- Email: [email protected]
A new design of foam spray nozzle used for precise dust control in underground coal mines
International Journal of Mining Science and Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
A new design of foam spray nozzle used for precise dust control in underground coal mines Han Fangwei a,⇑, Wang Deming b, Jiang Jiaxing b, Zhu Xiaolong b a b
College of Safety Science and Engineering, Liaoning Technical University, Huludao 125105, China Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 26 May 2015 Received in revised form 6 September 2015 Accepted 15 October 2015 Available online xxxx Keywords: Dust control Precise spray Arc jet Gas liquid ratio (GLR) Dust suppression efficiency
a b s t r a c t In order to improve the utilization rate of foam, an arc jet nozzle was designed for precise dust control. Through theoretical analysis, the different demands of foam were compared amongst arc jets, flat jets and full cone jets when the dust source was covered identically by foam. It is proved that foam consumption was least when an arc jet was used. Foam production capability of an arc jet nozzle under different conditions was investigated through experiments. The results show that with the gas liquid ratio (GLR) increasing, the spray state of an arc jet nozzle presents successively water jet, foam jet and mist. Under a reasonable working condition range of foam production and a fixed GLR, foam production quantity increases at first, and then decreases with the increase of liquid supply quantity. When the inner diameter of the nozzle is 14 mm, the best GLR is 30 and the optimum liquid supply quantity is 0.375 m3/h. The results of field experiments show that the total dust and respirable dust suppression efficiency of arc jet nozzles is 85.8% and 82.6% respectively, which are 1.39 and 1.37 times higher than the full cone nozzles and 1.20 and 1.19 times higher than the flat nozzles. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Dust is one of the main causes of hazards in the mining process in coal mines. It can lead to coal dust explosions and coal workers’ pneumoconiosis (CWP), and cause huge casualties and property losses. At present, coal dust explosions have been effectively controlled, but the number of pneumoconiosis deaths is far more than the total sum of deaths caused by mine fires, gas explosions and other mine accidents. For example, in China, the number of deaths caused by mine accidents was less than 1500 in 2012, but the people who died due to pneumoconiosis are more than 1800. Now, water sprays are widely used for dust prevention and suppression in the mining process in coal mines, but dust suppression efficiency is too low to meet with the actual demand [1–3]. Foam has an excellent effect on dust control with the mechanism of wetting, adhesion, interception, and settlement, etc. It is a type of dust control technology with significant research value and good prospects for application [4]. In recent years, practical applications and research on dust suppression by foam have been gradually increased in coal mines [5–7]. Field practice and research shows that dust suppression with foam faces an urgent problem: the cost
⇑ Corresponding author. Tel.: +86 13700196395.
is much higher, but foam utilization rate is low. In order to solve this problem, it is imperative to develop precise dust control technology. Through the precise use of foam spraying, the utilization rate of foam is increased, which decreases the cost. In research on precise dust control with foam, the spray pattern of the foam nozzle is an important research objective because it has a direct effect on the utilization rate of foam. The two conventional foam jets are ‘flat’ and ‘full cone’. Wang et al. designed a full cone nozzle and applied it to control dust at a heading face in the Xuehu Coal Mine in China [8]. The flat jet nozzle is widely used to control dust and harmful emissions. For instance, Singh M M and Laurito A W adopted a flat jet nozzle in their experiments on dust control with foam at a working face in a mine in Utah in the United States [9]. Ren designed a flat jet nozzle with a V-groove and carried out field applications at a heading face in the Xin’an Coal Mine in China [10]. Song designed two kinds of flat jets; one of them is shaped like a duck bill whilst the other is a flat jet composed of several full cone jets [11]. Another kind of foam nozzle-an ‘arc jet’ nozzle-was designed by Wang et al. Its stream cross section resembles an arc-shaped belt and, at present, the arc jet nozzle is being used in several coal mines in China. Firstly, through theoretical analysis, this paper will show that the arc jet is the best choice for achieving precise foam dedusting. Secondly, through experiment, the foam production
E-mail address: [email protected] (F. Han). http://dx.doi.org/10.1016/j.ijmst.2015.12.009 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
2
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
capability is studied to confirm the best operating conditions for foam production. Finally, the results of field tests of the new foam nozzle will be presented. 2. Advantages of arc jets 2.1. Framework of arc jet nozzle The newly-designed arc jet nozzle incorporates a main body combined with a diversion object. The diversion object is a semicone and the main body of the nozzle contains an inlet, outlet and an extended segment, as shown in Fig. 1. The extended segment controls the pattern of the arc jet, together with the diversion object, which can turn the foam into an arc jet. The foam jet produced by the arc jet nozzle is as shown in Fig. 2.
Fig. 2. Jet shape of arc jet nozzle.
