2-Bahrami

JRHS 2008; 8(1): 12-17

Copyright © Journal of Research in Health Sciences

Effects of Velocity and Particles Load on Efficiency of Cyclone in the Stone Crushing Units at Azendarian Area

Bahrami A (PhD)a, Qorbani F (MSc)a, Mahjub H (PhD)b, Aliabadi M (MSc)a

a Department  of Occupational Health, Hamadan University of Medical Science, Iran

b Department of Biostatistics & Epidemiology, Faculty of Health and Center for Health Research, Hamadan University of Medical Science, Iran

*Corresponding author: Dr AR Bahrami, E-mail: Bahrami@umsha.ac.ir

Received: 18 Decamber 2007; Accepted: 11 Jan 2008

Abstract

Background: The traditional cyclone has been developed for the removal of airborne silica particles from local exhaust ventilation (LEV). The objective of this research was to evaluate the effects of veloc­ity and particles load on efficiency of cyclone in the Stone Crushing Units at Azendarian Area.

Methods: After the designing and installing the traditional cyclone, downstream and upstream samples of the apparatus were obtained. The mass of all samples collected was determined gravimetrically us­ing EPA method with cascade Impactor.

Results: The relation between inlet total and respirable dust concentration to cyclone and cyclone over­all efficiency is statistically significant (P= 0.005) and the relation between inlet air velocity to cyclone and cyclone pressure loss is statistically significant (P= 0.002). There was a significant correla­tion between the inlet concentration loaded to cyclone and the efficiency of cyclone.

Conclusion: Increase of respirable dust concentration and also total concentration cause to increase effi­ciency of cyclone.

Keywords: Cyclone, Velocity, Particle, Concentration Load

Introduction

Cyclones are among the oldest types of in­dus­trial particulate control equipment and are still one of the most widely used of all in­dus­trial gas-cleaning devices. The main rea­sons for the widespread use of cyclones are that they are inexpensive to purchase, they have no moving parts, and they can be con­structed to withstand harsh operating condi­tions (1). Typi­cally, a particulate-laden gas en­ters tangen­tially near the top of the cy­clone; the gas flow is forced into a down­ward spiral simply be­cause of the cyclones shape and the tangen­tial entry. Centrifugal force and in­ertia cause the particles to move outward, collide with the outer wall, and then slide downward to the bot­tom of the de­vice. Near the bottom of the cyclone, the gas reverses its downward spiral and moves up­ward in a smaller inner spiral. The cleaned gas exits from the top through a “vortex­finder” tube, and the particles exit from the bottom of the cyclone through a pipe sealed by a spring-loaded flapper valve or rotary valve (1). The most commonly used cyclone designs are the 2D2D (2). Evaluations of cy­clone performance have long been studied to better understand and improve cyclone de­sign theory. Lapple (3) developed the Classi­cal Cyclone Design process (the CCD proc­ess) for designing cyclones and predicting their performance (emission concentrations and pressure drop). This model incorporated the number of effective turns, cut-point diame­ter, and a “generalized” fractional effi­ciency curve. For many situations, the Lap­ple model has been considered acceptable. Pre­vious results from research conducted at Texas A&M University (TAMU) (4) indi­cated that the Lapple methodology for predict­ing number of effective turns and the use of the “generalized” fractional efficiency curve in the CCD process yielded inaccurate re­sults.

We installed the LEV equipped with conven­tional cyclone with spray scrubber for the twenty nine factories from March 2004 to July 2006.

The objective of this research was to evalu­ate the effects of velocity and particles load on efficiency of cyclone in the Stone Crush­ing Units at Azendarian Area

Methods

Isokinetic sampling probes were used to meas­ure the representative upstream and down­stream particle concentrations of the cy­clones. The inlet and outlet concentrations of dust particles were measured using a probe connected to cascade impactor. For each test, the sampling time was 2 minutes. The mass of all samples collected was deter­mined gravimetrically using EPA method (5). Filters were equilibrated in desiccators for a minimum of 24 hours prior to tare and fi­nal weighing. A pitot tube with a manome­ter was used the monitoring of static pres­sure and velocity pressure in the site of sam­pling. Cyclone collection efficiency is one of the main parameters considered when evaluat­ing cyclone performance. There are two ways to calculate the overall collection ef­ficiency of a cyclone. Four parameters were required to develop cyclone fractional ef­ficiency curves. They were 1) inlet concen­tration, 2) inlet particle size distribu­tion (PSD), 3) emission concentration for each cyclone test, and 4) the PSD of dust emit­ted. The inlet and outlet concentrations for various size ranges were calculated using inlet and outlet dust concentrations and the fraction of particulate in those size ranges ob­tained from the Cascade Impactor. The out­let concentration was divided by the corre­sponding inlet concentration for each par­ticle size range and subtracted from one with the resulting values being the fractional ef­ficiency for each particle size range.

For comparing concentration of particle be­fore and after installation of local exhaust ven­tilation paired t-test was performed. ALSO pearson correlation coefficient was used to show the relationship between effi­ciency of cyclone and inlet concentration to cy­clone. Data analysis was performed using SPSS for windows.

Results

Figure 1 shows the scatter diagrams between con­centration load to cyclone and the effi­ciency of the cyclone. There was a signifi­cant correlation between the inlet concentra­tion loaded to cyclone and the efficiency of cy­clone with the increase in inlet concentra­tion, the efficiency was also increased (P< 0.05).

