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Temperature and humidity in headings at depth
Int. J. Rock Mech. Min. Sci. Vol. 4, pp. 55-70. Pergamon Press Ltd. 1967. Printed in Great Britain.
T E M P E R A T U R E A N D H U M I D I T Y IN H E A D I N G S AT DEPTH J. T. RUBEN* and D. F. SHARP~ Mining Research Establishment, Isleworth, Middx.
(Received 26 May 1966) Abstract---Details of thermal and psychrometric surveys of the ventilation air in three deep headings in British mines are presented. The results indicate the relative significance of factors controlling climatic conditions in headings. In particular the effect of moisture evaporation on underground temperatures is discussed. 1. INTRODUCTION SINCE coal has to be extracted from ever greater depths environmental conditions may be expected to have art increasingly important effect on the lengths to which headings can be driven. As art experimental approach to the investigation of this problem a number of surveys of climatic conditions were carried out in three deep headings in British mines, two having force ventilation and the other being exhaust ventilated. The results demonstrate typical conditions in headings and illustrate the relative importance of the various factors influencing mine climate. The observations provided information which was of use during the development of theoretical considerations by JORDAN [1] on heat transfer problems in headings. The surveys include measurements of air temperatures, moisture content and airflow. Effects of ventilation duct leakage are considered and a limited comparison is made between force- and exhaust-ventilation systems. 2. THE EXPERIMENTAL SITES
Observations were made in a heading at a depth within the range 2700-3000 ft at each of three collieries. In one of the force-ventilated headings, referred to here as heading A, six surveys were carried out; two surveys were made in the other force-ventilated heading (heading B) and also in the exhaust-ventilated heading (heading C). Detailed descriptions of the three headings are given below. 2.1 Heading A The six surveys in this heading were carried out over a period of one month, successive surveys being separated by a few days, thus showing the short-time variations in the quantities observed. The heading was level and situated at a depth of 3000 ft, where the virgin strata temperature was estimated to be 92°F. The heading was advancing at the rate of 15-20 yd per week through strata consisting mainly of sandstone with some layers of shale, and at the time of the surveys its total length was approximately 600 yd. The roadway was supported by 16 × 12 ft steel arches and lined with corrugated steel sheeting. Ventilation was provided by a * Present address : BICC Central Research and Engineering Division, Wood Lane, London, W.2. t Present address: Avco Corporation, Lycoming Division, Stratford, Corm, U.S.A. 55
56
S. T. RUBEN AND D. F. SHARP
single-stage axial flow fan of approximately 11 h.p. forcing air along a 24-in. duct to the face A diagram of the heading is shown in Fig. I. 2.2 Heading B The two surveys in this level heading were carried out with an interval between them a t three months. The depth of cover was 2950 ft and the estimated virgin strata temperature 91 °F. The heading was advancing at a rate of about 20 yd per week through sandstone with
~
00 F~n I
2 4 in Duct
-
.... I
__._f~-
/
~
....
......
~
-
,,'-r"
5yd .~.'~ J,,,~-'~,~ . . . . . . . . . ~85,td- ~
I 2
_. . . . J
6
5
3
Survey reference
AI A2 A3 A4 A5 A6
Distance between stations { v d )
30 33 58 63 78 90
13 8 8 8 8
I
FIG. I. Positions of measuring stations in heading A. a little shale and two coal seams. Its length at the time of the surveys was 750 and 950 yd. The arch dimensions were also 16 × 12 ft and there was a corrugated sheet steel lining. Ventilation was by a two-stage axial flow fan at the outbye end of a forcing 24-in. duct system. A diagram of the heading is shown in Fig. 2. Survey ntoke
I /I 2_J_J
u~
Fan
"5-vd
-p i
'
,
' /
20ya ~
~
160yd-~,,,,i
,~
- tSOy~
~
2 4 in. Duct
~ ' ~
,2
3 4
5yd
Survey
B 2
FIG. 2. Positions of measuring stations in heading B.
2.3 Heading C The two surveys in this exhaust-ventilated heading were separated by a period of five months. The measured virgin strata temperature, at a depth of 2740 ft, was 88°F. The heading passed through strata consisting predominantly of shale with a few thin coal seams. After 180 yd there was a junction with a subsidiary heading. At the times of the two surveys the length of the main branch was 415 and 765 yd and that of the subsidiary branch 160 and
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
57
445 yd. Surveys were made in both branches. Here again the roadway size was 16 x 12 ft and it was lined predominantly with corrugated steel sheet. A 24-in. diameter duct extended from the faces through doors at the outbye end of the heading into the upeast shaft, as shown in Fig. 3. No auxiliary fan was used, the pressure of the main mine fan being utilized.
.~\.. \ o \
6,5 I
\\\
I I=.
--
4 12,r'~td
~
7
i
3 2
) i -.-,'1',~,~
• il ?yd
~
¢'-~1 tl II
I/1
I/I
I/ I
'
,,
u~o~) s~f)
, e."%,\\\
<'." "
,'
)
. :X'..\,.X
÷,..¢X, \ ,,,,,
L I I1140yd Iii Iq I
FIG. 3. Positions of measuring stations in heading C.
3. EXPERIMENTAL
PROCEDURE
Measuring stations were established at intervals along each of the headings, their positions being shown in Figs. I-3. At each station the following quantities were measured by the means shown.
3.1In the duct O) Air velocity by pitot-static tube and manometer, and hence volume air flow, according to BS 1042:1943 [2]. (ii) Dry and wet bulb air temperatures by mercury-in-glass thermometers. (iii) Static pressure by manometer. 3.2 In the roadway (i) Dry and wet bulb air temperatures by Assmann psychrometer. (ii) Area and perimeter of roadway by M.R.E. protilograph [3]. In addition, the quantity of air discharged by the forcing systems was determined from measurements of air velocity made with an anemometer placed axially at the duct discharge [4], and the barometric pressure in the heading was measured. An estimate was also made of the volume of air released by any compressed-air operated machines, which augmented the ventilation in the return. Airflow at adjacent stations was used to determine the leakage coefficient of the duct between these stations. Leakage coefficient is defined [5] as the volume of air, in ft3/min, which leaks from 100 ft of duct at a uniform excess pressure of 1-in. water gauge. The measured
58
J. T. RUBEN AND D. F. SHARP
leakage was corrected for static pressure by dividing it by the square root of the mean of the pressures at each end of the section of duct considered. The dry and wet bulb air temperatures were used to evaluate the enthalpy (total heat) and moisture content* per pound of dry air according to BS 1339:1946 [6]. 4. RESULTS The results of the surveys, identified by the reference numbers in Table 1, are summarized in Tables 2, 3 and 4 for headings A, B and C respectively. Tables 2(a), 3(a) and 4(a) show the airflow in the duct at the ends of each section between two measuring stations, and the leakage coefficient of that section of duct; a figure is also given for the over-all leakage--in the TABLE 1. IDENTIFICATIONOF SURVEYS
Survey Length of Barometric Virgin strata Heading Systemof reference Survey heading pressurein temperature ventilation number date (yd) heading (°F) (in.Hg) A
Force
A1 A2 A3 A4 A5 A6
Jan. 19 Jan. 21 Feb. 2 Feb. 5 Feb. 12 Feb. 18
593 596 616 621 636 648
32.08 31"52 33'03 32.85 32"84 33'10
92
B
Force
BI B2
Mar. 19 June 10
741 938
33.07 32'44
9l
C Main Subsidiary Main Subsidiary
Exhaust CI CI C2 C2
Feb. Feb. July July
407 143 1035 415
33'10
88 16 16 14 14
31"59
forcing systems the percentage of the air entering the duct that is lost through leakage, in the exhaust-ventilated system the percentage of air leaving the duct that has entered it as a result of leakage. Tables 2(b), 3(b) and 4(b) show the dry bulb (DB) and wet bulb (WB) temperatures, moisture content and enthalpy per pound of dry air for the ventilating air in the duct and roadway at each measuring station. Since in the short distances between measuring stations the temperatures, moisture content and enthalpy of the air show only small increases, it is necessary to measure dry and wet bulb temperatures accurately if the results are to have any significance. The thermometers used for most of the present surveys were accurate to 0.2°F, corresponding to uncertainties of about 0.0002 lb/lb dry air in moisture content and 0.2 B.t.u./lb dry air in enthalpy. In some cases, however, a precision of only about 0.5°F in the temperature measurements was possible. * In this paper moisture content is expressed as the mass of water vapour associated with unit mass of dry air, given in the same units, i.e. lb water vapour per lb of dry air, or lb/lb. It is sometimes expressed in grains/lb; to convert to lb/lb the value in grains/lb should be divided by 7000. In the revised edition of the British Standard (BS 1339:1965) the term moisture content has been replaced by mixing ratio,
59
TEMPERATURE A N D HUMIDITY IN HEADINGS AT DEPTH
TABLe2(a). HEADINGA: AmrLOWANDLEAKAGE Duct Survey section Length reference (between (yd) number stations)
Airflow in duct section Beginning (fta/min)
End (fta/min)
Over-all Leakage leakage coefficient(percentage of air lost)
2-3 3-4 4-5 5-6
15 183 183 184
6900 6750 5100 4050
6750 5100 4050 3300
170 180 130 110
2-3 3-4 4-5 5-6
15 183 183 184
6250 6150 5500 4600
6150 5500 4600 3700
70 60 110 130
A3
2-3 3-4 4-5 5-6 6--7
15 183 183 184 58
5850 6750 5550 5550 5200
6750 5550 5550 5200 5100
120 130 0 60 130
25
A4
2-3 3-4 4-5 5-6
15 183 183 184
7300 7100 4700 4750
7100 4700 4750 4250
290 280 -80
42
A5
2-3 3-4 4-5 5--6 6--7
15 183 183 184 78
6950 6750 4800 4700 3850
6750 4800 4700 3850 3550
200 210 12 150 340
49
A6
2-3 3-4 4-5 5-7
15 183 183 274
6850 6650 4800 4700
6650 4800 4700 4650
220 190 10 5
31
A1
A2
52
41
The inaccuracy of the airflow measurements should be no greater than 2 per cent. Owing to leakage the airflow in a duct falls as the face is approached, but in two cases in the surveys (A4 and B2) an increase was actually indicated. This throws some doubt on the precision of the airflow measurements. As a result of the experience gained in these surveys the possibility of some improvements in the measuring techniques became apparent: (i) In any future surveys of this kind the position of the first measuring station in the roadway should be chosen to avoid any possibility of the mixing of intake and return air. A position about ten roadway diameters (say 50 yd) inbye of the end of the duct or of any roadway junction is suggested. (ii) There is some uncertainty in the discharge temperatures from the forcing ducts, resuiting from the entrainment of return air [7]. It is suggested that where possible discharge temperatures should be measured by an Assmann psychrometer held as far up the duct as possible. (iii) Wet bulb thermometers for use in duets suffer from radiation errors. Wet bulb temperatures of ducted air should therefore be measured by means of shielded duct thermometers [8].
