Types of Conveyors and Conveyor Systems
Types of Conveyors and Conveyor Systems
Conveyors and conveyor systems are mechanical devices or assemblies which transport materials with minimal effort. They are used for moving materials between two fixed work-stations. They are mainly used for moving bulk or unit load continuously or intermittently, uni-directionally from one point to another over fixed path. Their primary function is conveying of the material by the help of movement of some parts / components of the equipment. The equipment as a whole does not move. Conveyors come in many different varieties to suit the different products or materials which need to be transported.
Conveyors normally consist of a frame which supports rollers, wheels, buckets or belt, upon which materials move from one place to another. They can be powered by a motor, by gravity, or manually. The decision to provide conveyors is to be taken with care, since they are normally costly to install. Moreover, they are less flexible and, where two or more converge, it is necessary to coordinate the speeds at which the two conveyors move.
There are several types of conveyors and conveyor systems. These can be classified as (i) belt conveyors which can be flat, trough, closed, metallic, portable, or telescoping type, (ii) chain conveyors which can be apron or pan, slat, cross-bar or arm, car type / pallet, en-mass, carrier chain and flat-top, trolley, power and free, and suspended tray or swing-tray type, (iii) haulage conveyor which is a special class of chain conveyor in which load is pushed or pulled and the weight is carried by stationary troughs, surfaces or rail and which can be drag chain, flight or tow with over-head, flush-floor, and under-floor configuration, (iv) cable conveyor, (v) bucket conveyor which can be with gravity discharge, pivoted bucket, or bucket elevator type, (vi) roller conveyor which can be gravity, powered / driven, or portable type, (vii) screw conveyor, (viii) pneumatic conveying systems which can be pipe line, air-activated gravity (air slide), or tube type , and (ix) hydraulic conveying system.
A belt conveyor (Fig 1) is a material handling system which uses continuous belts to convey products or materials. It consists of an endless flat and flexible belt of sufficient strength, which is laid over two metallic flat pulleys at two ends, and which is driven in one direction by driving one of the two end pulleys. Material is placed on this moving belt for transportation. The belt is extended in an endless loop between two end-pulleys. Normally, one or both ends have a roll underneath. The conveyor belting is supported by either a metal slider pan for light loads where no friction is to be applied to the belt to cause drag or on rollers. Power is provided by motors which use either variable or constant speed reduction gears. The endless belt is kept tight by a belt tensioning arrangement.
The belts can be made from numerous materials, which correspond to the conditions under which the belt is to operate. Common conveyor belting materials include rubber, plastic, leather, fabric, and metal. Transporting a heavier load means a thicker and stronger construction of conveyor belting material is required. Belt conveyors are typically powered and can be operated at various speeds depending on the throughput required. The conveyors can be operated horizontally or can be inclined as well. They are troughed for bulk or large materials.
Fig 1 Belt conveyor and its important components
The general characteristics of belt conveyors are (i) they operate in one vertical plane, horizontally or with an inclination (up or down) depending on the frictional property of the load conveyed, (ii) for changing direction of the materials being conveyed, in the horizontal plane, more than one belt conveyors are needed, (iii) conveying capacity of the conveyor can be controlled by changing belt speed, (iv) they are normally employed for continuous flow of materials, and (v) metal / special belts can carry hot, abrasive or reactive materials. There are different types of belt conveyors. These are described below.
Flat belt conveyor – The active side of belt in this conveyor, remains flat supported by cylindrical rollers or flat slider bed. The conveyor is normally short in length and suitable for conveying unit loads like crates, boxes, packages, and bundles etc. in manufacturing, shipping, warehousing, and assembly operations. Flat belts are used conveniently for conveying parts between work-stations or in an assembly line in mass production of goods.
Troughed belt conveyor – In this conveyor, comparatively wide flat belt is supported on troughed carrying rollers or shaped supporting surface so that the two edges of the active side of the belt are elevated from the middle part to form a trough. This provides a higher carrying capacity than a flat belt of equal width for conveying bulk materials or those materials which can slide off flat belts. These conveyors are used in handling bulk materials of different classes. The return side of the belt is normally kept flat supported on cylindrical rollers.
The troughed conveyors which are used within a plant for moving bulk materials from one point to another point are normally termed as ‘normal’ or ‘transfer’ conveyors. These are comparatively of shorter lengths, and path of movements are in straight line in a horizontal or an inclined plane. The stresses in the belts are within limits of cotton fabric belts. However, troughed belt conveyors are frequently used for transportation of bulk materials over long distances, by means of a series of conveyors, over paths which are combination of inclines, declines and horizontal sections, following the natural contours of the ground. These are generally termed ‘long-centre’ conveyors.
There is no clear demarcation between a normal or long-centre conveyor. Long centre conveyors are those where belt tension is high warranting use of high tension quality belts with less belt stretch, and low acceleration through gradual starting controls for the drive. By using a number of conveyors in series, it is possible to change the direction of materials movement at the junction of two conveyors, called ‘transfer terminal’. Long-centre conveyors are used for jobs like (i) transportation of the output of mines to the processing plants, (ii) materials from shipping ports to the storage / transport loading sites, (iii) materials from outdoor storage yards to inside plants, and (iv) movement of materials between plants etc.
Closed belt conveyor – In a closed belt conveyor, the specially fabricated belt, after being loaded with the material, can be wrapped completely around the load. It essentially forms a closed tube moving along with the material. The advantages of a closed belt conveyor are (i) it can handle fragile materials safely and without breaking by reducing inter-particle collision, (ii) it can handle fine bulk materials without being swept by air though, it is not really air tight at loading and unloading points, (iii) it has ability to handle corrosive and reactive materials without contamination, and (iv) the tubed belt can travel around bends in more than one plane and hence versatile in layout. The lengths of these conveyors are normally limited. Different designs of closed belts have been manufactured and used in different countries. Different designs for closing two ends of the belt have been developed by different manufacturers.
Metallic belt conveyor – This is a flat belt conveyor where the flexible belt is replaced by a cold rolled carbon or stainless steel strip belt of thickness ranging from 0.6 mm to 1.2 mm. The ends of the steel strip are lap joint riveted with a single row of special wide flat head rivets. A steel belt runs flat on cylindrical idlers or runs troughed on resilient idlers (made by suitable length of springs). Apart from all rolled strip steel belts, wire-mesh, and belts of different designs have been used. The entire length is made up of short length sections. One of the designs is made up of flat wire spiral sections. The wire-mesh belts are more flexible and the design considerations are similar to that of a rubberized textile belt conveyor. Metallic strip belt conveyors are used for conveying hot and reactive loads. Wire-mesh belts are being widely used to handle unit and lump materials through furnaces (upto 1000 deg C temperature), as mobile base and also for wetting, cooling, dehydrating, and similar operations.
