Chapter 14

INFLUENCE OF RELATIVE HUMIDITY VARIATIONS ALONG MINE WORKINGS

ON AIR-FLOW TEMPERATURE

V. Kertikov

University of Mining and Geology Sofia, Bulgaria

ABSTRACT

In the paper the results from measurement of air flow temperature and relative humidity in downcast shafts and level workings in metal mines are presented and commented.

The results from alternative calculations for predicting air flow temperature along a downcast shaft, level working and upcast shaft are included. The main input data for alternative calculations are equal. There is a difference only in a change (increase or decrease) of relative humidity rate along the working (Δϕ).

Calculation results are discussed. They illustrate a high influence of the change of relative humidity (Δϕ) on the values of the predicted air temperature.

The main conclusion is drawn that exactness of predicting calculations of air temperature depends mainly on accuracy of supposed value of relative humidity changes (Δϕ).

KEYWORDS

Relative humidity, moisture content, thermal conductivity and specific heat of rock, virgin temperature of rock (VRT), increase or decrease in relative humidity by m', downcast shaft, upcast shaft

INTRODUCTION

It's well known that as a result of complex heat and humidity exchanges and other thermodynamical occurrences, air flow temperature and humidity in deep mines along the mine workings, as much as in time, have significant variations.

Those variations are directly influenced by several factors, as:

• virgin rock temperature (VRT),

• physical characteristics of the rock massif (heat transfer coefficient k_rock, specific heat C_rock and density ρ_rock),

• geometrical parameters of headings (length L, cross section F, equivalent diameter D),

• time for ventilation of heading τ,

• moisture and humidity of rock walls in the heading,

• flow ratio, temperature and relative humidity of fresh (income) air,

• angle of heading slope,

• power of local and linear heat sources,

• intensity of oxidation process relative to unit area of the heading.

Scientific investigations on this issue are conducted by many researchers and institutions (Ukraine, Russia, Poland, South African Republic, USA, England and others). These countries have rich theoretical and practical background which was used for development of methodologies and computer software for design of ventilation and air-conditioning in deep mines.

Knowledge of factors with most significant impact on microclimatic conditions is of crucial importance in selection of means and equipment for air-conditioning.

The investigation of distinct factors impact on the microclimatic conditions could be realized through the modeling and numerical prediction approach (numerical analysis).

The numerical approach is most convenient and applicable and in this paper will be discussed in details. This approach is based on execution of prognosis variation calculations for air temperature determination with variation of investigated parameter values for different variations.

As example, if impact of airflow ratio is investigated the practically possible values for airflow ratio (Q) are given and for every value of Q, prognosis of air temperature is calculated.

The paper presents investigation only of relative humidity impact on air temperature variations. This impact, as will be shown, is very significant and for this the investigations presented are completely justified.

Numerical investigations are conducted by use of TempFlow software for airflow temperature prognosis. It should be mention that humidity variation are given

through the variation (increase or decrese) of relative humidity of air by 1 m' of mine workings [Δϕ, %/m] (Kertikov 1992,1994).

Calculations and analysis will be presented separately for vertical downcast shaft, horizontal mine workings and upcast ventilation shaft. Basic thermal parameters of rock massif for all kind of workings are: k_rock =
3 w/m°C, C_rock= 800 J/kg°C, ρ_rock= 2700 kg/m³.

AIRFLOW TEMPERATURE AND HUMIDITY
IN THE DOWNCAST SHAFT

Results from measurements of airflow temperature and humidity in the downcast shaft

Results from measurements in two Bulgarian shafts are presented in figures 1 and 2. The measurements are conducted in different weather seasons (Kertikov, 1994). In the time of measurements the shafts have been in use as ventilation shafts for more than 5 years. The VRT₀ and VRT_H are primary rock temperatures at the heading (entry) and at the depth H from the heading of the shaft, consequently.

Characteristic of variations of temperature t and relative humidity ϕ are similar in several other shafts where smaller number of measurements are taken.

Figure 1. Temperature and humidity curves, obtained from the number 1^st shaft at VRT₀ = 15°C
and VRT_H = 31°C

Results from the measurements in all shafts show the following trends:

• increase in airflow relative humidity with maximum values (90-95%) at the down end of the shaft and there is no practical influence of income air relative humidity,

• airflow temperature also increases with exeption of the summer time measurements, when atmospheric relative humidity is low.

Figure 2. Temperature and humidity curves, obtained from the number 2^nd shaft at VRT₀ = 14°C and VRT_H = 34°C

The analysis of the results from the measurements with approximate values of Δϕ are given in the Table 1.

Table 1. Medium values of Δϕ in function of values of ϕ₀

Expected breach of values of ϕ0, %	Recommended values of Δϕ, %/m
40 - 60	0,060
60 - 70	0,040
70 - 80	0,020
80 - 90	0,010
90 - 100	0,005

Values of Δϕ from the Table 1 can be oriental used as an input data for prognosis of airflow temperature in the shaft for distinct atmospheric, geothermal and hydrological conditions.