J. T. RUBEN AND D. F. SHARP
60
TABLE 2(b). 1-IEADINGA: AIR TEMPERATURESAND MOISTURECONTENT In duct Survey reference number
In roadway
Station
D.B. Temp. (°F)
1" 2 3 4 5 6 7t 8~t
74"8 78"4 78'3 80"4 83.4 84"2 ---
61'5 62'8 62"8 63.4 64.2 64'8 ---
7"7 7"6 7"6 7'5 7-4 7"7
1" 2 3 4 5 6
76"0 78.8 78.2 81"4 86'4 84"6
62"5 63"3 63"2 64.0 64"5 64"8
8"3 8 "0 8-1 7'9 7.8 7.7
19"6 20"0 20'0 20"5 20"9 21"1
7t
--
A3
1" 2 3 4 5 6 7t
73"2 76"8 76"8 79"8 82"6 83"8 81"8
59"9 60"8 60"8 61 "2 62"9 63"9 64"9
6.8 6"4 6"4 6"0 6.4 6.7 7"9
17.4 17.8 17.7 18-0 19"2 19-8 20.6
A4
1" 2 3 4 5 6 7t
74"1 77"0 76"0 80"4 82"5 83"6 83"6
59'1 60"5 60"7 61 "5 62"9 63"4 63"4
6"2 6"3 6"5 6"1 6"5 6"8 6"6
16.9 17.7 17"8 18"3 19-2 19"6 19-6
A5
1* 2 3 4 5 6 7t
72"9 76"7 76"5 79"6 82'6 83'8 83'2
59"5 61"1 61"3 61 '0 63"1 64"0 63"8
6.7 6"7 6"8 6.0 6"6 6.9 6"9
17.1 18.0 18'2 18.0 19"4 20"0 19.7
1* 2 3 4 5 6 7t 8+*
74"0 77"3 77"1 82"1 83"2 84"5 84"0
60"0 61 "8 61"8 62"1 63"4 64"6 71 "5
6.7 6"8 6.9 6"9 6-6 7.0 11.9
17.4 18"4 18"3 18"4 19"5 20.2 25"6
AI
A2
A6
* Station before fan. t Duct discharge. Face.
W.B. Moisture Temp. content Enthalpy (°F) (10-alb/lb) (B.t.u./lb)
-
18'7 19"5 19"5 19"9 20"4 20"9
D.B. Temp. (°F)
W.B. Moisture Temp. content Enthalpy (°F) (10-31b/Ib) (B.t.u./Ib)
83"0 85.0 86.0 82"0
70-5 72.0 72.5 70.5
11'9 12.6 12'9 12.2
25.4 26'6 27.0 25'4
79.0
70.0
12'5
25"0
83"0 85.0 86'5 85.7
73"0 73"5 74.0 70"5
14'2 14"1 14.1 12.6
27.8 28.2 28"6 25'6
83'0
69"5
11-5
24"8
78.8 84-3 84"9 81 "8
66.4 70.8 70"8 68"9
9.6 11.4 11 "2 10-6
21 "8 25.0 25"0 23"6
81 "8
66"9
9"3
22"1
75.8 82"2 82"2 80"8
66"5 71 "8 70-4 67"2
10"4 12.8 11 "7 9"8
22"0 26.0 24"9 22"4
81 '8
66"8
9"2
22"1
77"5 82.1 82.9 80"8
67.5 72"8 72.6 71 '2
10.8 13'8 13.2 12"6
22.7 27"2 26.7 25.6
79'8
68"5
10.9
23"4
81"0 84-9 85-5 84.0
71"0 75-0 76'0 75-5
12-0 14-9 15-2 15.1
25.1 28"7 29-5 29.1
85.0
74"5
14.1
28.2
-
61
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH TABLE 3(a). ~ I N G
Survey reference number
B: AIRFLOW AND LEAKAGE
Duct section (between stations)
Length (yd)
Beginning (fta/min)
End (fta/min)
4-567-
5 6 7 9
159 204 155 313
8500 8050 6650 6400
8050 6650 6400 5700
36 98 30 120
4- 5 5- 7 7-11 11-12 12-13
159 359 160 150 95
7350 7100 6500 6750 6500
7100 6500 6750 6500 5400
22 22 -41 580*
B1
B2
Airflow in duct section I.~akage cocfllcient
Over-all leakage (percentage of air los0
33
27
* Duet damaged 10 yd from discharge end just prior to measurements being taken.
T ~ t ~ 3(b). HEADING B: A m TEMPERATURF~AND MOISTURECONTENT In duct Survey reference number
B1
B2
In roadway
D.B. Temp. (°F)
W.B. Temp. (°F)
1 2* 3 4 5 6 7 9t 10~
80"5 81"8 87.8 87"6 87"4 87-4 87"4 85"8
67"1 68"0 70.8 71.0 71.1 69.9 70"1 70"9
9"7 10"0 10.4 10.6 10.6 9"8 10"1 11"1
22.2 22.9 24.9 25-0 25"0 24"1 24"4 25.1
1 2* 3 4 5
83'4 85"2 91.2 91"2 91"2 90.1 89.4 87.8 87"8
73.0 74.1 76.3 76"1 76.1 75.6 75.7 75.6 75.2
13"6 14.0 14"4 13"9 13.9 13"9 14.3 14-6 14"3
27"3 28"2 30"1 29"6 29"6 29.3 29"5 29.5 29.1
Station
7 11 12 13t 14++ * Station before fan. t Duct discharge. Face.
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb)
D.B° Temp. (°F)
W.B. Temp. (°F)
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb)
87"3
75"4
14.3
29.0
87"2 87.7 87"7 87"7 85.8 83.1
76.1 76.1 75"3 75"4 71.6 71.9
14.9 14.8 14.1 14-2 11 "6 12"5
29"7 29.6 28.9 29"0 25.7 26"0
83-6 89"3 89.3 89"4 89"6 89-1 88.0 86.2 87-8 85.5
73.0 79.8 79"8 80"7 80.7 80"4 74-4 77.9 75.2 76.3
13.5 18"0 18"0 18"8 18-8 18"6 13"6 17-0 14"3 15"8
27"3 33.6 33"6 34"6 34.6 34"3 28-6 31.3 29.1 30"1
62
J. T. RUBEN AND D. F. SHARP TABLE 4(a). HEADING C: AIRFLOW AND LEAKAGE Duct section (between stations)
Survey reference number
Airflow in duct section Length (yd)
2"2-2' 1 4-3 2 '3"1"-2 If
CI
63 117 45
2-1
C2
Beginning (ft3/min)
135
2.3-2.1 8-7 7-3 2 .3"t"-21f
128 180 355 45
2-1
135
End (ft3/min)
Leakage coefficient
2000 1700 2950~k 3150f 7350
3150 2950 7350
1840* 800 1130
9850
440
2950 1750 2000 4550\ 4700f 10,650
4700 2000 4550 10,650
2390* 650 47 290
13,300
350
Over-all leakage (air leaked in as percentage of duct discharge)
62
65
N.B. Duct branches between stations 2 and 3. * High leakage caused by inferior duct which was subsequently replaced. TABLE 4(b). HEADING C" AIR TEMPERATURESAND MOISTURECONTENT In duct Survey reference number
C1
C2
D.B. Temp. (°F)
W.B. Temp. (°F)
1 2 3 4 5* 6~+
67"5 72"6 74"8 77"4 78'0
60"4 62"8 65'9 69-4 73"3
8"4 8"8 10"2 12'1 14'8
2.1 2.2* 2.3,+
71 "6 71 "0
60'0 59"4
1 2 3 7 8t 9*+
79'9 79.3 80"6 84.0 80"9
66"8 68.4 69'4 74.4 74'9
Station
2.1 2.3 2.4* 2.5++
!
.
78"4 79"8
.
.
69.8 65"3
In roadway
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb.)
Temp. (°F)
W.B. Temp. (°F)
17"6 19"3 21"4 24'1 27"3
65"0 65"0 66"5 74"0 78"0 77.5
57"0 57.0 58'0 62.5 73'3 73.0
6"9 6"8 7"3 8"2 14'8 14"9
15"5 15"4 16"2 19.0 27"3 27.2
7"3 7"0
17-4 17"0
63.0 71"0 75"0
59"0 59.5 68"0
8"7 7'1 11"6
16.9 17-1 23"0
10"2 11 "4 11"9 14"3 16"2
22"7 23.8 24"7 28.2 29-5
75"2 75'1 76"0 84"0 84"5 79"9
61"8 61"2 62"0 68"5 71.8 74"4
8'0 7"9 7'9 10'4 12"8 16"0
19"1 18"9 19"2 23"9 26"4 29,1
12"7 9"2
25"2 21 "6
76"5 79'9
62"4 65"4
8"0 9"2
19'5 21 '6
81"0
77"5
18"4
32"0
.
D.B.
Moisture content Enthalpy (10-Zlb/lb) (B.t.u2/lb)
I
N.B. Heading branches between stations 2 and 3. * Duct intake. t Duct intake situated between this station. and face, as shown in Fig. 3. :~ Face.