Portable Conveyor – Short length flat conveyors carried on a wheeled structure is termed portable conveyor. These are particularly useful for loading and unloading of trucks / transport vehicles. The inclination of the conveyor can normally be adjusted to suit application.
Chain or rope driven Belt conveyor – It is a special type of conveyor in which the specially designed belt is driven by a moving chain or rope, where belt only serves as load carrier, and motion is by a chain or a rope conveyor.
Submerged belt conveyor – It is a special type of conveyor in which a portion of the belt moves through a metallic trough (casing) which is filled with free flowing, powdered material at the loading end. The moving belt with holes makes the material flow to the unloading end of the trough.
The term chain conveyor means a group of different types of conveyors used in diverse applications, characterized by one or multiple strands of endless chains which travel entire conveyor path, driven by one or a set of sprockets at one end and supported by one or a set of sprockets on the other end. Materials to be conveyed are carried directly on the links of the chain or on specially designed elements attached to the chain. The load carrying chain is normally supported on the idle sprockets or the guide ways. The endless chains are kept tight by suitable chain tensioning device at the non-driven end (Fig 2).
Fig 2 Different types of conveyors
Since different types of chain conveyors are used in wide varieties of applications, it is not possible to have a set of common characteristics for all these chain conveyors. Chain, compared to the belts of the belt conveyors, has certain advantages as well as disadvantages. The major advantages are that the chain is easily wrapped around sprockets of small diameter, and the drive is positive i.e. no slippage takes place between chain and sprocket. The chain stretch is also little. The disadvantages of the chain are its high weight, high initial cost, higher maintenance cost and most importantly, limited running speed because of dynamic loading which come into play in chain-sprocket drive causing intensive wear at high speeds. Also, maximum length and maximum lift of chain conveyors are limited by the maximum allowable working tension of the chain used. There are different types of chain conveyors. These are described below.
Apron or pan conveyor – This is the most common type of chain conveyor. It consists of one or more strands of endless chain, normally link plate roller type, running in steel guides. Rollers ensure minimum pulling effort in the chain, while roller guides supported on the super-structure of the conveyor, carry the entire load of the materials and chains. The carrying surface of the conveyor is composed of a series of plates or shapes called ‘apron’, which are attached to the links of the chains through cleats. The bed created by the aprons is used for carrying bulk materials as well as unit loads. When the conveyor aprons have vertical flanges on all sides to form a pan like shape, if is specifically called a ‘pan conveyor’. Material carried by the apron is discharged over the sprockets at the driven end, and the conveyor chain with aprons comes back empty on its return Journey. These are normally slow speed conveyors with a speed range of 20 meters per minute (mpm) to 35 mpm.
Apron and pan conveyors are normally used to perform severe duties of conveying large quantities of bulk load such as coal, ore, slag, rock, and foundry sand etc. These are frequently used for feeding materials to large crushers, breakers, grinders, and similar machines. Specially designed aprons are used for conveying unit loads, coils, and hot forgings. Part of an apron conveyor can be run through a liquid or water bath for washing of the materials and then allow drainage of liquid from wet materials. Apron conveyors can have flexible layout to follow combined horizontal and inclined movement in the same vertical plane. Depending on the nature of materials to be conveyed, different designs of the apron and pan are used (Fig 2). Some of the common designs are (i) flat, spaced apron, (ii) corrugated apron, and (iii) special type.
Flat, spaced apron design conveyor is with rectangular shaped flat steel or wooden slat aprons with small gaps between them, providing a flat surface for carrying unit loads are specifically called ‘slat conveyor’. Some other designs of flat and spaced aprons are with cleats for carrying different shaped object.
Corrugated apron design conveyor is the most common type of apron, made of formed steel, with front and rear edges beaded so that one overlaps the other to form a continuous bed or trough. The overlaps are so made that during turning of the chain over sprockets, the apron ends move relative to each other without creating a gap for leakage of materials or a jamming of adjoining aprons or pans. Some of the aprons are plain while some are provided with overlapped vertical end plates to form pans. Corrugated aprons or pans can be fabricated or cast from gray or malleable iron. The pans are designated as leak proof (for carrying fines), shallow, and deep and hinged (for carrying chips, trimmings, scrap etc.). Deep pans can be used for carrying materials at an inclination of upto 45 degrees.
Apron conveyors of special types are used in special applications. Some of the typical examples are the three to four compartment cast-metal pans used for casting of pig iron. Beaded aprons are used in sugar mills. When deep loads are carried on an apron conveyor, stationary side plates called skirt plates are provided on both sides, fixed to the conveyor frame.
Cross-bar or arm conveyor – This type of conveyor consists of a single or two strands of endless chain, to which are attached spaced, removable or fixed arms (or cross members) from which materials are hung or festooned. The arms can be replaced by shelves / trays to support packages or objects to carry them in a vertical or an inclined path. Special arms are designed to suit specific load configuration. Depending on the design of arms, they are called by different names, some of which are (i) pendent conveyor, (ii) pocket conveyor, (iii) wire mesh deck conveyor, (iv) removable crossbar conveyor, (v) fixed cross-bar (or arm) conveyor, and (vi) swing tray conveyor.
Crossbar conveyors are used for conveying and elevating or lowering unit loads like barrels, drums, rolls, bags, bales, and boxes etc. The conveyors can be loaded / unloaded manually or at automatic loading / discharging stations. Cross-bar conveyors are also used in a wide range of process applications such as dipping, washing, spraying, drying, and assembly etc.
Car-type conveyor – This type of conveyor consists of a series of small platform cars, propelled by an endless chain, running on a closed track. Car-type conveyors can have vertical run-arounds over sprockets having horizontal axis. However, more frequently they are designed with horizontal run-arounds (carousels) over sprockets (or sheaves for rope drive) with vertical axis. This type of conveyor is also called a carousel conveyor or a pallet-type conveyor.
The track is placed more or less in a horizontal plane. The cars can either be permanently attached to the driving chain (or cable) or can be propelled by pusher dogs on chain or rope against lugs on cars. The driving chain is normally positioned at the bottom side of the cars, between the two track rails. Loads can be manually loaded / unloaded, or can be designed for automatic loading, and unloading through tilting of car top at unloading point.