Dust source
Cutting bit
2.2. Comparison between arc jet and other spray patterns Chinese mechanized mining equipment is comprised mainly of shearers and roadheaders, of which more than 90% of the cutting head (or drum) is in vertical-axis rotation, and therefore the source of dust presents an annular pattern, as showed in Fig. 3. Taking a vertical-axis roadheader as an example, three kinds of foam jet shapes can be analyzed: the full cone jet; the flat jet and the arc jet. Prior to making a comparison, the following assumptions can be made: nozzles are arranged equally in the same cycle at the root of the cutting arm; the number of nozzles is n, when n < 3; the three kinds of nozzles cannot all enclose the dust source. Therefore, according to objective reality, the number of nozzles should be a positive integer greater than 2. Suppose that the minimum thickness of foam that can effectively prevent the escape of dust is dmin , the largest circle diameter of the cutting head is dc , and the jet speed of foam is v. When using a full cone jet, flat jet or arc jet, the minimum total volume of the foam consumption is, respectively, V cone , V flat , V arc and the minimum total areas of their impact sections are Scone , Sflat and Sarc . (1) When using a full cone jet, the length of the strings confirmed by any two overlapping adjacent impact sections is the shortest path for the dust particles to exit the surroundings. The length of the string from point ‘a’ to point ‘b’ (which is shown in Fig. 4a) is dmin , taking the points ‘a’ and ‘b’ as points on a circle and making concentric circles for the largest circle of the cutting head. If L is the distance between the inner concentric circle and the largest circle of the cutting head, it can be seen from Fig. 4a that overlapping regions are formed by any two adjacent full cone impact sections. Therefore, the minimum value of the total impact section that is produced by the full cone jets can be
Cutting head Cutting arm
Coal face
Fig. 3. Schematic diagram of a dust source.
seen as being comprised of a ring-shaped region Storus , n overlapping regions Sov erlap and 2n remaining regions Sremain . Then,
V cone ¼ v Scone ¼ v ðStorus þ nSov erlap þ 2nSremain Þ
ð1Þ
(2) When using a flat jet, the impact section of a single flat jet is a belt-shaped region. The two adjacent flat jets should overlap, and the width of the impact section Lb should be dmin , as shown in Fig. 4b. The shortest path that the dust particles take through the impact section would appear at the overlapping region. Therefore, the length of the shortest escape path is dmin . Calculating the belt length La performed through the geometrical relationships, the minimum volume that is needed when using flat jets is:
V flat ¼ v Sflat ¼ v nLa Lb 180 180 þ dc tan ¼ nv dmin 2dmin sin n n
ð2Þ
(3) When using an arc jet, no matter how many nozzles are used, the total impact sections formed around the cutting head should be as shown in Fig. 4c. It is worth noting that two adjacent arc jets can theoretically have a seamless connection without overlapping. Then, the minimum volume that is needed by arc jets is:
V arc ¼ v Sarc
" 2 2 # 1 1 ¼ v p dc þ dmin p dc 2 2
¼ pv dmin ðdmin þ dc Þ
Fig. 1. Arc jet nozzle.
ð3Þ
(4) Comparisons: In order to compare V cone and V arc , half of the foam that goes through the overlapping regions and 2n crescent regions can be ignored. Introducing the fictitious minimum foam volume V cone when using full cone nozzles, we have:
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
3
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx (n-3) Belts
S torus
Impact section of arc jet
S remain
Impact section of full cone jet
a L
b
Impact section of flat jet
Largest circle of the cutting head
Lb
dc
Largest circle of the cutting head
dmin 360/n
d min
Largest circle of the cutting head
d min dc
dc
Soverlap La (a) Full cone jets
(b) Flat jets
(c) Arc jets
Fig. 4. Schematic diagram of a dust source covered by foam jets with different shapes.
V cone ¼ v Storus " 2 2 # 1 1 ¼ v p dc þ L þ dmin p dc þ L 2 2 ¼ pv dmin ðdmin þ dc þ 2LÞ
a bucket with foam; the experiment needed 4 measuring buckets with 0.05 m3 volumes. 3.2. Experimental processes
ð4Þ
Using V cone as a middle term and comparing V cone with V arc , we get:
V cone V cone ¼ nv ðSov erlap þ 2Sremain Þ > 0
ð5Þ
V cone V arc ¼ 2pv Ldmin P 0
ð6Þ
As shown in Fig. 4a, when V cone is compared with V arc , because two adjacent full cone impact sections must overlap in order to ensure that the thickness of the overlapping region is no less than dmin , and the full cone impact section is a circle, the side which is closest to the cutting pick cannot fit the largest circle of the cutting head. This leads to a gap between the full cone impact sections and the largest circle of the cutting head. However, the contour of the arc jet impact section can easily fit the largest circle of the cutting head. Even if full cone impact sections cross contacts with the cutting head, making the ring-shaped region Storus cling to the cutting head (L = 0), then V cone and V arc are equal and from Eqs. (5) and (6) we still get:
V cone > V arc
ð7Þ
When n is a positive integer greater than 2, comparing V flat and V arc functions using Matlab software, we get:
V flat > V arc
ð8Þ
To summarize: under the same condition of packing dust source, a minimum total volume of foam is required when using an arc jet, which proves that the arc jet is the best way to realize precise dust control using foam.
