the other. The propagationof the vortices is known as a Karman Street and can often be observeddownstream from projecting boulders in a river. The rate of

kg/m3.assuming an air density of

6-8

IO Pa,1.2

IO per cent accuracy inthe pressure reading is 10 Pa. Calculate the air velocity corre-sponding to a velocity pressure of

flPa, then the lowest pressure that will give a Examole. If a diaphragm pressure gauge can be read to the nearest

pitot-static tube in the turbulent airflows of anunderground system.

1.5m in length. Much smaller versions are availablefor use in ducts or pipes.

Modern pitot-static tubes reflect the total, static and velocitypressures of the airflow to an excellent degree of accuracy.Unfor-

tunately,the precision of the measurement depends also upon themanometer or pressure gauge connected to the tappings. This imposesa practical restriction on the lower limit of air velocity that canbe measured by a

(kg/m3)(see equation (14.52) for air density)

Pitot-static tubes vary widely in overall dimensions. Formeasuring air velocities in mine airways or at main fans, the longerstem may be some

= actual density of the air p

m /s (6.3)(from equation (2.17))

where

(2!;8:;

Furthermore, the velocity head is related to the actual velocity ofthe air, u

eqzition - Ps

(from

tappings will indicate the difference betweenthe total and static pressure, i.e. the velocity pressure:

Pv = Pt

ps. A pressure gauge or manometer con-nected across the two

pt. The outer tube is per-forated with a ring of small holes drilled at right angles to theshorter stem of the instrument and, hence, perpendicular to thedirection of air movement in the airway. This tube is, therefore,not influenced by the kinetic energy of the airstream and registersthe static pressure only,

6.2.8. Pitot-static tube

In Section 2.3.2. we discussed the concepts of total, static andvelocity pressures of a moving stream of fluid. A pitot-static tube,illustrated on Figure 6.2, can be used to measure all three. Thisdevice consists essentially of two concentric tubes. When heldfacing directly into an airflow, the inner tube is subjected to thetotal pressure of the moving airstream,

SPitot static tube

PI

Figure 6.2

IIII

II

III

III

IIII

II

III

equal areas

In this method, the cross-section of the duct or airway isdivided into subsections each of equal area. Figure 6.3 shows arectangular opening divided into 25 equal subsections similar inshape to the complete opening. Using a pitot-static tube or anemo-meter, the velocity at the centre of each subsection is measured.The mean velocity is then simply the average of the subsection velo-cities.

6- 9

pitot-static tubes may be left in place.

Method of

"fixed point traverse". Differing tech-niques of conducting such traverses vary in the number of observa-tions, locations of the instrument and treatment of the data. Threeof these techniques are described here. In all cases, the fixedpoint traverse method assumes that the distribution of flow over thecross-section does not vary with time. For permanent monitoringstations, a grid of multiple

pitot-static tube and the pressure gauge damps out the short termvariations. However, the cotton wool should not be so tightly tampedinto the tubing that the gauge reaction becomes unduly slow. Elec-tronic diaphragm gauges are often fitted with an internal dampingcircuit.

6.2.9 . Fixed point traverses

The rotating vane anemometer is an integrating device, accumula-ting the reading as it is traversed continuously across an airway orduct. Most other instruments for the measurement of air velocity,including the pitot-static tube, do not have this advantage but areconfined to giving a single spot reading at any one time. In orderto find the mean velocity in an airway from pitot-static tube readingsit is, therefore, necessary to take spot measurements at a number oflocations over the cross-section. This procedure is known by thecontradictory sounding term

longwall faces and trunk airways.

One of the difficulties observed when using a pitot-static tubefor the spot measurement of pressures or velocities in a turbulentairstream is the oscillation in the readings. A small wad of cottonwool inserted into the flexible pressure tubing between the

eva-sees, ventilation shafts, some

duc-ting and a few high velocity airways; primarily fan drifts and

= 4.08 m/s1.2

As the great majority of underground openings have air velocities ofless than 4 m/s, it is clear that the use of the pitot-static tubefor the measurement of air velocity is limited to ventilation

=

P

2 x 10

-= Vu pvI2 Solution.

froE several readingsdistributed over the subsection.

Measuring points for 2 fixed point traverse in arectangular opening. The velocity in each shadedsubsection should be the average

Figure 6.3

= number of the point counted outwards from the centre= diameter of the duct (m)

N = number of points across the diameter

6-1 0

:

(6.5)

where r = radius of point n from the centre

= D m

annulus on each diameter and may be calculated from

r

> 2.5No. of points 6 8 12

Table 6.1 Number of measuring points on each diameter ofa circular opening.

The locations of the points are at the centre of area of the relevant

- 2.5< 1.25 1.25

annuli,the method of equal areas

each of the same area. Readingsshould be taken at points across two diameters and the correspondingvelocity profiles plotted. Should those profiles prove to be skewedthen readings should be taken across two additional diameters. Thenumber of measuring points recommended on each diameter is given inTable 6.1. Figure 6.4 illustrates an 8 point traverse on each of 4diameters.

Diameter of duct (m)

ew8iA + 23 (6.4)where e is the exponential exponent, 2.7183.and A is the cross-sectional area (m)

The calculated number of points may then be rounded to a value thatis convenient for subdividing the cross-sectional area but shouldnever be less than 24. Correct positioning of the measuring instru-ment is facilitated by erecting a grid of fine wires in the airway torepresent the subsections.

In the case of circular openings,divides the circle into

= 100

not give thecorrect mean velocity. Secondly, it will be recalled that the velo-city gradient changes most rapidly near the walls. Hence, accuracywill be improved if the velocities for the subsections adjacent tothe walls and, especially, in the corners are determined from a num-ber of readings distributed within each of those subsections. Third,the number of subsections should increase with respect to the size ofthe airway in order to maintain accuracy. As a guide, an approxima-tion to the recommended number of points, n, for a rectangular openingmay be calculated from

n

velocitv at each subsection should be calculated. Averaging thevelocity pressure before employing equation (6.3) will

There are a few precautions that should be taken to ensuresatisfactory results. First, if a pitot-static tube is employed thenthe

Figure 6.4 Measuring positions for a 8 point traverse on4 diaaeters of a circular duct.

