Local Exhaust Ventilation

Guidelines on Design, Operation and Maintenance of Local Exhaust Ventilation Systems

2006

Occupational Safety and Health Specialist Department OSH Division Ministry of Manpower

Table of Contents

1

Introduction • •

2

Legal requirements Scope

Exhaust Hood • • • •

3

Ducting System • • • • • •

4

General Fan installation Fan capacity and static pressure

Stack •

7

General Classification and selection

Exhaust Fan • • •

6

General Duct construction Duct velocity Branches, elbows and transitions System details System resistance and balance

Air Cleaning Equipment • •

5

General Capture velocity Specific hood design Hood construction

General

Replacement and Re-circulated Air • • • •

General Rate of supply Makeup air system details Recirculation

2

8

Operation and Maintenance • • •

9

General Operation Maintenance

Testing • • •

General Measuring static pressure Measuring airflow in ducts

10 Annexes • • • • • • •

Annex 1 Capture velocities Annex 2 Metal thickness and classification Annex 3 Design duct velocities Annex 4: General classification of particulate collectors Annex 5: Comparison of pollution control equipment Annex 6: Sample visual inspection checklist Annex 7: Pitot traverse diagrams

3

Preface This set of Guidelines on Design, Operation and Maintenance of Local Exhaust Ventilation Systems provides information on good practices related to the design, fabrication, operation, testing and maintenance of local exhaust ventilation systems used for capture and exhaust airborne contaminants. It is intended for use by ventilation system designers, facility engineers, safety and health professionals, plant maintenance personnel, and persons having responsibility for ventilation system design, fabrication, operation, testing and maintenance. It was prepared by the Occupational Health Department, Ministry of Manpower in collaboration with the Department of Mechanical Engineering, National University of Singapore in 2003. The Guidelines updated and expanded the first edition on Design Guide for Local Exhaust Ventilation Systems in Factories, which was prepared by the then Industrial Health Division of the Ministry of Labour in 1983. These were reviewed by Jeff Burton and endorsed by the Committee on Management of Chemical Hazards in 2004. In preparing these guidelines, reference was made to the following publications. 1.

Industrial Ventilation. A Manual of Recommended Practice. American Conference of Governmental Industrial Hygienists. 25rd Edition 2003.

2.

Guide for Testing Ventilation Systems. American Conference of Governmental Industrial Hygienists. 1991.

3.

ANSI Z9.2-2001 - Fundamentals Governing the Design and Operation of Local Exhaust Ventilation Systems. American Industrial Hygiene Association. 2001.

4.

Air Pollution Engineering Manual. Air & Waste Management Association. 2nd Edition 2000.

5.

Design of Industrial Ventilation Systems. John L. Alden and John M. Kane. 5th Edition 1982.

6.

Industrial Ventilation Workbook. D. Jeff Burton. 4th Edition 2001.

7.

Handbook of Ventilation for Contaminant Control. Henry J. McDermott. 2nd Edition 1985.

8.

Maintenance, Examination and Testing of Local Exhaust Ventilation Systems, Health and Safety Executive. 1999.

4

Suggestions for improvement of the guidelines are welcome and should be sent to Occupational Safety and Health Specialist Department, Ministry of Manpower, 1500 Bendemeer Road, Singapore 339946, or e-mailed to: MOM OSHD/MOM/SINGOV@SINGOV

1.

