Study of the Efficiency of Dust Filters in Terms of Coal Mines

Mechanics, Materials Science & Engineering, May 2017

ISSN 2412-5954 28

1, a

1, b

1, c

1,d

1 a

sihc@yandex.ru

b

cheberiachkoyi@ukr.net

c

lenayavorskay@mail.ru

d

ruis@i.ua DOI 10.2412/mmse.81.48.85 provided by Seo4U.link

Keywords: Dust respirator, pressure drop, breathing resistance, dust content, particle size distribution.

ABSTRACT. The objective of the research is to determine protective properties of the filters of dust respirators being used in coal mines; the properties to be studied are obtained under laboratory conditions. It has been determined that penetration coefficient of dust-loaded filters gets worse along with the moisture increase in terms of volume filtration phase; however, the coefficient gets better in terms of the phase of surface accumulation of dust deposit at the expense of filter surface porosity reduction. It has been proved that the increase of dust concentration in the air of a working zone in terms of air moisture and high filtration rate within the initial phase helps improve protective properties; the effect is stipulated by the nonavailability of volume filtration phase. It is shown that a prefilter (made of special-purpose hydrophilic material with low fabric weight) being placed on a filter box prolongs the life of a main filter at the expense of the reduction of surface potential flowing off and as well as decrease in dust load.

Introduction. Working conditions in terms of coal enterprises are determined mostly by dust factor, i.e. they depend upon dust content in the air of a working zone, its generation type, and dust deposition near dust sources. In this context dust properties stipulates selection of the corresponding measures to deal with this problem.

Current regulatory documents on labor safety based on sanitary-hygienic standards or levels of permissible dust loads if the standards are impossible to meet. Moreover, miners are provided with respiratory protective devices. Their selection is regulated by several standards [1, 2]. It is considered that if filtering respirator is selected correctly, it can protect a worker against pneumoconiosis and dust bronchitis. Despite the fact that the use of respiratory protective devices is obligatory, the number of people with dust etiology diseases increases year by year. Thus, study of the efficiency of dust filtering respirators use under production conditions is of high importance and topicality.

Outlining of the unsolved problem. Polypropylene materials have become very popular to make filtering respiratory protective devices as the materials are characterized by slight initial breathing resistance, ease in their manufacturing and use, and high mechanical hardness. However, their protective properties depend greatly upon the available electrostatic charge. The research shows that in terms of linear filtration rates up to 0.006 m/s the efficiency of electrostatic mechanisms of aerosol particle catching reaches 80% of total action of all the mechanisms [3]. However, it is unstable and its behaviour can be unpredictable especially under the effect of high temperature and air moisture [4, 5]. Taking into account the fact that mine workings of coal mines are characterized by unfavorable climatic conditions, especially by high air moisture, there is the topical problem to research its effect

his is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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upon protective efficiency of filters made of polypropylene materials to determine its respiratoryprotection period.

Analysis of studies and publications. Publications concerning the estimation of the influence of air moisture upon the efficiency of aerosol catching by means of electret filters differ considerably. Some papers study deterioration of protective properties connected with the decrease of value of particle charges and fibers entering such conditions [4]. On the contrary, other papers do not admit such conclusion. For example, some authors [5] note that air moister has almost no effect on the penetration coefficient of aerosol particle being up to 100 nm in diameter. Experiments were carried out in terms of relative humidity from 40 % up to 90 %. However, there are studies demonstrating that catching of large particles improves if humidity increases; this effect is explained by the increase of capillary forces which increase their sticking to the material fibers [6]. There is a conclusion that the filters with the brought charge within the field of corona charge demonstrate worse protective features as in this case electrons on the fiber surface can be easily removed by means of water molecules. Electret filters are another case as they are obtained by means of electrospinning (when special-purpose more water-resistant coating is generated) [7]. Authors of the papers [8, 9] tried to find solution of the problem of electret life extending at the expense of introducing various composite additives into the structure of a filter. However, the studies have no clear answer concerning the ability of electrets filters to offer enough protection during one working shift under conditions of high humidity.