/42

I

Table 6.2 Positions of measuring points in a circular duct usingthe method of equal areas.

6-11

IIIIII IIIIII

.933/.979.8821 1.6441.7501.8231 .3561 .021~.067~.118~.177~.025~ (

1

I I I I I I II I I I I I12

.8951.96811.6771.8061 .323/ l.0321.1051.1941

1

I I I I I I II I I I I I8 I I

1 ~.044~.146~.296~~.704~.854~.956~

I I I I

6 I II

II I

pitot-static tube should be yawed slightly fromside to side until the orientation is found that gives the greatestreading of total or velocity pressure. The head of the instrumentis then aligned directly into the airstream.

No. ofmeasuringpoints oneach diameter

Fractions of one diameter measuredfrom side of duct

I I I I I I

Table 6.2 gives locations of points for 6, 8, and 12 point traversesin terms of fractions of duct diameter measured from one side.

Where a pitot-static tube traverse is to be conducted across aduct from the outside then a clamping device should be attached tothe outer surface of the duct to hold the pitot-static tube firmly inplace. The positions of measurement should be marked on the stem ofthe instrument using Table 6.2 or 6.3. After each relocation of themeasuring head, the

:able 6.3 Log-linear traverse positions of measuring pointsin a circular duct.

I

1.6261.7591.8171.8861.9251.986~.014~.075~.114~.183~.241~.374)

1

I I12

.978/ .34511.6551.8161.883

II I I

I II II

1.6791.8651.968

II I

.321/

I I I I I

I.0211.1171.184

.135

I I I I I8 I I

1.032)

each diameter

Fractions of one diameter measuredfrom side of duct

6 I I I

io. of

neasuringJoints on

Los-linear traverse

A more accurate method of positioning points of measurement alongthe diameters of a circular duct has been derived from a consider-ation of the logarithmic law equations that describe the velocityprofile for turbulent flow. The effects of observational errors areminimized when the points are located according to this method, knownas the log-linear traverse. The corresponding locations are given inTable 6.3.

In most hot

6-15

countour isevaluated and may be multiplied by the mean of the bounding velo-cities and the area scale factor to give the airflow for that band.Provided that the outermost contour is close to the walls then thevelocity at the walls may be taken as zero. The sum of all bandairflows gives the total flow for the airway.

6.2.10 Hot body anemometers

willWhen any heated element is placed in a moving fluid, heat energybe removed from it at a rate that depends upon the rate of mass

flow over the element.

In the hot wire anemometer a wire element is sited within a Smallopen ended cylinder to give the instrument a directional bias. Theelement forms one arm of a Wheatstone bridge circuit.

countour diagrams. However, the expense of such systems isseldom justified other than in research and testing laboratories.

By difference, the area of the band between each

plani-meter (if the scale drawing is large enough for good planimeteraccuracy) or by the rudimentary method of counting squares on thegraph paper. Fully automated systems have been devised to obtain thedata by scanning the actual cross-section and produce quantifiedvelocity

longwall face.

The area enclosed by each contour is determined either by

(iSOV&S) are constructed. Figure 6.5 showsan example of velocity contours obtained on a

equal velocity

10.

For difficult cases, the construction of velocity contours canprovide both a visual depiction of the flow pattern and also a meansof quantifying airflow. A scale drawing of the measurement cross-section is made on graph paper. A grid of fine wires is constructedin the airway to define the points of measurement. The number ofpoints should be not less than those defined in the previoussubsection. The greater the number of measurement points, the moreaccurate will be the result. The velocities at the correspondingpoints of measurement are entered on the graph paper and contourlines of

pv divided by (pv) are

greater than the maximum

pitot-static tube traverse is accept-able if more than 75 per cent of the velocity pressures

longwall faces, complex air-flow patterns may exist. A useful rule of thumb is that the averag-ing of spot velocities from a

evasee causes the air to move in the wrong direction within onepart of the cross-section. Similarly, in obstructed but high velo-city airways underground such as many

tlTextbookll advice is to choose measuringstations well away from obstructions, bends or changes in cross-section. Unfortunately, this is not always possible, especiallywhen measuring airflows at the inlets or outlets of fans. It is notuncommon to find that longitudinal swirl in a fan drift or re-entryin an

Velocity contours

One of the difficulties that besets ventilation engineers inmeasuring large scale airflows is that conditions are often not con-ducive to good accuracy.

.x

= actual air density at time of measurement.

Hot wire anemometers are particularly useful for low velocitiesand are reliable down to about 0.1 m/s. They are convenient forfixed point traverses in slow moving airstreams. If a hot wire ane-mometer is to be used in a gassy mine then a check should first bemade on the permissibility of that instrument for use in potentiallyexplosive atmospheres.

The Kata thermometer, described in Section 17.4.3.2 as a means ofmeasuring the cooling power of an airstream, can also be used as anon-directional device to indicate low air velocities, typically inthe range 0.1 to 1 m/s.

The main bulb of the Kata thermometer is heated until the alcohollevel is elevated above the higher of the two marks on the stem.When hung in the airstream, the time taken for the alcohol level tofall between the two marks, coupled with the Kata index for theinstrument and the air temperature, may be used to determine thenon-directional air velocity. The Kata thermometer is seldom usedfor underground work (except in South Africa) because of its fragil-ity.

6.2.11. Tracer gases

The rate at which injected gases are diluted provides a means ofmeasuring air volume flow without the need for a cross-sectionalarea. The method is particularly useful for difficult situationssuch as leakage flow through waste areas, main shafts and otherregions of high velocity and excessively turbulent flow, or totalflow through composite networks of airways.