Figure 1: The scatter diagrams between concentration load to cyclone and the efficiency of the cyclone

Figure 2 shows the scatter diagrams be­tween inlet velocity to cyclone and the effi­ciency of the cyclone. There was a signifi­cant correlation between the inlet velocity to cyclone and the efficiency of cyclone with the increase in inlet velocity, the efficiency was also increased (P< 0.05).

Figure   2: The scatter diagrams between inlet velocity to cyclone and the efficiency of the cyclone.

Figure 3 shows the scatter diagrams between respirable dust concentration and the efficiency of the cy­clone. There was a significant correlation be­tween the respirable dust concentration and the efficiency of cyclone with the increase in respirable dust concentration, the efficiency was also increased. The results show that rela­tion between inlet total and respirable dust concentration to cyclone and cyclone overall efficiency is statistically significant (P= 0.005). The results show that relation be­tween inlet air velocity to cyclone and dust collection efficiency is statistically nonsig­nificant (P= 0.116). However, the rela­tion between inlet air velocity to cyclone and cyclone pressure loss is statistically signifi­cant (P= 0.002). The results show that mean concentration of total dust  before and after starting ventilation system equal to 1628±322 mg/m3 and 8.33± 3.81 mg/m3 re­spectivly. Free silica rate of respirable dust in different workshops is from 81.3% to 97.5%.

Figure 3:  The scatter diagrams between respirable dust concentration and the efficiency of the cyclone.

Table 1 shows the mean of total dust before and after of installation of LEV in different sites of stone crushing units. Results show that the efficiency of LEV to control of parti­cles is greater than 99%. The average value of total dust emission from sources was 9.46 mg/m3 as compared to 1.24 mg/m3 respirable dust showing that 13.18% of total dust is res­pirable. No significant difference was ob­served for emission of particles among station­ary sites after installation of LEV.

Table 1: The mean of silica concentration before and after of LEV in different sites of grinder stone units

Site of sampling

No LEV

n=20

With LEV

n=20

ϕ1

Total dust

Mean±SD

Respirable dust

Mean±SD

Total dust

Mean±SD

Respirable dust

Mean±SD

Respirable quartz

Mean±SD


Hopper

1257.4±72.43

111.12±10.23

8.44±1.37

1.48±0.65

1.33±0.59

99.33

Rotary Grinder

2007.5±567.4

179.23±71.45

11.07±6.96

1.44±0.83

1.30±0.75

99.45

Screening I

1900.1±558.63

170.12±60.24

9.34±5.8

1.30±0.62

1.18±0.60

99.5

Screening II

1787.5±449.77

152.56±58.34

9.00±6.5

0.77±0.48

0.69±0.44

99.49

ϕ1 efficiency of LEV

Discussion

The cyclone fractional efficiency curve (FEC) relates percent efficiency to the parti­cle di­ameter and can be obtained from test data that include inlet and outlet concentra­tions and particle size distribution (PSDs).

There was a significant correlation between the inlet concentration loaded to cyclone and the efficiency of cyclone with the increase in inlet concentration, the efficiency was also in­creased. This can be related to striking of more particles to the walls of the cyclone (6) and probably the striking of particles to­gether inside the cyclone. Impaction of parti­cles to each other causes to lower velocity of particles and therefore their settlement in the bin of cyclone.

Cyclones, as the most cost-effective air pol­lu­tion device for particulate matter re­moval, have been studied for decades. Al­though many procedures for calculating collec­tion efficiency have been developed, cur­rent design practice either emphasizes past experience rather than an analytical de­sign procedure, or cannot accurately predict cy­clone collection efficiency.

In the literature, theories to predict cyclone ef­ficiency have been reported for many years. Lapple (3) developed a theory for cut-point (d50) based upon a force balance and rep­resentation of residence time with the air stream number of turns within a cyclone. The Lapple model is easy to use, but it can­not accurately predict cyclone collection effi­ciency. In 1972, Leith and Licht pre­sented another theory (back-mixing) for the study of cyclone collection efficiency (7). Their back-mixing theory suggests that the tur­bulent mixing of uncollected particles in any plane perpendicular to the cyclone axis produces a uniform uncollected dust concentra­tion through any horizontal cross sec­tion of a cyclone. According to the re­search conducted by Texas A & M univer­sity (TAMU) the 2D2D cyclone (traditional cy­clone) are the most efficient collectors for fine dust in another research, Wang et al re­ported that 2D2D cyclone had high effi­ciency for fine dust and large trash (8).

The inlet gas velocity of the cyclone was the variable with the greatest stronger influence on its efficiency: the higher the velocity, the higher the overall collection efficiency within the studied range. According to Hoff­man et al. (1995), vortex length increases with the increase in gas velocity, for a fixed cy­clone geometry. Therefore, besides increas­ing the centrifugal force, an increase in gas velocity also increases the effective col­lection area in the cyclone, and both ef­fects result in an appreciable improvement in collection efficiency (9). Yoshida et al. (1991) report an experimental and theoreti­cal study of the collection efficiency of a re­verse-flow cyclone, and they verified that large particles are collected in the upper part of the cyclone and small particles in the coni­cal section. This conclusion can be an indi­cation that the increase in ratio highet of cyclone decreases the collection area for fine particles (conical section), thereby affecting overall efficiency (10).

It is concluded from this study that increase of respirable dust concentration, velocity, and total concentration cause to increase effi­ciency of cyclone.

Acknowledgements

The authors thank the Union of Industrial units in Azandarian Area for supporting this re­search.

References

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