Introduction Local exhaust ventilation systems are widely used to control toxic gases, vapors, dusts, fumes and mists from various industrial operations and processes. A proper design of an exhaust ventilation system is necessary for the effective removal of airborne contaminants that would otherwise pollute the work environment resulting in health hazards, or nuisance, or cause air pollution. A local exhaust ventilation system usually consists of a number of separate exhaust hoods applied to several different operations and connected by a system of branch and main ducts to a central air cleaning device and common exhaust fan and discharge stack to the outside atmosphere. Each of these components requires separate consideration in the design of the overall system. Legal requirements Ventilation and removal of dust, fumes, etc are required under regulations 5 and 39 of the Workplace Safety and Health (General Provisions) Regulations. Regulation 5(2) stipulates that where gases, vapours or other impurities are generated in the course of any process or work carried out in a workplace which may be injurious to health, effective and suitable ventilation shall be provided for securing and maintaining the circulation of fresh air in the workplace, to render harmless so far as is practicable, all such gases, vapours or other impurities. Regulation 39(1) stipulates that where any process or work carried on in any workplace is likely to produce or give off any toxic, irritating or offensive dust, fume or other contaminants, all practicable measures shall be taken to protect persons employed in the workplace against inhalation of the dust, fume or other contaminants; and prevent their accumulation in the workplace. Regulation 39(2) specifies that the measures to be taken shall, where appropriate, include providing local exhaust ventilation to remove the dust, fumes or other contaminants at their sources of emission. Regulation 39(3) states that the local exhaust ventilation system shall be so designed, constructed, operated and maintained that dust, fume or other contaminants are safely and effectively removed at the source of generation and not dispersed or scattered in the surrounding air.

5

Scope This set of Guidelines on Design, Operation and Maintenance of Local Exhaust Ventilation Systems sets minimum requirements for the design and fabrication of industrial LEV systems used for the prevention and reduction of person exposure to harmful airborne substances in the work environment. It establishes requirements for the operation, testing and maintenance of LEV systems to assure continuous and satisfactory functioning of the systems. It also establishes requirements for replacement and re-circulated air. (a) LEV system designs and specifications should as far as is practicable conform to the requirements of these guidelines, or to other standards of good practice equal to or more stringent than these guidelines. (b) Persons designing, testing or maintaining an LEV system should be qualified by training and / or experience to perform the job.

2.

Exhaust Hood General (a) Exhaust hoods should be designed to effectively contain, receive or capture air contaminants. (b) Enclosing hoods should as far as possible be used to totally enclose emission sources. If this is not feasible, partial enclosures using baffles or flanges to increase hood control effectiveness should be considered. (c) Exterior or capture hoods should be placed as close to emission sources as possible. (d) Exhaust hoods should be designed and located such that the contaminants are removed before reaching the breathing zones of persons working near the hoods. (e) Exhaust hoods should be protected from cross-draft or other competing air movements such as open windows, free standing fans, walkways, etc. (f) Exhaust hoods should be designed, placed and operated to ensure even air flow into the hoods for consistent and reliable emission control. Capture velocity (g) Hood design should consider the capture or control velocity, flow-rate required, and hood static pressure for optimum performance.

6

(h) An adequate control or capture velocity should be selected to capture the contaminated air by causing it to flow into the exhaust hood. The recommended range of capture velocities is appended in Annex 1. (i) Hood flow-rate can be determined by theoretical or empirical calculation. (j) Hood static pressure can be determined from hood entry loss factor which can be obtained from manufacturers or suppliers of pre-built hoods or most ventilation technical handbooks. (k) Flanges or baffles should be provided wherever possible to eliminate airflow from contaminant-free zones. The flange width should be equal to X – 1/2 D where X = distance to desired capture point and D = duct diameter. Specific hood design (l) Specific hood design and operating criteria can be found in the American Conference of Governmental Industrial Hygienists (ACGIH) Industrial Ventilation Manual. Hood construction (m) Hoods should be at least 2 gauges heavier than the connecting duct, free of sharp edges and bends, and reinforced for stiffness. (n) A tapered transition piece between the hood and the exhaust duct should be provided if possible. (o) For cases where air temperature and corrosion problems are not severe, galvanized sheet metal can be used to construct hood. (p) For high temperatures of up to 480°C and over 480°C, black iron and stainless steel could be used respectively. For corrosive gases and vapors, corrosive resistant metals, polyvinyl chloride (PVC) or other plastics and coatings may be used. (q) A real-time hood performance monitor (e.g. a static pressure tap with manometer) should be provided if inadequate hood performance could result in hazardous conditions for persons using the hood.

3.