Hydrogen, nitrous oxide, carbon dioxide, ozone, radioactivekrypton 85 and sulphur hexafluoride have all been used with the lat-ter particularly suitable for leakage or composite flows. The gaschosen should be chemically inert with respect to the mineralization

6-16

Pm kg/m3)

andPC = air density at calibration (usually 1.2

= indicated air velocityUj_

Pm

where u = true air velocity

- (6.5)Ui PC

u =

wire anemometers, the temperature of the element is maintained con-stant by varying the electrical current passing through it as the airvelocity changes. In other designs, the current is kept constant andthe temperature (and, hence, electrical resistance) of the element ismonitored. Modern hot wire anemometers are compensated for varia-tions in ambient temperature and most also indicate dry bulb temper-ature. For precise work, readings should be corrected for air den-sity

(W/s)

6-17

Q% I -= MC IC

.prevailing temperatureand pressure

flowrate of gas at the monitoring station

= density of gas at the

= mass Pg S

--gg = m3M g

pg

giving

Mg

and

=- (fraction, by volume)Q

But

where

99

qg, and the airflow, Q.

C

flowrate of gas,

y time (t), graph is plotted as shownNow, the concentration C is given as the ratio of the

volume

(6.7)

Pg = density of tracer gas at ambient pressureand temperature

It is assumed that the volume flow of tracer gas is negligible com-pared with the airflow.

In the case of sluggish or composite flows, a known mass of thetracer gas, M (kg), is released as a pulse into the upstream airflow.At the downstream station,tored and a concentration,

the concentration of tracer gas is moni-

on Figure 6.6.(C)

(6.6)

m3

S

(M3/s)C = downstream concentration of tracer gas

(fraction by volume)

= airflow (m3/s)

Ti= volume flow of tracer gas

%Iwhere q

c = -

MgQ

QPg Q-= - = M g% I

concen-tration of the tracer gas. Then

or

C

Mg (kg/s). At a point suf-ficiently far downstream for complete mixing to havesamples of the air are taken to establish the steady

occurred,state

of the strata.

There are two techniques of using tracer gases for the measure-ment of airflow. For high velocity airways, the tracer gas may bereleased at a monitored and steady rate

time (s)

Figure 6.6 Concentration-time curve at a tracer gasmonitoring station.

(6.8)

the vast majority of airflows are determined as the product ofvelocity and a cross-sectional area, the accuracy of the air---flow depends equally upon the measured velocity and cross-sectional

area. There is little point in insisting upon meticulous proceduresfor the measurement of mean velocity unless the same care is appliedto finding the cross sectional area.

By far the most common method of measuring airway area is bysimple taping. This will give good results where the opening is ofregular geometric shape such as a rectangle or circle. Airflowmeasuring stations should, wherever possible, be chosen where theairway profile is well defined. The frames of removed ventilationcontrol doors provide excellent sites for airflow measurement.Shapes such as arched profiles or trapeziums may be subdivided intosimple rectangles, triangles and segments of a circle, and appropri-ate taped measurements taken to allow the area to be calculated.

Inevitably, there are many situations in which airflows must bedetermined in less well defined cross-sections. Several techniquesare available for determining the corresponding cross-sectional area.For shapes that approximate to a rectangle, three or more heightsand widths may be taped to find mean values of each. Care should betaken in such circumstances to make allowance for rounding at thecorners. This often occurs due to spalled rock accumulating on thefloor at the sides of airways.

6-18

m3/s

CrOSS-Sectional area

=

Measurement of

-

QM

Q

pg =-

"4dt, giving

0

or

6.2.12

Asa mean

MI

00

I

M, must also be equal to

dt

But the total mass of gas released,

; 3 PgQ I- =

= C dt0

1

al

I IHence, the complete area under the curve of C against t,

A more sophisticated technique is the offset method in whichstrings are erected that define a regular shape within the airway.These are usually two vertical and two horizontal wires encompassinga rectangle. Taping from the wires to the rock walls at frequentintervals around the perimeter allows a plot of the airway profile tobe constructed on graph paper.

The profilometer is a plane-table device. A vertical drawingboard is attached to a tripod in the middle of the airway. Tapedmeasurements made from the centre of the board to points around therock walls may be scaled down mechanically or manually to reconstructthe airway profile on the drawing board.

The photographic method entails painting a white line around theperimeter of the measuring station. A linear scale such as a sur-veyor's levelling staff is fixed vertically within the defined pro-file. A camera is located such that it is aligned along a longitu-dinal centre-line of the airway and with its lens equidistant fromall points on the painted line. These precautions reduce perspectiveerrors. The area within the white line may be determined by over-laying the resulting photograph with transparent graph paper.

These time consuming methods tend to be employed for permanentmeasuring stations rather than for temporary survey stations.

In all cases, the cross sectional area of conveyors, ducts orother equipment should be determined and subtracted from the overallarea of the airway.

6.3. PRESSURE SURVEYS

The primary purpose of conducting pressure surveys is to deter-mine the frictional pressure drop, p, that corresponds to the air-flow, Q, measured in each branch of a complete survey route. Thereare essentially two methods. The more accurate is the gauge and tubeor trailing hose method, in which the two end stations are connectedby a length of pressure tubing and the frictional pressure dropmeasured directly. The second method, of which there are severalvariations, involves observing the absolute pressure on a barometeror altimeter at each station.

Although tradition within individual countries tends to favourone or other of the two methods, both have preferred fields ofapplication. In general, where foot travel is relatively easybetween measuring stations, the gauge and tube method can beemployed. Where access is difficult as in multi-level workings or inshafts then the barometer method becomes more practicable.

6-19

Pazg + P when referred to a commondatum.

