Tech Corner | December 2017
How bulk solids flow patterns affect heat transfer in Solex heat exchangers:
Understanding bulk solids flow patterns is essential to troubleshooting issues with solids handling equipment. Subtle changes in how a material moves through your process equipment may drastically affect overall performance and operability of the equipment.
Solex heat exchangers are no exception to this phenomenon. In order to ensure uniform heating, cooling, or drying, each particle of material entering a Solex heat exchanger should have a similar residence time (total time spent in the equipment) to every other particle. To achieve uniform residence times, we design our equipment with discharge devices that provide uniform mass flow. Sometimes, whether due to wear on equipment, changes in product quality, or changes to processes, the flow pattern within an exchanger can change from uniform mass flow to mass flow, funnel flow, or even no flow (plugged). By understanding the differences between these conditions you can quickly find the root cause of the issue and get your process back to peak performance.
A no flow or plugged condition is often the easiest to identify. Whether due to mechanical bridging, the cohesive properties of a material, or caking, bulk solid material may begin to build up in process equipment. These plugs tend to extend throughout equipment over time and may eventually lead to a full blockage of the equipment. As the plugging extends through the equipment, bulk solid product flows more quickly through the remaining unplugged areas, resulting in a loss of residence time and therefor a substantial loss in performance. If the cause of the plugging is not addressed, the equipment may become fully plugged and no material will be able to be discharged from the equipment.
Funnel flow typically occurs in equipment which utilizes a transition hopper to decrease the footprint area above a discharge device, as would be the case in a conical hopper above a rotary valve. In a funnel flow pattern, a relatively small area of the equipment will see flowing product while the remainder of the product remains stationary. The stationary product may discharge when the equipment is fully emptied but you will typically see diminished, but stable, equipment performance similar to what may occur in a partially plugged situation.
When discussing bulk solids, mass flow is a condition where all particles within the equipment are in motion at the same time. This is an ideal condition for silos as it ensures that product is discharged from a silo in roughly the same order that it is introduced, reducing the chance of spoilage. Utilizing a mass flow pattern also changes how equipment is designed by allowing for more predictable calculations of stresses and reduced chances of shock loads from sudden shifts in product flow which can occur in a funnel flow situation. Equipment operating in a mass flow condition will typically perform well, but there may be signs of inconsistency within the discharged product, for example slight temperature gradients or differences in moisture content at different points across the product discharge stream.
Uniform Mass Flow
When operating in a uniform mass flow condition, not only are all of the bulk solids particles in a piece of equipment moving at the same time, as is the case in mass flow, they are also moving at the same velocity. This is a critical distinction as it ensures that each particle spends the same amount of time in the process equipment as every other particle, leading to exceptional uniformity of product at the discharge point.
How to Identify Flow Patterns in Solex Heat Exchangers
You can directly observe the flow pattern in a Solex heat exchanger by opening the inlet hopper inspection hatches and watching the product bed as the heat exchanger is emptied. The surface profile of the bulk solids in the heat exchanger should remain unchanged while the level decreases in the unit. Unfortunately, this type of observation often requires upsetting the operation of the equipment.
Sight glasses or transparent panels installed in the casing of the heat exchanger can also allow for direct observation of bulk solid flow. However, in addition to adding cost to fabrication, sight glasses may disrupt the flow you are trying to observe due to surface friction, electrostatic charge, or any small gap or ledge surrounding the sight glass.
In addition to direct observation, indirect identification of the flow pattern can be accomplished by marking a horizontal line around the exterior of the heat exchanger and measuring the temperature at different points along this line. The temperature measurements along this line can be used to identify different flow patterns as follows:
Identifying Uniform Mass Flow
If the temperature along a horizontal line on your heat exchanger is uniform, your heat exchanger is very likely operating with uniform mass flow. This is a good sign!
Identifying Mass Flow
If the temperatures at different points along a horizontal line around the exchanger are inconsistent but do not reach the temperature of the heat exchange fluid, you are likely encountering a non-uniform mass flow situation. In this case, there is often no immediate action required so long as your process is stable. At your next routine shut-down, take extra care to inspect the surfaces of any transition cones and the discharge device itself for damage, caked product, or wear which could be causing flow inconsistencies.
Identifying Funnel Flow and No Flow
If the temperatures at different points along a horizontal line around the exchanger are inconsistent and approach the temperature of the heat exchange fluid, there are likely areas where product is not flowing or flowing extremely slowly. This type of situation should be monitored closely, especially if the bulk solid being processed is subject to degradation or spoilage. In these situations, it is important that the exchanger is taken offline for maintenance as soon as possible and inspected to determine the cause of the poor flow condition prior to cleaning and restarting. If upon emptying the unit for inspection, there is little or no product left in the areas where inconsistent temperatures were noted, there is likely a funnel flow condition and the discharge device and any transitions may need to be examined more closely. If there is product still in the plate bank even after the unit has been run ‘empty’ there may be caking mechanisms, mechanical bridging, or excess debris to be dealt with before the equipment can be successfully operated.
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This entry was posted on December 7, 2017