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
63
5. D I S C U S S I O N
5.1 The effects of intake conditions and fan power The temperature of the air at the face of a heading is affected by the temperature of the intake air, by heat due to the fan in the case of a forcing ventilation system, by heat transfer from the strata to the intake air, by the presence of men and machines, by oxidation of coal and by evaporation of water which may extract part of the necessary latent heat from the air. One of the most important of these factors in headings at great depth is heat transfer from the strata, as a result of which the temperature of the intake air tends to approach the virgin strata temperature as it travels inbye. Any variations in intake temperature will be largely reproduced at the face of a short heading, but as the heading advances their effect will become progressively less important. The moisture content of intake air in a forcing duct will remain substantially constant, although as the air warms up its relative humidity will decrease. It is therefore important that, if it is desired to keep down the temperature and moisture content at the face of a heading, the air supplied to the intake should have a temperature and moisture content as low as possible, particularly in the early stages of driving the heading. Thus in the exhaust-ventilated heading C [Table 4(b)] the two intake temperatures differed by 10°F as a result of seasonal changes and at the inbye end of the main duct this difference was still as large as 6°F. In a forcing duct the fart increases the pressure of the air, and if this process is assumed to take place adiabatically it may be shown that the accompanying temperature increase is given approximately by A0.4 = 4.8 AP where A0A is the adiabatic increase in temperature in degrees Fahrenheit and AP the static pressure increase in inches of mercury. Owing to inefficiency of the fan the actual temperature increase, A0, is greater than A0a; the fan thermal efficiency, which is given by A0A _ 4.8 AP -A0 A0 can thus be evaluated. In the surveys in heading A the efficiency averaged 36 per cent and in heading B 48 per cent. Had it been possible to employ 100 per cent efficient fans in the two headings while maintaining at the observed level the air quantities delivered to the face the duct air temperatures at the beginning of the headings would have been reduced by 2°F and 3 °F respectively. An interesting feature of survey B2 is that the fan raised the air temperature in the duct to nearly virgin strata temperature, with the result that there was no rise in temperature as the air passed along the duct. Indeed, the temperature dropped as it approached the face, presumably as a result of water evaporating into the return air which cooled the return and hence the intake air, as discussed later. In force-ventilated headings the temperature rise due to the fan may on some occasions be obviated by using the pit ventilation pressure, rather than a fan, to produce ventilation. Fan heat has of course no effect on face temperatures in exhaust-ventilated headings. 5.2 Duct leakage In the course of these surveys some sixty measurements of airflow were carried out, enabling forty-four estimates of leakage coefficient for sections of duct to be made. The results
64
J. T. RUBEN AND D. F. SHARP
are presented in Tables 2(a), 3(a) and 4(a). Regarding leakage the N.C.B. Information Bulletin [5] states: For satisfactory and efficient ventilation of any underground heading the duct used should be so designed, installed and maintained that its leakage coefficient is not more than 20-30 ft3/min. There should be no difficulty in attaining this standard in normal conditions. The tables, however, show leakage coefficients ranging from 100 to 300 in heading A and from 20 to 120 in heading B. In the exhaust-ventilated heading C only one value below 290 was recorded while three of the nine values measured there exceeded 1000; two of these, however, were due to inferior ducting. The values obtained in heading A and, especially, in heading C are thus considerably higher than is usually considered acceptable. The duct installation in heading B, which was similar to that in heading A except that the joints were sealed with tape, was generally satisfactory. A leaky duct system greatly impairs the effectiveness of the ventilation. The quantity of air reaching the face is less than with a good system, and this can only increase dust and gas concentrations at the face because of the relative inability of the air to scour the face, as welt as increasing the temperature. To counteract the effects of this leakage more air is required at the fan. This necessitates higher pressure and consequently a more powerful fan, which in force-ventilated headings can only lead to higher initial temperatures. 5.3 Heat and moisture pick-up by the ventilating air The data gathered on moisture content and enthalpy are summarized in Tables 5, 6 and 7, which repeat the temperature, moisture content and enthalpy of the air at the intake, duct discharge (in the forcing headings), face and at the hottest point in the return. The Tables also show the increases in these quantities along the duct and roadway, and between the duct exit and face in the force-ventilated headings. One interesting fact demonstrated by these tables is the very large gain in moisture of the air between the duct discharge and the face of a force-ventilated heading. This confirms previous findings in another heading [9] and is explained by the work of SmJa~rLEWORTH[7]. Experiments with a ventilation model of a heading showed that the air discharged from the duct does not immediately return down the heading; a large proportion is entrained by the air issuing from the duct, establishing a circulatory airflow pattern between the duct exit and the face. Owing to the relatively long time the air spends in this region and to its low relative humidity after having been heated in the duct a large amount of water evaporates into it. The magnitude of the moisture pick-up between the duct exit and the face appears to depend on the nature of the activities at the face, as shown in Tables 5 and 6. In heading A moisture pick-up is least during the erection of arches and highest while loading and water spraying for dust suppression. It is not quite so high when only spraying, or when wet drilling. Only two figures are available for heading B, and these are inconclusive owing to the very different levels of moisture content in the duct on the two occasions. The moisture pick-up of the return air in heading A, while still appreciable, is usually less than in the small region between the duct exit and the face. In heading B, on the other hand, less moisture is picked up in this region than in the return, and this is probably because the discharged air has a much higher relative humidity in this heading than in heading A and is therefore less capable of taking up further moisture. In support of this explanation, the table shows that the larger the pick-up at the face of heading A (giving a higher relative humidity in the return) the smaller the moisture pick-up in the return.
t~
I
.~
Intake (3 in duct) Discharge (7 m duct) Face Max. return (4) Intake (3 in duct) Discharge (7 m duct) Fac¢ Max. return (4) Intake (3 in duct) Discharge (6 m duct) Face Max. return (5)
Max. return (4)
Intake (3 in duct) Discharge (6 in duct) Face Max. return (5) Intake 0 in duct) Discharge (6 in duct) Face Max. return (5) Intake (3 in duct) Discharge (6 m duct) Face
Position and station number
64"8 69.5 74.0 60"8 63"8 66.9 70-8 60-7 63"4 66.8 71-8 61"3 63"8 68-5 72"8 61"8 64"6 74"5 76"0
83"8 81 "8 84.3 76"6 83"6 81-8 82-2 76-5 83-2 79"8 82"1 77"1 84.5 85"0 85.5
64-8 70"0 72-5 63"2
62-8
(°F)
W.B. Temp.
84.6 83-0 86-5 76"8
84.2 79-0 86-0 78-2
78-3
Temp. (°F)
D.B. ~
* Virgin strata temperature = 92°F.
A6
A5
A4
A3
A2
AI
Survey reference number
7.0 14-1 15"2
6"9 10-9 13-8 6-9
6-6 9-2 12-8 6-8
6"7 9-3 11-4 6'5
7"7 11"5 14.1 6"4
7-7 12"5 12.9 8.1
7.6
Moisture content h (10 -3 lb/lb)
2"6 3"6
0"I 7"1 1"1
20"2 28-2 29-5
19.7 23.4 27-2 18-3
19"6 22.1 26"0 18.2
0-1
0-1 4.0 2.9
19-8 22-1 25-0 17.8
21"1 24"8 28"6 17"7
20"9 25.0 27"0 20"0
0-3 2"6 2"1
--0"4 3"8 2"6
0-1 4-8 0-4
19-5
1.9 8-0 1"3
1"5 3"7 3.8
1"8 2-5 3"9
2"1 2-3 2"9
1-1 3-7 3"8
1.4 4"1 2"0
Moisture Enthalpy increment Enthalpy increment Ah H AH (10 -a lb/lb) (B.t.u./lb) (B.t.u/lb)
0-06 0-95 0-91
0"07 1.16 0-82
0-06 1.12 0.99
0"15 1.22 0.78
--0-39 1.10 0.73
0"08 1"26 0-21
AH
1.1Ah
TABLE 5. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING A (FORCING) Mean
31 60 64
35 55 64 38
29 44 59 39
30 44 50 37
32 50 55 37
33 64 51 42
40
3-2
2.7
2-5
2.2
2.6
2.4
heat flow from rock Relative surface humidity (B.t.u./ (%) ft z hr) F ace
Spraying dirt with water Loading (Two loaders) and spraying
Erecting arches
Loading (One loader)
Loading (Two loaders)
Wet drilling
activity
o~
Intake (4 in duct) Discharge (13 in duct) Face Max. return (5)
B2
* Virgin strata temperature = 91 °F.
87.8 85.5 89.6
75.2 76-3 80.7
76.1
70.9 71.9 76.1
85.8 83-I 72-2
BI
91.2
71.0
87-6
number
once
Intake (4 in duct) Discharge (9 in duet) Face Max. return (4)
W.B. Temp. (°F)
D.B.* Temp. (°F)
Position and station number
Survey refer-
29"1 30'1 34"6
14"3 15,8 18"8
0"4 1"5 3"0
29-6
13"9
25"1 26-0 29"7
11"1 12"5 14"9
0"5 1"4 2-4
25'0
Moisture increment Enthalpy Ah H (10-31b/lb) (B.t.u./lb)
10"6
Moisture content h (10 -3 lb/lb)
--0"5 1"0 4"5
0"1 0-9 3"7
Enthalpy increment AH (B.t.u/lb)
--0'86 1"61 0"72
5'5 1'71 0"70
AH
1"1 Ah
TABLE 6. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING B (FORCING)
Mean
55 65 67
49
61 57 59
58
1'9
2.2
heat flow from rock Relative surface humidity fB.t.u./ ft~hr)
Face
Loading (Two loaders)
Wet drilling
activity
~7
Z
¢0
63.0 75.0 71.6 76.5 81.0 78-4
Intake (2" 1 in roadway) Face Duct return (2"1)
76.0 79-9 80.6
Intake (3 in roadway) Face Duct return (3)
Intake (2.1 in roadway) Face Duct return (2.1)
66"5 77.5 74-8
D.B.* Temp. (°F)
Intake (3 in roadway) Face Duct return (3)
Position and station number
* Virgin strata temperature = 88 °F.