6-20

ps =

pv =2

and static pressure

U2However, velocity pressure

Pa

p1)2 Pa (6.11)

(See Section 3.4.1 for a fuller explanation of this equation)

- (P2 - z2)9 - Pa(z1 Pa + = F12 = Pa P12 - u22)(U12

F12 (6.10)

Applying these conditions to equation (6.9) gives

Pa = P12

l/V. Furthermore, the frictional pressure drop referred tothat density is given by equation (2.46) as

= pa

(J/W)

If we assume a linear variation in air density between stations 1 and2 then we can adopt an arithmetic mean value of density for the air-way,

F12 = work done against frictionm3/kg

andVP)(m/s2)

specific volume of air (= ;=

(6.9)

where u = air velocity (m/s)Z = height above mine datum (m)

= gravitational acceleration

J/kgF122 1

VdP +s

Z2)g =- (ZlU12_U2 2 2

+

1 and 2, and containing no fan,

tappings of pitot-statictubes sited at the end stations. In practice, of course, the tubingand instrumentation are all within the airway. Let us deal firstwith the essential theory of the method before discussing the prac-tical procedure of gauge and tube surveying.

Theorv

From the steady-flow energy equation (3.25) for an airway betweenstations

6.3.1. Gauge and tube surveys

Figure 6.7 illustrates the principles of gauge and tube survey-ing. A pressure gauge is connected into a length of tubing whoseother ends are attached to the total head

nethod.& tube Measuring the frictional pressure drop between twostations by the gauge

a = high pressure tapping

b = low pressure tapping

Figure 6.7

Pa, then it must be located at

slope and resistance,In shafts or other airways of constant

this is very close to the midpoint. However,this is usually difficult to arrange and it is more practicable tolocate the pressure gauge either at the top or bottom of the shaft.A correction must then be applied to the reading in order to arriveat the frictional pressure drop referred to the mean air density.

6-21

PaIn the great majority of cases no further calculation is

required. This explains why the gauge and tube technique is termeda direct method of measuring the frictional pressure drop in anairway.

To this time, the gauge and tube technique has seldom been usedfor shafts or highly inclined airways although the often difficulttask of measuring shaft resistance would be greatly facilitated byleaving a length of small bore pressure tubing permanently in theshaft. Where the ends of the tube are at significantly differentelevations a complication does, however, arise. It is found that inthese circumstances, the reading depends upon the location of thegauge and increases as the elevation of the gauge decreaseswithin the airway. This phenomenon occurs because the air within thetubing is stationary and, hence, not affected by friction. Thepressure at all points within the tube is, therefore, slightly higherthan outside the tube at corresponding elevations. If the tempera-ture and moisture content inside and outside the tube are the same atcorresponding points it follows that the mean density in the tubemust be a little higher than that in the airway.

If the gauge is to indicate the frictional pressure drop referredto the mean density in the airway,the position of mean density.

-p12 Pstp12 (standardized) =

pst, in order to compare or compound it withfrictional pressure drops measured in other airways then the correc-tion is given as

AP in Figure 6.7.If that measured frictional drop is to be referred to a standardvalue of air density,

p12, referred tothe mean density in the airway between the pitot-static tubes isgiven simply as the pressure gauge reading shown as

ps) as sensed by the total headtapping of a pitot-static tube.

This shows that the frictional pressure drop,

(pv + pt1 pt2 (6.12)

where pt = total pressure

Ps2) Pa(Pv2 + - PSI)(Pvl + P12 =

Hence, equation (6.11) may be written as

= density in tube at position c

6-22

pc

Pt = (Mean density in tube)2

where

PC)(PI +

=2

(Mean density in airway)Pa (PI + P2 )

- Pa)2

(6.15)

If we substitute for

AZg(Pt p12 = + - = AP E:pau22) - (u12 - u22) - P2 q2 (PI

tterrorl' in the gauge reading becomes

p2) (6.14)2

Hence, the

- (PI Pa + Az g Pa += P12 uz2)- ml2

pa, is given as(6.11), the frictional pressure drop in the air-

way, referred to the airway mean density,

- P2)2

(6.13)

Now, from equation

(PlPt +u22

+ AZ g - P2 uJ_2 Pl

- Pb =PC

?

Then AP =

p2 + pb = u22p2

= mean air density within the tube.

And, if the gauge is at the elevation of station 2

pt

Pt2

where P = barometric (static) pressureu = air velocityAZ = difference in elevation between stations 1 and 2

and

Pl + + AZ g PC =U12Pl

PC must equal the totalpressure at station 1 plus the pressure due to the head of static airPb within the tube

Pb* However, the pressure - PC AP = Pb. The gauge

reads PC and that at the low pressure tapping is

Consider, again, Figure 6.7 with the gauge at the lower extremityof the tubing. The pressure in the tubing at the high pressure tap-ping is

ascen-tional slope. However, for this application, stations 1 and 2 remainthe top and bottom locations respectively with air flowing from 2 to1.

6-23

upcast shaft or

2Pl 4

Again, the kinetic energy term involving u values can usually beneglected.

The same equations may be employed for an

PI)+ (6.19)- u22) (P2 (U12 +

p12 = AP(top) 1 +~1AZg

top of the shaft or slope, similarreasoning leads to

P2When the gauge is located at the

2 - (6.18)

P21

z 9 A p12 = AP

Pa. To be precise, this latter term should beapplied even when the airway is level. However, it is normallyinsignificant and may be ignored for practical purposes, giving

p2 tothe mean density

p1 and

PI)4

is the result of converting the velocity pressures at

- (p2u22)W I2 +

(6.15), while

2P2arises from the difference in mean air density between the airway andthe tubing (equation

~2Azg

2P2 4

for a reading taken at the base of the shaft. (6.17)

This is the full form of the equation that allows the reading on thegauge at the base of the shaft or slope to be corrected to mean den-sity for the airway. The term

PI)- (p2- +u22)(q2 + ~2

1Azg

E = AP- p12 = AP

PI)4

giving

- 2p2

(p 2= (6 .1 6)E

u22)(u12 + ~2AP AZ g

P2 (for the same temperature at c and b)P2

then, after some algebraic simplification, we obtain

- PC = P2 + and AP

1 waterproof field book and pencils

6-24

.

I pocket barometer and 1 whirling hygrometer.

. 2 or 3 cans of spray paint for station marking. Chalk or indus-trial type crayons can also be used.

