DIRECT VERSUS INDIRECT HEATING TECHNOLOGY

Quite simply, direct heating technology involves a bulk solid

material coming into direct contact with a heating gas to change

the nal temperature of the material. On the other hand, indirect

heating technology does not allow the product to come in

contact with air or gas or any other heating agent to change

the nal temperature. Indirect heat transfer uses conduction,

radiation or convection to achieve temperature change. There are

various types of technologies used in both the direct and indirect

rapidly. As the product (ex. powder, fertilizer, sugar) enters the

drum dryer, the rotating mechanism lifts the material up onto

a series of internal ns lining the outer walls of the dryer. As

the material ows toward the discharge end, it falls off the ns

and passes through the hot gas stream within the rotary drum,

removing moisture. The gas owing through the drum can ow in

either a co-current or counter-current direction.

Rotary drum indirect

heat transfer

Rotary drums can also be used in niche applications as an indirect

rotates, heating the outer shell and in turn the material inside.

Fluid bed direct heat transfer

Another heat transfer technology is the uid bed. This technology

works well when a precise control of temperature and residence

time is required such as in polymer, chemical and pharmaceutical

industries. Fluid beds force air through perforated steel beds

that uidizes free owing bulk solids. As material is added to

one end of the uidized bed, it moves gradually from one end of

moving between the material particles, removing the moisture

and changing the nal temperature. Air has two functions in uid

bed technology; rst it creates the uidized state of the material

enabling the product to ow and second, it either heats or dries

the material through direct contact.

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SOLEX THERMAL SCIENCE INC.

250, 4720 – 106 AVE. S.E.

directly heat or cool the free owing material passing through

the equipment. The gas and bulk solid (ex. powder) are both fed

into the heat exchanger in two streams, at different temperatures.

The gas can either ow counter-current to the bulk solid, fed from

the bottom upwards, as the material ows down or cross-current,

in a perpendicular fashion across the material as it ows down.

For successful operation, adequate heat transfer, uniform ow,

proper outlet size, reliable gas ow and sufcient unit volume

are required. Unlike rotary drums or uid beds, bulk solid direct

heat exchangers have no moving parts, lowering installation and

systems, large fans, extensive ducting and emission stacks. Pre-

heating the air, as well as processing and cleaning the air, requires

high energy consumption. Rotary drums and uid beds can

consume over 600kW of energy for a direct heating application

of 100 tph to achieve a 25oC temperature change. While these

direct heat transfer systems have been in place for decades,

they are proving to be an inefcient use of resources, with a high

energy consumption.

The direct heat transfer systems also results in high dust and

emissions. All emissions must be cleaned before being released

exchanger. This unit uses a counter-current ow of heating water

or uid running through the hollow plates to achieve indirect

heat transfer through conduction. As a free owing bulk solid (ex.

sugar, fertilizer, powder) ows downward between the plates at

longer residence time, heat transfer occurs between the plates

and the material.

There are numerous advantages to using indirect heat transfer

• Increased energy efciency, as there is no need to use air in this

preheating boiler feed water or elsewhere within the plant.

Alternatively, waste energy from other plant processes can be

used as a heating uid within the indirect heat exchanger for a

cooling process.

Fluid Bed

Cooler

4 - 5

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retrots and new facility installs.

• Similar to the direct heat exchanger, this indirect system

has no moving parts, leading to decreased maintenance

energy consumptions in direct heating processes. While no air is

required in indirect heat exchangers, reducing energy costs and

allowing for the implementation of energy reuse and recovery

systems. With signicant efciencies in energy consumption

and process expenditure, indirect heat exchange systems are

becoming a more viable solution for mineral, agricultural and

To learn more about indirect heat exchange systems and how

they can be used in heating and drying free owing bulk solid

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