The anatomy of a moving bed heat exchanger
A look at the infrastructure that’s behind the science of indirect heat transfer of bulk solids
They represent the framework behind plate-based moving bed heat exchangers (MBHEs) – the bones, so to speak.
For as much science and engineering as there is behind the indirect heating, cooling and drying of bulk solids, there’s just as much thought put to the infrastructure and components in these MBHEs to get the job done.
So let’s dig deeper into what makes them tick. For this discussion, we’ll break down the MBHEs that Solex specializes in into three sections:
- Inlet hoppers
- Plate banks
- Discharge feeders
Starting at the top and working our way down . . .
The inlet hopper is the top section of the heat exchanger. Its key function is to uniformly distribute the product as it enters into the plate bank. The inlet hopper is also designed to provide surge capacity to accommodate normal plant fluctuations in product flow.
Inlet hoppers are modular in design, but can also be an integral part of the plate bank casing. This is simply known as an integral inlet hopper, and is used for certain applications.
The inlet hopper is made up of a number of different components:
The job of the inlet hopper casing is to contain the bulk solid product within the heat exchanger. It also forms the structure to which other sub-components are attached.
The cover contains inspection hatches, instrumentation ports, vent connections and product inlet connection.
Inspection hatches & access door
The hatches allow operators to safely view the interior of the inlet hopper during operation, while the access door allows for maintenance access. These openings can also be used to remove screen sections or remove foreign material that enters the heat exchanger but is too large to pass through the plate banks.
The purpose of the vent is to allow any injected air or gas – as well as any air, gas, or dust that enters the heat exchanger via the incoming product stream – to exit the unit. The vent also ensures the operating pressure of the heat exchanger is maintained at the desired set-point (often atmospheric pressure, but not always). It is typically connected to a dust collection system to prevent any dust entrained with the vent gas from contaminating the workplace . An adjustable vent damper may be included.
Tie-back lugs (not shown here) are used when there is a risk of seismic activity that could compromise the structural integrity of the heat exchanger. By securing the tie-back lugs to the plant structure, seismic loads can be transferred from the heat exchanger to the plant structure in a controlled manner. Meanwhile, lifting lugs are used to transport the inlet hopper during construction.
The level probe is used to determine the height of product in the inlet hopper. During normal operations, product levels in the inlet hopper are maintained to ensure the plate banks are completely covered with product. The operation of the discharge feeder and the supply feed system are controlled by the level probe.
This is where the free-flowing granular solids enter the heat exchanger. The product inlet will be connected to the plant’s feed supply system.
The screen prevents oversized material (e.g., lumps) from entering the plate bank section where it can cause damage and/or blockages. The screen must be periodically cleared.
Product bypass (not shown)
Inlet hopers may be equipped with a product bypass port depending on the configuration and control of the feed supply system. The by-pass prevents overfilling and allows oversized material accrued on the screen to bypass the unit with minimal operator intervention.
Product distribution device (not shown)
Product distribution devices are used to provide more uniform distribution of the incoming product within the inlet hopper and minimize size segregation. The distribution device will also help reduce the wear on the heat exchanger plates when there is frequent starting and stopping of operations. The distribution device can also provide additional protection to the level probe by diverting incoming product away from the probe.
Clean-in-place nozzle (CIP) (not shown)
The CIP nozzle(s) can be installed to support cleaning of the heat exchanger when it is required due to the nature of the product.
Instrumentation ports allow operators to monitor various product and operating conditions (e.g., temperature, pressure) as is required.
The plate bank is the section of a heat exchanger that houses the plates that provide for the indirect transfer of heat to/from the product from/to the working fluid. Plate banks are typically modular in construction and may be configured vertically in series, depending on the duty requirements of the heat exchanger.
The plate banks are made up many different components:
The casing forms the outer shell of the heat exchanger. In addition to containing the product, the casing is integral to the structural design of the heat exchanger, such as the base frame support channel.