. Short lengths of flexible tubing to connect the pitot-statictubes and gauge to the main tubing. Metal connectors and clampsshould also be carried in case it becomes necessary to repairdamage to the main tubing.

. A continuous length of nylon or good quality plastic tubingbetween 100 and 200m in length. The tubing should be mechanicallystrong so that it can withstand being run over by rubber tyredvehicles or being dragged under doors without permanent damage.An internal diameter from 2 to 3 mm is convenient. Larger tubingmay become difficult to handle while the waiting time of trans-mission of a pressure wave may become unduly long if the tube istoo narrow. The tube should be pressure tested before and afterthe survey.

. A range of diaphragm pressure gauges varying from a full scaledeflection of 100 Pa to the highest pressure developed by any fanin the system. The gauges should be calibrated in the horizontalposition against a primary manometer immediately prior to animportant survey. The use of diaphragm gauges rather thaninclined manometers has greatly improved the speed of gauge andtube surveying.

1.25m in length. Shorterinstruments may be employed for small airways or for use in ducts.

. 2 pitot-static tubes, approximately

Pm = mean barometric pressure in the shaft,2

Practical Procedure

The procedure for conducting a gauge and tube survey commences byassembling the equipment and calibrating the gauges. For conve-nience, a list of the required equipment is given here, together withsome explanatory comments:

Pl + P2where

Pl- for the gauge at the top (6.21)x p12 = AP pm

- for the gauge at the bottom (6.20)P2

and

x p12 = AP pm

1962:-

Approximate but simpler relationships for the correctionsrequired due to gauge location in a shaft were derived by Hinsley in

posi-

6-25

pitot-static tube at each station or substation and walking for-ward. However, it is preferable that the observers exchange

tappings and the pitot-static tube without requiring undueforce.

To make the observation, both pitot-static tubes are held facinginto the airflow, away from the body of the observer and at a posi-tion between one half and two thirds the height of the airway. Thegauge is observed until the reading becomes constant. This may taketwo to three minutes depending upon the length and diameter of themain tube. Light tapping of the fingers may assist in overcoming anyslight frictional resistance of the diaphragm or linkages within thegauge. On completing the gauge reading, the leading observer shouldindicate that fact to the trailing observer either by cap-lamp sig-nals or by a tug on the tube. The barometric pressure, wet and drybulb temperatures are also read and booked by the leading observertogether with the distance between observers. In most cases this isthe known length of the main tube. For shorter distances, themeasuring tape or other means should be used to determine the actuallength.

The final duty of the leading observer is to paint or chalk anindicator mark on the rail or airway side. A second tug on the tubeor a cap-lamp signal indicates that it is time to move on. Theleading observer walks forward, dragging the tube behind him. Whenthe trailing observer reaches the indicator mark he simply stops,grasping the main tube firmly.

The procedure is repeated for each tube length until the nextmain (junction) station is reached. The leading observer is keptbusy while the trailing observer has little to do other than holdinga

pitot-static tube is similarly connected to thepressure tubing. The flexible tubing used for connections should beof an internal diameter that fits snugly on to the main tube, thegauge

tappingsby a short length of tubing. The gauge is then connected in-linebetween the main tube and the total head tapping of a leadingpitot-static tube as illustrated on Figure 6.7. At the rear posi-tion, the second

. and a sharp knife.

The route of the traverse and sites of main junction stationsshould have been established before commencing the observations(Section 6.4). Two persons are required for a gauge and tubesurvey. It is helpful to have the 100 Pa gauge fixed within a boxwith a transparent top,and side holes for extended pressuretappings. Straps around the waist and neck of the observer hold thegauge in a horizontal position.

At the starting station, the pressure tubing is unwound and laidout along the airway in the direction of the second main station. Atthe forward position, the zero setting of the gauge is checked and, ifnecessary, adjusted by connecting the high and low pressure

1OOm flexible measuring tape

Tool kit containing screwdrivers, adjustable spanners (wrenches)

. 1

. the frictional pressure drop between the two stations atthe prevailing airflow.

AS the elevations and velocities can be measured independently, itfollows that the barometric readings can be used to determine thefrictional pressure drop.

6-26

. the air velocities

. the difference in elevation between the stations

10m length of flexible tubingfor this purpose. It takes only a few seconds to attach a gauge ofthe required range. If in doubt concerning the range, a high pres-sure gauge should be used first to establish an approximate pressuredifference, then exchanged for a more appropriate instrument. It isusually sufficient to measure the static pressure across a door.Hence, the two ends of the tubing should be protected against thevery local air velocities that sometimes occur from leakage close toa door. A practical way of doing this is simply to insert the end ofthe tube into one's pocket.

6.3.2. Barometer and altimeter surveys

If the absolute static pressures are measured on barometers atthe two ends of a subsurface airway then the difference between thosetwo measured pressures will depend upon

pitot-static tube(s) may be employed. Inthis case, an anemometer should be held at the position of thepitot-static tube(s) to measure the local velocity. The correspond-ing velocity pressures should be calculated and applied as a correc-tion to the gauge reading in order to determine the frictional dropin total pressure.

Care should be taken at all times to ensure that the pitot-statictubes do not become clogged by dust or other debris. Similarly, inwet conditions, it is vital to take precautions against water enter-ing any tube. Pitot-static tubes or the open ends of pressure tubingshould never be allowed to fall on to the floor during a traverse.

During the course of a pressure traverse, check readings shouldbe taken of the pressure differences across doors between airways.It is convenient to carry a separate

tions only in alternate shifts rather than during any one day. Anexperienced team can progress along a traverse route fairly quickly.Indeed, using modern equipment, it is usually the measurement ofairflows by the accompanying airflow team rather than frictionalpressure drops that dictates the overall speed of the survey (seeSection 6.4).

