has no submerged sprockets. In the catenary screen(see Fig. 5-3c), the rake is held against the rack by the weight of the chain. If heavy obiects become jammed in the bars. the rakes pass over them instead of has a relatively large"footprint"and thus re Continuous Belt Screen. The continuous belt screen is a relatively new development for use in screening applications in the United States. It is a continuous, self-cleaning screening belt that removes fine and coarse solids(see Fig. 5-3d). A large number of screening elements(rakes) are attached to the drive chains the number of screening elements depends on the depth of the screen channel. Because the creen openings can range from 0.5 to 30 mm, it can be used as either a coarse or a fine screen. Hooks n the belt el Design of Coarse Screen Installations. Considerations in the design of screening installations include(D) location:(2)approach velocity: (3)clear openings between bars or mesh size: (4) headloss through the Because the purpose of coarse screens is to remove large objects that may damage or clog downstream equipment, in nearly all cases. they should be installed ahead of the grit chambers. If grit chambers are placed before screens, rags a terial could foul the grit cham ber collector mechanisms rap around air piping and settle with the grit. If grit is pumped, further fouling or clogging of the pumps will likely occur In hand-cleaned installations, it is essential that the velocity of approach be limited to approximately 0. 45 s at average flow to provide adequate screen area for accumulation of screenings between raking operations. Additional area to limit the velocity may be obtained by widening the channel at the screen gging the screen, the upstream head will increase. submerging new areas for the flow to pass through. The structural design of the screen should be adequate to prevent collapse if it becomes plugged completely For most mechanically cleaned coarse screen installations, two or more units should be installed so that one unit may be taken out of service for maintenance. Slide gates or recesses in the channel walls for the insertion of stop logs should be provided ahead of, and behind, each screen so that the unit can dewatered for screen maintenance and repair If only one unit is installed, it is absolutely essential that a bypass channel with a manually cleaned bar screen be provided for emergency use. Sometimes the manually cleaned bar screen is arranged as an overflow device if the mechanical screen should become inoperative, especially during unattended hours. An appI mize solids To prevent the pass-through of debris at peak flowrate screen should not exceed 0.9 m/s Headloss throu cleaned coarse screens is typically limited to about 150 mm by operational controls. Hydraulic losses through bar screens are a function of approach velocity and the velocity through the bars. The headloss through coarse screens can be estimated using the following equation where hr= headloss. m C= an empirical discharge coefficient to account for turbulence and eddy losses, typically 0.7 for a clean screen and 0.6 for a clogged screen V= velocity of flow through the openings of the bar screen, m/s v=approach velocity in upstream channel, m/s g=acceleration due to gravity, 9 18 m/s The headloss calculated using above equation applies only when the bars are clean. Headloss increases with the degree of clogging. The buildup of headloss can be estimated by assuming that a part of the open space in the upper portion of the bars in the flow path is clogged Although most screens use rectangular bars, optional shapes, i.e., "teardrop"and trapezoidal, are available For the optional shapes, the wider width dimension is located on the upstream side of the bar rack to make it easier to dislodge materials trapped between the bars. The alternative shapes also reduce headloss Screenings from the rake mechanism are usually discharged directly into a hopper or container or into a screenings press. For installations with multiple units, the screenings may be discharged onto a conveyor or into a pneumatic eiector system and transported to a common screenings storage hopper. As an alterative, screenings grinders may be used to grind and shred the screenings. Ground screenings are then5-4 has no submerged sprockets. In the catenary screen (see Fig. 5-3c), the rake is held against the rack by the weight of the chain. If heavy objects become jammed in the bars, the rakes pass over them instead of jamming. The screen, however, has a relatively large "footprint" and thus requires greater space for installation. Continuous Belt Screen. The continuous belt screen is a relatively new development for use in screening applications in the United States. It is a continuous, self-cleaning screening belt that removes fine and coarse solids (see Fig. 5-3d). A large number of screening elements (rakes) are attached to the drive chains; the number of screening elements depends on the depth of the screen channel. Because the screen openings can range from 0.5 to 30 mm, it can be used as either a coarse or a fine screen. Hooks protruding from the belt elements are provided to capture large solids such as cans, sticks, and rags. Design of Coarse Screen Installations. Considerations in the design of screening installations include (1) location; (2)approach velocity;(3)clear openings between bars or mesh size; (4) headloss through the screens; (5) screenings handling processing, and disposal; and (6) controls. Because the purpose of coarse screens is to remove large objects that may damage or clog downstream equipment, in nearly all cases, they should be installed ahead of the grit chambers. If grit chambers are placed before screens, rags and other stringy material could foul the grit chamber collector mechanisms, wrap around air piping, and settle with the grit. If grit is pumped, further fouling or clogging of the pumps will likely occur. In hand-cleaned installations, it is essential that the velocity of approach be limited to approximately 0.45 m/s at average flow to provide adequate screen area for accumulation of screenings between raking operations. Additional area to limit the velocity may be obtained by widening the channel at the screen and by placing the screen at a flatter angle to increase the submerged area. As screenings accumulate, partially plugging the screen, the upstream head will increase, submerging new areas for the flow to pass through. The structural design of the screen should be adequate to prevent collapse if it becomes plugged completely. For most mechanically cleaned coarse screen installations, two or more units should be installed so that one unit may be taken out of service for maintenance. Slide gates or recesses in the channel walls for the insertion of stop logs should be provided ahead of, and behind, each screen so that the unit can be dewatered for screen maintenance and repair. If only one unit is installed, it is absolutely essential that a bypass channel with a manually cleaned bar screen be provided for emergency use. Sometimes the manually cleaned bar screen is arranged as an overflow device if the mechanical screen should become inoperative, especially during unattended hours. An approach velocity of at least 0.4 m/s is recommended to minimize solids deposition in the channel. To prevent the pass-through of debris at peak flowrates, the velocity through the bar screen should not exceed 0.9 m/s.Headloss through mechanically cleaned coarse screens is typically limited to about 150 mm by operational controls. Hydraulic losses through bar screens are a function of approach velocity and the velocity through the bars. The headloss through coarse screens can be estimated using the following equation: 2 2 1 ( ) 2 L V v h C g − = where hL = headloss, m C = an empirical discharge coefficient to account for turbulence and eddy losses, typically 0.7 for a clean screen and 0.6 for a clogged screen V = velocity of flow through the openings of the bar screen, m/s v = approach velocity in upstream channel, m/s g = acceleration due to gravity, 9.18 m/s2 The headloss calculated using above equation applies only when the bars are clean. Headloss increases with the degree of clogging. The buildup of headloss can be estimated by assuming that a part of the open space in the upper portion of the bars in the flow path is clogged. Although most screens use rectangular bars, optional shapes, i.e., "teardrop" and trapezoidal, are available. For the optional shapes, the wider width dimension is located on the upstream side of the bar rack to make it easier to dislodge materials trapped between the bars. The alternative shapes also reduce headloss through the rack. Screenings from the rake mechanism are usually discharged directly into a hopper or container or into a screenings press. For installations with multiple units, the screenings may be discharged onto a conveyor or into a pneumatic ejector system and transported to a common screenings storage hopper. As an alterative, screenings grinders may be used to grind and shred the screenings. Ground screenings are then