The science behind moving bed heat exchangers
A primer to better understanding how the equipment handles free-flowing granular solids
Author: Lowy Gunnewiek
Moving bed heat exchangers (MBHE, MBHX) are found in a wide variety of applications where the primary function is to heat or cool free-flowing granular solids. As the name implies, there is a “bed” of granular solids that flows by gravity down past solid surfaces that act as the interface for thermal energy transfer to or from a working fluid such as water.
MBHEs are generally simple and robust pieces of equipment with few moving parts and low energy requirements. However, a detailed understanding of bulk solid material properties – and the uniform mass flow of solids – is required to achieve effective and efficient heat transfer.
Inside the box
The flow of solids through the heat exchanger is controlled by a discharge feeder, and there are a variety of devices that are commonly used for this purpose.
The separation of the moving bed of solids from the heat transfer working fluid can be provided by vertically oriented plates (e.g., pillow plates, dimple plates, plate-coils, etc.), vertically oriented tubes or horizontally oriented round or oval shaped tubes. For a MBHE based on a vertically oriented tube configuration, the solids can flow either inside the tubes or outside the tubes. Depending on the configuration of the heat exchanger, the heat transfer fluid flow configuration can be counter-flow, co-flow or cross-flow.
Heating, cooling and drying
Achieving the desired heating or cooling of the solids also requires managing the gas (typically air) that occupies the pore space in the moving bed of solids. This an important aspect of the MBHE design, and some type of aspiration system will be required.
For example, during cooling, the dew point temperature needs to be carefully controlled as condensation can cause caking and fouling inside the heat exchanger and/or reduce the quality of the product. During heating, moisture may be released from the solids. This moisture must also be removed from the heat exchanger to prevent fouling, caking and other operational problems.
For some applications of MBHEs, it may be desirable to do more than just heating or cooling. This could include drying the solids or preventing certain chemical reactions from taking place. For these applications, the management of the pore space in the moving bed of solids is highly engineered and controlled.
The transport phenomena occurring inside a MBHE are intricate and complex. Some of these include momentum and conduction energy transfer for a solid particle in contact with a wall and with other particles, the convection, conduction, radiation and diffusion processes that takes place in the moving bed and many more.
In addition, the thermal properties of both the solids and the gas will be a function of the temperature in the heat exchanger, as will be the mechanical properties for the solids.
Experts in MBHE design
Given this complexity, the design of MBHEs must be based on empirical data and experience. With more than 30 years of MBHE design experience and more than 800 heat exchangers installed, Solex Thermal Science has developed the tools and know-how to be experts in MBHE design. With an extensive proprietary material property database and an in-house developed software design and sizing tool validated using empirical data, customers can be confident in the heat exchange solution that Solex provides.
Ready to talk specifics? Connect with a Solex representative today.
Lowy Gunnewiek, Chief Executive Officer
Lowy joined Solex as an independent director in 2018, and became CEO in 2019. He has more than 30 years of experience as a senior executive and professional engineer in the international energy, mining and minerals and infrastructure sectors. He provides leadership in corporate direction and strategy, and is responsible for providing best-in-class technology solutions for Solex’s clients.
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