Airflow temperature prognosis in downcast shafts (numerical analysis)

These calculations are executed with following input data: shaft diameter D = 6.5 m, shaft dept H = 1000 m, airflow ratio Q =150 m³/s, initial rock temperature at the shaft entry VRT₀ = 15°C, geothermal degree σ = 0,033°C/m, time in use τ = 600 days, atmospheric pressure at the shaft entry P_at = 100000 Pa, temperature of income air t₀ = 25°C.

Figure 3. Temperature and humidity prognosis curves

at ϕ₀ = 60%

The variation of airflow humidity is analyzed through the 6 following hypothesis:

1- without moisture evaporation (“dry” heat exchange)

2- Δϕ = - 0.010%/m

3- Δϕ = 0%/m

4- Δϕ = 0.01%/m

5- Δϕ = 0.03%/m

6- Δϕ = 0.05%/m.

The temperature curves shown at Figure 3 illustrated large influence of the Δϕ at airflow heating down the shaft. By experts conclusion it can be adopted that airflow humidity variation are most probably between Δϕ = 0.01%/m and Δϕ = 0.03%/m, (hypothesis 4 and 5). With this adoption airflow temperature increase will be between 0 and 3°C.

Figure 4. Temperature and humidity prognosis curves

at ϕ₀ = 80%

Two first hypothesis can be excluded as they have very little chance to occur in real terms. Last (6) hypothesis is possible in real terms, but only in shafts with wet walls.

The Figure 4 presents the results from airflow temperature prognosis with same input data, but with increased relative humidity of income air (ϕ0 = 80%).

Temperature curves from Figures 3 and 4 show that with same increase of relative humidity Δϕ the airflow temperature down the shaft shows following trends:

• in cases when airflow down the shaft doesn't reach the condition of total saturation (ϕ < 100%), the temperate increase at the shaft end is smaller at ϕ₀=80% compared to the temperature increase at the ϕ₀=60% (hypothesis 2,3 and 4, Figure 4);

• in cases when the airflow has reach condition of total saturation (ϕ = 100%), the temperate increase at the shaft end is bigger at ϕ₀=80% compared to the temperature increase at the ϕ₀= 60% (hypothesis 5 and 6, Figure 4);

In order to obtain more detailed evaluation of relative humidity impact of income airflow on temperature increase of airflow moving down the shaft, the hypothesis are analyzed for following values of ϕ₀ consequently 60%, 70%, 80%, 90% and 100%. The Figure 5 shows the results.

Temperature curves given at Figure 5, obtained at Δϕ = 0.05%/m, illustrated following trends:

• the airflow temperature decreses when air is moving down the shaft to the point when condition of total saturation has been reached,

• after the point where condition of total saturation has reached the airflow temperature incasing with same grade for all possible conditions.

Figure 5. Temperature and humidity prognosis curves

at ϕ₀ 60 %, 70%, 80%, 90%, and 100%

AIRFLOW TEMPERATURE AND HUMIDITY
IN HORIZONTAL WORKINGS

Results from measurements

The results from measurement in two horizontal workings are given.

The first working (Figure 6) is about 2000 m long, cross section is 7 m², VRT is 37^oC (Rodope region) and it's ventilated for more than 2 years. Airflow average velocity is 1.5 m/s. The workings is directly connected to the downcast shaft and for this the airflow humidity at the beginning of the working is very high (over 95 %). The results shown are for measurements conducted in two different days.

 Second working (Figure 7) has the cross section of 7 m². This working is build through the compact environment and has no lining (support). VRT at the beginning of working is 45^oC and at the end of the working the VRT is 34^oC. The walls of this working are visually evaluated as wetted. Average airflow velocity is 1.6 m/s. Measurements are conducted in two different days through the fall season by the doc.dr. Dimiter Kralev.

Figure 6. Temperature and humidity curves for 1-st horizontal working, obtained from the measurements

The airflow temperature increase in this working is completely in accordance with the theory of heat exchange between the airflow and surrounding rocks. There is no principle in relative humidity variation along the workings. Still, moisture content increasing - for first working for about 12 g/kg, and for second working for about 6 g/kg.

Figure 7. Temperature and humidity curves for 2-nd horizontal working obtained from the measurements

The measurements conducted in many different workings metal mines also confirmed the conclusions for non-principle character of relative humidity variation along the workings. In one and the same working measurements conducted in different days gave different trends of relative humidity variations. In general, there could be stated that relative humidity increase or decrease (Δϕ) is between -0.03 and +0.03 %/m.

In most cases when ϕ₀ is over 90% the relative humidity decreases along the working or has the same value as ϕ₀. Opposite to this, when ϕ₀ is under 90% the relative humidity has the same value as ϕ₀ or increase along the workings.

Airflow temperature prognosis in horizontal working

Input data for prognosis calculations are: equivalent diameter D = 4.5 m, length L = 1000 m, airflow quantity Q = 30 m³/s, VRT = 48°C, time in use τ = 300 days, relative humidity at the beginning of the working ϕ_{0 =} 80%, atmospheric pressure P_at = 110000 Pa, temperature of income air t₀ = 25°C.

