design of dust extraction system

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Information about design of dust extraction system

Published on September 30, 2008

Author: rcdohare


Design of Pollution Control Equipment : Design of Pollution Control Equipment Project and Presentation by Ms. Kakoli Shaw Mr. R.C.DOHARE Introduction : Introduction Workplace environment has direct impact on a employee’s health. Particulate matter is a major air pollutant in the industrial workplace environment. Major harmful effects of Respirable Suspended Particulate Matter (RSPM) are bronchitis, red eyes, respiratory symptoms and aggravated asthma. De-dusting systems are used for abatement of such potential health hazard as a part of industrial hygiene. Design of a more efficient and effective de-dusting system as compared to the existing one has been done using ACGIH guidelines for Industrial Ventilation. Limit Values of Dust (TWA/8 hrs) : Limit Values of Dust (TWA/8 hrs) Related Research Work : Related Research Work 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. Related Research Work (contd) : Related Research Work (contd) 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. Site Visit Findings : Site Visit Findings With focus on status of work zone area, following observations were made Dust generation during handling and transfer of powdered raw material, material unloading and vehicle movement. Fugitive emissions generation due to dust leakage from conveyors and conveyor transfer points. Too much dust in ambience Dust control being done by dry cyclone based dust extraction systems. Problems in Existing Dust Control System : Problems in Existing Dust Control System Low cleaning efficiency Higher dust emission from stack Ducts get frequently choked Inadequate suction at the hoods Puffing taking place at suction points Work zone is dusty Dust deposited on floor of work place Loss of precious material Adverse effects on workers’ health Breakdowns and repetitive problems for long duration Site Pollutant Characteristics : Site Pollutant Characteristics Reacts with water to form Ca (OH) 2 and lots of heat Reacts with CO2 to form CaCO3 Exposure causes severe eye irritation, burning, skin irritation and respiratory irritation. Concentration Limits Basics of Industrial Ventilation : Basics of Industrial Ventilation 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. Components Hood Ductwork Air cleaning device Fan Stack Selection criteria for appropriate dust collection equipment : Selection criteria for appropriate dust collection equipment Following factors are considered Contaminant concentration Gas stream characteristics Contaminant characteristics Energy consideration Dust disposal Efficiency required 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 : 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. Changes made Review of suction air quantity based on ACGIH norms Designing a number of suction hoods and their location Designing a bag house filter Designing a stack of suitable height Design Philosophy : Design Philosophy 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. Design Calculations : Design Calculations DATA OF EXISTING SYSTEM FOR ONE JUNCTION HOUSE BELT CONVEYOR BELT WIDTH: 1.2 metre BELT SPEED: 1.5 m/sec HEIGHT OF FALL: 3 metre 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 m3/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 m3 / hr x 1.2 = 3360 m3 / 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 m3 / hr = 6720 m3 / hr Design Calculations (contd) : Design Calculations (contd) Since the height of fall is more than 1 metre, we add an extra exhaust air of 1700 m3 / hr at the other end. It is proposed to install three suction hoods as shown. Design Calculations (contd) : Design Calculations (contd) Air quantity from hood H1 = air quantity from hood H2 Thus, the air quantity to be sucked from hood H3 = 1700 m3 / hr The total Suction Air Quantity Q to be considered for further designing of components of dust extraction system = Q1 + Q2 + Q3 = 6720 + 6720 + 1700 = 15140 m3 / hr HOOD DESIGN FOR HOOD 1 AND 2 The suction hood is fabricated from MS plates. The capture velocity of a hood ranges from 0.8 m/sec for fine powder to 2.5 m/sec for grains and 3 m/sec for lumps. Air Quantity = 6720 m3 / hr = 1.87 m3 / sec Now, air quantity Q = area A x capture velocity Taking capture velocity of hood to be 1 m/sec 1.87 m3 / sec = A x 1 m / sec Area of hood = 1.87 m2 Design Calculations (contd) : Design Calculations (contd) For a square section, Area = Side2 Hood size = 1.87 m x 1 m Selecting a canopy hood and considering the belt width, the dimensions are obtained as shown. 