Ducting System General

7

(a) The equipment of the exhaust system should be located to permit, as far as possible, a symmetrical layout of pipes about the central fan, to minimize inequality in airflow resistance in the branches. (b) The shortest lengths of straight ductwork should be used; long runs of small diameter duct should be avoided except when transport of dust is required. All unnecessary elbows, tees or entries should be avoided. (c) Exhaust duct takeoff from the hood should, wherever possible, be located in the line of normal contaminant travel. (d) Ductwork should be located so that it is readily accessible for inspection, cleaning and repairs; ductwork should be protected against external damage. (e) For large and shallow hoods, multiple takeoffs may be used to attain the desired distributions of exhaust airflow. Interior baffles or filter banks should also be used to attain satisfactory air distribution. Duct construction (f) All exhaust system should be constructed of new materials and installed in a permanent and workmanlike manner. Duct supports of sufficient capacity should be provided to carry the weight of the system. (g) The interior of all ducts should be smooth and free from obstructions, especially at joint, elbows, and bends. (h) Round duct should be used for the construction of the exhaust system. Rectangular ducts, if used, should be as square as possible and be two gauges heavier than round ducts and reinforced to prevent collapsing at any static pressure possible in the duct. (i) Ducts should be constructed of galvanized sheet steel riveted and sealed, or black iron welded, flanged or gasketed, except where corrosive gases or mists or other factors render such metals impractical. Galvanized construction is not recommended for temperatures above 200oC. (j) For corrosive conditions, corrosive resistant metals, PVC or other plastics or coatings may be used for duct construction. (k) The actual metal thickness for round industrial ducts will vary with the diameter of the duct, the concentration and abrasiveness of the contaminants, static pressure, reinforcement, and span between supports. The recommended range of metal thickness for ducts of local exhaust systems for non-corrosive applications is appended in Annex 2.

8

Duct velocity (l) Duct velocities should be sufficient to prevent the settling of dry aerosols. The recommended minimum duct velocities are appended in Annex 3. (m) When condensable vapors are to be exhausted, the effects of cold temperatures on the exhaust duct should be considered and provisions should be made to prevent or clean / remove unwanted or uncontrolled condensation. Branches, elbows and transitions (n) All branches should enter the main duct at gradual expansions at an angle not exceeding 45o and preferably 30o or less. Connections should be to the top or side of the main and not directly opposite each other. (o) Elbows and bends should be at a minimum of 2 gauges heavier than straight length ducts of equal diameter and have a centerline radius of curvature of at least 2 and preferably 2.5 times the pipe diameter. (p) Transitions in mains and sub-mains should be tapered in duct enlargement and contraction. The taper should be at least 5 units long for each unit change in diameter. System details (q) Static pressure losses throughout the LEV system should be determined before fans are selected. (r) The duct design should allow for vibration and expansion. Flexible connection should be made between the duct and the fan or air-cleaning unit. The flexible coupling should be mounted on the outside of the duct at the inlet side of the fan; inside on the outlet side. (s) Where contaminants are likely to settle or collect in duct work, clean out opening should be provided in horizontal runs of ducts carrying dust-laden air and, especially, near elbows and junctions. (t) Where condensation may occur, the duct system should be liquid-tight and provisions made for proper sloping and drainage. (u) Where blast gates are used for airflow adjustment or system balance, they should be placed near the connection of branch to main duct, and means of locking should be provided after the adjustments have been made. (v) Within six duct diameter of the fan inlet, the diameter of the main duct should be approximately equal to the fan inlet diameter.

9

(w) Hoods and ductwork should not be added to an existing system unless such additions were specifically provided for in the original design or such additions are approved by a person qualified to perform system design and such additions will not reduce the performance of the existing system. System resistance and balance (x) In a multiple branch system, the desired airflow between the branches should be properly distributed by Static Pressure Balance Method (also known as the Velocity Pressure Method). Blast Gate Adjustment Method for balancing an exhaust system is not encouraged. A balanced design ventilation system does not require blast gates or dampers to control airflow as the system is designed to operate at desired flows without further balancing after construction, i.e., it is balanced on paper during design.

4.