Each major junction of airways should be a main station withinthe gauge and tube traverse. At each of those junctions, thepitot-static tube should be held at the centre of the junction. Ifhigh turbulence causes excessive fluctuations on the gauge then thestatic tapping(s) on the

(P2/PI)2

6-27

ln RTI - Z2)g - (ZIup

F12 = + -

T2TI = or, for isothermal flow where

T2/TI

(see equation (8.1)) (6.24)

J/ W2 ln

TI)- - R(T2Z2)g - (ZIP2/P1

F12 = + u -22 ln - U12

as:-

ai-\.";,;

Again, we commence with the steady flow energy equation for anairway between stations 1 and 2, and containing no fan. In the usualcase of polytropic flow, the energy equation gives the work doneagainst friction

.~~~ t; ~;~;'-',~J ~~,~~ ~~ ~~~~~~~~~~'~~~~~_~~ ;: ,~~-~~~~~~~~~~~~ '.-

i.5 the United States.early$se of altimeters a the use of a head of water to indicate frictional pressure

drops that led to the /of air

/

become_identicalL. It would seem to have been the concept of a head

s?7;t4.In-most mining countries, barometric pressure surveys are carried

out using direct indicating barometers. In the United States, alti-meters are commonly employed. However, if equation (6.22) is used toconvert the altimeter readings to pressure units then the two methods

Jur SC& 4\- 9 2!.~ Q sL~~L C!LZ ECU> I-A ~ - h~ Ii k a+ have dck~tsrs ; caTed s Q & s7

ck .r,-I i-- p>

kPa

2.03 98.782

1.52 98.800

6-28

PC OC!

(6.26)

ElevationZm

2652

2573

Velocity ControlU barometer

m/st,

14:05 104.61 17.2 14.2

Temperatures

13:42 103.75 15.6 13.0

kPa

-PC

Example. The following two lines are an excerpt from a barometerfield book

StationNo.

1

2

Time Traversebarometer

P

AP, Pl

l), itcan be shown that a more accurate value of the correction is givenas

AP,, may, of course, be positive or negative. Byassuming a series of polytropic processes connecting the controlbarometer (subscript c) to the traverse barometer (subscript

AP, (6.25)

The correction,

P1 +

AP, during thetime elapsed while moving from station 1 to station 2, then the ini-tial value, PI, should be corrected to

pg (equation (6.22)) todetermine the barometric pressure, P, before using equation (6.24).

A complication arises if the barometric pressures at the twostations are not read simultaneously. In this case, the surfaceatmospheric pressure may change during any time interval that occursbetween readings at successive stations. If the atmospheric pressureat a fixed control station is observed to increase by

F12 Pa(see, also, equation (6.10))

In the case of an altimeter, the indicated altitude, h, should bemultiplied by the calibration value of

Pa = p12

p12, referred to anygiven air density, pa.

FI2, can be determined. This, in turn, canbe converted into a frictional pressure drop,

;=air velocity at the barometer (m/s)mean gas constant (from equation (14.14)

As all parameters are measurable in these relationships, the workdone against friction,

=and

= elevation of barometer location (m)= absolute temperature (degrees Kelvin)

Z

(kPa)T

where P = barometric pressure

J/W

6- 29

= 1 01 . 6

- 674.30 J/kg= 0. 9 1 + 744.99

ln(290.35/288.75)(2652-2573)9.81-288.474(17.2-15.6)

2

ln(104.61/103.769)F12 = +

1.522)- (2.032

T2 = 273.15 + 17.2 = 290.35 KTI = 273.15 + 15.6 = 288.75 K

and

T2/TI

where

TI)2 ln

- - R(T2Z2)g - (ZlF12 = +P2/PIu2 2 ln - q2

equation (equation (6.24)) then gives

kPa

The steady flow energy

= 103.75 + 0.019 = 103.769

= 103.75 + 0.018 x98.782

PC

103.75

-AP, PlPI +

kPa

equation (6.26) gives the corrected reading at station 1 as

- 98.782 = 0.018AP, = 98.800

P2f the control barometer registered an increase in atmosphericpressure of

Pl and

OC.

During the time period between taking barometer readings

OC with anarithmetic mean of 288.474 J/kg

R2 = 288.517 J/kg OC and

X)

giving RI = 288.431 J/kg

(I +

x1 = 0. 0091 08 kg/kg dry air

and x2 = 0.009777 kg/kg dry air

Equation (14.14) then indicates the corresponding gas constants as

287.04 + 461.5 XR =

psychrometric equations given in Section 14.6,the moisture contents of the air at stations 1 and 2 were calculatedto be

Using the

for main ventilating shafts, the most accurateresults are obtained by taking readings at intervals down the shaft(Section 8.2.2).

There are essentially two methods of handling the natural vari-ations in atmospheric pressure that occur during the course of thesurvey. One technique is to maintain a barometer at a fixed controlstation and to record or log the readings at intervals of about 5minutes. The second method is the leapfroq procedure in which bothbarometers are used to take simultaneous readings at successive sta-tions. After each set of readings, the trailing barometer is broughtup to the forward station where the two barometers are checkedagainst each other and reset if necessary. The trailing barometer isthen moved on to assume the leading position at the next station.

The traverse procedure commences by the observers synchronizingtheir watches. If the control station method is used, the controlshould be established in a location that is reasonably stable withrespect to temperature and not subject to pressure fluctuations fromfans, ventilation controls, hoists or other moving equipment. A

6-3 0

ca.libration cabinet can be con-structed with the internal pressure controlled by compressed airfeeds and outlet valves. In addition to pressure calibration, theinstruments should be checked for temperature compensation and creepcharacteristics. Modern instruments are stable over the range oftemperatures normally encountered in mines and adapt to a change inpressure within a few minutes.

For an underground barometer traverse only the main junctionsneed be considered as measurement stations. Intermediate substations,as required in the gauge and tube technique, are normally unneces-sary. However,

. 2 or 3 cans of spray paint to mark station numbers

The microaneroids should be calibrated against a primary barome-ter prior to an important survey. A

. waterproof field books and pencils

. 1 2m measuring tape1 anemometer.2 accurate watches.1 whirling or aspirated psychrometer.