3. Performance experiment of arc jet nozzle 3.1. Experimental system In order to investigate the capacity of the new foam nozzle, an experimental system was constructed as showed in Fig. 5. The system consists of an air compressor, water tank, high pressure pump, foaming machine, high-pressure hose and arc jet nozzle with an inner diameter of 14 mm. During the experiment, liquid and gas flows were determined by electromagnetic and vortex flowmeters. The foaming agent used in this experiment was type KYFPJ-II, which was mixed with water in the tank at a concentration of 1%. The quantity of foam was calculated from the time taken to fill
The experimental processes were introduced as follows (1) Each part of the experimental system was connected according to Fig. 5, and was tested to ensure that the system worked properly. (2) The liquid flow rate Q l was adjusted to 0.5 m3/h and the ratio of gas liquid ratio (GLR) was maintained at 5. The buckets were filled with foam (if foam was produced), and the filling time was recorded as t 1 , t 2 , t3 and t 4 . If foam was not produced, the state of the jet was recorded. (3) The GLR was changed to 10, 15, 20, 25, 30, 35, 40, 45 and 50. Then step (2) was repeated. (4) The liquid flow rate Q l was increased to 1.0 m3/h, 1.5 m3/h, 2.0 m3/h and 2.5 m3/h and steps (2) and (3) were repeated. (5) The total quantity of foam,Q , was calculated and the experimental data was then analyzed. 3.3. Results and discussion 3.3.1. Relationship between the GLR and the state of the jet The experimental results show that with the GLR increasing, the spray of the arc jet nozzle presented, successively, a water jet, foam and mist in the same liquid supply. That is because the GLR is low. The foaming liquid cannot effectively froth and becomes a water jet. As shown in Fig. 6, when the GLR is 5, the foaming liquid cannot effectively froth and the state of the jet is a water jet. As shown in Fig. 7, when the GLR is 30, foam can be effectively produced and, as Fig. 8 shows, when the GLR reaches 50, the arc jet produces atomization owing to the rupture of the bubbles under the air atomization mechanism by the higher gas supply quantity. 3.3.2. Relationship between GLR and foam production quantity Under reasonable working conditions, over the range of foam production and at different liquid supply rates, foam production quantity increases at first, and then decreases with increasing GLR, as shown in Fig. 9. Therefore, under reasonable working conditions and over the range of foam production, there is an optimum proportion between the foam quantity and the GLR. When the liquid quantity is maintained in the range 0.5–2.5 m3/h, the GLR remains between 25 and 35, which can be effective in producing foam by an arc jet nozzle. 3.3.3. Relationship between the liquid supply quantity and foam production quantity Fig. 10 shows the relationship between the liquid supply quantity and the foam production quantity, with the GLR between 25
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
4
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Air compressor Vortex flowmeter High pressure pump Electromagnetic flowmeter
Water tank Distributor
Foaming machine
High-pressure hose
Measuring bucket 4
Measuring bucket 3
Measuring bucket 2
Measuring bucket 1
Nozzle
Fig. 5. Schematic diagram of experimental system.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 6. Spray patterns of the arc jet nozzle when the GLR is 5.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 7. Spray patterns of the arc jet nozzle when the GLR is 30.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 8. Spray patterns of the arc jet nozzle when the GLR is 50.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
5
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
Foam production quantity (m3/h)
40
types of nozzles, the experimental conditions must be consistent; the water supply must be maintained at 1.5 m3/h, the gas supply must be maintained at 45 m3/h and the added ratio of foaming agent should be 1%. Test steps are: (1) Install foam de-dusting system on the roadheader and ensure that the system works properly. (2) Measure the total dust concentrations (TDC) and the respirable dust concentrations (RDC) around the driver’s position without any dust suppression. (3) Install four arc fan nozzles equally around the end of the cutting arm. Measure TDC and RDC. (4) Change the nozzles to full cone and flat nozzles respectively. Then repeat step (3). The field trial results are presented in Table 1. As can be seen from Table 1, when the arc jet nozzle is used, the average de-dusting efficiency for total dust and respirable dust reached 85.8% and 82.6% respectively. These values are 1.39 times and 1.37 times higher than with full cone nozzles and 1.20 and 1.19 times higher than with flat nozzles. It can therefore be concluded that arc jet nozzles produce the best de-dusting performance with the same consumption of foam.
Liquid supply quantity 0.5 m3/h Liquid supply quantity 1.0 m3/h Liquid supply quantity 1.5 m3/h Liquid supply quantity 2.0 m3/h Liquid supply quantity 2.5 m3/h
35 30
25
20 15 20
25
30
35
40
45
GLR
Fig. 9. Relationship between GLR and foam production quantity.