Air Cleaning Equipment General a) Air cleaning equipment should be compatible with all the components of the local exhaust ventilation system. b) Fire safety and explosion control must be considered when designing or selecting an air-cleaning device. c) Collection rate, capacity, and resistance of the air cleaner should remain as constant as possible throughout its daily operating cycle and be nearly independent of entering dust, fume, or vapor concentration. Classification and selection (d) The selection of air cleaning equipment is based on the characteristics of the air / gas stream, and the nature and quantity of the contaminants. Air cleaning equipment and devices are classified and compared in Annexes 4 and 5. (e) The degree of outflow air cleanliness should satisfy the National Standards for Air Pollutants prescribed by the Pollution Control Department of the National Environment Agency (NEA). (f) Handling and disposal of collected materials or effluents from the air cleaning equipment should meet NEA’s requirements and should not create a hazard to persons handling the materials.

10

5.

Exhaust Fan General (a) An exhaust fan must be selected to produce the rate of airflow required by the exhaust system. The flow must be developed against the total system resistance, including pressure losses through the hoods, branch and main exhaust ducts and accompanying fittings such as elbows, branch-main junctions as well as those incurred through air cleaners and discharge piping. (b) The exhaust fan should be located near the middle of an array of exhaust hoods rather than at the end if possible; high static pressure or suction branch ducts should be located near the fan if possible. (c) The exhaust fan should be located downstream of the air cleaning equipment to protect it against any corrosive action of the gas or vapours or any abrasive action of the dust which is being collected, and as close to the discharge point as possible. (d) The preferred location for an exhaust fan is outdoors, normally on the roof. Fan location should be chosen such that noise is not problem. (e) A straight duct section of at least 6 equivalent duct diameters and 3 equivalent duct diameters should be used when connecting to the fan inlet and outlet respectively before any bend or fittings. Where this is impracticable due to space constraints, the associated pressure loss must be accounted for. (f) Fan selection should consider long-term contaminant effects on the fan and the fan wheel. Where severe conditions of abrasion or corrosion are present, special linings or metals could be used in fan construction. Fan blades might need to be cleaned periodically. (g) Fan serving systems with air cleaners or plenum design should be selected such that the system operating point (interception of airflow rate Q and fan total pressure FTP) lies on a steep forward portion of the fan characteristic curve. Fan installation (h) A flexible sleeve or band should be incorporated onto the fan inlet and outlet ducts to minimize vibration of the ductwork. The flexible coupling should be mounted on the outside of the duct at the inlet side of the fan; or on the inside of the duct at the outlet side of the fan. (i) Fan and motor should be firmly mounted on a sound foundation or structural support. If vibration isolator supports are to be used, the fan and motor should

11

be mounted solidly to a common, rigid base and the vibration isolators placed between the base and the structural support. (j) Safe means should be provided to allow the wheel of an exhaust fan to be examined without removing the connecting ducts. This provision may be, however, waived for smaller fans used in non-corrosive and non-dusty atmospheres. (k) When exhaust systems are used to handle flammable gases or vapors or combustible dust, fan blades and casings should be made of non-sparking material, and the motor should be placed outside the combustible region or be of explosion-proof design. Electrical bonding and grounding should also be provided for all fan parts. Fan capacity and static pressure (l) The fan must have a capacity not less than the sum of the originally estimated airflow rates (using Blast Gate Adjustment Method) or the corrected flow rates (using Velocity Pressure Method) for all the exhaust hoods. (m) A fan of the proper size and operating speed should be selected from the rating table published by the fan manufacturer based on the airflow rate and static pressure required or as estimated by design.

6.

Stack General (a) Exhaust stacks should be vertical and terminated at a point where height or air velocity would preclude re-entry of the contaminated air into the work environment. (b) Weather cap on discharge stack is not recommended. Stack head design should be used. (c) Local exhaust ventilation stack outlets should be sufficiently (3 m recommended) above adjacent air intakes and / or roof lines if they are within 15 m of the stack. (d) The stack outlet velocities should be high enough (15 m/s recommended) to prevent downwash and rain from entering the stack.

7.