C2
Subsidiary heading C1
C2
Main heading C1
Survey reference number
62"4 77"5 69"8
59"0 68.0 60-0
62.0 74.4 69.4
58"0 73"0 65"9
W.B. Temp. (°F)
8"0 18"4 12.7
8"7 11"6 7"3
7.9 16.0 11-9
7-3 14"9 10.2
Moisture content h (10 -a lb/lb)
10"4 --5"7
2"9 --4.3
8-1 --4.1
7"6 --4-7
19"5 32"0 25"2
16"9 23"0 17"4
19.2 29-1 24-7
16"2 27"2 21"4
12"5 --6-8
6"1 --5.6
9-9 --4-4
11.0 --5"8
Moisture Enthalpy increment Enthalpy increment Ah H AH (10 -a lb/lb) (B.t.u./lb) (B.t.u./lb)
0-90 0.92
0-51 0"83
0.88 1.00
0.74 0"87
AH
1.1Ah
TABLE 7. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING C (EXHAUSTING) Mean
44 85 75
78 69 49
44 77 56
58 81 61
1.2
0.5
1.5
2.0
heat flow from rock surface Relative (B.t.u./ humidity ft~hr) %
Loading (Two loaders)
No operations at face Loadingat station 2.2
No operatiom in progress
Wet drilling
Face Activity
~0
>
68
J. T. RUBEN AND D. F. SHARP
In contrast to a force-ventilated heading the intake air in an exhaust-ventilated heading picks up moisture from the roadway surface along its entire length. The resulting moisture content and relative humidity of the air at the face of heading C were very much higher than for the two force-ventilated headings in spite of the recirculation effect in the latter. The temperature and moisture content of the return air in the duct of the exhaust-ventilated heading fell as a result of the leakage of intake air into the duct. The euthalpy of moist air is the sum of its sensible heat and the latent heat of the water vapour in unit mass of dry air, and is given by H ---- (0 -- 32) (0.24 q- 0.45 h) -q- 1075 h
B.t.u./lb dry air
where 0 is the dry bulb temperature in degF and h the moisture content in pounds of water per pound of dry air. If the temperature of the air is increased by A0 °F and its moisture content by Ah millipounds per pound, the resulting change in enthalpy will be AH ---- (0.24 + 0-45 h) A0 q- ~
1000
/ Ah
___ (0.24 q- 0.45 h) A0 + 1.1 Ah. The second term is that part of the enthalpy increase due to the latent heat of the water evaporated. The column in Tables 5-7 headed 1.1 Ah/AH therefore gives approximately the proportion of the total enthalpy increase which appears as latent heat. If this is unity the dry bulb temperature is unchanged. When it is less than unity the dry bulb temperature has increased, while a value greater than unity indicates a temperature decrease. The Tables show that where moisture pick-up was possible, that is outside the ducts, a very large proportion of the gairt in enthalpy, usually over 70 per cent, is in the form of latent heat. Between the duct discharge and the face of the forcing headings the ratio 1.1 Ah/AHis greater than unity. This is a consequence of the high rate of moisture evaporation in this region which will be further discussed later. Neglecting the initial heat content (enthalpy) of any gained moisture, which is small compared to its latent heat of vaporization, the change in enthalpy of the moist air over a roadway section is equal to the thermal flow from the surrounding rock provided there are no local heat sources in the roadway. Tables 5, 6 and 7 show a value for the rate of heat flow into the headings per unit area of rock surface, calculated from the gain in enthalpy of the air passing the intake point. If the effects of moisture are ignored, the rate of heat flow into a heading from the surrounding strata averaged over the total surface area of the heading would be expected to be a maximum when the heading is commenced and then to decrease gradually, since the heat flux from a freshly exposed surface will be greater than that from art old surface. In addition, in a force-ventilated heading the air at the face will be cooler than elsewhere in the roadway and will therefore be capable of extracting more heat from the strata. The longer the heading the smaller is the relative importance of the face region. As the face advances, less air will reach the face as a result of increased duct resistance and leakage. All these features would lead one to expect that, at least in the absence of moisture, the heat flow rate per unit area averaged over the length of the heading would decrease with time. While this is the case in heading B, the figures for heading A show a trend in the opposite direction. However, as the length of this heading increased by less than 10 per cent in the short periods covered by the surveys the decrease in mean flux expected in the dry case would be small and the increased heat flow rate actually observed may be accounted for by
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
69
consideration of the effects of moisture pick-up, which also showed a tendency to increase. Two possible physical mechanisms causing this increase in heat flow from the strata may be envisaged. Evaporation of moisture on the airway surface may occur, which will have the effect of reducing the surface temperature; this will increase the temperature gradient and hence the heat flow from the surrounding strata into the heading at the location of the evaporation. Secondly, if part of the heat of evaporation is extracted from the air, causing a local depression of dry bulb temperature, this will lead to an increased heat flux as the air travels along the roadway. It is therefore seen that evaporation of water does not necessarily mean a deterioration in environmental conditions, since although there will be an increase in thermal flux from the strata this is, of course, expended in the evaporation process. Thus a more rapid cooling of the surrounding strata occurs without causing higher air temperatures. When substantial evaporation takes place and part of the latent heat is derived from the air there will be a local suppression of any potential increase in dry bulb temperature. In fact an actual reduction in air temperature was usually observed between the duct exit and the face of the force-ventilated headings where in the dry case a large increase would have been expected owing to the heat from freshly exposed uncooled strata. A subsequent result of this is to produce some cooling of the intake air in the duct sections nearer the face since the duct acts as a counter-flow heat exchanger. This can lead to the maximum duct air temperature occurring before the end of the duct. In survey B2, where the fan raised the air temperature to that of the virgin strata, this cooling effect on the duct air is particularly marked, as it is to a lesser extent in survey B1. In both these cases heat exchange between intake and return air resulted in a continuous drop in the duct air temperature from the fan to the discharge.
5.4 A comparison of forcing and exhausting ventilation systems The relative merits of forcing and exhausting ventilation systems may now be considered. The main advantage of a forcing system is that relatively dry, cool air is delivered to the end of the duct. Although a large moisture pick-up then generally takes place between the duct exit and the face, the temperature and moisture content of the air at the face may be expected to be below those in a comparable exhaust-ventilated heading. In the forcing system the temperature drop resulting from the moisture pick-up is concentrated in the face region, whereas in an exhaust-ventilated system it occurs along the intake roadway and is partially neutralized by the time the air reaches the face. However, air temperatures in the roadway outbye of the face will often be lower in an exhaust-ventilated heading than in a forceventilated one. A further advantage of a forcing ventilation system is that it may be possible to exercise some control over the intake air. To minimize temperature rise the duct may be lagged (only sections near the fan since those nearer the face may experience a cooling effect as described previously) or in extreme cases the intake air may be refrigerated. If the moisture pick-up between the duct exit and the face becomes excessive, in that the cooling effect on the air is offset by a large rise in humidity, it may be possible to redesign the duct system to prevent this. Such control over the intake air cannot readily be exercised in an exhaustventilated heading. It can be concluded that, from a climatic point of view, a forcing ventilation system is always preferable, given an efficient fan.
70
J. T. RUBEN AND D.F. SHARP
6. CONCLUSIONS Large increases in the moisture content of the ventilation air were observed. The thermal energy involved in the evaporation processes represents a large fraction (usually over 70 per cent) of the enthaply increase of the air over any length of roadway. The moisture evaporation usually causes an increase in the overall thermal flux from the surrounding strata, thus promoting more rapid cooling of the rock without causing higher air temperatures. If high evaporation rates occur in a confined region there may be a local suppression of the rise of the roadway air temperature, or even a reduction, owing to some of the latent heat being taken from the air. This may also have a cooling effect on the air in the duct near its discharge end. It is therefore evident that moisture gained by the air may have an advantageous result on mine climatic conditions provided that it does not become so excessive that favourable temperature effects are offset by large humidity increases. Cool, dry air should be supplied to the intake of headings since a large proportion of any change in intake conditions appears at the face in all but the longest headings. The inefficient use of fans which gives rise to unnecessary heating of the air should be avoided. Very high values of duct leakage were recorded in two of the three headings. A reduction of leakage would lead to improvements in face conditions. From a climatic point of view a forcing system for ventilating headings is preferable to an exhausting one. Acknowledgement--This work was carried out as part of the research programme of the Mining Research Establishment with the co-operation of specialist ventilation engineers on some of the measurements. The views expressed are those of the authors and not necessarily those of the National Coat Board.
REFERENCES 1. JORDAND. W. The numerical solution of underground heat transfer problems--Ill. The calculation of temperature distribution in dry and wet force-ventilated headings, Int. J. Rock Mech. Min. ScL 2, 365-387 (1965). 2. British Standard 1042:1943, Code for Flow Measurements. 3. NORraOV~RE. W. A mechanical device for use in measuring the cross-sectional area of mine airways, Colliery Engng 34, 417-420 (1957). 4. NORTr~OWRE. W. Auxiliary ventilation air measurements, Colliery Engng 35, 422-428 (1958). 5. The Ventilation o f Headings, National Coal Board Information Bulletin 60/220 (1960). 6. British Standard 1339:1946, Humidity of the Air. 7. SHUa~rLEwORrHSrmlLAE. H. Ventilation at the face of a heading, studies in the laboratory and underground, Int. J. Rock Mech. Min. Sci. 1, 79-92 (1963). 8. BLANCHARDM. ]-I. and RUBENJ. T. Wet bulb thermometers for use in ducts, Colliery Engng 41, 233-234 (1964). 9. HrrCHCOCKJ. A., Jo~.s C. and TEAL~R. Studies in an air-conditioned heading at Snowdown Colliery, Colliery Engng 35, 165-168; 204-209 (1958).