Car-type chain conveyors are particularly used for carrying heavy or irregular shaped large objects like moulds in foundries, coils in rolling mills etc. These conveyors are conveniently used to combine different processing operations during transportation of the loads. Rolled coils can be cooled, liquid metals can be solidified in moulds, assembly of components can be achieved, and testing inspection can be performed etc. The conveyors with horizontal run-arounds can be arranged to move in any straight or irregular shaped path in the same horizontal plane, hence called contour type, which makes them very suitable for use in different process plants for picking up and delivery of materials from and to desired locations of the plant. On horizontal run-arounds, a load not removed continues to move with the conveyor. This gives an obvious advantage of using a short conveyor for accomplishing long duration processes (drying, cooling etc.) and irregular processes (foundry, testing etc.). Horizontal carousel conveyor normally occupies larger floor space.
Carrier chain and flat-top chain conveyor – Carrier chain conveyor consists of one or more number of endless chains to which can be attached one of the many different attachments for the purpose of carrying unit materials or objects. In several cases, the materials are conveyed while being directly in contact with the chain / chains. These conveyors have a broad application in practically all the fabricating and the processing industries. Different designs of attachments are used for different types of materials. Carrier chain conveyors are normally classified into two basic types namely (i) rolling type, and (ii) sliding type.
In case of rolling-type carrier chain conveyors, the chains are provided with rollers moving on tracks for minimum of friction. The materials are supported on the attachments. In a variation of this type of conveyor, the rollers can be used for supporting the objects while the chain acts as the connecting and propelling link for the rollers. The rollers can be shaped to accommodate curved faced objects or can be flat-faced to carry objects with flat surfaces. Rotation of the carrying rollers frequently causes the objects to move at a higher velocity than that of the chain. In case of sliding-type carrier chain conveyor, the loads are carried directly on one or more chains, while the individual chain slides on a track or surface or a trough. Attachments or specially designed links can be used to suit the loads.
Flat-top chain conveyor is a particular group of carrier chain conveyors, which can be rolling or sliding type, with specially designed chain links or with flat plate attached to the chain links so as to provide a continuous, smooth, level top surface to carry small articles like bottles, and cans, etc. at a high speed. These conveyors are widely used in canning and bottling plants. Different types of chains and / or attachments are used such as hinged-joint continuous flat top sliding type, plate-top sliding or rolling type, crescent-shaped plate top type. The crescent plate design is particularly suitable for carousel-type operation to turn in a horizontal curve, a typical example being the baggage handling conveyors in the arrival section of an airport.
Trolley conveyor – These conveyors consist of a series of trolleys supported from an overhead endless track and propelled by an endless chain or cable, with the loads normally suspended from the trolleys Fig 3). This is one of the most versatile types of chain conveyors which can work in horizontal and inclined paths, vertical curves and horizontal turns to follow complicated routes. Different structural members are used as track for overhead trolley-conveyor which includes I-beam, double angles, T-rails, steel bars, pipes and fabricated sections. However, I-beam is the most common track. These tracks are laid at a higher level, suspended from roof, building structures or hung from floor-mounted columns, and routed around obstacles. Overhead operation allows free floor space and no interference with equipment or traffic at the floor level. For this reason, trolley conveyors are frequently being called overhead conveyors.
Normally two wheeled trolleys or more wheeled trolleys with load bar between them for handling large loads are used. Loads are suspended from carriers bolted to the trolley bracket. Hooks and trays are the most common carriers. As the trolleys can move in three dimensions, this type of conveyor is extensively used for carrying materials continuously through different processes, like cleaning, washing, painting, drying, baking, degreasing, sand blasting etc. These conveyors can be, and normally are, used as a storage conveyor, at the same time as a processing and delivery conveyor. The carriers can be loaded and unloaded en-route, at one or more points of the conveyor run, either manually or automatically. According to the method by which load is conveyed, trolley conveyors are further classified into following three types as given below.
Load-carrying trolley conveyor is the first and the main type, in which the trolley and the load carriers are permanently fixed to the pulling chain. The second type is load-propelling trolley conveyor, in which the trolleys with load carriers travel on track being pushed by pusher dogs attached to the pulling chain or chain trolley. The special advantage of this load-propelling conveyor (also called pusher trolley conveyor is the capacity to divert the load carriers from the main track to a branch track for achieving different operational requirements. The third type is the load towing trolley conveyor in which the trolleys are permanently secured to the pulling member, and specially designed hooks or rods from the trolley engage and tow floor mounted trucks carrying the load. In this case, the conveyor can be made very light as the load is basically carried on the floor, but the advantage of free floor / working space is lost. This particular type of trolley conveyor is also classified as overhead tow conveyor.
At horizontal turns or vertical curves, where the trolley conveyor changes direction, special care is taken to keep the pulling chain from becoming slack or making kink. At turns, the chain can be supported by a series of rollers or by a suitable sized sprocket. At vertical curves, normally the slope is limited to 30 degrees and while going down it starts with a dip down (that is gradually changing slope to the desired angle) and the opposite requires a dip up. At vertical curves, stops are sometimes used to prevent runaway of trolleys and loads if the chain breaks. The advantages of an overhead trolley conveyor can be summarized as (i) movement is three dimensional and easily adopted to changes in direction, (ii) large length with one or multiple drives, (iii) free floor space, (iv) small power consumption, (v) little maintenance, and (vi) high salvage value.
Fig 3 Types of conveyors
Power and free conveyor – These conveyors are basically a special design of the ‘load-propelling or pusher trolley’ conveyors. In a normal pusher trolley conveyor the non-powered trolleys, supported from a monorail, carry the load and are pushed by dogs / pushers attached to the chain trolleys mounted on a separate track. A power and free conveyor (Fig 3) is one in which the power trolleys run directly above the free trolleys, which run in double channel track, and arrangements are made such that at desired points the non-powered load carrying trolleys can be engaged to or disengaged from the power trolleys. The power trolley dogs / pushers are rigid attachments on the trolleys or chain. They engage or disengage with the free trolleys by switching them in from a branch line to the mainline, and by horizontal turns and vertical curves in the power line. The switching operations can be made mechanically or through actuation of pneumatic cylinder synchronous with movement of power trolleys.
