How Does Direct Heat Work?

The Louisville Rotary Direct Heat Dryer is a continuous type dryer which operates primarily through convective heat transfer.

Wet material, which is to be dried, is fed into one end of a rotating drum and travels to the other end where it discharges as dry product into a transfer conveyor. Heated air and usually some combustion gases are introduced into the dryer. As the process material travels the length of the drum, it is picked up by lifter flights, which are attached to the drum shell. These flights are designed to shower the material so that the hot gas can make contact and transfer the heat from the heated air into the material being dried. As the material and the water are heated, the water vaporizes and is swept out by the gas stream.


Components in a direct heat rotary dryer include:

1. Process air heater to provide the hot gas.

2. Rotating drum to tumble and lift the material into the gas stream.

3. Air-exhaust system to remove the vapor-laden air.


Rotary direct heat dryers are used to dry and heat a wide range of both organic and inorganic products.

direct heat dryer
direct heat dryer Louisville Dryer

Process Air Heater:

The thermal heat load which drives a direct heat dryer is carried in a hot gas. This gas can include air or a combination of air and combustion products.


Low-Temperature Drying:

In some cases, the material to be dried is sensitive to high temperature or cannot tolerate the presence of combustion products. Some specific examples include the polymers, certain plastics and live organisms such as yeast. It is customary for such processes to utilize steam coils to heat the thermal gas. Air is blown through a steam coil which heats the air. The heated air is then induced into the dryer to deliver the heat. The air temperature is easily controlled by varying the steam pressure.


In most direct heat dryers a dry combustion system is employed to provide the hot gas. The combustion air heater usually includes:

1. Refractory lined combustion chamber

2. Burner with combustion air blower

3. Process air fan (or quench air fan)

The dryer gas temperature is determined based on the temperature limits of the processed material. The burner and combustion air fan are selected to provide the calculated heat load or BTU value. The process air fan delivers the quantity of air sufficient to cool or quench the hot gas to the desired dryer gas temperature.

As the evaporation occurs in the dryer, the gas stream is further cooled. The air and water vapor are induced from the dryer to the air pollution control system and usually exhausted to atmosphere.

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