. 2 microaneroid barometers (or altimeters) of equal precision

= 122

(See, also, Sections 8.2.2. and 8.3.3. for further examples)

Practical procedure

A barometric survey can be conducted with one observer at eachstation although an additional person at the traverse stationsfacilitates more rapid progress. The equipment required is as fol-lows:-

Fl2~ = 1 01 . 6 x 1 . 2p12 =

and frictional pressure drop referred to standard density becomes

.Thethethe

equipment. In particular, it is inadvisable to rely upon manufac-turers' calibrations of vane anemometers or diaphragm pressuregauges. If the equipment required for calibration is unavailablelocally then the work may be carried out by a service organization orthe instruments returned to the manufacturers for customized cali-bration. The calibration is normally produced as a table of correc-tions against indicated readings and taped on to the side of theinstrument or carrying case. Interpolation from the table can be

6-31

. .

PQ, of each branch. AS airflows and, hence, frictional pres-sure vary with time in an operating subsurface facility, it followsthat p and Q should, ideally, be measured simultaneously in any givenairway. Typically, there are two observers measuring airflows andanother two involved in the pressure survey. The two teams mustliaise closely.

6.4.1. Initial planning

A pressure-volume survey should be well planned and managed.practical work for a major survey commences a week or two beforeunderground observations by assembling, checking and calibrating

airpowerloss,

p/Q2, and R =

0.5m.

The anemometer should be employed to measure the air velocity atthe position of the barometer. There is no need to conduct ananemometer traverse. AS shown in the example given in the previoussubsection, the effect of air velocity is usually small compared withthe other terms in the steady-flow energy equation_

6.4. ORGANIZATION OF PRESSURE-VOLUME SURVEYS

The preceding two sections have discussed the techniques ofmeasuring volume flows, Q, and frictional pressure drops, p, separa-tely. It will be recalled that the results of the two types of sur-vey will be combined to give the resistance,

pre-equivalent to that of the traverse barometer.

each traverse station the following readings are logged:

date, barometer identification and name of observernumber and location of stationtimebarometer readingwet and dry bulb temperaturesanemometer reading

The location of each station should be correlated with surveyorsplans to determine the corresponding elevation. The traversebarometer should be held at the same height above the floor ateach station in order that its elevation can be ascertained to within

location on surface near the top of a downcast shaft and shaded fromdirect sunlight is usually satisfactory. A recording barometer maybe employed at the control station, but only if it provides a

- then it is necessary to take thosemeasurements in one of the airways only. However, to obtain thetotal airflow in that composite branch of the network, it will benecessary to take flow measurements at corresponding points in each ofthe parallel airways. Airflow measuring stations should be selectedand marked on the plan and, also, on the walls of the airway.

The subsequent employment of survey data for ventilation networkanalysis and forward planning (Chapters 7 and 9) should be kept inmind during the management of ventilation surveys. The identifica-tion number assigned to each network junction should give an indica-tion of the location of the junction within the mine. In multi-levelworkings, for example, the first integer of station numbers may beused to indicate the level.

A pre-survey briefing meeting should be held with all observerspresent. Each observer should be fully trained in survey procedures,use of the instruments and techniques of observation. The traverseroutes and system of station identification should be discussed,together with an outline schedule covering the days or weeks requiredto complete the survey.

6.4.2. Survey management

During production shifts, the airflows and pressure drops in anunderground mine are subject to considerable variation due to movementof equipment, changes in resistance in the workings and opening ofventilation doors. Hence, the best time for ventilation surveys iswhen the mine is relatively quiescent with relatively few peopleunderground. During the period of a survey, the observers shouldbe prepared to work at weekends and on night shifts_

6-32

- and the gauge and tube method is employedfor the pressure measurements

carried out at the time of measurement so that the reading, correctionand corrected observation can all be logged immediately_

The mine plan should be studied carefully and the routes of thesurvey selected. A full mine survey will include each major venti-lation connection to surface and the infrastructure of airways thatcomprise the primary ventilation routes. Subsidiary survey routesmay be appended to include individual working districts or to extenda data bank that exists from previous surveys. The routes should bechosen such that they can be formulated into closed traverse paths orloops within the ventilation network of the mine. Branches thatconnect to the surface close through the pressure sink of the atmos-phere. A main loop in a large mine may take several days to survey.However, each main loop should be divided into smaller subsidiaryloops each of which can be closed within a single day of surveying.

A reconnaissance of the mine should be carried out, travellingthrough all airways selected for the primary traverses andestablishing the locations of main stations. These are normally atjunctions of the ventilation system. Where two or more airways areadjacent and in parallel

com-

6-33

Statesthat the algebraic sum of standardized pressure drops around anyclosed loop must also be zero, having taken fans and natural ventila-tion pressures into account. In a near level circuit, the closingerror of a pressure loop may be expressed as the actual closuredivided by the sum of the absolute values of the measured frictionalpressure drops around the loop. This should not exceed 5 per cent.The check measurements of pressure differentials across doors areinvaluable in tracing or distributing observational errors. In thecase of loops involving significant changes in elevation such asshaft circuits, the sum of standardized pressure drops will be a

zero. Kirchhoff II Kirchhoff'I requires that the algebraic

sum of airflows entering any junction is

Kirchhoff's Laws should be checked both at the timeof observations wherever practicable and, also, during the datatransposition at the end of each shift. These laws are discussedfully in Chapter 7. Briefly,

Although the frictional pressure drop and the airflow should,ideally, be measured simultaneously in each leg of the traverse, thisis often not practicable. Nevertheless, the teams should stay fairlynear to each other so that there is a minimum delay between the twosets of measurements in a given branch. With experienced observersthe teams maintain close liaison, assisting one another and alwaysbeing conscious of the activity of the other team. This avoids theinfuriating situation of the pressure team opening doors to take acheck reading while the airflow team are in the middle of an anemo-meter traverse. Friendships have been known to suffer on suchoccasions.

