Conveying is the method of transferring materials from one place to another generally by mechanical means. The use of conveying process are generally in all stages of manufacturing process including harvest and transportation of raw materials to store, preparation of food, movement of food or produce within the factory and movement of finished product to warehouse. It can also be used for presentation of products for selling. Efficient handling is the organized movement of materials in correct quantities to and from correct place accomplished with minimum time, labor, waste, expenditure and with maximum safety.Benefits of good conveying system:
1. Good utilization of labor
2. Good utilization of space
3. Reduce wastage
4. Improve stack control
5. Improve productivity
6. Lesser damage to product
7. Low risk of accident
8. Improved working condition
It is important to plan material handling system at the initial design layout of factory in order to maximize its benefits. There are large number of conveyor designs produced to meet specific application but all types can cover only a fixed function or operations. Some examples of conveyors are described below.
Pneumatic conveyor: pneumatic conveying is an excellent method for transportation of powders which are easily flow able. The conveying system uses a gas, usually air to transport the material. It consist of system pipes through which powders or small food particles are suspended in recirculated air and transported at up to 25 – 35 m/sec in pipes of 5 – 90 cm diameter.
The air velocity is critical in this type of conveyor as too low velocity causes settling of particles and block the pipe, whereas if it is too high, there is risk of abrasion and damage to internal pipe surface. Generation of static electricity by movement of food is the potential problem that would result in dust explosion when conveying powders. It is prevented by earthing the equipment or venting or using explosion contaminants ex; inert gas and other suppression techniques. Usually air pressure is maintained at 700 kpa (7 atm). Pneumatic conveying can be done by following methods
1. Pneumatic conveying under vacuum
2. Pneumatic conveying under pressure – vacuum condition
3. Pneumatic conveying by fluidization method
Belt conveyor: These are the endless belt which are held under tensions between two rollers one of which is driven. The belts may be stainless steel mesh, rubber or composite material made by canvas, polyethene or polyester. Flat belts are used to carry packaged food whereas trough shaped are used for loose shaped materials; example flour and liquid egg. In some designs belt moves on roller for easy conveying. Belt conveyors are universal transportation equipment to carry heavy loads as high as 5000 ton per hour (tph) and belt travels with speed of 5 m/s.
It is normally not possible to change the direction of flow with belt conveyors. To change direction, another belt conveyor in changed direction is used. In some cases, slat are used to bend the direction. Belt conveyors are also used to with magnetic pulley arrangement to separate magnetic materials and iron pieces if there are any from food material, while transporting the material.
Advantage of belt conveyor
1. High machine efficiency
2. Lower product damage
3. Easily cleanable so that materials are not mixed when the same conveyor are used for different products
4. Less noise, low power requirement and large carrying capacity.
Disadvantage of belt conveyor:
1. Limit to angle of inclination
2. High initial capital cost
3. Non portable
Screw conveyor: Screw conveyors are used for transportation of solids up to maximum capacity of 4.7 m3/min over long or short distance. Preferably they are used for short distance within the process plant or for feeding of raw material into hopper at predetermined rate. They consist of trough or long channel into which the screws in the form of helicoid flights rotate by means of electric motor. Generally screws rotate with low rpm. The material is fed at one end. As the screw rotates, the flight push forward the material in the screw conveyor. The flow rate in the conveyor depends upon the pitch of the flight. One rotation will move the material forward by one pitch length.
The rpm of the shaft indicates the number of pitch lengths moved in one minute. The screw conveyors are frequently used as feeders for feeding the raw materials for the process plants like feeding of wheat from storage silo into grinding mill to make powder. They are also used to convey bulk dry materials like sugar flour or grains and non-abrasive materials like pulps or comminuted meat over distances.
Advantage of screw conveyor:
1. Have uniform flow rate
2. Totally enclosed, prevention from outer contamination
3. Flexibility with inclination
Disadvantage of screw conveyor:
1. They are generally limited to 6 m length
2. Higher frictional forces, resulting excessive power consumption.
Bucket elevators: bucket elevators are used in food processing industry for lifting the solid food materials either vertically or in inclined position. It consists of series of buckets connected to an endless belt or chain. The belt or chain will be moving and hence bucket will also move upward. Spacing of the bucket and speed of conveyor (15 – 100 m/min) control the flow rate of material.
As the bucket moves upward, bottom bucket occupies the position of feeding point. Thus the movement continues. The material discharge could be by centrifugal force or by simply dumping tilting buckets. Bucket elevators are commonly used for free flowing materials like sugar, beans, salt, cereals etc. These are extensively used in wheat milling industry or paddy processing units for lifting the cereals from ground level to feed into the mills at higher level.
Application of conveyors in food processing industries:
Animal products processing:
1. Conveying slaughtered carcass — overhead monorail
2. Conveying fish and shrimp and prawns — hydraulic conveyor
3. Conveying packaged unit — belt/ chain conveyor
Cereals and pulses
1. Lifting paddy in rice mills — bucket elevators
2. Conveying wheat and other cereals in grinding mills — bucket elevators/ screw conveyors
3. Conveying flour for dough mixing — screw conveyor / pneumatic conveyor
Fruits and vegetables processing
1. Conveying fruits from storage to processing section — hydraulic conveyor
2. Conveying clean fruit to pulping unit — belt conveyor
3. Conveying final product to storage — roller conveyor / cranes/ trolleys
Spices and condiments processing
1. Conveying dried spices to grinding unit — pneumatic conveyor
2. Conveying ground spices to processing unit — screw conveyors
Confectionary and convenience food
1. Conveying sugar, flour, milk powder, cocoa powder — belt conveyor
2. Conveying final product to packing unit — belt conveyor
Miscellaneous application
1. Transportation of final packaged cartons — roller / belt conveyor
2. Feeding raw material from silos to processing operation — screw conveyor/ vibratory conveyor
Useful links:
How monorail works? https://www.youtube.com/watch?v=li4Y2Xnbuj4
How skate wheel conveyor works?
Different types of belt conveyors
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.