T E M P E R A T U R E A N D H U M I D I T Y IN H E A D I N G S AT DEPTH J. T. RUBEN* and D. F. SHARP~ Mining Research Establishment, Isleworth, Middx.
(Received 26 May 1966) Abstract---Details of thermal and psychrometric surveys of the ventilation air in three deep headings in British mines are presented. The results indicate the relative significance of factors controlling climatic conditions in headings. In particular the effect of moisture evaporation on underground temperatures is discussed. 1. INTRODUCTION SINCE coal has to be extracted from ever greater depths environmental conditions may be expected to have art increasingly important effect on the lengths to which headings can be driven. As art experimental approach to the investigation of this problem a number of surveys of climatic conditions were carried out in three deep headings in British mines, two having force ventilation and the other being exhaust ventilated. The results demonstrate typical conditions in headings and illustrate the relative importance of the various factors influencing mine climate. The observations provided information which was of use during the development of theoretical considerations by JORDAN [1] on heat transfer problems in headings. The surveys include measurements of air temperatures, moisture content and airflow. Effects of ventilation duct leakage are considered and a limited comparison is made between force- and exhaust-ventilation systems. 2. THE EXPERIMENTAL SITES
Observations were made in a heading at a depth within the range 2700-3000 ft at each of three collieries. In one of the force-ventilated headings, referred to here as heading A, six surveys were carried out; two surveys were made in the other force-ventilated heading (heading B) and also in the exhaust-ventilated heading (heading C). Detailed descriptions of the three headings are given below. 2.1 Heading A The six surveys in this heading were carried out over a period of one month, successive surveys being separated by a few days, thus showing the short-time variations in the quantities observed. The heading was level and situated at a depth of 3000 ft, where the virgin strata temperature was estimated to be 92°F. The heading was advancing at the rate of 15-20 yd per week through strata consisting mainly of sandstone with some layers of shale, and at the time of the surveys its total length was approximately 600 yd. The roadway was supported by 16 × 12 ft steel arches and lined with corrugated steel sheeting. Ventilation was provided by a * Present address : BICC Central Research and Engineering Division, Wood Lane, London, W.2. t Present address: Avco Corporation, Lycoming Division, Stratford, Corm, U.S.A. 55
56
S. T. RUBEN AND D. F. SHARP
single-stage axial flow fan of approximately 11 h.p. forcing air along a 24-in. duct to the face A diagram of the heading is shown in Fig. I. 2.2 Heading B The two surveys in this level heading were carried out with an interval between them a t three months. The depth of cover was 2950 ft and the estimated virgin strata temperature 91 °F. The heading was advancing at a rate of about 20 yd per week through sandstone with
~
00 F~n I
2 4 in Duct
-
.... I
__._f~-
/
~
....
......
~
-
,,'-r"
5yd .~.'~ J,,,~-'~,~ . . . . . . . . . ~85,td- ~
I 2
_. . . . J
6
5
3
Survey reference
AI A2 A3 A4 A5 A6
Distance between stations { v d )
30 33 58 63 78 90
13 8 8 8 8
I
FIG. I. Positions of measuring stations in heading A. a little shale and two coal seams. Its length at the time of the surveys was 750 and 950 yd. The arch dimensions were also 16 × 12 ft and there was a corrugated sheet steel lining. Ventilation was by a two-stage axial flow fan at the outbye end of a forcing 24-in. duct system. A diagram of the heading is shown in Fig. 2. Survey ntoke
I /I 2_J_J
u~
Fan
"5-vd
-p i
'
,
' /
20ya ~
~
160yd-~,,,,i
,~
- tSOy~
~
2 4 in. Duct
~ ' ~
,2
3 4
5yd
Survey
B 2
FIG. 2. Positions of measuring stations in heading B.
2.3 Heading C The two surveys in this exhaust-ventilated heading were separated by a period of five months. The measured virgin strata temperature, at a depth of 2740 ft, was 88°F. The heading passed through strata consisting predominantly of shale with a few thin coal seams. After 180 yd there was a junction with a subsidiary heading. At the times of the two surveys the length of the main branch was 415 and 765 yd and that of the subsidiary branch 160 and
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
57
445 yd. Surveys were made in both branches. Here again the roadway size was 16 x 12 ft and it was lined predominantly with corrugated steel sheet. A 24-in. diameter duct extended from the faces through doors at the outbye end of the heading into the upeast shaft, as shown in Fig. 3. No auxiliary fan was used, the pressure of the main mine fan being utilized.
.~\.. \ o \
6,5 I
\\\
I I=.
--
4 12,r'~td
~
7
i
3 2
) i -.-,'1',~,~
• il ?yd
~
¢'-~1 tl II
I/1
I/I
I/ I
'
,,
u~o~) s~f)
, e."%,\\\
<'." "
,'
)
. :X'..\,.X
÷,..¢X, \ ,,,,,
L I I1140yd Iii Iq I
FIG. 3. Positions of measuring stations in heading C.
3. EXPERIMENTAL
PROCEDURE
Measuring stations were established at intervals along each of the headings, their positions being shown in Figs. I-3. At each station the following quantities were measured by the means shown.
3.1In the duct O) Air velocity by pitot-static tube and manometer, and hence volume air flow, according to BS 1042:1943 [2]. (ii) Dry and wet bulb air temperatures by mercury-in-glass thermometers. (iii) Static pressure by manometer. 3.2 In the roadway (i) Dry and wet bulb air temperatures by Assmann psychrometer. (ii) Area and perimeter of roadway by M.R.E. protilograph [3]. In addition, the quantity of air discharged by the forcing systems was determined from measurements of air velocity made with an anemometer placed axially at the duct discharge [4], and the barometric pressure in the heading was measured. An estimate was also made of the volume of air released by any compressed-air operated machines, which augmented the ventilation in the return. Airflow at adjacent stations was used to determine the leakage coefficient of the duct between these stations. Leakage coefficient is defined [5] as the volume of air, in ft3/min, which leaks from 100 ft of duct at a uniform excess pressure of 1-in. water gauge. The measured
58
J. T. RUBEN AND D. F. SHARP
leakage was corrected for static pressure by dividing it by the square root of the mean of the pressures at each end of the section of duct considered. The dry and wet bulb air temperatures were used to evaluate the enthalpy (total heat) and moisture content* per pound of dry air according to BS 1339:1946 [6]. 4. RESULTS The results of the surveys, identified by the reference numbers in Table 1, are summarized in Tables 2, 3 and 4 for headings A, B and C respectively. Tables 2(a), 3(a) and 4(a) show the airflow in the duct at the ends of each section between two measuring stations, and the leakage coefficient of that section of duct; a figure is also given for the over-all leakage--in the TABLE 1. IDENTIFICATIONOF SURVEYS
Survey Length of Barometric Virgin strata Heading Systemof reference Survey heading pressurein temperature ventilation number date (yd) heading (°F) (in.Hg) A
Force
A1 A2 A3 A4 A5 A6
Jan. 19 Jan. 21 Feb. 2 Feb. 5 Feb. 12 Feb. 18
593 596 616 621 636 648
32.08 31"52 33'03 32.85 32"84 33'10
92
B
Force
BI B2
Mar. 19 June 10
741 938
33.07 32'44
9l
C Main Subsidiary Main Subsidiary
Exhaust CI CI C2 C2
Feb. Feb. July July
407 143 1035 415
33'10
88 16 16 14 14
31"59
forcing systems the percentage of the air entering the duct that is lost through leakage, in the exhaust-ventilated system the percentage of air leaving the duct that has entered it as a result of leakage. Tables 2(b), 3(b) and 4(b) show the dry bulb (DB) and wet bulb (WB) temperatures, moisture content and enthalpy per pound of dry air for the ventilating air in the duct and roadway at each measuring station. Since in the short distances between measuring stations the temperatures, moisture content and enthalpy of the air show only small increases, it is necessary to measure dry and wet bulb temperatures accurately if the results are to have any significance. The thermometers used for most of the present surveys were accurate to 0.2°F, corresponding to uncertainties of about 0.0002 lb/lb dry air in moisture content and 0.2 B.t.u./lb dry air in enthalpy. In some cases, however, a precision of only about 0.5°F in the temperature measurements was possible. * In this paper moisture content is expressed as the mass of water vapour associated with unit mass of dry air, given in the same units, i.e. lb water vapour per lb of dry air, or lb/lb. It is sometimes expressed in grains/lb; to convert to lb/lb the value in grains/lb should be divided by 7000. In the revised edition of the British Standard (BS 1339:1965) the term moisture content has been replaced by mixing ratio,
59
TEMPERATURE A N D HUMIDITY IN HEADINGS AT DEPTH
TABLe2(a). HEADINGA: AmrLOWANDLEAKAGE Duct Survey section Length reference (between (yd) number stations)
Airflow in duct section Beginning (fta/min)
End (fta/min)
Over-all Leakage leakage coefficient(percentage of air lost)
2-3 3-4 4-5 5-6
15 183 183 184
6900 6750 5100 4050
6750 5100 4050 3300
170 180 130 110
2-3 3-4 4-5 5-6
15 183 183 184
6250 6150 5500 4600
6150 5500 4600 3700
70 60 110 130
A3
2-3 3-4 4-5 5-6 6--7
15 183 183 184 58
5850 6750 5550 5550 5200
6750 5550 5550 5200 5100
120 130 0 60 130
25
A4
2-3 3-4 4-5 5-6
15 183 183 184
7300 7100 4700 4750
7100 4700 4750 4250
290 280 -80
42
A5
2-3 3-4 4-5 5--6 6--7
15 183 183 184 78
6950 6750 4800 4700 3850
6750 4800 4700 3850 3550
200 210 12 150 340
49
A6
2-3 3-4 4-5 5-7
15 183 183 274
6850 6650 4800 4700
6650 4800 4700 4650
220 190 10 5
31
A1
A2
52
41
The inaccuracy of the airflow measurements should be no greater than 2 per cent. Owing to leakage the airflow in a duct falls as the face is approached, but in two cases in the surveys (A4 and B2) an increase was actually indicated. This throws some doubt on the precision of the airflow measurements. As a result of the experience gained in these surveys the possibility of some improvements in the measuring techniques became apparent: (i) In any future surveys of this kind the position of the first measuring station in the roadway should be chosen to avoid any possibility of the mixing of intake and return air. A position about ten roadway diameters (say 50 yd) inbye of the end of the duct or of any roadway junction is suggested. (ii) There is some uncertainty in the discharge temperatures from the forcing ducts, resuiting from the entrainment of return air [7]. It is suggested that where possible discharge temperatures should be measured by an Assmann psychrometer held as far up the duct as possible. (iii) Wet bulb thermometers for use in duets suffer from radiation errors. Wet bulb temperatures of ducted air should therefore be measured by means of shielded duct thermometers [8].