1.87 m 1 m Design Calculations (contd) : Design Calculations (contd) DUCT DESIGN 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 m3 / hr = 1.87 m3 / sec Q = x velocity where D is the diameter of duct. Taking duct velocity to be 22 m/sec Q = x 22 D = 328 mm A duct of size 325 mm is selected. Design Calculations (contd) : Design Calculations (contd) HOOD DESIGN FOR HOOD 3 Air Quantity = 1700 m3 / hr = 0.472 m3 / sec Now, air quantity Q = area A x capture velocity 0.472 m3 / sec = A x 1 m / sec Area of hood = 0.472 m2 A square canopy hood of 0.7 side is considered. DUCT DESIGN Air Quantity = 1700 m3 / hr = 0.472 m3 / sec Diameter comes out to be 165 mm. A duct of size 165 mm is selected. SYSTEM CAPACITY Considering 20 % design cushion for leakage, ageing, etc System Capacity = 15140 m3 / hr x 1.2 = 18000 m3 / hr Design Calculations (contd) : Design Calculations (contd) BAG FILTER SYSTEM Inlet dust load as measured = 10 gm / Nm3 As per latest CPCB norms, Stack norms = 150 mg / Nm3 Designing shall be done for a lower value of 100 mg/Nm3 Bag filter efficiency = Bag filter efficiency = 99% Air to cloth ratio is generally taken as 90 m/hr. Air quantity coming to the bag filter system = 18000 m3 / hr Cloth area = = 200 m2 Selecting bags of diameter 160 mm and height 3.6 m as per standard bags available in the market; Cloth area / bag = = π x 0.16 m x 3.6 m = 1.8 m2 Number of bags required = = 111 or say 112. Design Calculations (contd) : Design Calculations (contd) DESIGN OF ID FAN 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. Design Calculations (contd) : Design Calculations (contd) STACK DESIGN 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 5 m3 / hr = x 15 m/sec D = 650 mm Design Calculations (contd) : Design Calculations (contd) INSTRUMENTATION AND ELECTRICS An electrical panel and the power will be made available by a suitable feeder up to the motor control center (MCC). The following instruments also need to installed 1. Flow measurement of fan 2. Temperature measurements of incoming air in duct, inlet of bag filter 3. Differential pressure measurements across bag filter SAFETY ASPECTS The following safety aspects need to be kept in mind during the design and operation of the dust extraction system. 1. Bag filter casing design shall withstand shut off head of fan 2. The fan and bag filter shall be interlocked with the belt conveyor system so that the fan will suck air only when the belt conveyor is running. 3. The hopper below the bag filter shall have minimum 8 hrs of storage capacity. 4. Lightning arrester, orange and white stripes and aviation lamps shall be installed on the stack. 5. Port holes, ladder and platform for stack monitoring shall be provided with all the stacks and be maintained in good condition. Design Calculations (contd) : Design Calculations (contd) UTILITY REQUIREMENTS 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. Conclusion and Recommendations : Conclusion and Recommendations 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. CONCLUSIONS 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. Conclusion and Recommendations : Conclusion and Recommendations RECOMMENDATIONS 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. Conclusion and Recommendations : Conclusion and Recommendations 6. Hopper heaters, zero speed switches, level switches and DP switches which ensure the smooth and trouble free operation of the system can be used. 7. Dust masks meeting the NIOSH N95 rating should be made compulsory for workers. 8. Full clothing to cover arms and legs, gloves, safety glasses or face shield should be provided to workers handling lime. 9. Eye wash and shower station should be readily available. 10. Offline cleaning should be preferred as high level particulate emission takes place during on-line cleaning process. THANKS : THANKS

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