Unlike in a load-propelling conveyor, where the side pusher is to be so arranged that switching is always done on the side away from the power conveyor pusher arm, the power and free conveyor can switch load on both sides of the power trolley track. Through power and free conveyor, it is possible to switch off loads to branch lines, to alter load spacing in various sections of the conveyor, to stop the loads for making inspection, work repair or storage etc. Another advantage of this conveyor is that the power conveyor can frequently be kept out of the processing zone like oven, painting booth or other undesirable location. The special features of automatic dispatching, switching and transfer have resulted in remarkable savings in labour and manufacturing cost in a number of different process industries like automobiles, wire rod mill, foundry, and graphite anode handling in aluminum pot shop etc.
From design point of view, one interesting aspect is the mechanism used for engagement and disengagement of the pusher and the free trolley. A popular design provides two counter-weighted tilting dogs at the top of the free trolleys, so that as the power pusher attachment approaches the free trolley, it pushes one dog down to pass over it and engages the second dog. When first dog is released, it tilts back to its initial position and becomes a hold-back. Depending on the direction of travel or the inclination of the track, either of the dogs can become pusher or hold-back. In certain designs, the pusher units are supported against springs, which have sufficient rigidity to push a loaded carrier. However, when the carriers are stopped against a manual or automatic stop, the pusher spring is compressed and the pusher slides over the dogs on the free trolleys. This design of a pusher is called reversing spring pusher.
Suspended tray conveyor – It is also known as ‘swing-tray conveyor’. These conveyors consist of two strands of chains between which are pivot mounted a series of trays to carry in-process movement of various unit loads (forged components, boxes etc.) along complex contours comprising horizontal and vertical paths in one vertical plane. As the trays are pivot mounted from the links of the chains, the trays along with their loads always remain suspended vertically irrespective of the path of the chain. Suspended tray conveyors are loaded on vertical sections manually or automatically by specially designed loading devices. These conveyors are particularly used for raising / lowering of loads between floors, convey materials between processing equipment, carry loads without transfer between inter-linked horizontal and vertical sections. The conveyor can be used for carrying load through processing stations like drying, and pickling chambers etc.
The design of the trays is adapted to the requirements of loads and method of loading / unloading.The trays can be flat or curved. When the trays are made of steel plates in the shape of buckets for carrying powdered or granular bulk load, the particular conveyor is called ‘pivoted bucket conveyor’. The bulk material is fed into the buckets on the lower horizontal section and carried through various sections without transfers, and hence is not crushed en-route. The pivoted buckets are discharged at the upper horizontal section automatically by tippers or dischargers. The buckets are fitted with projecting curved cams or guide rollers, which on coming in contact with the arms of tippers, tip the buckets. Pivoted bucket conveyors are used in power plants in carrying coal and ashes, in cement mills, ceramic industry, and stone crushing plants etc.
Haulage conveyor is a special group of chain conveyors. As the name implies, the material is dragged, pushed or towed by means of a chain or chains, making use of flights or surfaces which are parts of the chain themselves. The weight of the material is normally carried by stationary troughs, surfaces, or wheeled trucks / dollies on rails / floor. In certain designs, the chain can be replaced by cables. These conveyors are run at slow speed (15 mpm to 60 mpm), being built for heavy duty, and need little maintenance. However, the chains undergo wear under heavy tension and work in one direction only. Haulage conveyors are normally classified into (i) drag conveyor, (ii) flight conveyor and (iii) tow conveyor.
Drag chain conveyor – It is a conveyor having one or more endless chains, which slides in a track or tracks, resting at the bottom of a trough, and materials resting directly on the chain are carried by the chain links. The layers of materials above the chain level are moved by the cohesiveness with the material below. The troughs or sliding base surface can be made of steel, concrete or even wood. The chain tracks are frequently made from steel channels. These conveyors normally work in the same horizontal plane with little inclines, for movement of bulk materials, hot materials, abrasive materials, logs / timber, and packages etc. Even cars can be moved in a car assembly line by putting two wheels on one chain. A few of the typical designs of drag chain conveyors are described below.
In ‘multiple strands drag chain conveyor’ the multiple parallel chains can be spaced widely upto a few meters apart and can be used to transfer long objects like lumber, or hot steel sections etc. placed across multiple strands. This type of conveyor is widely used in transferring hot steel sections in hot bar and section rolling mills. All chains move at same speed. The ‘pusher-bar conveyor’ consists of two strands of sliding endless chains connected by spaced crossbars. Unit loads resting on a slider bed are pushed by these cross bars. In case of ‘wide-chain drag conveyor’ it consists of a single strand large width chain which can move a bed of bulk materials along a troughed path. The space between one set of link and bottom sliding plate form individual pockets for the bulk materials. These conveyors are widely used for handling refuse materials like clinkers, ashes, sawdust, and also bulky materials like wood chips, coal etc.
Flight conveyor – it is a conveyor comprising of one or more endless strands of chain with spaced transverse flights or scrapers attached which push granular bulk material along a shaped trough. The material can be loaded at any point into the trough and discharge can also be affected at various points through openings in the trough floor, closed by sliding gates. Both upper and lower strand can be used for transporting materials in opposite directions. These conveyors normally work at speed range of 30 mpm to 50 mpm to handle free flowing materials of small to moderate size to move them in both the directions. These are used for handling coal, ashes, sand, gravel, ore, wood chips, saw dust, chemicals, grains, and cereals etc., normally for loading bunkers and bins and also used under floor for removal of metal chips / cut pieces. One flight conveyor can handle two or more materials simultaneously by making two or more material flow troughs / channels side by side and designing the flights to match individual troughs. These conveyors are built rugged for long life and low maintenance.
When a two-chain flight conveyor have flights made in the shape of fixed buckets, such a flight conveyor is called ‘gravity discharge bucket’ conveyor, also called ‘V-bucket’ conveyor. These can fill up buckets in the horizontal section, move through vertical sections carrying bulk material and then discharge the material in another horizontal trough section at a higher level for filling up of bins etc.
Tow conveyor – This conveyor consists of a single strand endless chain which tows floor / track mounted trucks, dollies or cars on which the materials are placed. Tow conveyors are normally used for handling of unit loads like boxes, barrels, crates, cartons, in the warehouse, in assembly lines and for intra-plant movement. Though the tow conveyors follow a fixed path, the carts can be detached easily from the conveyor and moved to other points. Tow conveyors are classified into following three groups.