Foam production quantity (m 3/h)
40
32
GLR 25
24
5. Conclusions
GLR 30 GLR 35 16 0
1
2
3
Liquid supply quantity (m 3/h)
Fig. 10. Relationship between liquid supply quantity and foam production quantity.
and 35. It is found that the foam production quantity increases at first and then decreases with increasing liquid supply quantity. The main reason is that, with an increase in the liquid supply, the airflow easily mixes with the foaming liquid, so that the amount of foam increases. Due to the increasing volume of liquid and gas and the limit in gas–liquid mixing space, the internal pressure of the nozzle increased sharply. Under the effect of a variety of atomization mechanisms, foam gradually shows an atomization trend and consequently the foam quantity is reduced. When the GLR is 30 and the liquid quantity is 1.5 m3/h, the foam quantity reaches a maximum. As four nozzles were used, it can be concluded that the best working condition for a single nozzle is a liquid quantity is 0.375 m3/h and a GLR of 30.
(1) In this paper, a theoretical comparison is made on the minimum volume of foam required when using the full cone jet, flat jet and arc jet. The results show that the arc jet is the best choice for achieving precise foam de-dusting. Under the same condition of an enclosed dust source, the arc jet requires the least amount of foam. (2) Foam production performance of an arc jet nozzle is researched via laboratory experiment. It was found that, with the same amount of liquid supply, the shape of the jet presents, successively, water jet, foam jet and mist with increasing GLR. Under a reasonable working range of foam production, foam quantity increases at first, and then decreases with increasing GLR. The best single nozzle condition is when the fluid supply quantity is 0.375 m3/h, and the GLR is 30. (3) Under the same working conditions, the field experiment shows that the total dust and respirable dust suppression efficiency of arc jet nozzles are 85.8% and 82.6% respectively. These values are 1.39 times and 1.37 times higher than in the case of full cone nozzles and 1.20 and 1.19 times higher than with the flat nozzles. That is to say, arc jet nozzles significantly improve the efficiency of foam de-dusting.
4. Field test of an arc jet nozzle The field test, which demonstrated the de-dusting performance of an arc jet nozzle, was carried out in 22611 roadway. This roadway was driven by an EBZ160 roadheader belonging to the underground coal mine Guandi, located in central China. In order to demonstrate the scientific de-dusting performance of different
Acknowledgment This study was supported by the National Natural Science Foundation of China (No. 51474216).
Table 1 Dust removal efficiency of different nozzles. Parameter
Measured data (mg/m3)
Average value (mg/m3) Average de-dusting efficiency (%)
No spray
Spray by flat nozzle
Spray by full cone nozzle
Spray by arc fan nozzle
TDC
RDC
TDC
RDC
TDC
RDC
TDC
RDC
793.0 786.0 791.0 790.0
362.0 355.0 359.0 358.7
225.0 234.0 211.0 223.3 71.7
102.0 119.0 106.0 109.0 69.6
289.0 294.0 321.0 301.3 61.9
143.0 129.0 156.0 142.7 60.2
116.0 105.0 115.0 112.0 85.8
63.0 69.0 55.0 62.3 82.6
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
6
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
References [1] Cheng WM, Nie W, Zhou G, Zuo QM. Study of dust suppression by atomized water from high-pressure sprays. J China Univ Mining Technol 2011;40 (2):185–9. [2] Nie W, Cheng WM, Zhou G, Xie J, Cui XF. Experimental study on atomized particle size as affected by airflow disturbance at the heading face. J China Univ Mining Technol 2012;41(3):378–83. [3] Xie YS, Fan GX, Dai JW, Song XB. New respirable dust suppression systems for coal mines. J China Univ Mining Technol 2007;17(3):321–5. [4] Page SJ, Volkwein JC. Foams for dust control. Eng Mining J 1986;187(10): 50–2. [5] Ren WX, Wang DM, Kang ZH, Lu XX. A new method for reducing the prevalence of pneumoconiosis among coal miners: foam technology for dust control. J Occup Environ Hygiene 2012;9(4):D77–83.