Formulation of the research objective. Study of the interrelation between penetration coefficient of polypropylene material filters and value of electrostatic charge on their fibers under conditions of high air humidity to determine the period of respiratory protection life.

Materials and research methods. The research involved three different types of filters for nondisposable respirators complying with the second class of protection according to DSTU EN 143:2002 (confirmed by quality certificates). They were made of polypropylene filtering material with 2.5 micron mean fiber diameter, 4 mm filtering layer thickness and following fabric weight: first filter 55 g/m2 (type second filter 45 g/m2 (type B); third filter 40 g/m2 (type C).To reduce testing error of respirator protective efficiency and filters air flow resistance was determined first to select the ones with similar parameters to use during experiments.

Following parameters were determined under laboratory conditions: - penetration coefficient of filters respirator during clogging using the formula [4]

,%

(1)

where 1 and 2 are aerosol concentration within undermask area of respirator and in a test chamber, mg/m3, respectively; value of surface charge on filter surface using the formula [10] 2V

where

/S, Kl/m2

(2)

is dielectric air penetration, 2

V

is dielectric penetration of electret material; is potential of electret surface, V;

S is the area of measuring electrode, m2; pressure drop on filters in terms of 30 dm3/min air flow using the formula [10] MMSE Journal. Open Access www.mmse.xyz

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,

(3)

where ni is indication of filters resistance using micromanometer, Pa; n0 is the initial resistance of the measuring system, Pa; K1 is the adjustment coefficient for temperature and atmospheric pressure.

Tests were carried out to determine the effect of relative humidity and dustiness of the air on the penetration coefficient and pressure drop of filters in terms of laboratory conditions. Levels of relative humidity were from 60% to 80% and dust concentration in the chamber was 250 300 mg/m3. Each test was being carried out until pressure drop on a filter was 100 Pa. While dusting coefficient of penetration in terms of sodium chloride test-aerosol (with size-consist being from 0.2 micron up to 2 micron) was being determined at regular intervals. Paper [10] described in detail its method of determination. Filter was weighed before and after the tests to determine total weight of water and amount of dust on it. When the tests were completed the filter was left to dry at room temperature within 24 hours. After drying it was weighed again to obtain the mass of the deposited dust.

Special-purpose stand was used for dusting (Fig. 1) that allowed simulating real working process with the dust available in the air of a working zone. Dusty air was blown through the filter with 30 dm3/min air flow.

Fig. 1. General view of a testing stand: 1, 9 compressed air supply; 2 prefilter to purify compressed air; 3 ejector; 4 diffuser; 5 cyclonic collector; 6 flowrate meter; 7 exhauster fan; 8 outlet fitting; 10 dust generator; 11 dust distributor; 12 damping plate; 13 testing chamber.

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The stand operates as follows. Ejector is used to deliver dust into a testing chamber from a generator.

Diffuser available in the structure of a testing chamber allows equal dust and airflow distribution within a chamber providing its laminarity.

Filter is mounted on a proper adaptor in a testing chamber. Dust concentration is measured using sampling instrument with AFA filter being placed near test specimen. Dust concentration is determined according to the formula:

,

where m1 is the mass of AFA filter, mg; m2

is the mass of AFA filter with dust after sample collection, mg;

Q is the air flow, dm3/min.; t is the time of air sample collection, min.

Potential on filter surfaces were determined experimentally to calculate the value of electrostatic charge. The plant consists of two blocks (measuring and indicating) (Fig.2). It allows measuring effective voltage on electret surface up to 1000 V. Principle of its operation is based on the use of as electron-beam wobbulator as a null-indicating instrument with the displaying of compensation voltage on a digital voltmeter. Before the test started, the filter was unfolded; then a measuring electrode was placed on its internal surface. For the purpose of experiment repeatability areas of electrode placing were marked (Fig. 3). Further, filter was folded with the help of matrix (Fig. 4) and after the dusting it was unfolded again to determine surface potential within the same areas.