Heat transfer fluid manifolds
These fluid manifolds deliver the heat transfer fluid coming from plant systems to the individual plates via flexible hoses. From the outlet of the plates, the fluid manifolds also collect the working fluid coming from each of the plates, again via flexible hoses, and return it to the plant systems. There is typically an inlet and an outlet manifold per plate bank.
Flexible hoses (not shown)
The hoses connect the manifolds to the plates. They allow for various plant operation-induced movements between the exchanger and manifold, thermal expansion. Importantly, they allow individual plates to be isolated and replaced, if needed.
Sealing assemblies (not shown)
Sealing assemblies are used to seal any openings in the casing to prevent product and or air/gas leakage, especially where the flexible hoses are attached to the plate nozzles. The sealing assemblies allow for relative movement between the plates, casing and manifold.
The plate are where the exchange of thermal energy between the solids and working fluids take place. Typically, the heat exchangers are designed to operate with the working fluid flowing counter-current to the downward flow of solids. The plates are fabricated by welding together two metal sheets, inflating them to create the flow passage for the working fluid, and with an inlet and outlet nozzle provided, connected to the manifold by the flexible hose.
Like the inlet hopper, the plate banks also are provided with an access door for inspection and removal of the plates, if necessary, as well as any maintenance and cleaning that may be required.
Support channel/plate supports
The support channel forms part of the integrated plate bank structure, distributing the weight of the plates from the plate supports to the structure of the casing and subsequently the plant structure. In addition to supporting the physical weight of the plates filled with working fluid, the structure must also accommodate the friction load from the solids flowing between the plates.
Plate supports, meanwhile, are located at the bottom of the plate bank and directly support the weight of the plates. They also act as spacers to ensure the required spacing between individual plates is maintained. The spacers themselves can be divided into two categories: fixed spacers, located at the top and rear of the plates, and removable spacers, located just inside the exchanger plate access door.
Manifold side access door
This door is located between the two manifolds on the rear of the plate bank. The door may be used for inspection and cleaning of the plate bank.
Inter-bank spool pieces (not shown)
Inter-bank spool pieces are used to connect the manifolds in series when needed. For example, the outlet manifold will typically be connected to the inlet fluid manifold of a plate bank module located above it.
Anti-abrasion plate caps
Anti-abrasion plate caps are installed along the top of the plates, typically in the uppermost plate bank of heat exchangers. They are used with highly abrasive materials (e.g. sand). The caps are considered sacrificial and prevent unnecessary wear of the heat exchange plates.
Sealed manifold covers
The covers are installed over the manifolds to prevent any air/gas that escapes past the nozzle seal assemblies from entering the plant atmosphere, and vice versa.
Air/gas injection headers/ports
Air/gas injection into the heat exchanger is provided when the dew point moisture profile in the heat exchanger needed to be managed (e.g., condensation during cooling.)The headers are typically connected to perforated pipe that extend the full width of the plate bank and deliver the air/gas into the moving bed of solids. Headers are equipped with clean-outs to allow for period removal of dust or debris, which may be deposited during normal operation of the heat exchanger.
Instrumentation ports (not shown)
The plate bank typically contains ports for temperature monitoring instruments above and below the plate bank, as well as pressure and temperature monitoring of the heat exchange fluid in the manifolds. Other instrumentation and viewing ports may be added as needed.
The discharge feeder, including any transition from the plate banks to the discharge feeder, is an integral component of a MBHE. This is because it controls not only the rate of flow through the heat exchanger, but also the nature of the flow. And for effective and efficient performance, uniform mass flow through the exchanger is a must.
For more information about discharge feeders, we encourage you to revisit our earlier blog post on the different options we use to create the conditions of uniform mass flow, depending on the application. Or learn more about our lineup of our standalone discharge feeders at our dedicated portal.
Note: Some installations might require the external mounting of insulation, which allows for improved operating efficiency, reduces the possibility of condensation or provides personnel protection in case of high-operating temperatures.
Do you have additional questions about how our heat exchange technology? Or do you need to assistance with servicing and maintenance? Our experts are here to help maximize operational availability.
Contact a member of our team today!
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This entry was last updated on 2023-10-10
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