Figure 8. Temperature and humidity prognosis curves for horizontal working

The variation of relative humidity is analyzed through the 8 following hypothesis:

1- "dry" heat exchange

2- Δϕ = - 0.010%/m

3- Δϕ = - 0.005%/m

4- Δϕ = 0.0%/m

5- Δϕ = 0.005%/m

6- Δϕ = 0.01%/m

7- Δϕ = 0.025%/m

8- Δϕ = 0.05%/m

The results from calculations are shown in Figure 8.

In first 6 hypothesis airflow temperature increases along the working. In hypothesis 7 and 8 as a result of intensive evaporation of humidity, the airflow temperature decreases along the working until the point where condition of total saturation is reached. After this point the intensity of evaporation is smaller and airflow temperature has increased.

First hypothesis is unreal, and all other cases are practically possible. Values of temperature prognoses at the end of workings are between 24.34^oC (hypothesis 8) and 28.51^oC (hypothesis 2). The airflow temperature grade of increase is -0.66^oC and 3.51^oC, consequently.

AIRFLOW TEMPERATURE AND HUMIDITY IN UPCAST SHAFTS

Our practical experience gives solid confirmation of well known trends of airflow relative humidity increasing in downcast shafts. In almost all of the downcast shafts observed airflow has reached the condition of total saturation (ϕ = 100%).

With the results from calculations presented below the characteristics of airflow temperature and humidity variations in the upcast shafts are shown.

Calculations are executed as example at the upcast shaft with following input data: shaft diameter D = 6.5 m, shaft depth H = 1000 m, airflow quantity Q = 150 m³/s, time in use τ= 600 days, geothermal grade σ = 0,033°C/m, VRT₀ at the shaft base 48^oC, atmospheric pressure P_at = 110000 Pa, temperature of income air at the shaft base t₀ = 30°C.

With the upcast movement of the air in general there is a trend of airflow temperature decreasing. This is a result of pressure and rock massive temperature decreasing. In this condition relative humidity rapidly increases and state of total saturation is reached.

The following 5 hypothesis are analyzed:

1- "dry" heat exchange and with upcast movement

of dry air (ϕ₀ = 0%)

2- ϕ₀ = 70%

3- ϕ₀ = 80%

4- ϕ₀ = 90%

5- ϕ₀ = 100%

By interpretation of temperature and humidity curves (Figure 9) the following conclusions are stated:

1. Airflow temperature is decreasing with same grade to the point of total saturation (curves for hypothesis 2,3 and 4)

2. After the point where condition of total saturation is reached the airflow temperature decreasing but with smaller grade which is the same for all 4 hypothesis

3. As the relative humidity at the shaft base increases temperature prognoses at the shaft exit also increase. This increasing of airflow temperature at the shaft exit is a result of airflow heating from the higher latent heat in more humid air.

Figure 9. Temperature and humidity prognosis curves for upcast shaft

CONCLUSION

The results from the measurements of airflow temperature and humidity presented in this paper show the following trends:

• well known trends for airflow temperature and humidity increasing at the air movement through the downcast shafts are confirmed,

• trend of airflow humidity increasing through the downcast shafts is clearly marked,

• there is no principle trends in airflow humidity variations through the horizontal workings.

Results from numerical calculations obtained for air movement through the downcast shafts and horizontal workings show:

• when airflow humidity increasing with smaller grades Δϕ, the bigger increase of airflow temperature is noted (Figures 3, 4 and 8),

• for higher enough values of airflow humidity grades Δϕ after reaching the conditions of total saturation, airflow temperature rapidly increases (Figures 3, 4, 5, and 8),

• except the values of Δϕ, the values of relative humidity of income air ϕ₀ give the biggest influence of the airflow temperature variations (Figure 5).

General conclusions from calculation for upcast shafts are:

• trend of airflow temperature decreasing and relative humidity increasing of airflow through the upcast shafts is clearly marked,

• after the reaching of condition of total saturation the grade of airflow temperature decreasing is smaller (Figure 9),

• increasing of relative humidity at the upcast shaft base give smaller cooling of airflow through the shaft (Figure 9).

For airflow velocities higher then 2-3 m/s and long enough time in use of workings most important parameters which influence the airflow temperature variations are Δϕ and ϕ₀. Precision of the prognosis calculations is most dependable on the precision in determination of Δϕ and ϕ₀.

REFERENCES

Kertikov V.P., 1992. Influence of Relative Humidity Change Along the Mine Opening on the Air Flow Temperature. Journal "Minno delo i geologie", Sofia, No 4, pp. 27-31

Kertikov V.P., 1994. Studying the Influence of Some Factors on the Ventilation Flow Temperature in Vertical Shafts. “Proceedings, 16th World Mining Congress, Sofia. Book 4, pp. 95-102

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I SZKOŁA AEROLOGII GÓRNICZEJ 1999

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90

PROCEEDINGS OF THE 7^TH INTERNATIONAL MINE VENTILATION CONGRESS

89

INFLUENCE OF RELATIVE HUMIDITY VARIATIONS ALONG MINE WORKINGS

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