24-10-2016, 11:55 AM
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INTRODUCTION
Drying is defined as removal of the moisture from material by the application of heat and it is accomplished by transfer of the moisture from the surface into an vapour phase.
Drying is when we reduce the moisture in the grain until it becomes safe to store the grain. Dried grain can be stored much longer than wet grain.
Drying is commonly the last stage in a manufacture process.
It is the final removal of water from material (usually by heat)
The rotary drier is basically a cylinder, inclined slightly to the horizontal, which may be rotated, or the shell may be stationary, and an agitator inside may revolve slowly.
2)In either case, the wet material is fed in at the upper end, and the rotation, or agitation, advances the material progressively to the lower end, where it is discharged.
Air flow may be parallel or counter current.
3) This method is suitable for sticky materials.
4) Damage caused by impact & abrasion in drier restrict this method to relatively a few foods.
Double drum dryer
1)Food is spread over the surface of heated drum.
2) These drums are made of hollow steel drums heated internally by pressurized steam at 120- 170ºC.
3) A thin layer of feed is uniformly spread over the outer surface, by spraying
4)Usually drying time taken for one complete revolution is 20 sec to 3 minutes.
5)A dried deposit can be scrapped off with the help of flexible or adjustable knife.
6) It may regarded as conduction drying.
Fluidized bed dryers
1) Food material is maintained & suspended against gravity in an upward-flowing air stream. The air acts as the drying medium.
2) maximum surface area of food is made available for drying.
3) Heat is transferred through convection.
4)Fluidized bed dryers are compact & have good control over drying conditions.
5) They have relatively high thermal efficiencies and high drying rates.
6) They are limited to small particulate foods that are capable of being fluidized without excessive mechanical damage such as: peas, diced or sliced vegetables, powders etc.
Spray Dryer
It is a method of producing a dry powder from a liquid or slurry by rapidly drying with hot air.
This is preferred method of drying of many thermally sensitive materials such as foods.
The spray dryer provides a large surface area for heat and mass transfer by atomizing the liquid to small droplets.
These are sprayed into a stream of hot air (150- 300ºC), so that each droplet dries to a solid particle.
Advantages of the spray drying process
1)The droplets are small, giving a large surface area for heat transfer, so that evaporation is very rapid.
2) The characteristic particle gives the product ready solubility.
3) The powder will have a uniform and controllable particle size.
4) The product is free-flowing and is especially convenient for tablet manufacture.
5) Labour costs are low, the process yielding a dry, free-flowing powder from a dilute solution, in a single operation with no handling.
Disadvantages of Spray Dryer
It is bulky and large equipment.
It is not always easy to operate.
Tunnel dryers:
Tray or trolleys move through a tunnel where heat is applied and vapors removed.
Product moves through tunnel at a rate required to maintain residence time needed for dehydration.
Typically a 20 m tunnel with a capacity of 5000 Kg of food.
Ability to dry large quantity of food in a relatively short time (5-16 hours) made it widely used.
Conveyor belt Dryer
Continuous conveyor dryers are up to 20 m long and 3 m wide.
Food (mostly fruits & vegetables) is dried on a belt 5-15 cm deep.
INTRODUCTION
Drying is defined as removal of the moisture from material by the application of heat and it is accomplished by transfer of the moisture from the surface into an vapour phase.
Drying is when we reduce the moisture in the grain until it becomes safe to store the grain. Dried grain can be stored much longer than wet grain.
Drying is commonly the last stage in a manufacture process.
It is the final removal of water from material (usually by heat)
The rotary drier is basically a cylinder, inclined slightly to the horizontal, which may be rotated, or the shell may be stationary, and an agitator inside may revolve slowly.
2)In either case, the wet material is fed in at the upper end, and the rotation, or agitation, advances the material progressively to the lower end, where it is discharged.
Air flow may be parallel or counter current.
3) This method is suitable for sticky materials.
4) Damage caused by impact & abrasion in drier restrict this method to relatively a few foods.
Double drum dryer
1)Food is spread over the surface of heated drum.
2) These drums are made of hollow steel drums heated internally by pressurized steam at 120- 170ºC.
3) A thin layer of feed is uniformly spread over the outer surface, by spraying
4)Usually drying time taken for one complete revolution is 20 sec to 3 minutes.
5)A dried deposit can be scrapped off with the help of flexible or adjustable knife.
6) It may regarded as conduction drying.
Fluidized bed dryers
1) Food material is maintained & suspended against gravity in an upward-flowing air stream. The air acts as the drying medium.
2) maximum surface area of food is made available for drying.
3) Heat is transferred through convection.
4)Fluidized bed dryers are compact & have good control over drying conditions.
5) They have relatively high thermal efficiencies and high drying rates.
6) They are limited to small particulate foods that are capable of being fluidized without excessive mechanical damage such as: peas, diced or sliced vegetables, powders etc.
Spray Dryer
It is a method of producing a dry powder from a liquid or slurry by rapidly drying with hot air.
This is preferred method of drying of many thermally sensitive materials such as foods.
The spray dryer provides a large surface area for heat and mass transfer by atomizing the liquid to small droplets.
These are sprayed into a stream of hot air (150- 300ºC), so that each droplet dries to a solid particle.
Advantages of the spray drying process
1)The droplets are small, giving a large surface area for heat transfer, so that evaporation is very rapid.
2) The characteristic particle gives the product ready solubility.
3) The powder will have a uniform and controllable particle size.
4) The product is free-flowing and is especially convenient for tablet manufacture.
5) Labour costs are low, the process yielding a dry, free-flowing powder from a dilute solution, in a single operation with no handling.
Disadvantages of Spray Dryer
It is bulky and large equipment.
It is not always easy to operate.
Tunnel dryers:
Tray or trolleys move through a tunnel where heat is applied and vapors removed.
Product moves through tunnel at a rate required to maintain residence time needed for dehydration.
Typically a 20 m tunnel with a capacity of 5000 Kg of food.
Ability to dry large quantity of food in a relatively short time (5-16 hours) made it widely used.
Conveyor belt Dryer
Continuous conveyor dryers are up to 20 m long and 3 m wide.
Food (mostly fruits & vegetables) is dried on a belt 5-15 cm deep.