Fig. 2. Scheme of the stand to measure electrostatic potential on the surface of filtering material.

Production research of the filters was carried out in terms of Western Donbas mines. Miners of the following professions participated in the experiment: machine operator, assistant of machine operator, and mineworker. They were given respirators of RPA type with filters tested previously under laboratory conditions. It should be noted that half masks were selected according to facial parameters. There were no additional trainings for workers participating in the research concerning the proper use of respiratory protective equipment except for the ones envisaged by the initial-training program. Experiments lasted for 4 hours. While measuring, air humidity and temperature within each working place were controlled by means of -4 aspirator; dust concentration was also determined by means of IZSA dust counter. After the experiment, filters were placed into individual packets for MMSE Journal. Open Access www.mmse.xyz

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the following measuring of their dust-loaded weight and determining surface electrostatic charge and penetration coefficient under laboratory conditions.

Fig. 3. Points of electrode placing to measure electrostatic charge on the surface of filtering material.

Fig. 4. Matrices to form pleated filters of conelike blank part for a respirator.

The results. Table 1 demonstrates results of penetration coefficient, surface potential, mass and pressure drop of filters measuring before the tests as for their dustiness in terms of their compliance with DSTU EN 143 requirements.

Table 1. Average values of penetration coefficient of filters under laboratory conditions.

Filter Mark

Filter weight, g

Breathing resistance, Pa in terms of 30 dm3/min air flow

Value of the surface potential, V

Coefficient of penetration in terms of sodium chloride, %

Type

6.45

27

455

0.226

Type B

6.25

23

415

0.511

Type C

6.55

17

376

0.806

Tables 2 and 3 show the results of experimental research to study the dependence of pressure drop and penetration coefficient of the filters carried out in the laboratory under various testing conditions simulating mine working air (marked 1 to 5 on Fig. 5a).

The analysis of the obtained data has shown that under certain conditions high air humidity along with coal dust can deteriorate considerably the quality of the filters. We can see that just in 30 min their operational properties weaken significantly. Similar results have been obtained in the research.

However, it cannot be said definitely that the tested filters will protect the workers properly under production conditions. More research is required; first of all it is necessary to explain why air humidity along with dust effect filtering properties.

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Table 2. Average values of penetration coefficient and pressure drop of filters in terms of various testing conditions in 30 min.

Filter mark

Testing conditions characterized by the temperature in a testing chamber, t, 0C, humidity available dustiness 2, mg/m3 23 0

23 0 200 mg/m3

28 0

28 0

37 0

200 mg/m3

200 mg/m3

Type

0.25/27

1.2/55

4.3/29

6.2/55

8.6/61

Type B

0.5/23

3.8/45

5.5/25

9.1/46

13.2/52

Type C

0.8/17

5.2/37

7.1/22

12.3/39

15.3/44

Note: numerator indicates coefficient of penetration, denominator indicates pressure drop If the obtained results are studied thoroughly, it will be clear that if there are changes only in climatic conditions then breathing resistance shows almost no growth (testing conditions 1 and 3); in this context coefficient of penetration has grown almost tenfold. Such deterioration of filtering properties at constant filtration rate and similar particle size distribution can be explained by the weakening of the action of electrostatic mechanism of particle catching due to charge flowing-off. Fig.6 shows the dependence of penetration coefficient of filters upon the value of electrostatic charge. It is seen that the filters with charge being less than 2.5 nC /cm2 will not correspond to DSTU EN 143:2002 requirements. Increase in air temperature and humidity as well as deposition of charged dust particles are also the factors deteriorating coefficient of penetration. The research shows that filtering polypropylene material with fabric weight not less than 45 g/m2 at the available charging system is not efficient in filter production so only filters of A type are used in the further research.