J. T. RUBEN AND D. F. SHARP
60
TABLE 2(b). 1-IEADINGA: AIR TEMPERATURESAND MOISTURECONTENT In duct Survey reference number
In roadway
Station
D.B. Temp. (°F)
1" 2 3 4 5 6 7t 8~t
74"8 78"4 78'3 80"4 83.4 84"2 ---
61'5 62'8 62"8 63.4 64.2 64'8 ---
7"7 7"6 7"6 7'5 7-4 7"7
1" 2 3 4 5 6
76"0 78.8 78.2 81"4 86'4 84"6
62"5 63"3 63"2 64.0 64"5 64"8
8"3 8 "0 8-1 7'9 7.8 7.7
19"6 20"0 20'0 20"5 20"9 21"1
7t
--
A3
1" 2 3 4 5 6 7t
73"2 76"8 76"8 79"8 82"6 83"8 81"8
59"9 60"8 60"8 61 "2 62"9 63"9 64"9
6.8 6"4 6"4 6"0 6.4 6.7 7"9
17.4 17.8 17.7 18-0 19"2 19-8 20.6
A4
1" 2 3 4 5 6 7t
74"1 77"0 76"0 80"4 82"5 83"6 83"6
59'1 60"5 60"7 61 "5 62"9 63"4 63"4
6"2 6"3 6"5 6"1 6"5 6"8 6"6
16.9 17.7 17"8 18"3 19-2 19"6 19-6
A5
1* 2 3 4 5 6 7t
72"9 76"7 76"5 79"6 82'6 83'8 83'2
59"5 61"1 61"3 61 '0 63"1 64"0 63"8
6.7 6"7 6"8 6.0 6"6 6.9 6"9
17.1 18.0 18'2 18.0 19"4 20"0 19.7
1* 2 3 4 5 6 7t 8+*
74"0 77"3 77"1 82"1 83"2 84"5 84"0
60"0 61 "8 61"8 62"1 63"4 64"6 71 "5
6.7 6"8 6.9 6"9 6-6 7.0 11.9
17.4 18"4 18"3 18"4 19"5 20.2 25"6
AI
A2
A6
* Station before fan. t Duct discharge. Face.
W.B. Moisture Temp. content Enthalpy (°F) (10-alb/lb) (B.t.u./lb)
-
18'7 19"5 19"5 19"9 20"4 20"9
D.B. Temp. (°F)
W.B. Moisture Temp. content Enthalpy (°F) (10-31b/Ib) (B.t.u./Ib)
83"0 85.0 86.0 82"0
70-5 72.0 72.5 70.5
11'9 12.6 12'9 12.2
25.4 26'6 27.0 25'4
79.0
70.0
12'5
25"0
83"0 85.0 86'5 85.7
73"0 73"5 74.0 70"5
14'2 14"1 14.1 12.6
27.8 28.2 28"6 25'6
83'0
69"5
11-5
24"8
78.8 84-3 84"9 81 "8
66.4 70.8 70"8 68"9
9.6 11.4 11 "2 10-6
21 "8 25.0 25"0 23"6
81 "8
66"9
9"3
22"1
75.8 82"2 82"2 80"8
66"5 71 "8 70-4 67"2
10"4 12.8 11 "7 9"8
22"0 26.0 24"9 22"4
81 '8
66"8
9"2
22"1
77"5 82.1 82.9 80"8
67.5 72"8 72.6 71 '2
10.8 13'8 13.2 12"6
22.7 27"2 26.7 25.6
79'8
68"5
10.9
23"4
81"0 84-9 85-5 84.0
71"0 75-0 76'0 75-5
12-0 14-9 15-2 15.1
25.1 28"7 29-5 29.1
85.0
74"5
14.1
28.2
-
61
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH TABLE 3(a). ~ I N G
Survey reference number
B: AIRFLOW AND LEAKAGE
Duct section (between stations)
Length (yd)
Beginning (fta/min)
End (fta/min)
4-567-
5 6 7 9
159 204 155 313
8500 8050 6650 6400
8050 6650 6400 5700
36 98 30 120
4- 5 5- 7 7-11 11-12 12-13
159 359 160 150 95
7350 7100 6500 6750 6500
7100 6500 6750 6500 5400
22 22 -41 580*
B1
B2
Airflow in duct section I.~akage cocfllcient
Over-all leakage (percentage of air los0
33
27
* Duet damaged 10 yd from discharge end just prior to measurements being taken.
T ~ t ~ 3(b). HEADING B: A m TEMPERATURF~AND MOISTURECONTENT In duct Survey reference number
B1
B2
In roadway
D.B. Temp. (°F)
W.B. Temp. (°F)
1 2* 3 4 5 6 7 9t 10~
80"5 81"8 87.8 87"6 87"4 87-4 87"4 85"8
67"1 68"0 70.8 71.0 71.1 69.9 70"1 70"9
9"7 10"0 10.4 10.6 10.6 9"8 10"1 11"1
22.2 22.9 24.9 25-0 25"0 24"1 24"4 25.1
1 2* 3 4 5
83'4 85"2 91.2 91"2 91"2 90.1 89.4 87.8 87"8
73.0 74.1 76.3 76"1 76.1 75.6 75.7 75.6 75.2
13"6 14.0 14"4 13"9 13.9 13"9 14.3 14-6 14"3
27"3 28"2 30"1 29"6 29"6 29.3 29"5 29.5 29.1
Station
7 11 12 13t 14++ * Station before fan. t Duct discharge. Face.
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb)
D.B° Temp. (°F)
W.B. Temp. (°F)
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb)
87"3
75"4
14.3
29.0
87"2 87.7 87"7 87"7 85.8 83.1
76.1 76.1 75"3 75"4 71.6 71.9
14.9 14.8 14.1 14-2 11 "6 12"5
29"7 29.6 28.9 29"0 25.7 26"0
83-6 89"3 89.3 89"4 89"6 89-1 88.0 86.2 87-8 85.5
73.0 79.8 79"8 80"7 80.7 80"4 74-4 77.9 75.2 76.3
13.5 18"0 18"0 18"8 18-8 18"6 13"6 17-0 14"3 15"8
27"3 33.6 33"6 34"6 34.6 34"3 28-6 31.3 29.1 30"1
62
J. T. RUBEN AND D. F. SHARP TABLE 4(a). HEADING C: AIRFLOW AND LEAKAGE Duct section (between stations)
Survey reference number
Airflow in duct section Length (yd)
2"2-2' 1 4-3 2 '3"1"-2 If
CI
63 117 45
2-1
C2
Beginning (ft3/min)
135
2.3-2.1 8-7 7-3 2 .3"t"-21f
128 180 355 45
2-1
135
End (ft3/min)
Leakage coefficient
2000 1700 2950~k 3150f 7350
3150 2950 7350
1840* 800 1130
9850
440
2950 1750 2000 4550\ 4700f 10,650
4700 2000 4550 10,650
2390* 650 47 290
13,300
350
Over-all leakage (air leaked in as percentage of duct discharge)
62
65
N.B. Duct branches between stations 2 and 3. * High leakage caused by inferior duct which was subsequently replaced. TABLE 4(b). HEADING C" AIR TEMPERATURESAND MOISTURECONTENT In duct Survey reference number
C1
C2
D.B. Temp. (°F)
W.B. Temp. (°F)
1 2 3 4 5* 6~+
67"5 72"6 74"8 77"4 78'0
60"4 62"8 65'9 69-4 73"3
8"4 8"8 10"2 12'1 14'8
2.1 2.2* 2.3,+
71 "6 71 "0
60'0 59"4
1 2 3 7 8t 9*+
79'9 79.3 80"6 84.0 80"9
66"8 68.4 69'4 74.4 74'9
Station
2.1 2.3 2.4* 2.5++
!
.
78"4 79"8
.
.
69.8 65"3
In roadway
Moisture content Enthalpy (10-alb/lb) (B.t.u./lb.)
Temp. (°F)
W.B. Temp. (°F)
17"6 19"3 21"4 24'1 27"3
65"0 65"0 66"5 74"0 78"0 77.5
57"0 57.0 58'0 62.5 73'3 73.0
6"9 6"8 7"3 8"2 14'8 14"9
15"5 15"4 16"2 19.0 27"3 27.2
7"3 7"0
17-4 17"0
63.0 71"0 75"0
59"0 59.5 68"0
8"7 7'1 11"6
16.9 17-1 23"0
10"2 11 "4 11"9 14"3 16"2
22"7 23.8 24"7 28.2 29-5
75"2 75'1 76"0 84"0 84"5 79"9
61"8 61"2 62"0 68"5 71.8 74"4
8'0 7"9 7'9 10'4 12"8 16"0
19"1 18"9 19"2 23"9 26"4 29,1
12"7 9"2
25"2 21 "6
76"5 79'9
62"4 65"4
8"0 9"2
19'5 21 '6
81"0
77"5
18"4
32"0
.
D.B.
Moisture content Enthalpy (10-Zlb/lb) (B.t.u2/lb)
I
N.B. Heading branches between stations 2 and 3. * Duct intake. t Duct intake situated between this station. and face, as shown in Fig. 3. :~ Face.