Replacement and Re-circulated Air

12

General (a) If the local exhaust system is utilized in space where there is no or inadequate natural ventilation, makeup air should be supplied to the enclosed space to replace the air being exhausted. (b) Mechanical makeup air should be filtered at the air intake to protect ventilation system equipment and to ensure that the makeup air is clean. (c) A monitoring system should be provided to signal any malfunction of the makeup air system if the malfunction could adversely affect the performance of the local exhaust ventilation system. Rate of supply (d) To maintain a slight negative pressure in an area to control fugitive emissions and/or prevent migration of contaminants to other areas of the plant or building, the exhaust rate should be more than the supply rate by up to 10%. (e) To maintain a slight positive pressure in an area to prevent intrusion of dust into clean areas, the supply rate should be more than the exhaust rate by up to 10%. (f) In cases where the local exhaust ventilation systems vary the exhaust airflow over time, the makeup air volume flow rate should track the exhaust airflow rates to maintain proper pressure relationship. Makeup air system details (g) The supply air should be located such that clean air is first passed over the people and then to the contaminated area, where it will be removed by the local exhaust ventilation system. The flow should also be from normal temperature areas to high heat process areas to provide some cooling. (h) The makeup air intakes should be located so that no contaminated air from nearby exhaust stacks or any sources of air contaminants is drawn into the makeup air system. (i) Makeup air should be introduced into the “living zone” of the area, generally 2.4 to 3.0 m from the floor. (j) Supply air locations and velocities should be selected to avoid high velocity drafts on hooded processes or on the workers themselves. Recirculation

13

Recirculation of exhausted air is discouraged, especially for systems handling toxic contaminants or high concentrations of any material. However, due to the practicality of operating some of the larger exhaust system, as well as the cost of increasing energy consumption for many smaller exhaust systems, recirculation may be deemed acceptable as a last resort in some circumstances under the following conditions: (k) The physical, chemical and toxicological properties of the contaminants in the air stream to be re-circulated must be identified and assessed. Exhaust air containing substances whose toxicity is unknown or for which there are no established permissible exposure levels should not be recirculated. (l) The effects of a recirculation system malfunction must be considered. Recirculation should not be permitted if a malfunction could result in exposure levels that might cause permanent damage or significant physiological harm. (m) The availability of an effective air cleaner must be determined. An air cleaner capable of removing airborne contaminants to achieve acceptable workplace concentrations must be available. (n) The effects of minor airborne contaminants should be considered. Recirculation should not cause a concentration of minor contaminants to reach an unacceptable level. (o) Recirculation systems should incorporate a monitoring system that gives an accurate warning or signal capable of initiating corrective action or process shutdown before harmful concentrations of the recirculated contaminants build up in the workplace.

8.

Operation and Maintenance General (a) Employees working with LEV systems and maintenance personnel responsible for LEV systems should be instructed on the proper operating procedure and reasons for the installation. (b) Every individual testing, operating, maintaining and redesigning the local exhaust ventilation system should have access to the most recently updated plans and specifications for the local exhaust ventilation system. (c) Lock-out, tag-out programmes for both electrical power sources and mechanical energy sources should be established. Typically, the fan wheel should be locked out during installation or maintenance of the fan.

14

Operation (d) A programme of safe operating procedures based on the needs of the system and the process should be established and maintained. (e) No process or equipment on which the exhaust ventilation has been installed for the protection of the employees should be operated when the exhaust system is not functioning properly. (f) Every local exhaust ventilation system handling particulate matter should be operated with inlets to the system open unless the system was specifically designed for safe operation with some inlets closed. Using dampers for any reason except balancing and shutoff during maintenance is usually discouraged. Maintenance (g) A programme of scheduled maintenance tailored to the needs of the system should be established. The responsibility for scheduled maintenance and oversight should be entrusted to a single, qualified individual who should maintain a logbook of maintenance for references. (h) Manufacturers’ recommendations for the maintenance of local exhaust ventilation system components should be included in the maintenance schedule. The maintenance schedule, in general, should include the clearing of any blockages in the ductwork and the cleaning of the air cleaning equipment on a regular basis.