The results of experimental research to determine the effect of air humidity upon the value of electric potential of filters confirm the aforementioned statement. On average, process of flowing-off may last from several days up to several months. The research shows that under standard conditions charge decrease on filters made of polypropylene material takes one-two months; after that it stays the same for a considerable period of time. At the same time if air humidity increases up to 95%, then the process of charge flowing-off is accelerated (Fig.7). It is considered that when water molecules fill in certain part of electret surface, electroconductive channel evolves which reduces sharply the value of surface potential.

Fig. 6. Curves of the dependence of penetration coefficient of polypropylene filters with various fabric weight of fibers upon the value of surface electrostatic charge.

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Fig. 7. Curves of electrostatic charge flowing-off at 95 % air humidity and 26 0 temperature (1) and 60 % and 23 0 respectively (2) on polypropylene filters.

Fig. 8. Curves characterizing change of filters coefficient penetration in terms of sodium chloride test aerosol within certain period of time.

However, due to different reasons the process can take too much time; thus, within the certain intervals of filter operation it is possible to have stable result in terms of high air humidity as well (Fig. 8). In particular, in terms of low filtration rates the initial stage shows improvement of protective properties owing to the precipitation of aerosol water particles on fibers and porosity reduction.

Accumulation of coal dust on fibers will result almost in the same effect. It is confirmed by the change of the penetration coefficient of filter in terms of its dusting (Fig. 9). It should be also noted that in time protective efficiency of filters will improve. Accumulation of aerosol and moisture which reduce porosity of filtering material also helps improve the protective properties.

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Fig. 9. Curves characterizing change of filters coefficient penetration in terms of sodium chloride test aerosol within certain period of time.

Fig. 10. Dependence of changes in filters coefficient penetration in terms of sodium chloride test aerosol upon dust content.

If we compare dependence of filters coefficient penetration in terms of sodium chloride test aerosol upon the amount of the accumulated dust with the curves of changes in pressure drop in term of dust deposition, then we can say that the improvement of protective properties will be possible only when the volume phase of dust deposition is over (Fig. 10, 11). It is characterized by nonlinearity explained by the deposition of dust particles within a filtering layer. In this context increase in air humidity reduces the duration of this phase (Fig. 11).

Value of respirators coefficient penetration within the initial operation phase can be decreased at the expense of placing prefilter made of special-purpose hydrophilic material with low fabric weight, e.g.

SMMS, in front of a filter box (Fig. 12). Tests have shown that most moisture amount and coarse fraction of dust deposit right on it; it is confirmed by sharp growth of its mass relative to main filter (Table 4). It allows not only making respirator service life longer but reducing flowing-off of electrostatic charge on filters to improve protective properties of half masks in general (Fig.13). It means that the use of such additional prefilters will make it possible to prolong service life of respirators.

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Fig.11. Curves of dependence of pressure drop upon dust content in terms of different air humidity obtained at 30 dm3/min filtering rate.

Fig. 12. Prefilter being mounted on a respirator filter box.

Table 4. Average values of filter coefficient of penetration with the mounted prefilter.

Parameters

Test conditions characterized by the temperature in a testing chamber, t, 0C, humidity, %, and available dustiness, 2, g/m3 23 0

28 0 mg/m

3

, 200 mg/m

3

37 0

, 200 mg/m3

Initial mass of a main filter, g

6.33

6.27

6.24

Initial mass of a prefilter, g

1.51

1.43

1.62

General coefficient of penetration, %

0.35

1.12

3.63

Mass of a main filter with dust and moisture, g

6.58

6.48

6.49

Mass of a prefilter with dust and moisture, g

1.86

2.01

2.04

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Fig. 13. Curves characterizing change in filters coefficient penetration in terms of sodium chloride test aerosol within a certain period of clogging time.

Tests of filters under production conditions within one shift have shown that it is the increase of dust concentration in the air of a working zone in combination with high humidity and pressure drop that results in the increase of protective properties of a filter (Table 1).

