Thermal drying is a unitary operation where liquids are removed from a product by addition of heat.
The resulting product can be completely free of moisture or not depending on the requirements of the process.
As a consequence that so many different products can be processed by thermal drying, different types of dryers have been developed to treat them.
Direct heat dryers, where the heat transfer occurs by convection of hot air or gas inside a rotary cylinder, are the most common and used thermal dryers.
Hot air or gas flows through the cylinder and then transfers to the product, warming it and evaporating the water, therefore temperature of heating media inside the dryer is the mean parameter which control the process.
In convective rotary dryers the product is dispersed in the hot air or gas by means of lifting flights welded to the cylinder and equally spaced circumferentially.
Lifting flights are of different shape which assures the wet product cascading from the top of the cylinder is exposed to the flow of hot gas. Flights shape is studied to optimize the thermal transfer from hot gases to wet product to reduce the size of the cylinder.
At feed end direct heat dryers are equipped with spiral flights to assure the quick movement of wet product in the initial part of cylinder before coming in the active length of the dryer.
The rotary dryer can operate both in concurrent or countercurrent flow arrangement.
Permissible velocities of gases inside the cylinder are selected as a function of specific gravity and particle size of the product to be dried.
Direct Rotary Dryers can use as heating media:
- hot gases, for drying sand, clay and mineral and chemical products
- hot air, produced by heat exchangers, for the drying of heat sensible products and which cannot come in direct contact with flue gases.
Products that can be processed by convective rotary dryers:
- Fertilizers (ammonium sulfate, ammonium nitrate, potassium nitrate, copper sulfate)
- Minerals (sand, pumice, clay, limestone, basalt, rutile, fluorite, chromite, borax, barite, mercury concentrate, sea salt)
- Polymers (polyvinyl, resins)
- Blast furnace slags
- Choline chloride
- Brass and stainless steel chips
- Carbon coke
- Municipal and industrial sludges
The rotary unit consists principally of a revolving cylinder, slightly inclined to the horizontal,
supported in two riding rings resting on two trunnion rolls each, with the trunnion roll bearings
mounted on structural steel bases.
On either side of one of the riding rings is an adjustable thrust roll for holding the cylinder in its longitudinal position.
The cylinder is rotated by means of a girt gear meshing with a spur pinion mounted on the slow speed shaft of a speed reducer.
The high shaft of the reducer is directly coupled to an electric motor.
The interior of the cylinder is fitted with spiral flights at the feed end to quickly move the feed product into the active section, where longitudinal parallel lifting flights pick up the product and cascade it in thin even sheets, so that it will dry most advantageously.
The cylinder is arranged so that the hot gases and product pass continuously in same directions, that is cocurrent flow.
The time of passage of the product thru the rotary unit is controlled by the slope of the cylinder to the horizontal the rotational cylinder speed, and the velocity of the gases thru the cylinder.
At the discharge end, the cylinder is enclosed by a breeching with seals.
The breeching is hoppered and flanged for attaching the Purchaser’s receiving equipment.
At the feed end, the cylinder is provided with an inlet head equipped with feed chute and seals.
The front of the breeching is flanged for connection to the hot gases generator.
Both discharge and feed inlet head are supported by structural steel supports.
The hot gases for the drying process are produced in a temperature controlled gas generator equipped with fuel burner.
Combustion chamber is refractory lined and combustion air cool outside shell of the chamber and at the end of the chamber conditioning air is added to hot gases to keep operating temperature requested.
The system is automatically controlled and flame supervision is provided for the burner.
The exhaust gases and removed water vapour are dedusted and cleaned in accordance with local regulation and vented to atmosphere.