[6] Wang HT, Wang DM, Ren WX, Lu XX, Han FW, Zhang YK. Application of foam to suppress rock dust in a large cross-section rock roadway driven with roadheader. Adv Powder Technol 2013;24(1):257–62. [7] Wang HT, Wang DM, Lu XX, Gao QC, Ren WX. Experimental investigations on the performance of a new design of foaming agent adding device used for dust control in underground coal mines. J Loss Prevent Process Indust 2012;25 (6):1075–84. [8] Wang K, Zhou FB, Zhang RG. Application of foam de-dusting technology in Xuehu coal mine. Coal Eng 2011;8:52–4. [9] Singh MM, Laurito AW. Field tests of a foam dust suppressant system with longwall shearers. WV: Engineers International Inc.; 2004. [10] Ren WX. Research on the theory and technology for dust control by foam in underground coal mines. Xuzhou: China University of Mining & Technology; 2009. p. 78–88. [11] Song JG. Research and practices on dust control technology with carrier compressed air foam. Coal Eng 2010;9:82–5.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
A new design of foam spray nozzle used for precise dust control in underground coal mines Han Fangwei a,⇑, Wang Deming b, Jiang Jiaxing b, Zhu Xiaolong b a b
College of Safety Science and Engineering, Liaoning Technical University, Huludao 125105, China Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 26 May 2015 Received in revised form 6 September 2015 Accepted 15 October 2015 Available online xxxx Keywords: Dust control Precise spray Arc jet Gas liquid ratio (GLR) Dust suppression efficiency
a b s t r a c t In order to improve the utilization rate of foam, an arc jet nozzle was designed for precise dust control. Through theoretical analysis, the different demands of foam were compared amongst arc jets, flat jets and full cone jets when the dust source was covered identically by foam. It is proved that foam consumption was least when an arc jet was used. Foam production capability of an arc jet nozzle under different conditions was investigated through experiments. The results show that with the gas liquid ratio (GLR) increasing, the spray state of an arc jet nozzle presents successively water jet, foam jet and mist. Under a reasonable working condition range of foam production and a fixed GLR, foam production quantity increases at first, and then decreases with the increase of liquid supply quantity. When the inner diameter of the nozzle is 14 mm, the best GLR is 30 and the optimum liquid supply quantity is 0.375 m3/h. The results of field experiments show that the total dust and respirable dust suppression efficiency of arc jet nozzles is 85.8% and 82.6% respectively, which are 1.39 and 1.37 times higher than the full cone nozzles and 1.20 and 1.19 times higher than the flat nozzles. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Dust is one of the main causes of hazards in the mining process in coal mines. It can lead to coal dust explosions and coal workers’ pneumoconiosis (CWP), and cause huge casualties and property losses. At present, coal dust explosions have been effectively controlled, but the number of pneumoconiosis deaths is far more than the total sum of deaths caused by mine fires, gas explosions and other mine accidents. For example, in China, the number of deaths caused by mine accidents was less than 1500 in 2012, but the people who died due to pneumoconiosis are more than 1800. Now, water sprays are widely used for dust prevention and suppression in the mining process in coal mines, but dust suppression efficiency is too low to meet with the actual demand [1–3]. Foam has an excellent effect on dust control with the mechanism of wetting, adhesion, interception, and settlement, etc. It is a type of dust control technology with significant research value and good prospects for application [4]. In recent years, practical applications and research on dust suppression by foam have been gradually increased in coal mines [5–7]. Field practice and research shows that dust suppression with foam faces an urgent problem: the cost
⇑ Corresponding author. Tel.: +86 13700196395.
is much higher, but foam utilization rate is low. In order to solve this problem, it is imperative to develop precise dust control technology. Through the precise use of foam spraying, the utilization rate of foam is increased, which decreases the cost. In research on precise dust control with foam, the spray pattern of the foam nozzle is an important research objective because it has a direct effect on the utilization rate of foam. The two conventional foam jets are ‘flat’ and ‘full cone’. Wang et al. designed a full cone nozzle and applied it to control dust at a heading face in the Xuehu Coal Mine in China [8]. The flat jet nozzle is widely used to control dust and harmful emissions. For instance, Singh M M and Laurito A W adopted a flat jet nozzle in their experiments on dust control with foam at a working face in a mine in Utah in the United States [9]. Ren designed a flat jet nozzle with a V-groove and carried out field applications at a heading face in the Xin’an Coal Mine in China [10]. Song designed two kinds of flat jets; one of them is shaped like a duck bill whilst the other is a flat jet composed of several full cone jets [11]. Another kind of foam nozzle-an ‘arc jet’ nozzle-was designed by Wang et al. Its stream cross section resembles an arc-shaped belt and, at present, the arc jet nozzle is being used in several coal mines in China. Firstly, through theoretical analysis, this paper will show that the arc jet is the best choice for achieving precise foam dedusting. Secondly, through experiment, the foam production
E-mail address: [email protected] (F. Han). http://dx.doi.org/10.1016/j.ijmst.2015.12.009 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
2
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
capability is studied to confirm the best operating conditions for foam production. Finally, the results of field tests of the new foam nozzle will be presented. 2. Advantages of arc jets 2.1. Framework of arc jet nozzle The newly-designed arc jet nozzle incorporates a main body combined with a diversion object. The diversion object is a semicone and the main body of the nozzle contains an inlet, outlet and an extended segment, as shown in Fig. 1. The extended segment controls the pattern of the arc jet, together with the diversion object, which can turn the foam into an arc jet. The foam jet produced by the arc jet nozzle is as shown in Fig. 2.
Fig. 2. Jet shape of arc jet nozzle.