9.

Testing General (a) Every ventilation system should be thoroughly inspected and tested upon completion of installation to determine that its installation is in accordance with the design specifications and drawings. The initial test should include measuring the volumetric flow rate, hood static pressure, fan static pressure, and fan speed, as well as determining the pressure drops across air cleaning equipment The initial test acts as a baseline for periodic maintenance testing and rapid isolation of system failures when a malfunction occurs. The test is also necessary to verify the setting of blast gates, fire dampers, and other airflow control devices which may be part of the system. (b) The test data should be compared with design specifications. If adjustments or alterations are made to the system in order to meet the specifications or performance criteria, the system should be retested.

15

(c) Visual inspections for physical damage (e.g. leaking or corroded duct) and proper operation of components (air cleaner, exhaust fan, damper, etc) should be carried out at least once a month. A sample of an inspection checklist is appended in Annex 6. (d) Periodic tests (including static pressure and airflow measurements) of at least once every 12 months should be made throughout the life of the system to ensure continuing performance. Tests should also be carried out whenever major modifications are made to the system, or when complaints of poor performance are made by operating personnel. (e) The measured static pressures and volumetric flow rates should be compared with that of the initial test. If corrections or alterations are made to the system, a new initial test should be conducted to verify system performance. (f) Makeup air system should be included in the local exhaust ventilation testing procedure. As naturally or mechanically supplied air must be provided to the workspace, the performance of the makeup air system should be tested with adjustments for flow, direction, and supply air system components. (g) All instruments used for testing must be calibrated, frequently if specified, especially direct-reading meters as they are easily impaired by shock, dust, high temperatures, and corrosive atmospheres. (h) Records of testing and measurements should be kept. Measuring static pressure Static pressure can be measured by a vertical manometer, inclined manometer, Pitot tube or other pressure measuring devices. (i) For hood static pressure measurement, a static pressure hole of 12 mm diameter, drilled at least 2 to 4 pipe diameters downstream in a straight section of duct, should be provided at each hood opening. (j) Static pressure holes should be provided for pressure measurement at both the inlet and the outlet of the air-cleaning device. The resistance or pressure drop across the air cleaning equipment should be determined and compared with the manufacturer’s data for maintenance purpose. (k) Static pressure holes should be provided for pressure measurements at both the inlet and outlet of the fan. The fan manufacturer’s specifications on location of measurement should be followed. Measuring airflow in ducts

16

Airflow rate in a duct is obtained by multiplying the average air velocity and the cross-sectional area of the duct. The air velocity is determined by the velocity pressure which can be measured by Pitot traverse method. (l) A Pitot traverse across the diameter of a duct can be performed to measure airflow in the duct. The traverse should be made at a location of at least 6 duct diameters downstream and 2 duct diameters upstream from any major disturbance such as damper, elbow or branch entry. If the traverse is made less than 6 duct diameters downstream and 2 duct diameters upstream, another traverse at a second location should be made and checked if there is an agreement within 10% of the readings obtained at the two traverses. If there is an agreement, the average of the two readings should be used. Where the variation exceeds 10%, a third location should be selected and the two airflows in the best agreement averaged and used. (m)For round ducts, two traverses across the diameter of the duct at right angles to each other can be made. For ducts 15 cm in diameter or smaller, two 6-point traverses should be made. For ducts 15 to 120 cm in diameter, at least two 10-point traverses should be made. Above 120 cm or for smaller ducts where large velocity variations are suspected, two 20-point traverses should be made. The locations of the measuring points are selected such that the duct is divided into equal annular areas, not equal distant points along the duct diameter. Refer to Annex 7 for measurement locations. (n) For a long, small and straight round duct section, the average duct velocity pressure can be estimated by multiplying the measured centerline duct velocity pressure by 0.81. A velometer can also be used for centerline duct velocity measurement; the average duct velocity can be estimated by multiplying the measured duct centerline velocity by 0.9. (o) For rectangular ducts, the cross-section should be divided into a number of equal areas and the velocity pressure reading is measured at the center of each area. At least 16 readings should be taken, but the distance between measuring points should not exceed 15 cm. Refer to Annex 7 for measurement locations. (p) A Pitot tube cannot be used for measuring velocities less than 3.0 m/s.

