Direct heating is as simple as it sounds. This technology involves a bulk solid material making contact with direct heat to change the temperature of the material, while indirect heating technology does not allow the product to come in contact with air, gas, or any other heating agent to change the final temperature. However, as simple as the difference sounds between these two manufacturing processes, there are several advantages and disadvantages to both.
Direct Heating Technology
Direct bulk solid heat exchangers use vertical, enclosed hopper bins or silos that allow for the injection of gas, most commonly oxygen, to directly heat or cool the free-flowing material passing through the equipment. The gas and products, such as powder, are both fed into the heat exchanger in two streams at different temperatures.
The air can then either flow to the bulk solid, fed from the bottom upwards, or travel across the material as it flows down. Unlike other processes, bulk solid direct heat exchangers have no moving parts, lowering installation and maintenance costs.
When it comes to direct contact heat transfer systems, large amounts of air are required for the process to achieve sufficient heating, cooling, or drying. This requirement results in the need for air handling 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 fluid beds can consume over 600kW of energy for a direct heating application to achieve a certain temperature change.
Unfortunately, many of these direct heat transfer systems also result in high dust and emissions. In order to comply with pollution and environmental controls, all emissions must be cleaned before being released back into the atmosphere.
Indirect Heating Technology
As technology has advanced, a more energy-efficient and cost-effective method has been introduced, using indirect heat transfer to heat free-flowing bulk solids. This unit uses a flow of heating water or fluid running through the hollow plates to achieve indirect heat transfer through conduction. As free-flowing bulk solids, such as sugar or fertilizer, flow downward, the heat transfer occurs between the plates and the material. Unlike direct heating, indirect heating technology is environmentally responsible, as there are minimal emissions released during the indirect heat transfer process.
The water that is used during this process can often be repurposed for other manufacturing processes. Manufacturers like Solex Thermal Science use their innovative design to provide better, more efficient solutions to heat exchanger technology issues. Check them out today to see examples of heat exchanger improvements and developments (and how this innovator produces nearly zero emissions, uses 90% less energy, and produces a superior end product at a lower cost for their customers).
Evidently, the primary difference between direct and indirect heat transfer systems is the process and amount of air and energy required to achieve heat transfer. Heating, processing, cooling, and cleaning air results in high energy consumptions during direct heating processes.
With indirect heating technologies, no air is required, reducing energy usage. As technology continues to advance and more manufacturers develop innovative processes to significantly reduce energy consumption, indirect heat exchange systems are becoming a more viable solution for mineral, commercial agricultural, and powder industries.