Mechanically Cleaned Bar Screens. The design of mechanically cleaned bar screens has evolved over the years to reduce the operating and maintenance problems and to improve the screenings removal Many uns include extensive use of corrosion-resistant materials including stainless steel and plastics( ABS. etc). Me ally cleaned bar screens are divided into four principal types: (1)chain dri (2) reciprocating rake, Continous (3)catenary, and (4) chain scraper continuous Raking tynes Bar reck Cable-driven screens were used extensively in the past but largely have been replaced in wastewater applications by the other types of screens re motor Examples of the different types necha ntlvent cover ng Typical mechanically clean screens Guide tra (a)front clean, front (reciprocating rak (acontinuous belt Chain-Driven Screens. Chain driven mechanically cleaned bar screens can be divided into categories based on whether the screen is raked to clean from the front(upstream) side or the back(downstream )side and whether the rakes return to the bottom of the bar screen from the front or back. Each type has its advantages and disadvantages, although the general mode of operation is similar. In general, front cleaned, front return screens(see Fig. 5-3a) are more efficient in terms of retaining captured solids, but they are less rugged and are susceptible to jamming by solids that collect at the base of the rake. Front cleane front return screens are seldom used for plants serving combined sewers where large objects can jam the rakes. In front cleaned, back return screens, the cleaning rakes return to the bottom of the bar screen on the downstream side of the screen, pass under the bottom of the screen, and clean the bar screen as the rake rises The potential for jamming is minimized, but a hinged plate, which is also subject to jamming is required to seal the pocket under the screen In back cleaned screens, the bars protect the rake from damage by the debris. However, a back cleaned screen is more susceptible to solids carryover to the down-stream side, particularly as rake wipers wear out. The bar rack of the back cleaned, back return screens is less rugged than the other types because the top of the rack is unsupported so the rake tines can pass through. Most of the chain-operated screens share the disadvantage of submerged sprockets that require frequent operator attention and are difficult to maintain. Additional disadvantages include the adjustment and repair of the heavy chains, and the need to dewater the channels for inspection and repair of submerged parts Reciprocating Rake(Climber)Screen. The reciprocating-rake-typo bar screen(see Fig 5-3b)imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. Most screen designs utilize a cogwheel drive mechanism for the rake. A major advantage is that all parts requiring m above the waterline and can be easily inspected and maintained without dewatering the channel. The front cleaned, front return feature minimizes solids carryover. The screen uses only one rake instead of multiple rakes that are used with other types of screens. As a result, the reciprocating rake screen may have limited capacity in handling heavy screenings loads, particularly in deep channels where a long"reach"is v. The nigh overhead clearance required to accommodate the rake mechanism can limit its use in retrofit applications Catenary Screen. A catenary screen is a type of front cleaned, front return chain driven screen, but it 5-35-3 Mechanically Cleaned Bar Screens. The design of mechanically cleaned bar screens has evolved over the years to reduce the operating and maintenance problems and to improve the screenings removal capabilities. Many of the newer designs include extensive use of corrosion-resistant materials including stainless steel and plastics(ABS, etc). Mechanically cleaned bar screens are divided into four principal types: (1) chain driven, (2) reciprocating rake, (3) catenary, and (4) continuous belt. Cable-driven bar screens were used extensively in the past but largely have been replaced in wastewater applications by the other types of screens. Examples of the different types of mechanically cleaned bar screens are shown on Fig. 5-3 Fig 5-3 Typical mechanically cleaned coarse screens: (a)front clean, front return chain-driven; (b)reciprocating rake, (c)catenary, (d)continuous belt Chain-Driven Screens. Chain driven mechanically cleaned bar screens can be divided into categories based on whether the screen is raked to clean from the front (upstream) side or the back (downstream) side and whether the rakes return to the bottom of the bar screen from the front or back. Each type has its advantages and disadvantages, although the general mode of operation is similar. In general, front cleaned, front return screens (see Fig. 5-3a) are more efficient in terms of retaining captured solids, but they are less rugged and are susceptible to jamming by solids that collect at the base of the rake. Front cleaned, front return screens are seldom used for plants serving combined sewers where large objects can jam the rakes. In front cleaned, back return screens, the cleaning rakes return to the bottom of the bar screen on the downstream side of the screen, pass under the bottom of the screen, and clean the bar screen as the rake rises. The potential for jamming is minimized, but a hinged plate, which is also subject to jamming, is required to seal the pocket under the screen. In back cleaned screens, the bars protect the rake from damage by the debris. However, a back cleaned screen is more susceptible to solids carryover to the down-stream side, particularly as rake wipers wear out. The bar rack of the back cleaned, back return screens is less rugged than the other types because the top of the rack is unsupported so the rake tines can pass through. Most of the chain-operated screens share the disadvantage of submerged sprockets that require frequent operator attention and are difficult to maintain. Additional disadvantages include the adjustment and repair of the heavy chains, and the need to dewater the channels for inspection and repair of submerged parts. Reciprocating Rake (Climber) Screen. The reciprocating-rake-typo bar screen (see Fig. 5-3b) imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. Most screen designs utilize a cogwheel drive mechanism for the rake. A major advantage is that all parts requiring maintenance are above the waterline and can be easily inspected and maintained without dewatering the channel. The front cleaned, front return feature minimizes solids carryover. The screen uses only one rake instead of multiple rakes that are used with other types of screens. As a result, the reciprocating rake screen may have limited capacity in handling heavy screenings loads, particularly in deep channels where a long "reach" is necessary. The nigh overhead clearance required to accommodate the rake mechanism can limit its use in retrofit applications. Catenary Screen. A catenary screen is a type of front cleaned, front return chain driven screen, but it