The ‘overhead tow conveyor’ has already been described as ‘load towing trolley conveyor’ under ‘trolley conveyor’. In case of the ‘flush-floor tow conveyor’, the materials are transported on rail bound carts, moved by one strand of endless chain moving in a fixed guide flushed with the floor. The carts are connected to the conveyor chain by removable link like chain with hook, removable dog and rigid drawbar which engage the trolleys and push them. In case of the ‘under floor tow conveyor’ an endless chain is installed below floor level either supported by trolleys or sliding in a channel or angle track. The floor mounted carts are connected to the conveyor by retractable pin through a narrow slot in the floor, to pick up connecting device on the chain. Under floor tow conveyors work at higher speeds than overhead ones. These are widely used in variety of applications including moving automobiles, wash racks, in manufacturing assembly lines, warehouses, freight handling terminals etc. The major limitations are track clogging with refuse and difficult access to maintenance. These conveyors have to be planned before constructing the new building where they have to be installed.
Cable conveyors form a distinct group of materials handling conveyors to transport people and bulk materials in load carrying buckets, using overhead moving cables and / or wire ropes and are composed of one or more spans from the loading point to the discharge point / points covering long distances upto several kilometers. These conveyors are also known as ‘ropeways’ or ‘aerial tramways’. These are normally used for carrying minerals from mines to their processing stations. The characteristic features and advantages of these conveyors are (i) as loads are moved at a substantial height from the ground, shortest route between the terminals can be followed independent of the ground contour, (ii) wide varieties of materials including human passengers can be transported, (iii) cost of operation is comparatively less than other transportation systems, (iv) materials are moved between distantly located points without the need of re-handling, and (v) materials can be automatically discharged at the desired point, hence eliminating use of an auxiliary discharging system.
From design point of view there are mainly two distinct systems of aerial tramways / ropeways, the ‘bi-cable’ and the ‘mono-cable’ system. In the bi-cable system one or more (normally two) stationary high-tensioned track cables are used over which carriers are placed from which hang the load carrying buckets. For a continuous bi-cable system, two stationary cables are needed. The carriers are pulled by an endless traction or pulling rope moving continuously in one direction. The loaded carriers move from the loading terminal to the discharge terminal, while the empty ones move in the opposite direction. For reversible bi-cable system one track cable is sufficient. In the mono-cable tramways system, one endless moving wire rope is used for supporting as well as moving the carriers.
Monorail tramways is another type of cable conveyor in which the load carriers run on suspended rails, and moved by a moving wire rope. This is essentially a trolley chain conveyor where the traction chain is replaced by a wire rope. These are used for towing load between much shorter distances.
A cable conveyor basically consists of (i) one or more cables / wire ropes, one of which is driven which pulls the load, (ii) a number of load carrying buckets or carriers which are hung through hangers from wheeled (2-wheel or 4-wheel type) carriages, (iii) loading and discharge terminals, (iv) intermediate towers for supporting the cable / rope, and (v) drive arrangement. Selection of size of traction rope or carriage supporting static rope is based on the estimated maximum tension in the rope. The construction of rope is selected such that it provides a smooth wearing surface for the carriage wheels, provides better gripping of the carrier with the rope and whose outer layers of wires do not unlay and get loosened.
The carriages are generally of 2-wheel or 4-wheel type, the later being used for heavier load. The rigid hangers are connected with traction rope through grips. These grips can be of (i) compression type which operate on toggle principle, (ii) screw type, or (iii) weight-operated type which depends on the weight of the loaded carriers for its gripping capacity. The carriers are normally buckets of different types to suit specific nature of the load. The usual bucket designs are (i) trunnion mounted rotary dump buckets which can be overturned easily for discharging, (ii) end dump buckets for discharging load from one end, and (iii) bottom dump buckets for discharging load from the bottom.
Loading or discharge terminals are structural platforms built at the height of the cables / ropes, where arrangements are provided for detaching and attaching the carrier from and to the traction rope by operating the grips, and taking them onto a loop of rail on the platform. At the loading terminal the empty buckets are loaded automatically from storage bins through proper feeders, while at the unloading terminal the buckets are emptied. Rope tensioning arrangement is provided at the discharge terminal while rope driving system is placed at the loading terminal. Cable conveyor drive consists of a suitable power unit coupled to a speed reduction gear box and the driving sheave. The diameter of the driving sheave depends on the rope size and pressure of the rope on the sheave groove. Driving sheave can be rubber-lined. Grip sheaves with toggle jaws spaced around a segmented rim have been used for increasing driving power of the sheaves.
These conveyors convey bulk loads in bucket shaped vessels which are attached to a system of moving chains or belt. These are normally classified as (i) gravity discharge bucket conveyor, (ii) pivoted bucket conveyor and (iii) bucket elevator. Gravity discharge bucket conveyor also called V-bucket conveyor has been described under ‘flight conveyor’ and pivoted bucket conveyor has been described under ‘suspended tray conveyor’.
Bucket elevators (Fig 3) are used for conveying bulk materials from a lower level to a higher level. They are powered equipment for conveying bulk materials in a vertical or steep inclined path, consisting of an endless belt, or chain / chains to which metallic buckets are fixed. With the flexible belt / chain, the buckets move uni-directionally within a casing and collect bulk materials at bottom end of the equipment and deliver it at the top end.
A typical bucket elevator has the major parts consisting of (i) an endless pulling member (flat belt or chain), (ii) driving and take up pulleys or sprockets at top and bottom respectively, mounted on bearings and blocks, (iii) metal casing covering the entire elevator and consisting of head at the top, boot at the bottom and intermediate sections, all joined at flanges by fasteners, (iv) buckets, normally made out of sheet metal, which are attached at definite pitch to the pulling member by fasteners (screw and nuts, riveted etc.), (v) drive at the top consisting of an electric motor, gearbox, and couplings, (vi) hold back brake attached to the top pulley / sprocket shaft, to prevent reverse motion of the elevator when drive is stopped, (vii) feed hopper attached to the boot for feeding materials to the elevator, (viii) delivery / discharge spout fixed with the top part of the casing, through which the material is discharged, (ix) manholes provided in the casing to check operations of the elevator, and (x) guides and guide sprockets provided for belt and chain respectively to keep them in a straight path.
Bucket elevator is simple and reliable equipment widely used in process plants for lifting of bulk materials like lime stone, foundry sand, coke, coal, grain, dry chemicals and many more. It is normally low cost equipment requiring little floor space. However, it is not suitable for large sized (100 mm or above), hot or sticky materials since buckets are normally loaded by scooping action in the boot section and discharge from the buckets by centrifugal force or gravity. Chain type, with one or two chain, elevators are suitable for a speed range of 0.6 meters per second (mps) to 1.55 mps, while belt type are recommended to be used in a speed range of 1.15 mps to 2.85 mps. Bucket elevators are classified based on bucket spacing and mode of discharge of materials. They are of the following three basic types.