Dust source
Cutting bit
2.2. Comparison between arc jet and other spray patterns Chinese mechanized mining equipment is comprised mainly of shearers and roadheaders, of which more than 90% of the cutting head (or drum) is in vertical-axis rotation, and therefore the source of dust presents an annular pattern, as showed in Fig. 3. Taking a vertical-axis roadheader as an example, three kinds of foam jet shapes can be analyzed: the full cone jet; the flat jet and the arc jet. Prior to making a comparison, the following assumptions can be made: nozzles are arranged equally in the same cycle at the root of the cutting arm; the number of nozzles is n, when n < 3; the three kinds of nozzles cannot all enclose the dust source. Therefore, according to objective reality, the number of nozzles should be a positive integer greater than 2. Suppose that the minimum thickness of foam that can effectively prevent the escape of dust is dmin , the largest circle diameter of the cutting head is dc , and the jet speed of foam is v. When using a full cone jet, flat jet or arc jet, the minimum total volume of the foam consumption is, respectively, V cone , V flat , V arc and the minimum total areas of their impact sections are Scone , Sflat and Sarc . (1) When using a full cone jet, the length of the strings confirmed by any two overlapping adjacent impact sections is the shortest path for the dust particles to exit the surroundings. The length of the string from point ‘a’ to point ‘b’ (which is shown in Fig. 4a) is dmin , taking the points ‘a’ and ‘b’ as points on a circle and making concentric circles for the largest circle of the cutting head. If L is the distance between the inner concentric circle and the largest circle of the cutting head, it can be seen from Fig. 4a that overlapping regions are formed by any two adjacent full cone impact sections. Therefore, the minimum value of the total impact section that is produced by the full cone jets can be
Cutting head Cutting arm
Coal face
Fig. 3. Schematic diagram of a dust source.
seen as being comprised of a ring-shaped region Storus , n overlapping regions Sov erlap and 2n remaining regions Sremain . Then,
V cone ¼ v Scone ¼ v ðStorus þ nSov erlap þ 2nSremain Þ
ð1Þ
(2) When using a flat jet, the impact section of a single flat jet is a belt-shaped region. The two adjacent flat jets should overlap, and the width of the impact section Lb should be dmin , as shown in Fig. 4b. The shortest path that the dust particles take through the impact section would appear at the overlapping region. Therefore, the length of the shortest escape path is dmin . Calculating the belt length La performed through the geometrical relationships, the minimum volume that is needed when using flat jets is:
V flat ¼ v Sflat ¼ v nLa Lb 180 180 þ dc tan ¼ nv dmin 2dmin sin n n
ð2Þ
(3) When using an arc jet, no matter how many nozzles are used, the total impact sections formed around the cutting head should be as shown in Fig. 4c. It is worth noting that two adjacent arc jets can theoretically have a seamless connection without overlapping. Then, the minimum volume that is needed by arc jets is:
V arc ¼ v Sarc
" 2 2 # 1 1 ¼ v p dc þ dmin p dc 2 2
¼ pv dmin ðdmin þ dc Þ
Fig. 1. Arc jet nozzle.
ð3Þ
(4) Comparisons: In order to compare V cone and V arc , half of the foam that goes through the overlapping regions and 2n crescent regions can be ignored. Introducing the fictitious minimum foam volume V cone when using full cone nozzles, we have:
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
3
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx (n-3) Belts
S torus
Impact section of arc jet
S remain
Impact section of full cone jet
a L
b
Impact section of flat jet
Largest circle of the cutting head
Lb
dc
Largest circle of the cutting head
dmin 360/n
d min
Largest circle of the cutting head
d min dc
dc
Soverlap La (a) Full cone jets
(b) Flat jets
(c) Arc jets
Fig. 4. Schematic diagram of a dust source covered by foam jets with different shapes.
V cone ¼ v Storus " 2 2 # 1 1 ¼ v p dc þ L þ dmin p dc þ L 2 2 ¼ pv dmin ðdmin þ dc þ 2LÞ
a bucket with foam; the experiment needed 4 measuring buckets with 0.05 m3 volumes. 3.2. Experimental processes
ð4Þ
Using V cone as a middle term and comparing V cone with V arc , we get:
V cone V cone ¼ nv ðSov erlap þ 2Sremain Þ > 0
ð5Þ
V cone V arc ¼ 2pv Ldmin P 0
ð6Þ
As shown in Fig. 4a, when V cone is compared with V arc , because two adjacent full cone impact sections must overlap in order to ensure that the thickness of the overlapping region is no less than dmin , and the full cone impact section is a circle, the side which is closest to the cutting pick cannot fit the largest circle of the cutting head. This leads to a gap between the full cone impact sections and the largest circle of the cutting head. However, the contour of the arc jet impact section can easily fit the largest circle of the cutting head. Even if full cone impact sections cross contacts with the cutting head, making the ring-shaped region Storus cling to the cutting head (L = 0), then V cone and V arc are equal and from Eqs. (5) and (6) we still get:
V cone > V arc
ð7Þ
When n is a positive integer greater than 2, comparing V flat and V arc functions using Matlab software, we get:
V flat > V arc
ð8Þ
To summarize: under the same condition of packing dust source, a minimum total volume of foam is required when using an arc jet, which proves that the arc jet is the best way to realize precise dust control using foam.
