A guide to optimizing flow control in bulk solids heat exchangers
Different discharge methods and the role they play in indirect heat transfer technology
Authors: Lowy Gunnewiek, Peter Menchenton, Caroline Richard and Francisco Castellano
So what is the big deal about flow control in bulk solids heat exchangers? With gravity providing the motive force to get the solid granules, pellets, beans, seeds or particles from the top of the heat exchanger to the bottom discharge, you would think its all pretty straightforward and simple.
Unfortunately, that is not reality. A great deal of effort goes into ensuring uniform mass flow conditions are achieved. That is, the bulk solids must move at a constant and uniform velocity from the top of the heat exchanger down to the bottom.
And why is uniform mass flow important?
For indirect heat transfer technology, uniform mass flow plays a critical role in ensuring all the product that moves through a heat exchanger has the same residence time – and, this ultimately ensures the product has a uniform and consistent temperature profile at the outlet.
There are numerous factors and operating conditions that affect uniform mass flow, with the method of discharging material having the biggest role to play. Particle size, particle characteristics and throughput rates rate are just some of the criteria that must be considered when determining the optimal method of discharging solids form the heat exchanger.
The presentation of the bulk solids to the discharge feeder plays a significant role in achieving the desired flow behaviour, as well as how you transition from the bottom of the heat exchanger to the discharge feeder. In addition, spatial constraints and the equipment used to convey the bulk solids after passing through the heat exchanger should be considered when determining the type of equipment needed to do the job.
Here’s a quick guide on some of the more popular and proven discharge options available today. For more information, contact a Solex representative.
Also referred to as a louvered-style, the vibrating discharge feeder accepts product under a ledge formed by a top louver and forms a pile on top of another louver below. When vibration is initiated, the pile flows off the bottom louver. Imagine that as the feeder moves back and forth, the product is “sliced” off the edge of the louver.
Several factors impact the flow rate in a vibrating discharge, including frequency, amplitude, overlap-to-gap ratio and angle of the bottom louver. This type of device is best suited for free-flowing granules and non-cohesive powders that can fluidize. Due to the louver positions being fixed (based on specific product characteristics), a vibratory discharge cannot accommodate large changes in product characteristics (particle size, angularity, moisture, etc.) or changes in product (changing from one material to another).
This is a type of vibrating discharge with louvers that can be rotated. Because the opening between the louvers is adjustable, the Hogan design is better situated to accommodate changing products or characteristics.
However, the addition of more moving parts makes it more costly to maintain than a fixed louver vibrating discharge. It also has difficulty handling low flow rates because of its shallow minimum louver angle.
In this case, the product flows by gravity through a slot formed by the gate. The gate discharge regulates the flow of product by adjusting the width of that slot opening – meaning the wider the opening, the higher the flow rate it can accommodate (one of its primary advantages).
The main disadvantage is it doesn’t fare well for products with a large size distribution, or for finer powders. However, it is simple to operate and maintain.
Gate discharge (inset, inside of gate)
The slide gate is formed by two parts: one fixed and another that slides in a back and forth motion. The fixed part is made up of small hoppers that guide the product toward the ladder-shaped sliding part that, due to its horizontal back and forth movement, loads and unloads the product in the pockets existing between the bars. The ladder is powered by an electric actuator that allows total control of both the speed and amplitude of the oscillations, and can even operate intermittently. Movement is slow so wear is minimal and the throughput is directly proportional to the number of cycles per unit of time.
The slide gate is recommended for low flow rates, as well as when there are height limitations. In the event of a power failure, there is no product free flow. It also works with any particle size or product angle of repose.
Slide gate discharge
The Oszillomat discharge consists of a series of horizontal square beams that rotate back and forth along their length. The beams are connected via linkages to move in unison based on the position of a linear actuator (e.g., hydraulic cylinder). When the beams are at “home position,” their edges are in contact and bulk solids cannot pass through. By moving the linear actuator back and forth, the beams rotate, creating gaps that allow solids to pass through. By controlling the speed and position of the linear actuator, the flow rate of bulk solids can be controlled.
For solids with poor flow characteristics, the linear actuator can be continually moved, causing the gaps between the beams to grow and shrink. This allows the beams to actively agitate the product bed to promotes the flow of the bulk solid. As a “live bottom” feeder that can extend across the entire footprint of a heat exchanger, the Oszillomat is ideally suited to products that would otherwise require very steep mass flow cone designs or situations where available height is limited.
A screw discharge, unsurprisingly, looks like a typical screw. In bulk solids applications requiring mass flow conditions, screws feeders feature flighting that continuously expands along the length of the discharge to continuously and uniformly pull the product from the heat exchanger.
This type of discharge device is typically used where there are long, narrow openings or the granular solids need to be conveyed as it is discharges from the heat exchanger. It provides good flow rate control and can be designed many ways depending on product characteristics. On the flip side, uniform mass flow is sensitive to the flight design and shaft diameter.
The cone is a simple and primarily static discharge device. For this device, bulk solids flow from the bottom of the heat exchanger into a cone that sits above a conveying device with a specified gap between the bottom of the cone and the conveyor. It is similar to the circle discharge in that the gap size and speed of the conveyor controls the rate of discharge. The gap size can be adjusted to control the flow rate of solids through the heat exchanger.
This type of discharge can be used when the bulk solids are fully free flowing and there is adequate height to accommodate the cone at the bottom of the discharge. The cone requires little to no energy to operate and is virtually maintenance free.
This discharge device consists of a cylindrical column that sits above a flat plate, with a controlled opening between the column and the flat plate. The product flows through the column onto the flat plate. The plate is rotated causing the product to flow through the gap between the column and the plate and into a doughnut-shaped chamber from where it is discharged to a conveying system.
The gap between the column and the plate is adjustable, as is the speed of the rotor – making a circle discharge ideal for a variety of products and flow rates.
A rotary valve provides excellent control for materials that may exhibit fluidization or flooding behaviours (where bulk solids flow like liquids). Consisting of a fixed housing enclosing a rotating, lobed shaft, rotary valves provide flow rate control while also providing a seal between the heat exchanger and the downstream material-handing system. This is a requirement when using pneumatic conveying systems or when it is important to restrict the ingress of ambient air into the heat exchanger.
The shaft of a rotary valve is typically driven by an electric motor via a gear reduction system. The rate of discharge of the bulk solid can be controlled by adjusting the speed of the drive motor. Rotary valve discharge capacity is governed by the size of the rotary valve (diameter of the rotor), the depth of the lobes (pocket volume), and the speed of rotation.
Although rotary valves can be used for a wide range of throughputs, they are an excellent option for extremely small flow rates.
Did you know?
Solex has more than 30 years of experience in bulk solids heat transfer technology. We have a proven track record of success with more than 600 installations in more than 50 countries. Our experience spans across industries, including everything from fertilizer to foundry sand, oilseeds to olive pits and sugar to sand.
Ready to talk specifics about your application? Contact a Solex team member today.
This entry was last updated on 2021-5-31
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