Centrifugal discharge elevators – In this elevator, the buckets are spaced at a regular pitch to avoid interference in loading and discharging. The charging of buckets is by scooping action and the discharge is by centrifugal action. These elevators are normally used in vertical configuration and used for practically all types of free flowing, small lump materials like grain, coal, sand, clay, sugar, dry chemicals etc. Both belt and chain can be used and the speed of these elevators range between 1.1 mpm to 2 mpm.
Positive discharge elevators – These elevators are similar to the centrifugal discharge type excepting that the buckets are side-mounted on two strands of chains (i.e. buckets lie between two strands of chains), and are provided with a pair of two snub sprockets under the head sprockets to invert the buckets for complete discharge. The speed of the elevator can be slow in the range of 0.6 mpm to 0.67 mpm. These elevators are used for light, fluffy, sluggish and slightly sticky materials. The feeding is through scooping or digging by the buckets. An inclined elevator is particularly suitable for perfect gravity discharge.
Continuous discharge elevators – In these elevators, V-type buckets are used without any gap between them. These elevators are employed for handling larger lumps and materials which can be difficult to handle by centrifugal discharge. The charging of the buckets is by direct filling. The discharge is by directed gravity i.e. when the buckets pass over head wheel, the flanged end of the preceding bucket act as a chute to deliver materials gently to the discharging spout. This type of charging and discharging is particularly effective for handling fragile materials. Both belt and chain are used as pulling medium. Speed is generally kept low. These elevators are used vertically or inclined. When inclined, special supports are provided for belt / chain in the return run, and wider casing is provided to allow for return run sag.
A roller conveyor (Fig 2) supports unit type of load on a series of rollers, mounted on bearings, resting at fixed spacings on two side frames which are fixed to stands or trestles placed on floor at certain intervals. A roller conveyor essentially coveys unit loads with at least one rigid, near flat surface to touch and maintain stable equilibrium on the rollers, like slabs, billets, plates, rolled stock, pipes, logs, boxes, crates, and moulding boxes etc. The spacing of rollers depend on the size of the unit loads to be carried, such that the load is carried at least by two rollers at any point of time.
Roller conveyors are used for conveying almost any unit load with rigid riding surface which can move on two or more rollers. These are particularly used between machines, buildings, in warehousing as storage racks, docks, foundries, rolling mills, manufacturing, assembly and packaging industry. They are also used for storage between work-stations and as segment of composite handling system. However, the limitations of rollers conveyors are that they can be best used for objects with rigid flat surfaces, and for movement to relatively short distances. They need side guards to retain the loads from falling off. Gravity roller conveyors have the risk of accelerating loads. Roller conveyors are classified into two groups according to the principle of conveying action. These are (i) not powered or idle roller conveyor, and (ii) powered or live roller conveyor.
In a not powered roller conveyor, the rollers are not driven or powered from an external source. The loads roll over the series of rollers either by manual push or push from an endless moving chain or rope fitted with pusher dogs, rods, or clamps. Normally these conveyors operate at horizontal plane, but at times a gentle slope is given to these conveyors to aid motion of the loads. An inclination of 1.5 % to 3 % ensures the load to roll by gravity. Such conveyors are termed ‘gravity roller conveyor’.
The not powered roller conveyor consists of series of rollers, the frame on which the rollers are placed and the stands also called the trestles, on which the framework rests. Because of simplicity of design, competitive cost and trouble free operation, these conveyors are used extensively in handling unit loads in workshops or process plants to convey articles from one working station to another. Not powered roller conveyors are frequently used as a storing platform and as such are often termed as roller table. These are also used in stores as storing racks and in loading bays for loading / unloading materials from carriages. A gentle slope can be provided in the conveyor to aid movement of the loads on idle rollers. These gravity roller conveyors are used to convey load in one direction only. The conveyors can have a curved section to change direction. Material movement between two levels can be done by an inclined or a spirally formed gravity roller conveyor. The spiral form increases the length of the conveyor and thereby controls the velocity of the articles moving down the conveyor.
A number of features can be incorporated in a not powered conveyor to satisfy different functional requirements. Some of these are (i) double-row roller conveyor which is used to convey wide and heavy loads and in which in place of one long and proportionately large diameter roller, two smaller diameter rollers with lengths less than half of the larger roller are used, (ii) curved sections in which the rollers are arranged radially and which are used for changing direction of the conveyor in the horizontal plane, (iii) switches which are the devices used to change the normal direction of the load or divert the load, from the conveyor are called switches, and (iv) stops which are placed at the end of the conveyor to physically stop the moving loads from falling off the conveyor end. Disappearing stops can be placed at desired intermediate points in the path of a roller conveyor to stop the moving articles at such points, if needed. Disappearing stops can be moved up or down from the top level of the roller by suitable mechanism.
In a powered roller conveyor, all or a selected number of rollers are driven by one or a number of motors depending on the selected drive arrangement. The driven rollers transmit motion to the loads by friction. The powered roller conveyors can be installed at a slightly inclined position, upto 10 degree up or upto 17degree down. The load can be moved in either direction by changing the direction of rotation of the rollers, where these are called reversing conveyors.
In a powered roller conveyor, also called ‘live roller conveyor’, all or a few of the rollers are driven by one or multiple motors through associated transmission system. The loads on the roller conveyor are moved by the frictional force caused between the loads and the driven rollers supporting the loads. Powered roller conveyors are intensively used in heavy process plants like rolling mills to feed heavy and at times hot metal to or to take the material in the mill to various other process equipment. The roller conveyors can be reversing type or non-reversing type.
Major classification of powered roller conveyor is based on the type of drive arrangement employed. When one motor drives more than one or all the driven rollers, it is called group or multiple drive roller conveyors. In group drive, normally only one motor with suitable transmission arrangement is used to drive all the driven rollers. For a long conveyor, or from other considerations, more than one motor can be used, each driving a group of rollers in different sections of the conveyor. The transmission of power from the motor to the rollers varies widely depending on use. In a heavy duty non-reversing conveyor, bevel gear transmission arrangement can be used. In an alternative design the transmission of power can come to one roller, and the other driven rollers can be connected to this driven roller by series of sprockets and chains.
In a light duty powered roller conveyor, the rollers can be driven by one endless flat belt driven below the rollers, and supported by idle rollers such that the belt touches all the rollers and transmit power to them by friction. This is, unlike others, not a positive drive. When each of the driven rollers is driven by an individual motor, it is called individual drive. These motors can be high speed motors transmitting motion through a reducing gear. Alternatively, specially designed slow speed hollow rotor shaft motors are used which are directly coupled to the roller shaft. With the availability of better electrical control systems, individually driven roller conveyors are getting more popular particularly for reversing duty.
