S.M.T.K Samarakoon and S.Thiruchelvam (2006) showed in their research results that about 41% and 12% of the total population living within 500 m and 1000 m distance from the lime kiln area, respectively were experiencing high health costs due to exposure to lime dust.
Ali B.A, Ballal S.G,Alba A.A et al (1998) showed in their epidemiological study that particulate matter like cement dust causes increased risk of respiratory diseases.
A case study in alumina industry by P.K.Pattajoshi (2006) revealed that permissible limit values exceeded in locations like lime plant bunker area and recommendation of a good dust extraction was given.
C.M.Hammond (1980) stressed the importance of further research in design of local exhaust ventilation hoods.
Krieger,Dames and Moore(1995) showed in their risk assessment project that fugitive emissions of a lime factory contributed to air pollution.
Many such studies have been cited in project which show that particulate matter is an occupational health hazard and emphasize on the use of dust captive systems for effective management of occupational health in any industry.
Industrial ventilation is a method of controlling worker exposure to airborne toxic chemicals or flammable vapors by exhausting contaminated air away from the work area and replacing it with clean air.
Local exhaust ventilation is used in industries to capture dusts.
Air cleaning device
Selection criteria for appropriate dust collection equipment
Following factors are considered
Gas stream characteristics
Comparing all the factors for various dust collectors, bag filter was selected for the proposed dust extraction system.
Design of Bag Filter Based Dust Extraction System
Dust is generated in work zone area during material transfer by conveyor belt.
As compared to the present dust control system, a better and more efficient dust extraction system is designed for conveyor belt ventilation.
Review of suction air quantity based on ACGIH norms
Designing a number of suction hoods and their location
Several empirical formulae are available for calculations and design. Amongst them, the design guidelines and criteria given by the American Conference of Governmental Industrial Hygienists (ACGIH ® ) in “Industrial Ventilation – A Manual of Recommended Practice” are most widely followed by engineers to design local ventilation systems.
The ACGIH ventilation manual contains dozens of design plates of ventilation systems for specific industrial applications that have been used to control emissions. The next slide is the ACGIH design plate for conveyor belt ventilation.
MATERIAL: LIMESTONE (~ 50 mm size) LIME (less than 3 mm)
Calculation of suction velocity
According to ACGIH Design Norms for conveyor belt ventilation; Suction air quantity Q is greater than or equal to 2800 m 3 /hr/m belt width for belt speeds over 1 m/sec. Considering 20 % design cushion for leakage air entrance from either side of the hood, the total volume of air to be considered for designing the components of the dust extraction system may be
Air Quantity Q = 2800 m 3 / hr x 1.2 = 3360 m 3 / hr
However as the material is lime powder, which is very dry and dusty, a material factor of 2 is considered.
Suction Air Quantity Q = 3360 x 2 m 3 / hr = 6720 m 3 / hr
Duct velocity is in the range of 18 to 22 m/sec for horizontal ducts and 14 to 18 m/sec for vertical ducts. The inspection and cleaning doors are to be provided at suitable locations in the ducting layout. The duct shall be of circular cross section.
Suction Air Quantity = 6720 m 3 / hr = 1.87 m 3 / sec
The type of fan for dust extraction application shall be centrifugal type with provision of direct type. The impeller of the fan shall be backward curved radial type. Noise level at 1 m distance from fan and motor assembly shall be limited to 85 db. For this purpose, the noise silencer shall be provided at the fan outlet side.
Capacity required = 18000 m3 / hr
Static Pressure = 300 mm WC
Rating of motor =
Air Quantity = 18000 m3 / hr
H = 300 mm of water column
Assuming Fan efficiency = 0.8 Motor efficiency = 0.95 Derating Factor is 0.9 for 50° C
Rating of motor = = 23.64 kW
Motors available in market are of standard horsepower. Thus the rating of electric motor shall be 30 kW and speed 1500 rpm.
Norms are that the stack height should be at least 30 m or 15 m above the nearest tallest building in the immediate surrounding area of the stack. The building nearest to the place of proposed stack is 35 m in height.
Thus stack height will be 50 metre
Air Quantity = 18000 m3 / hr = 5 m3 / hr
Air Quantity = x efflux velocity of stack
Efflux velocity of stack is taken to be 15 m/ sec for effective dispersion of dust emissions from the stack
The following utilities are needed to be set up alone with the dust extraction system.
1. All electrical supplies and works including wiring, cabling etc and power facility at 40 kW 3 phase connection, 415 V 50 Hz
2. Compressed air facility of 20 m3/hr at pressure 6-8 kg/cm2 is required for bag cleaning
3. Industrial water facility
4. Fire Fighting water facility
5. Drinking water facility
6. Handling and hosting facilities- moonbeam with electric hoist shall be provided for motor and fan maintenance
7. Screw conveyors to remove captured dust from the bottom of the hoppers under the fabric filter and (if used) mechanical collector. Alternatively, air conveying (pneumatic) systems and direct dumping into containers can be used for dust removal from the hoppers.
This project was commenced to design an appropriate de-dusting system for dust extraction during material transfer in an industry. This project addresses the basic necessity of a healthy work zone area i.e. occupational hygiene for greater productivity.
1. Bag filter based de-dusting system has been suggested with cleaning efficiency of 99%. Latest CPCB norms of 100 mg / Nm3 (stack emissions) has been considered in line with latest industrial trends following Air Act, CREP etc
2. The total dust extraction system for the junction house consists of 3 suction hoods, ductwork, butterfly dampers at appropriate locations, a modular casing entry, off-line, pulse-jet bag filter, a centrifugal ID fan and a self supported stack.
3. Proper dust conveying velocities have been selected to avoid dust settling in ducts.
4. Sufficient cushion has been kept in the air volume calculations to maintain proper suction at hoods.
5. The designed dust extraction system not only keeps the work atmosphere free from air pollution but also aids in precious material (lime) being recovered, which can be ploughed back into the process.
In addition to the more efficient de-dusting system designed for air pollution control in work zone, the following steps are recommended to be brought into practice in the Lime Shop as routine job.
1. Provision should be made to spray water on the feeders before charging the raw material into shaft kilns in Lime Shop. This would ultimately result in less dust generation, healthy environment for the workers and stack emissions inside the norms.
2. Stationary and mobile super suckers or industrial vacuum cleaners should be utilized regularly as per requirements in various areas and floors for better housekeeping.
3. In house training program on pollution control systems at the Shop and feedback clarification should be done for the workers.
4. Scheduled and routine maintenance of the pollution control systems should be carried out.
5. Handling of lime, a sticky and hygroscopic material, is an operationally difficult task. Special moisture repellant polypropylene filter bags can be used .