17

18

Annex 1 Capture Velocities

Condition of Dispersion of Contaminants Released with practically no velocity into quiet air Released at low velocity into moderately still air Active generation into zone of very rapid air motion Released at high velocity into zone of rapid air motion

Capture Velocity m/s

Examples

Evaporating from tanks; degreasing

0.25 - 0.5

Spray booths; intermittent container filling welding, plating, pickling

0.5 - 1.0

Spray painting in shallow booths; barrel filling; conveyor loading; crushers

1.0 - 2.5

Grinding; abrasive blasting, tumbling

2.5 - 10

In each category above, a range of capture velocity is shown. The proper choice of values depends on the following factors.

Upper End Range

Lower End Range (1)

Room air current minimal

(1)

Disturbing room air current

(2)

Contaminant of low toxicity

(2)

Contaminants of high velocity

(3)

Intermittent, low production

(3)

High production, heavy use

(4)

Large hood – large air mass in motion

(4)

Small hood - local control ones

19

Annex 2 Metal Thickness and Classification Classification of ducts for Local Exhaust Ventilation systems for noncorrosive applications.

Class 1

Light duty for nonabrasive applications such as replacement air and general ventilation.

Class 2

Medium duty for applications with moderately abrasive particulates in light concentrations such as woodworking and grain dust.

Class 3

Heavy duty for applications with highly abrasive particulates in low concentrations such as abrasive cleaning and sand handling.

Class 4

Extra heavy duty for applications with highly abrasive particles in high concentrations such as canopying systems in heavy industrial plants.

Range of Metal Thickness Standard Gauge for Steel Duct Diameter of Straight Duct

Class 1

Class 2

Class 3

Class 4

100 mm to 200 mm

22-20

22-18

16

14

>200 mm to 450 mm

22-12

22-12

16-11

14-11

>450 mm to 760 mm

18-7

16-7

16-6

14-6

>760 mm

14-2

14-2

12-2

12-2

20

Annex 3 Design Duct Velocities

Nature of Contaminant

Example

Design Velocity m/s

Vapours, gases, smokes

All vapours gases & smoke

6.0 - 10.0

Fumes

Zinc and aluminium oxide fumes

7.0 - 10.0

Very fine, light dust

Cotton lint, wood, flour, litho powder

10.0 - 12.5

Dry dusts and powders

Fine rubber dust, Bakelite molding powder dust, jut lint, cotton, dust, shavings (light), soap dust, leather shaving.

12.5 - 17.5

Average Industrial dust

Sawdust (heavy and wet), grinding dust, buffing lint (dry), wool, jute dust, coffee beans, shoe dust, granite dust, silica flour, general material handling, brick cutting, clay dust, foundry (general), limestone dust, packaging and weighing asbestos dust in textile industries

17.5 - 20.0

Heavy dusts

Metal turnings, foundry tumbling barrels and shakeout, sand blast dust, wood blocks, hog waste, brass turnings, cast iron boring dust, lead dust

20.0 - 22.5

Heavy or moist dusts

Lead dusts with small ships, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky) Quick-lime dust

22.5 & above

21

Annex 4 General Classification of Particulate Collectors

Control Device

Class

Force

Particle Diameter for 90% Removal in microns 50

Settling Chamber

Mechanical

Gravity

Impingement Separator Cyclone (Small Diameter) Cyclone (Large Diameter) Bag house Panel Filters Mat Filters Deep Filter Beds Spray Chamber Packed Tower Cyclone Scrubber

Mechanical

Initial Impingement

25

Mechanical

Centrifugal

>5

Mechanical

Centrifugal

25

Filtration Filtration Filtration Filtration Scrubber Scrubber Scrubber

>1

Venturi

Scrubber

Inertial Impingement + Electrostatic + Diffusional Inertial Impingement + Electrostatic + Diffusional Inertial Impingement + Electrostatic + Diffusional Inertial Impingement + Electrostatic + Diffusional Inertial Impingement