Roller conveyor can be a portable roller conveyor which is a short (upto 7 m) section of roller conveyor mounted on legs and at times with wheels. These can be shifted from one place to another and adjusted in height or inclination for loading and unloading of trucks. The portable roller conveyor can be idle or driven. Driven conveyor is frequently through an endless belt.
A screw conveyor (Fig 3) consists of a continuous or interrupted helical screw fastened to a shaft which is rotated in a U-shaped trough to push fine grained bulk material through the trough. The bulk material slides along the trough by the same principle a nut prevented from rotating. The bulk material moves in a rotating screw. The load is prevented from rotating with screw by the weight of the material and by the friction of the material against the wall of the trough. A screw conveyor is suitable for any pulverized or granular non viscous material, and even at high temperature. The conveyor is particularly suitable for mixing or blending more than one material during transportation, and also for controlling feed rate of materials in a processing plant. Abrasion and consequently certain amount of degradation of the material is unavoidable, hence it is not suitable for brittle and high abrasive materials. It is also not suitable for large-lumped, packing or sticking materials.
The screw shaft, if short (upto 5 m), is supported at two ends. But for longer shafts (upto 40 m to 50 m), they are supported by bearing hangers, at intermediate points. The shaft can be solid or hollow. Hollow shafts are lighter and can be easily joined to make a long shaft. The screw shaft is driven at one end, and the design can permit discharge of material from the bottom or one end. Opposite handed screw at two sides cause the centre discharge.
The U-shaped fabricated trough is generally covered at the top to avoid particulate pollution. The bottom portion of the trough is of circular cross section matching the diameter of the screw. Normally a radial gap of 10 mm to 20 mm is kept between the screw and the trough, depending on size of the screw. Screws of different constructional design and style are used. Continuous screws are normally made from 4 mm to 8 mm sheet steel circular section with a hole corresponding to the size of the shaft. One radial slit is made in this section, and then formed into one pitch of the screw. The section is welded to the shaft and welded or riveted to each other to form the entire length of the screw. The screw can also be cast integral with the shaft. The paddle type flights consist of cast straight or curved segments fixed to the shaft. A ribbon screw is fixed to the shaft by means of radial rods.
The drive unit comprises of an electrical motor, gear box and couplings. Material is fed through the feed hopper fixed on the trough cover. A number of discharge sprouts with rack gears for closing and opening as needed, are provided. Screw conveyors are normally operated horizontally or at a small inclination (10 degrees to 20 degrees). However, there are special designs where the load is moved vertically up or at a small angle to vertical. These are called vertical screw conveyors.
Pneumatic conveying systems
Pneumatic conveying is the process of conveying granular / powdered materials by floating the materials in a gas, mainly air, and then allowing it to flow to the destination through a closed pipe. The operating principle common to all types of pneumatic conveying is that motion is imparted to the material by a fast moving stream of air. Hence, any pneumatic conveyor consists of an air supply equipment (blower or compressor), pipelines, product storages, air lock feeders and dust filters.
Pneumatic conveying system (Fig 4) is used for delivery of non-sticky, dry materials via pipelines to various storage or process points which are economically inaccessible by conventional conveyors. The major advantages of using pneumatic conveying system are (i) materials can be picked from one or more points and can be delivered to one or more points in a plant or even outside to a different plant, (ii) conveying of materials take place through air tight piping and auxiliary system and hence neither pollutes the environment nor the materials get contaminated with foreign materials, (iii) offers plant and operator safety in handling fine powdery materials which can be toxic in nature or fire prone, (iv) offers a flexible system in which the conveying pipe lines can be routed and rerouted with little efforts as per demand of the operations, (v) makes possible unloading of materials from ships, barges, transport vessels directly to storage bins, (vi) is self cleaning system, preventing accumulation of materials in the conveying system and hence, the same installation can be used for conveying different materials, (vii) offers a low maintenance cost system, (viii) offers a lower cost materials handling system compared to handling and storage of bulk materials, and (ix) can be operated automatically and can easily be integrated into manufacturing processes as feeders.
There are also some disadvantages which are (i) types of materials suitable for pneumatic conveying is limited to materials which are dry, granulated, pulverized, crushed etc. and essentially free flowing, (ii) friable or too abrasive materials are not suitable to be transported by pneumatic conveyors, (iii) movement of transportation is fixed (uni-directional), (iv) relatively high energy consumption per unit weight of materials transported, and (v) length of pneumatic conveyors are limited. Vacuum systems are limited to 500 m while high pressure systems up to 2 kms or marginally more. Pneumatic conveyors are broadly classified into three groups, based on application namely (i) pipeline conveyor, (ii) air-activated gravity conveyor (air slide), and (ii) tube conveyor.
Pipeline conveyor – Further classification of pipeline conveyors is made on the basis of air pressure used. In low pressure system, the operating air pressure is normally limited to 1 atmosphere gauge. This system is restricted to short distances (upto 500 m) and small flow rate. This system is further sub-classified into (i) positive pressure system, (ii) negative pressure (or suction) systems, and (iii) combined negative-positive pressure (or combination) system. In medium pressure system the air pressure is in the range of 1 atmosphere gauge to 3 atmosphere gauge. In high pressure system the air pressure is in the range of 4 atmosphere gauge to 7 atmosphere gauge. Medium and high pressure systems are essentially positive pressure type systems.
Fig 4 Pneumatic conveying systems
A positive low pressure pipeline system (Fig 4) is one in which a positive air flow, created by a positive displacement blower, effectively transport slow flowing materials over a distance upto 500 m. The major characteristic of this system is that it can pick up material from one source and discharge the same to more than one point. A positive displacement blower sucks air from the atmosphere through a suction filter and delivers pressurized air into the conveyor pipe line. Material to be conveyed is introduced into the pipeline from the feeding bin / hopper through different types of air lock feeder at controlled rate. Air lock feeders are used at the outlet port of the feeding silos for discharge of materials into the pipeline, without being blown out from the top of such silos because of high air pressure. The materials so introduced are immediately air laden and blown to one or a number of receiving bins / silos through diverting valves, also called change over valves.
The diverting valves are generally designed for one inlet and two outlets, but multiple outlets are also possible. The valves are operated either manually or through remote controlled actuator. The air, after the material gets discharged at the receiving hoppers, is made to pass through an appropriate dust removal system, before it is allowed to escape back to atmosphere. The principal advantage of this system is that the blower does not handle dusty air. However, the major disadvantage of the system is the possible leakage of high pressure air along with materials to the surroundings.
