Dryers

Question 1

What is the difference between direct and indirect heating

Answer 1

In direct heating the material is heated mainly by convection from the surrounding air
With indirect heating the heat is transferred by conduction through the material

Question 2

Why is material dried

Answer 2

• reduce cost of transport
• to make the material more suitable for handling
• to provide definate properties
• to avoid the presence of moisture which may lead to corrosion

Question 3

Explain vacuum operation

Answer 3

Where lengthy drying times are used, it s necessary to operate under vacuum in order to maintain evaporative temperatures at acceptable levels

Question 4

Explain the operation of tray and tunnel dryers

Answer 4

Material is placed on a series of trays, which may be heated from below by steam coils and drying is carried out by circulation of air over the material

Question 5

Explain the operation of rotary dryers

Answer 5

Long cylindrical shell mounted on rollers and at a slight angle so that the material fed in travels through the cylinder under gravity

Question 6

Explain the operation of drum dryers

Answer 6

A solution of slurry is run on to a steam heated drum which is slowly rotating.

Question 7

Explain the operation of spray dryers

Answer 7

The mixture is sprayed into a vessel through which a current of hot gases is passed. This gives a large interfacial area and high rate of evaporation

Question 8

Explain the operation of pneumatic dryers

Answer 8

The solid is introduced into the dryer by some form of mechanical feeder. Hot gases from a furnace are passed into the bottom of the dryer, and these pick up the particles and carry them up the column. The stream of particles leaves the dryer through a cyclone separator and the hot gases pass out the system.

Question 9

Explain the operation of fluidised bed dryers

Answer 9

Heated air or gas is passed via a plenum chamber and diffuser plate into the fluidized bed of material from which is passes to a dust separator

Question 10

Explain the operation of turbo shelf dryers

Answer 10

Used for handling sticky materials. The wet solid is fed in a thin layer to the top member of series of annular shelves each made of a number of segmental plates with slots between them. These shelves rotate and the material is pushed through a slot to the shield below by suitably placed arms. After repeated movements the solid leaves at the bottom of the dryer.