The analysis of the obtained data has demonstrated that the increase of filters test aerosol penetration coefficient is not crucial. Even in the worst case they will provide sufficient protective level.

Summary. Because of the research the following facts have been determined. Respirators with polypropylene filters give sufficient protection for the workers operating during a shift (with not more than 6.0% coefficient of penetration and up to 70 Pa pressure difference). However, protective properties of polypropylene filters are not constant value as they depend on the operating conditions; thus under conditions of high humidity and temperature their parameters worsened in time at the expense of decrease of the value of surface electrostatic charge.

Table 5. Values of filters parameters after their testing under production conditions.

Parameters of filter groups worn by the miners

Dust concentration, mg/m3

Mass of the dust deposited on a filter, g

Pressure drop, after clogging, Pa

Coefficient of penetration, , %

Cutter-loader operator

.4

0.

.24

.6

0.

.05

Assistant operator

.1

0.

.31

.3

3.

.11

Mineworker

.4

0.

.16

.4

5.

.31

It has been determined that penetration coefficient of dusty filters deteriorates along with the increase of humidity within the phase of volume filtration while when there is the phase of surface accumulation of dust deposit this coefficient improves at the expense of reduction of filter surface porosity. It has been proved that on the contrary the increase of dust concentration in the air of a working zone in terms of the available air humidity and high filtration rate at the initial phase favours the improvement of protective parameters; such effect is stipulated by the nonavailability of the phase of volume filtration. It has been shown that the prefilter (made of special-purpose hydrophilic material MMSE Journal. Open Access www.mmse.xyz

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with low fabric weight) being placed on a filter box increases the working life of a main filter at the expense of the decrease of surface potential flowing-off and dust load.

The research being carried out requires further specification of the dependence of penetration coefficient upon surface electrostatic charge.

References [1] SRARP 0.00-1.04Committee of Ukraine on Industrial Safety, Labour Protection and Mines Inspectorate under order No. 331 of 28.12.2007.

[2] DSTU EN 529:2006 Respiratory protective devices. Recommendations on the selection, use, and maintenance. Instruction (EN 529:2005, IDT).

[4] J Kuzmin, Yu.I., Pshchelko, N.S., Sokolova, I.M., Zakrzhevskiy, V.I. The percolation behaviour of electret at presence of water condensation. In: Proc. of the 8th Int. Symp. on Electrets, Ed. by J. Paris, France. P. 124-129.

[5] Romay, F.J. Experimental study of electrostatic capture mechanisms in commercial electret filters, Aerosol Science and Technology, Vol. 28 (1998) Nr 3, pp. 224-234. DOI 10.1080/02786829808965523 [6] Ackley, M.W., Degradation of electrostatic filters at elevated temperature and humidity, Filtration and Separation, Vol. 22 (1992) Nr 4, pp. 239-242.

[7] Moyer, E. S. & Stevens, G. A., Worst Case aerosol testing parameters: II. Efficiency dependence of commercial respirator filters on humidity pretreatment. American Industrial Hygiene Association Journal, Vol. 50 (1989), pp. 265-270.

[8] Chen C.C., M. Lehtimaki and K. Willeke: Aerosol penetration through filtering facepieces and cartridges. Am. Ind. Hyg. Assoc. J. 53: 566 574 (1992). DOI 10.1080/15298669291360166 Vergleich zu Messungen an Filtern aus dem Betrieb; Chem. Ing. Techn.; 84, No. 6, 808-812 (2012).

[10] V.I. Golinko, S.I. Cheberiachko, V.Ye. Kolesnik, .S. Ishchenko (2004), Analysis and estimation of the protective efficiency of filtering respirators, Scientific Herald of Naional mining University, No. 12., pp. 33 36.

[11] S.I. Cheberyachko, D.I. Radchuk, Y.I. Cheberyachko, M.O. Ziborova (2016). On Development of a New Filtering Half-Mask. Mechanics, Materials Science & Engineering, Vol 4. doi: 10.13140/RG.2.1.3389.4802

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