In the negative pressure (or suction) type of conveyor system (fig 4), a positive displacement blower creates a vacuum in the conveyor pipe line, which causes the material to be sucked through one or multiple nozzles and conveyed to a receiving hopper. The conveyed material is separated from the air stream and collected in the hopper. The air is normally made to pass through a dust collector to be freed from remaining finer particles of materials and then released to the atmosphere. Easy flowing materials like cereals, grains, powdery materials etc. are conveyed in suction conveyors. In this system, materials from multiple storage points can be collected and transported to a single storage point.
The major advantage of negative pressure conveyor system this system is that at leakage points, air from surroundings enters the system, and hence air pollution through materials leakage is virtually zero. Hence, this system is particularly useful for toxic and very fine powdered materials. However, the major limitation is that, if the air is not totally separated from the conveyed materials, dust laden air passes through the blower and tends to damage it. Moreover, the distance and volume of conveying is also limited because the actual vacuum created inside the pipeline is frequently not below 0.3 atmospheres absolute.
In the combined negative-positive pressure (or combination) system (Fig 4), the principles of both positive pressure system and suction system are employed. This type of pneumatic conveyor is particularly employed when materials from more than one point have to be picked up and simultaneously delivered to multiple delivery points. For this system generally one positive delivery blower is used. Two separate blowers can also be used for the suction line and pressure line of the combination system. The suction side of the blower conveys material from the storage points to an intermediate receiving station. The suction air passes through this intermediate receiving station and then through a cyclone separator and a further dust cleaner, if needed, and enters the blower. The compressed air is then thrown into the pressure delivery pipe line, into which the material from the intermediate storage is discharged through necessary air lock feeder. From there, material is conveyed to the multiple storage silos as per a positive pressure system already discussed.
Medium and high pressure systems are basically similar to a positive pressure system in its operational principles. The difference is the range of air pressure used. Because of use of increased air pressure, the medium and high pressure systems principally differ from a low pressure system in two aspects. Firstly, the type of air supply equipment varies according to the pressure selected. Secondly, the feeding device used is to be different to make the system leak proof. A single stage rotary compressor is used for a medium pressure pneumatic conveying system while a double stage rotary compressor is normally used for a high pressure system. The medium pressure system normally operates continuously while a high pressure system can be used for continuous, intermittent or batching operations.
In medium pressure system normally screw feeder (also termed screw pump) is used for feeding materials from the feeding hopper / silo to the conveyor pipe. Screw feeders are suitable for safe (no leakage of air through hopper) operations upto an air pressure of 2.5 atmospheres gauge to 3 atmospheres gauge. For high pressure system, feeding of material is done through blow tank (also called chamber feeder) type line charger. In these medium and high pressure systems, the materials flow takes place in the dense phase i.e. the mass flow ratio of material particles to air is over 15. The low pressure systems operate at dilute phase i.e. mass flow ratio being less than 15.
Selection of pneumatic handling system depends on properties of materials like bulk density, particle size, material characteristics like moisture content, abrasiveness, corrosiveness, fragility etc. and other factors like distance of conveyance, environmental considerations etc. The air velocity necessary for pneumatic conveying depends on the bulk density and size of the material.
Air-activated gravity conveyors (air slide) – Dry powdered materials, when aerated acquires fluidity and can move along a plane having a slope of only 4 % to 5 %. This principle is adopted in an air slide for short distance movement of powdery materials at a fairly controlled rate. The air slide consists of an inclined covered metallic trough with a longitudinal porous partition in between. Over the partition the material moves. Low-pressure air is allowed to enter from the bottom of the porous partition. The air fluidizes the powdered / pulverized material and makes it flow along the inclined partition due to gravity.
Air slides are normally used for controlled movement of materials i.e. as feeders. For their simple design, low costs, low power consumption, high handling capacity combined with small overall dimensions, simple erection and adjustment and absence of moving and wearing parts, they are, in many cases, preferred over screw or vibrating feeders. Materials such as cement, ash, coal dust etc. can be effectively conveyed by air slides.
Tube conveyor – When small piece goods in standardized containers, called carriers, are transported pneumatically over short distance through pipe, it is called tube conveyor (Fig 4). This type of transportation of piece goods is used for sending samples to the laboratories and in telegraph offices, banks, stores, and newspaper offices. The pipes of these installations can be round (upto 100 mm diameter) or elliptical. The carriers are moved at speeds range of 6 mps to 12 mps.
Moving bulk materials along pipes or channels (troughs) in a stream of water is called ‘hydraulic conveying’. The mixture of materials and water is termed as pulp. Pump is used for conveying of pulp through pipe under pressure. In channels the conveying takes place down the inclination due to gravity.
A hydraulic conveying system normally consists of a mixer where the material and water is mixed to form the requisite pulp. Depending on starting size of the bulk material, the materials can have to be crushed / ground in a crushing plant and screening facility. The prepared pulp is then pumped by a suitable pumping and piping system. In certain installation a suitable recovery system can be incorporated at the delivery end for dewatering the material. The most important consideration in a hydraulic conveying system is that the material is not to get settled and choke the pipeline.
Hydraulic conveyors are used in many industries, mining operations and construction works. Some of the popular uses are to dispose ash and slag from boiler rooms, deliver materials from mines and sand and water to fill up used mines, to remove slag from concentration plants, to quench, granulate and convey furnace slag to disposal points, to move earth and sand in large construction projects and for land filling etc.
The advantages of using hydraulic conveyors are (i) high capacity of materials can be conveyed over a considerable length (tens of kilometers), (ii) comparatively simple and low running cost equipment are used, (iii) conveying can be combined with other processes like cooling, quenching and granulating of molten slags, washing etc., (iv) flexibility in selection and subsequent modifications to route, (v) conveying process is normally safe and easily controllable, and (vi) low maintenance costs. The disadvantages of hydraulic conveying system are (i) materials which can be handled are quite limited, only smaller sized bulk materials which do not react or get dissolved in water can be conveyed in a hydraulic conveyor, (ii) cannot be used in cold conditions when water can freeze, (iii) increased humidity when operating in close environment, (iv) disposal or recirculation of water is frequently difficult and / or costly, (v) crushing and mixing are added power intensive operations, (vi) degradation of some materials due to attrition, and (vii) choking of pipelines particularly at bends / fittings.