Immediately following each shift the two teams should check allcalculations carried out underground, transcribe the results of thatshift's work from the field books to clean log sheets and, also, onto a large scale copy of a mine map. Positions of measured airflowsand pressure drops should be reviewed by both teams to ensure compa-tibility of those measurement locations and to correlate identifica-tion of station numbers. Any difficulties encountered during theshift should be discussed. The final half hour or so of each workingday may be spent in reviewing the ground to be covered in the fol-lowing shift and the allocation of individual duties.

6.4.3. Quality assurance

It is most important that control is maintained over the qualityof all aspects of an important ventilation survey, from initial cal-ibration of the instruments through to the production of finalresults. Field books or booking sheets should be laid out clearlysuch that persons other than the observers can follow the recordingof observations and calculations carried out underground. All cal-culations should be checked by someone other than the originator.Most of the calculations involved in ventilation surveys are quitesimple and may be carried out on a pocket calculator. The exceptionis for barometric surveys where a verified program for a personalcomputer is very helpful. Commercially available spreadsheetsoftware can readily be adapted for this purpose.

Adherence to

psychrometricconditions throughout the traverse paths.

6-34

variation in pollu-tant levels and, secondly, it enables zones of emission of gases,dust, heat and humidity to be identified_ Measurements of gasconcentration are often made as a normal part of a pressure-volumesurvey in a gassy mine. Similarly the observations of barometricpressure, wet and dry bulb temperature made during a pressure surveymay be used to compute and plot the variations in

the

air-power losses may be shown on a colour-coded map in order to highlightsections of airways that are particularly expensive to ventilate.

6.5. AIR QUALITY SURVEYS

While pressure-volume surveys are concerned with the distributionof airflow around a ventilation system, the subsurface environmentalengineer must also maintain control of the quality of that air, i.e.the concentrations of gaseous or particulate pollutants, and thetemperature and humidity of the air. Such measurements may be madeto ensure compliance with mandatory standards and with a regard forthe safety and health of the workforce.

Details of the techniques of measuring and quantifying levels ofdust, gas concentrations and climatic conditions are given in Chapter23, 11 and 14 respectively. A set of such measurements made in asystematic manner around a continuous path is known as an air qualitysurvey. This procedure may be employed for two reasons. First, itprovides a means of tracking and quantifying

airpower losses andfriction factors, and also provides a data base from which a computermodel of the mine ventilation network can be constructed (Chapter 9).Additionally, the resistances, resistance per unit length and

effects. Thelatter may be determined independently from temperature and pressuremeasurements as discussed in Section 8.3.

It is vital that good records are kept of each phase of a survey.The survey team leader should maintain a detailed diary of the acti-vities and achievements of each working day. This should include theclean log sheets of results transcribed from the field books at theend of each shift.

The conclusion of a major survey should see the establishment ofa spreadsheet type of data bank or the extension of an existing databank, holding the frictional pressure drop and corresponding airflowfor every branch included in the survey. Other details such as thedate of the observation, names of observers, instrument identifica-tion and dimensions of the airway may be included_ The data bank maythen be used to calculate airway resistances,

ventilating bination of observational errors and natural

longwallface. 2nd U.S. Mine Ventilation symposium, Reno,NV. pp. 531-542

Mine Ventilation Society of South Africa, (1982). EnvironmentalEngineering in South African Mines. Mine Ventila-tion Society of S. Africa, Kelvin House,2 Hollard St., Johannesburg, S. Africa.

National Coal Board, (1979). Ventilation in Coal Mines: A Handbookfor Colliery Ventilation Engineers_

6-35

M-J., (1985). The resistance to airflow on a

- reproduced Jnl. MineVentilation Society of S. Africa, Vol 9, No. 3,Sept. 1980

McPherson,

M-J., (1969). A new treatment of mine barometer surveys.Mining Engineer, Vol 129, No. 109, pp. 23-34

McPherson, M.J., and Robinson, G. (1980). Barometric survey of shaftsat Boulby Mine, Cleveland Potash Ltd., Trans. Instof Min. and Met. Vol 89

(1964). An enquiry into the principles and mutualinteraction of natural and fan ventilation.The Mining Engineer, Vol 124. N O. 49, pp. 63-78

McPherson,

F-B.,

F-B., (1962). The assessment of energy and pressure lossesdue to air-flow in shafts, airways and mine circuitsThe Mining Engineer, Vol 121. N O. 23, pp. 761-777

Hinsley,

W-J., (1982). Mine Ventilationand Air Conditioning. Wiley Interscience

Hinsley,

H-L., Mutmansky, J. and Wang, Hartman

W-1

N-E. Atlanta, GA. 30329, USA

British Standard 848 Part 1 (1963). Methods of Testing Fans forGeneral Purposes. British Standards Institution,2 Park St., London

ASHRAE, 1791 TullieCircle,

ASHRAE Handbook (1985). Fundamentals Volume.

-System Performance in the Field.

American National Standard (1985). Laboratory Methods of Testing Fansfor Rating. AMCA, 30 West University Drive,Arlington Heights, IL 60004

Control Assoc. Inc. (1976). AMCA Application Manual(Part 3). A guide to the Measurement of Fan

BIBLIOGRAPHY

Air Movement and

6.1. PURPOSE AND SCOPE OF VENTILATION SURVEYS6.2. AIR QUANTITY SURVEYS6.2.1. Rotating vane anemometer6.2.2. Moving Traverses6.2.3. Fixed point measurement6.2.4. Density correction6.2.5. Swinging vane anemometer (velometer)6.2.6. Vortex-shedding anemometer6.2.7. Smoke tubes6.2.8. Pitot-static tube6.2.9. Fixed point traverses6.2.10. Hot body anemometers6.2.11. Tracer gases

6.3. PRESSURE SURVEYS6.3.1. Gauge and tube surveys6.3.2. Barometer and altimeter surveys

6.4. ORGANIZATION OF PRESSURE-VOLUME SURVEYS6.4.1. Initial planning6.4.2. Survey management6.4.3. Quality assurance

6.5. AIR QUALITY SURVEYSBIBLIOGRAPHY