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
63
5. D I S C U S S I O N
5.1 The effects of intake conditions and fan power The temperature of the air at the face of a heading is affected by the temperature of the intake air, by heat due to the fan in the case of a forcing ventilation system, by heat transfer from the strata to the intake air, by the presence of men and machines, by oxidation of coal and by evaporation of water which may extract part of the necessary latent heat from the air. One of the most important of these factors in headings at great depth is heat transfer from the strata, as a result of which the temperature of the intake air tends to approach the virgin strata temperature as it travels inbye. Any variations in intake temperature will be largely reproduced at the face of a short heading, but as the heading advances their effect will become progressively less important. The moisture content of intake air in a forcing duct will remain substantially constant, although as the air warms up its relative humidity will decrease. It is therefore important that, if it is desired to keep down the temperature and moisture content at the face of a heading, the air supplied to the intake should have a temperature and moisture content as low as possible, particularly in the early stages of driving the heading. Thus in the exhaust-ventilated heading C [Table 4(b)] the two intake temperatures differed by 10°F as a result of seasonal changes and at the inbye end of the main duct this difference was still as large as 6°F. In a forcing duct the fart increases the pressure of the air, and if this process is assumed to take place adiabatically it may be shown that the accompanying temperature increase is given approximately by A0.4 = 4.8 AP where A0A is the adiabatic increase in temperature in degrees Fahrenheit and AP the static pressure increase in inches of mercury. Owing to inefficiency of the fan the actual temperature increase, A0, is greater than A0a; the fan thermal efficiency, which is given by A0A _ 4.8 AP -A0 A0 can thus be evaluated. In the surveys in heading A the efficiency averaged 36 per cent and in heading B 48 per cent. Had it been possible to employ 100 per cent efficient fans in the two headings while maintaining at the observed level the air quantities delivered to the face the duct air temperatures at the beginning of the headings would have been reduced by 2°F and 3 °F respectively. An interesting feature of survey B2 is that the fan raised the air temperature in the duct to nearly virgin strata temperature, with the result that there was no rise in temperature as the air passed along the duct. Indeed, the temperature dropped as it approached the face, presumably as a result of water evaporating into the return air which cooled the return and hence the intake air, as discussed later. In force-ventilated headings the temperature rise due to the fan may on some occasions be obviated by using the pit ventilation pressure, rather than a fan, to produce ventilation. Fan heat has of course no effect on face temperatures in exhaust-ventilated headings. 5.2 Duct leakage In the course of these surveys some sixty measurements of airflow were carried out, enabling forty-four estimates of leakage coefficient for sections of duct to be made. The results
64
J. T. RUBEN AND D. F. SHARP
are presented in Tables 2(a), 3(a) and 4(a). Regarding leakage the N.C.B. Information Bulletin [5] states: For satisfactory and efficient ventilation of any underground heading the duct used should be so designed, installed and maintained that its leakage coefficient is not more than 20-30 ft3/min. There should be no difficulty in attaining this standard in normal conditions. The tables, however, show leakage coefficients ranging from 100 to 300 in heading A and from 20 to 120 in heading B. In the exhaust-ventilated heading C only one value below 290 was recorded while three of the nine values measured there exceeded 1000; two of these, however, were due to inferior ducting. The values obtained in heading A and, especially, in heading C are thus considerably higher than is usually considered acceptable. The duct installation in heading B, which was similar to that in heading A except that the joints were sealed with tape, was generally satisfactory. A leaky duct system greatly impairs the effectiveness of the ventilation. The quantity of air reaching the face is less than with a good system, and this can only increase dust and gas concentrations at the face because of the relative inability of the air to scour the face, as welt as increasing the temperature. To counteract the effects of this leakage more air is required at the fan. This necessitates higher pressure and consequently a more powerful fan, which in force-ventilated headings can only lead to higher initial temperatures. 5.3 Heat and moisture pick-up by the ventilating air The data gathered on moisture content and enthalpy are summarized in Tables 5, 6 and 7, which repeat the temperature, moisture content and enthalpy of the air at the intake, duct discharge (in the forcing headings), face and at the hottest point in the return. The Tables also show the increases in these quantities along the duct and roadway, and between the duct exit and face in the force-ventilated headings. One interesting fact demonstrated by these tables is the very large gain in moisture of the air between the duct discharge and the face of a force-ventilated heading. This confirms previous findings in another heading [9] and is explained by the work of SmJa~rLEWORTH[7]. Experiments with a ventilation model of a heading showed that the air discharged from the duct does not immediately return down the heading; a large proportion is entrained by the air issuing from the duct, establishing a circulatory airflow pattern between the duct exit and the face. Owing to the relatively long time the air spends in this region and to its low relative humidity after having been heated in the duct a large amount of water evaporates into it. The magnitude of the moisture pick-up between the duct exit and the face appears to depend on the nature of the activities at the face, as shown in Tables 5 and 6. In heading A moisture pick-up is least during the erection of arches and highest while loading and water spraying for dust suppression. It is not quite so high when only spraying, or when wet drilling. Only two figures are available for heading B, and these are inconclusive owing to the very different levels of moisture content in the duct on the two occasions. The moisture pick-up of the return air in heading A, while still appreciable, is usually less than in the small region between the duct exit and the face. In heading B, on the other hand, less moisture is picked up in this region than in the return, and this is probably because the discharged air has a much higher relative humidity in this heading than in heading A and is therefore less capable of taking up further moisture. In support of this explanation, the table shows that the larger the pick-up at the face of heading A (giving a higher relative humidity in the return) the smaller the moisture pick-up in the return.
t~
I
.~
Intake (3 in duct) Discharge (7 m duct) Face Max. return (4) Intake (3 in duct) Discharge (7 m duct) Fac¢ Max. return (4) Intake (3 in duct) Discharge (6 m duct) Face Max. return (5)
Max. return (4)
Intake (3 in duct) Discharge (6 in duct) Face Max. return (5) Intake 0 in duct) Discharge (6 in duct) Face Max. return (5) Intake (3 in duct) Discharge (6 m duct) Face
Position and station number
64"8 69.5 74.0 60"8 63"8 66.9 70-8 60-7 63"4 66.8 71-8 61"3 63"8 68-5 72"8 61"8 64"6 74"5 76"0
83"8 81 "8 84.3 76"6 83"6 81-8 82-2 76-5 83-2 79"8 82"1 77"1 84.5 85"0 85.5
64-8 70"0 72-5 63"2
62-8
(°F)
W.B. Temp.
84.6 83-0 86-5 76"8
84.2 79-0 86-0 78-2
78-3
Temp. (°F)
D.B. ~
* Virgin strata temperature = 92°F.
A6
A5
A4
A3
A2
AI
Survey reference number
7.0 14-1 15"2
6"9 10-9 13-8 6-9
6-6 9-2 12-8 6-8
6"7 9-3 11-4 6'5
7"7 11"5 14.1 6"4
7-7 12"5 12.9 8.1
7.6
Moisture content h (10 -3 lb/lb)
2"6 3"6
0"I 7"1 1"1
20"2 28-2 29-5
19.7 23.4 27-2 18-3
19"6 22.1 26"0 18.2
0-1
0-1 4.0 2.9
19-8 22-1 25-0 17.8
21"1 24"8 28"6 17"7
20"9 25.0 27"0 20"0
0-3 2"6 2"1
--0"4 3"8 2"6
0-1 4-8 0-4
19-5
1.9 8-0 1"3
1"5 3"7 3.8
1"8 2-5 3"9
2"1 2-3 2"9
1-1 3-7 3"8
1.4 4"1 2"0
Moisture Enthalpy increment Enthalpy increment Ah H AH (10 -a lb/lb) (B.t.u./lb) (B.t.u/lb)
0-06 0-95 0-91
0"07 1.16 0-82
0-06 1.12 0.99
0"15 1.22 0.78
--0-39 1.10 0.73
0"08 1"26 0-21
AH
1.1Ah
TABLE 5. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING A (FORCING) Mean
31 60 64
35 55 64 38
29 44 59 39
30 44 50 37
32 50 55 37
33 64 51 42
40
3-2
2.7
2-5
2.2
2.6
2.4
heat flow from rock Relative surface humidity (B.t.u./ (%) ft z hr) F ace
Spraying dirt with water Loading (Two loaders) and spraying
Erecting arches
Loading (One loader)
Loading (Two loaders)
Wet drilling
activity
o~
Intake (4 in duct) Discharge (13 in duct) Face Max. return (5)
B2
* Virgin strata temperature = 91 °F.
87.8 85.5 89.6
75.2 76-3 80.7
76.1
70.9 71.9 76.1
85.8 83-I 72-2
BI
91.2
71.0
87-6
number
once
Intake (4 in duct) Discharge (9 in duet) Face Max. return (4)
W.B. Temp. (°F)
D.B.* Temp. (°F)
Position and station number
Survey refer-
29"1 30'1 34"6
14"3 15,8 18"8
0"4 1"5 3"0
29-6
13"9
25"1 26-0 29"7
11"1 12"5 14"9
0"5 1"4 2-4
25'0
Moisture increment Enthalpy Ah H (10-31b/lb) (B.t.u./lb)
10"6
Moisture content h (10 -3 lb/lb)
--0"5 1"0 4"5
0"1 0-9 3"7
Enthalpy increment AH (B.t.u/lb)
--0'86 1"61 0"72
5'5 1'71 0"70
AH
1"1 Ah
TABLE 6. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING B (FORCING)
Mean
55 65 67
49
61 57 59
58
1'9
2.2
heat flow from rock Relative surface humidity fB.t.u./ ft~hr)
Face
Loading (Two loaders)
Wet drilling
activity
~7
Z
¢0
63.0 75.0 71.6 76.5 81.0 78-4
Intake (2" 1 in roadway) Face Duct return (2"1)
76.0 79-9 80.6
Intake (3 in roadway) Face Duct return (3)
Intake (2.1 in roadway) Face Duct return (2.1)
66"5 77.5 74-8
D.B.* Temp. (°F)
Intake (3 in roadway) Face Duct return (3)
Position and station number
* Virgin strata temperature = 88 °F.