3. Performance experiment of arc jet nozzle 3.1. Experimental system In order to investigate the capacity of the new foam nozzle, an experimental system was constructed as showed in Fig. 5. The system consists of an air compressor, water tank, high pressure pump, foaming machine, high-pressure hose and arc jet nozzle with an inner diameter of 14 mm. During the experiment, liquid and gas flows were determined by electromagnetic and vortex flowmeters. The foaming agent used in this experiment was type KYFPJ-II, which was mixed with water in the tank at a concentration of 1%. The quantity of foam was calculated from the time taken to fill
The experimental processes were introduced as follows (1) Each part of the experimental system was connected according to Fig. 5, and was tested to ensure that the system worked properly. (2) The liquid flow rate Q l was adjusted to 0.5 m3/h and the ratio of gas liquid ratio (GLR) was maintained at 5. The buckets were filled with foam (if foam was produced), and the filling time was recorded as t 1 , t 2 , t3 and t 4 . If foam was not produced, the state of the jet was recorded. (3) The GLR was changed to 10, 15, 20, 25, 30, 35, 40, 45 and 50. Then step (2) was repeated. (4) The liquid flow rate Q l was increased to 1.0 m3/h, 1.5 m3/h, 2.0 m3/h and 2.5 m3/h and steps (2) and (3) were repeated. (5) The total quantity of foam,Q , was calculated and the experimental data was then analyzed. 3.3. Results and discussion 3.3.1. Relationship between the GLR and the state of the jet The experimental results show that with the GLR increasing, the spray of the arc jet nozzle presented, successively, a water jet, foam and mist in the same liquid supply. That is because the GLR is low. The foaming liquid cannot effectively froth and becomes a water jet. As shown in Fig. 6, when the GLR is 5, the foaming liquid cannot effectively froth and the state of the jet is a water jet. As shown in Fig. 7, when the GLR is 30, foam can be effectively produced and, as Fig. 8 shows, when the GLR reaches 50, the arc jet produces atomization owing to the rupture of the bubbles under the air atomization mechanism by the higher gas supply quantity. 3.3.2. Relationship between GLR and foam production quantity Under reasonable working conditions, over the range of foam production and at different liquid supply rates, foam production quantity increases at first, and then decreases with increasing GLR, as shown in Fig. 9. Therefore, under reasonable working conditions and over the range of foam production, there is an optimum proportion between the foam quantity and the GLR. When the liquid quantity is maintained in the range 0.5–2.5 m3/h, the GLR remains between 25 and 35, which can be effective in producing foam by an arc jet nozzle. 3.3.3. Relationship between the liquid supply quantity and foam production quantity Fig. 10 shows the relationship between the liquid supply quantity and the foam production quantity, with the GLR between 25
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
4
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Air compressor Vortex flowmeter High pressure pump Electromagnetic flowmeter
Water tank Distributor
Foaming machine
High-pressure hose
Measuring bucket 4
Measuring bucket 3
Measuring bucket 2
Measuring bucket 1
Nozzle
Fig. 5. Schematic diagram of experimental system.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 6. Spray patterns of the arc jet nozzle when the GLR is 5.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 7. Spray patterns of the arc jet nozzle when the GLR is 30.
(a) 0.5 m³
(b) 1.0 m³
(c) 1.5 m³
(d) 2.0 m³
(e) 2.5 m³
Fig. 8. Spray patterns of the arc jet nozzle when the GLR is 50.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
5
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
Foam production quantity (m3/h)
40
types of nozzles, the experimental conditions must be consistent; the water supply must be maintained at 1.5 m3/h, the gas supply must be maintained at 45 m3/h and the added ratio of foaming agent should be 1%. Test steps are: (1) Install foam de-dusting system on the roadheader and ensure that the system works properly. (2) Measure the total dust concentrations (TDC) and the respirable dust concentrations (RDC) around the driver’s position without any dust suppression. (3) Install four arc fan nozzles equally around the end of the cutting arm. Measure TDC and RDC. (4) Change the nozzles to full cone and flat nozzles respectively. Then repeat step (3). The field trial results are presented in Table 1. As can be seen from Table 1, when the arc jet nozzle is used, the average de-dusting efficiency for total dust and respirable dust reached 85.8% and 82.6% respectively. These values are 1.39 times and 1.37 times higher than with full cone nozzles and 1.20 and 1.19 times higher than with flat nozzles. It can therefore be concluded that arc jet nozzles produce the best de-dusting performance with the same consumption of foam.
Liquid supply quantity 0.5 m3/h Liquid supply quantity 1.0 m3/h Liquid supply quantity 1.5 m3/h Liquid supply quantity 2.0 m3/h Liquid supply quantity 2.5 m3/h
35 30
25
20 15 20
25
30
35
40
45
GLR
Fig. 9. Relationship between GLR and foam production quantity.