Wet Inertial (Mechanical) Orifice

Scrubber

Inertial Impingement

5

Scrubber

Inertial Impingement + Centrifugal

5

Single-stage High Voltage Two-stage Low Voltage

Electrostatic Precipitators Electrostatic Precipitators

Electrostatic Attraction

>1

Electrostatic Attraction

>1

22

>1 10 1 25

Annex 5 Comparison of Pollution Control Equipment

Device

To Control

Advantages

Disadvantages

Costs

Examples

Mechanical Separators

Medium to large diameter particles

1) Low initial cost 2) Simple construction 3) Erase of operation 4) Use as precleaners

1) Low efficiency 2) Erosion of components 3) Cannot remove small particles 4) Large space requirements

Low initial cost

1) Gravity Chambers 2) Impingement Separators 3) Cyclone Collectors

Filtration Devices

Dusts, fumes

1) High collection efficiency on small particles 2) Moderate power requirements 3) Dry disposal

1) High costs 2) Large space requirements 3) Must control moisture and temperature of a gas stream

High costs

1) Fabric Filters 2) Mat Filters 3) Ultra-filters

Wet Collectors

Hightemperature, moisture-laden gases

1) Constant pressure drop 2) Elimination of dust removal problems 3) Compact design

1) Disposal of waste water may be expensive and troublesome

Moderate

1) Spray Chambers 2) Cyclone, Orifice, Venturi Scrubbers 3) Mechanical Centrifugal Collectors

Electrostatic Precipitators

All sizes of 1) High particles-even efficiency very small 2) Dry dust mists which collection form free3) Low pressure running liquids drop 4) Can collect mists and corrosive acids

1) Often requires precleaner 2) Large space requirements 3) Cannot collect some high/low resistivity materials 4) High initial cost

High initial 1) Single-stage costs-low Precipitators operating & 2) Two-stage low Precipitators maintenanc e costs

23

Device

To Control

Gas Absorbers

Highly odorous, radioactive or toxic gases

Combustion Incinerators

Odours, plume opacity, carbon monoxide, organic vapours

Advantages

Disadvantages

Costs

1) Contaminant solvent may be recovered

1) High equipment & operating costs 2) Corrosion 3) Contamination

High equipment and operating costs

1) Fixed Bed 2) Regenerative

1) Capable of reaching high efficiency operation

1) Must burn additional fuel or add catalyst

Vary widely depending upon application

1) Direct flame 2) Catalytic combustion

2) Catalytic combustion reduces NOx pollutants

2) Incomplete combustion can further complicate original problem

24

Examples

Annex 6 Sample Visual Inspection Checklist Exhaust Hood □ □ □

Is there any physical damage such as corroded surfaces? Is there any cross draft or turbulence air currents at the hood face? Are the contaminants captured by the hood during normal operation?

Ducting System □ □ □

Is there any physical damage such as dents or holes in the duct? Is there any blockage in the duct by contaminants? Are the damper or blast gate settings correct?

Air Cleaning Equipment □ □

Is there any physical damage such as leakage? Is the waste material handling satisfactory?

Exhaust Fan □ □ □ □ □ □

Is the blade worn out or corroded? Is the direction of rotation correct? Is the rotation speed sufficient? Is the fan belt slipping? Is there any leakage at the flexible sleeve? Is there excessive noise or vibration?

Stack □ □

Is the stack exit velocity sufficient? Is dispersion hindered in any possible way?

Makeup Air System □ □ □

Is contaminated air recirculated? Is the makeup air sufficient? Is there any interference with the capture velocity?

25

Annex 7

10 Point Pitot Traverse in a Circular Duct

6 Point Pitot Traverse in a Circular Duct

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Pitot Traverse Points in a Rectangular Duct

26

0.043D

0.146D

0.296D

0.704D

0.854D

0.957D

0.854D 0.918D 0.974D

0.774D

0.685D

0.342D

0.226D

0.026D 0.082D 0.146D

Pitot Traverse Diagrams