C2
Subsidiary heading C1
C2
Main heading C1
Survey reference number
62"4 77"5 69"8
59"0 68.0 60-0
62.0 74.4 69.4
58"0 73"0 65"9
W.B. Temp. (°F)
8"0 18"4 12.7
8"7 11"6 7"3
7.9 16.0 11-9
7-3 14"9 10.2
Moisture content h (10 -a lb/lb)
10"4 --5"7
2"9 --4.3
8-1 --4.1
7"6 --4-7
19"5 32"0 25"2
16"9 23"0 17"4
19.2 29-1 24-7
16"2 27"2 21"4
12"5 --6-8
6"1 --5.6
9-9 --4-4
11.0 --5"8
Moisture Enthalpy increment Enthalpy increment Ah H AH (10 -a lb/lb) (B.t.u./lb) (B.t.u./lb)
0-90 0.92
0-51 0"83
0.88 1.00
0.74 0"87
AH
1.1Ah
TABLE 7. THE GAIN IN MOISTURE AND ENTHALPY IN HEADING C (EXHAUSTING) Mean
44 85 75
78 69 49
44 77 56
58 81 61
1.2
0.5
1.5
2.0
heat flow from rock surface Relative (B.t.u./ humidity ft~hr) %
Loading (Two loaders)
No operations at face Loadingat station 2.2
No operatiom in progress
Wet drilling
Face Activity
~0
>
68
J. T. RUBEN AND D. F. SHARP
In contrast to a force-ventilated heading the intake air in an exhaust-ventilated heading picks up moisture from the roadway surface along its entire length. The resulting moisture content and relative humidity of the air at the face of heading C were very much higher than for the two force-ventilated headings in spite of the recirculation effect in the latter. The temperature and moisture content of the return air in the duct of the exhaust-ventilated heading fell as a result of the leakage of intake air into the duct. The euthalpy of moist air is the sum of its sensible heat and the latent heat of the water vapour in unit mass of dry air, and is given by H ---- (0 -- 32) (0.24 q- 0.45 h) -q- 1075 h
B.t.u./lb dry air
where 0 is the dry bulb temperature in degF and h the moisture content in pounds of water per pound of dry air. If the temperature of the air is increased by A0 °F and its moisture content by Ah millipounds per pound, the resulting change in enthalpy will be AH ---- (0.24 + 0-45 h) A0 q- ~
1000
/ Ah
___ (0.24 q- 0.45 h) A0 + 1.1 Ah. The second term is that part of the enthalpy increase due to the latent heat of the water evaporated. The column in Tables 5-7 headed 1.1 Ah/AH therefore gives approximately the proportion of the total enthalpy increase which appears as latent heat. If this is unity the dry bulb temperature is unchanged. When it is less than unity the dry bulb temperature has increased, while a value greater than unity indicates a temperature decrease. The Tables show that where moisture pick-up was possible, that is outside the ducts, a very large proportion of the gairt in enthalpy, usually over 70 per cent, is in the form of latent heat. Between the duct discharge and the face of the forcing headings the ratio 1.1 Ah/AHis greater than unity. This is a consequence of the high rate of moisture evaporation in this region which will be further discussed later. Neglecting the initial heat content (enthalpy) of any gained moisture, which is small compared to its latent heat of vaporization, the change in enthalpy of the moist air over a roadway section is equal to the thermal flow from the surrounding rock provided there are no local heat sources in the roadway. Tables 5, 6 and 7 show a value for the rate of heat flow into the headings per unit area of rock surface, calculated from the gain in enthalpy of the air passing the intake point. If the effects of moisture are ignored, the rate of heat flow into a heading from the surrounding strata averaged over the total surface area of the heading would be expected to be a maximum when the heading is commenced and then to decrease gradually, since the heat flux from a freshly exposed surface will be greater than that from art old surface. In addition, in a force-ventilated heading the air at the face will be cooler than elsewhere in the roadway and will therefore be capable of extracting more heat from the strata. The longer the heading the smaller is the relative importance of the face region. As the face advances, less air will reach the face as a result of increased duct resistance and leakage. All these features would lead one to expect that, at least in the absence of moisture, the heat flow rate per unit area averaged over the length of the heading would decrease with time. While this is the case in heading B, the figures for heading A show a trend in the opposite direction. However, as the length of this heading increased by less than 10 per cent in the short periods covered by the surveys the decrease in mean flux expected in the dry case would be small and the increased heat flow rate actually observed may be accounted for by
TEMPERATURE AND HUMIDITY IN HEADINGS AT DEPTH
69
consideration of the effects of moisture pick-up, which also showed a tendency to increase. Two possible physical mechanisms causing this increase in heat flow from the strata may be envisaged. Evaporation of moisture on the airway surface may occur, which will have the effect of reducing the surface temperature; this will increase the temperature gradient and hence the heat flow from the surrounding strata into the heading at the location of the evaporation. Secondly, if part of the heat of evaporation is extracted from the air, causing a local depression of dry bulb temperature, this will lead to an increased heat flux as the air travels along the roadway. It is therefore seen that evaporation of water does not necessarily mean a deterioration in environmental conditions, since although there will be an increase in thermal flux from the strata this is, of course, expended in the evaporation process. Thus a more rapid cooling of the surrounding strata occurs without causing higher air temperatures. When substantial evaporation takes place and part of the latent heat is derived from the air there will be a local suppression of any potential increase in dry bulb temperature. In fact an actual reduction in air temperature was usually observed between the duct exit and the face of the force-ventilated headings where in the dry case a large increase would have been expected owing to the heat from freshly exposed uncooled strata. A subsequent result of this is to produce some cooling of the intake air in the duct sections nearer the face since the duct acts as a counter-flow heat exchanger. This can lead to the maximum duct air temperature occurring before the end of the duct. In survey B2, where the fan raised the air temperature to that of the virgin strata, this cooling effect on the duct air is particularly marked, as it is to a lesser extent in survey B1. In both these cases heat exchange between intake and return air resulted in a continuous drop in the duct air temperature from the fan to the discharge.
5.4 A comparison of forcing and exhausting ventilation systems The relative merits of forcing and exhausting ventilation systems may now be considered. The main advantage of a forcing system is that relatively dry, cool air is delivered to the end of the duct. Although a large moisture pick-up then generally takes place between the duct exit and the face, the temperature and moisture content of the air at the face may be expected to be below those in a comparable exhaust-ventilated heading. In the forcing system the temperature drop resulting from the moisture pick-up is concentrated in the face region, whereas in an exhaust-ventilated system it occurs along the intake roadway and is partially neutralized by the time the air reaches the face. However, air temperatures in the roadway outbye of the face will often be lower in an exhaust-ventilated heading than in a forceventilated one. A further advantage of a forcing ventilation system is that it may be possible to exercise some control over the intake air. To minimize temperature rise the duct may be lagged (only sections near the fan since those nearer the face may experience a cooling effect as described previously) or in extreme cases the intake air may be refrigerated. If the moisture pick-up between the duct exit and the face becomes excessive, in that the cooling effect on the air is offset by a large rise in humidity, it may be possible to redesign the duct system to prevent this. Such control over the intake air cannot readily be exercised in an exhaustventilated heading. It can be concluded that, from a climatic point of view, a forcing ventilation system is always preferable, given an efficient fan.
70
J. T. RUBEN AND D.F. SHARP
6. CONCLUSIONS Large increases in the moisture content of the ventilation air were observed. The thermal energy involved in the evaporation processes represents a large fraction (usually over 70 per cent) of the enthaply increase of the air over any length of roadway. The moisture evaporation usually causes an increase in the overall thermal flux from the surrounding strata, thus promoting more rapid cooling of the rock without causing higher air temperatures. If high evaporation rates occur in a confined region there may be a local suppression of the rise of the roadway air temperature, or even a reduction, owing to some of the latent heat being taken from the air. This may also have a cooling effect on the air in the duct near its discharge end. It is therefore evident that moisture gained by the air may have an advantageous result on mine climatic conditions provided that it does not become so excessive that favourable temperature effects are offset by large humidity increases. Cool, dry air should be supplied to the intake of headings since a large proportion of any change in intake conditions appears at the face in all but the longest headings. The inefficient use of fans which gives rise to unnecessary heating of the air should be avoided. Very high values of duct leakage were recorded in two of the three headings. A reduction of leakage would lead to improvements in face conditions. From a climatic point of view a forcing system for ventilating headings is preferable to an exhausting one. Acknowledgement--This work was carried out as part of the research programme of the Mining Research Establishment with the co-operation of specialist ventilation engineers on some of the measurements. The views expressed are those of the authors and not necessarily those of the National Coat Board.
REFERENCES 1. JORDAND. W. The numerical solution of underground heat transfer problems--Ill. The calculation of temperature distribution in dry and wet force-ventilated headings, Int. J. Rock Mech. Min. ScL 2, 365-387 (1965). 2. British Standard 1042:1943, Code for Flow Measurements. 3. NORraOV~RE. W. A mechanical device for use in measuring the cross-sectional area of mine airways, Colliery Engng 34, 417-420 (1957). 4. NORTr~OWRE. W. Auxiliary ventilation air measurements, Colliery Engng 35, 422-428 (1958). 5. The Ventilation o f Headings, National Coal Board Information Bulletin 60/220 (1960). 6. British Standard 1339:1946, Humidity of the Air. 7. SHUa~rLEwORrHSrmlLAE. H. Ventilation at the face of a heading, studies in the laboratory and underground, Int. J. Rock Mech. Min. Sci. 1, 79-92 (1963). 8. BLANCHARDM. ]-I. and RUBENJ. T. Wet bulb thermometers for use in ducts, Colliery Engng 41, 233-234 (1964). 9. HrrCHCOCKJ. A., Jo~.s C. and TEAL~R. Studies in an air-conditioned heading at Snowdown Colliery, Colliery Engng 35, 165-168; 204-209 (1958).