Foam production quantity (m 3/h)
40
32
GLR 25
24
5. Conclusions
GLR 30 GLR 35 16 0
1
2
3
Liquid supply quantity (m 3/h)
Fig. 10. Relationship between liquid supply quantity and foam production quantity.
and 35. It is found that the foam production quantity increases at first and then decreases with increasing liquid supply quantity. The main reason is that, with an increase in the liquid supply, the airflow easily mixes with the foaming liquid, so that the amount of foam increases. Due to the increasing volume of liquid and gas and the limit in gas–liquid mixing space, the internal pressure of the nozzle increased sharply. Under the effect of a variety of atomization mechanisms, foam gradually shows an atomization trend and consequently the foam quantity is reduced. When the GLR is 30 and the liquid quantity is 1.5 m3/h, the foam quantity reaches a maximum. As four nozzles were used, it can be concluded that the best working condition for a single nozzle is a liquid quantity is 0.375 m3/h and a GLR of 30.
(1) In this paper, a theoretical comparison is made on the minimum volume of foam required when using the full cone jet, flat jet and arc jet. The results show that the arc jet is the best choice for achieving precise foam de-dusting. Under the same condition of an enclosed dust source, the arc jet requires the least amount of foam. (2) Foam production performance of an arc jet nozzle is researched via laboratory experiment. It was found that, with the same amount of liquid supply, the shape of the jet presents, successively, water jet, foam jet and mist with increasing GLR. Under a reasonable working range of foam production, foam quantity increases at first, and then decreases with increasing GLR. The best single nozzle condition is when the fluid supply quantity is 0.375 m3/h, and the GLR is 30. (3) Under the same working conditions, the field experiment shows that the total dust and respirable dust suppression efficiency of arc jet nozzles are 85.8% and 82.6% respectively. These values are 1.39 times and 1.37 times higher than in the case of full cone nozzles and 1.20 and 1.19 times higher than with the flat nozzles. That is to say, arc jet nozzles significantly improve the efficiency of foam de-dusting.
4. Field test of an arc jet nozzle The field test, which demonstrated the de-dusting performance of an arc jet nozzle, was carried out in 22611 roadway. This roadway was driven by an EBZ160 roadheader belonging to the underground coal mine Guandi, located in central China. In order to demonstrate the scientific de-dusting performance of different
Acknowledgment This study was supported by the National Natural Science Foundation of China (No. 51474216).
Table 1 Dust removal efficiency of different nozzles. Parameter
Measured data (mg/m3)
Average value (mg/m3) Average de-dusting efficiency (%)
No spray
Spray by flat nozzle
Spray by full cone nozzle
Spray by arc fan nozzle
TDC
RDC
TDC
RDC
TDC
RDC
TDC
RDC
793.0 786.0 791.0 790.0
362.0 355.0 359.0 358.7
225.0 234.0 211.0 223.3 71.7
102.0 119.0 106.0 109.0 69.6
289.0 294.0 321.0 301.3 61.9
143.0 129.0 156.0 142.7 60.2
116.0 105.0 115.0 112.0 85.8
63.0 69.0 55.0 62.3 82.6
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009
6
F. Han et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
References [1] Cheng WM, Nie W, Zhou G, Zuo QM. Study of dust suppression by atomized water from high-pressure sprays. J China Univ Mining Technol 2011;40 (2):185–9. [2] Nie W, Cheng WM, Zhou G, Xie J, Cui XF. Experimental study on atomized particle size as affected by airflow disturbance at the heading face. J China Univ Mining Technol 2012;41(3):378–83. [3] Xie YS, Fan GX, Dai JW, Song XB. New respirable dust suppression systems for coal mines. J China Univ Mining Technol 2007;17(3):321–5. [4] Page SJ, Volkwein JC. Foams for dust control. Eng Mining J 1986;187(10): 50–2. [5] Ren WX, Wang DM, Kang ZH, Lu XX. A new method for reducing the prevalence of pneumoconiosis among coal miners: foam technology for dust control. J Occup Environ Hygiene 2012;9(4):D77–83.
[6] Wang HT, Wang DM, Ren WX, Lu XX, Han FW, Zhang YK. Application of foam to suppress rock dust in a large cross-section rock roadway driven with roadheader. Adv Powder Technol 2013;24(1):257–62. [7] Wang HT, Wang DM, Lu XX, Gao QC, Ren WX. Experimental investigations on the performance of a new design of foaming agent adding device used for dust control in underground coal mines. J Loss Prevent Process Indust 2012;25 (6):1075–84. [8] Wang K, Zhou FB, Zhang RG. Application of foam de-dusting technology in Xuehu coal mine. Coal Eng 2011;8:52–4. [9] Singh MM, Laurito AW. Field tests of a foam dust suppressant system with longwall shearers. WV: Engineers International Inc.; 2004. [10] Ren WX. Research on the theory and technology for dust control by foam in underground coal mines. Xuzhou: China University of Mining & Technology; 2009. p. 78–88. [11] Song JG. Research and practices on dust control technology with carrier compressed air foam. Coal Eng 2010;9:82–5.
Please cite this article in press as: Han F et al. A new design of foam spray nozzle used for precise dust control in underground coal mines. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2015.12.009