Jonathan Thorn | Schenck Process
This article focuses on the positive and negative reactions that effective air velocity has on a dilute-phase pneumatic conveying system. This article compares using static velocity control versus dynamic velocity control in a pneumatic conveying system and the impacts each control type has on blower speed, blower power, and effective velocity.
Whether you’re processing pet food, human food, or plastic pellets using a pneumatic conveying system, air velocity plays a large role in how effectively and efficiently your system operates. In dilute phase, as part of the design process, it’s important to determine your particular material’s minimum entrainment velocity (also known as pickup velocity), which is the minimum air velocity at which the material and air move within the system. Then effective air velocity, which is a value greater than the minimum entrainment velocity, is applied to the system. The generated airflow from a blower or other air-movement device combined with the cross-sectional area of a pipe and line pressure create the effective air velocity that interacts with the process material. The effective air velocity picks up and transports each particle while it’s suspended in the air, creating a dilute-phase flow. Pneumatic conveying systems also move material via dense-phase flow, but for the purpose of this article, we will be focusing on dilute-phase flow only.
Dilute-phase flow has been used over many years on thousands of systems. Each time dilute-phase flow was used, the most important aspect to ensuring proper system operation was that the system’s effective air velocity was greater than the material’s minimum entrainment velocity. It didn’t matter if the effective air velocity was 5 percent or 50 percent greater; the system would work just as well at either rate with the user unaware of the excess. A system operating at an excessively high velocity is a possible unintended outcome when using this pneumatic conveying design method, unfortunately, but we’ll go over that issue later.
Duty and process operation
The pneumatic conveying dilute-phase design process ensures that the air mover, usually a positive-displacement (PD) blower like the one shown in Figure 1, is operating faster than required to perform the duty. The term duty is used to describe the value operators use to define the requirements for conveying a particular material at a certain rate across a given distance. As any of these variables change, the pressure and airflow required to operate the system change as well. However, in the design process, generally only the most-extreme or worst-case duty (most difficult-to-move material, greatest transfer rate, longest conveying distance) is considered and equipment is selected with this factor in mind. The parameters at which to run the equipment and components in your pneumatic conveying system, such as the base operating speed of a blower, is determined with the worst-case conditions in mind, and without an operator adjustment after installation, the equipment may operate this way for its entire service life. Therefore, nearly every dilute-phase system can operate at a speed greater than what’s required to perform the duty, and for many systems, the extra speed is excessive. This is the cumulative effect of the effective air velocity and the worst-case considerations.
The classic dilute-phase system would normally be designed and operated at full speed with across-the-line power of 50 or 60 hertz. Until recently, adding a variable-frequency drive (VFD) for blower speed control came with a significant cost. Unless there was a specific process need for changing blower speed, many projects didn’t integrate speed controls due to the cost. At full speed, the blower would operate the system reliably across a range of material feedrates and conveying distances if required. However, running the blower at full speed when a lesser speed will do results in greater-than-planned air velocities, potentially from 5 to 20 percent greater, in the conveying line. Generally, the system would continue working as expected and the elevated velocities wouldn’t be evident to the operator.
Reduce velocity, reduce risk
As we’ve established, elevated velocities can occur as part of the dilute-phase conveying system’s initial design or by operating the system below the maximum design conditions. Air velocity matters in a dilute-phase flow system because the effective air velocity is a strong contributing factor in equipment abrasion, material degradation, material buildup in conveying lines, and overall process power consumption. Each of these potential system issues can pose problems in a pneumatic conveying system.
Abrasion. Normally, abrasiveness is considered a material property, and materials can be abrasive, somewhat abrasive, or nonabrasive. There’s no exact definition but rather an aggregate of experience regarding how quickly equipment wears out around specific materials. To estimate abrasiveness, a material can be assigned to the hardness scale and compared to another material. Other custom or standard tests can be conducted to put the material on another relative scale, but what matters most is how the material interacts with the process equipment. In pneumatic conveying, equipment abrasion occurs because of a combination of the material properties and the conveying velocity. A highly abrasive material moving at a low velocity will cause less equipment abrasion than a somewhat abrasive material moving at a high velocity. Therefore, in nearly all cases, reducing the system’s velocity will minimize the risk of abrasion. Conversely, a lack of control over the system’s velocity will unnecessarily increase the risk of abrasion. There are many examples of seemingly harmless materials, which appear soft and nonabrasive, wearing holes in pipes and elbows; usually, the culprit can be traced back to excessive air velocity.
Degradation. Degradation isn’t necessarily a material property, but materials can be considered fragile or friable and, as a result, degrade when they’re handled roughly. In pneumatic conveying, degradation happens when the air-entrained materials interact with hard surfaces such as pipe walls. The particle momentum and resulting impact breaks larger material pieces into multiple smaller pieces. Again, there’s no exact definition for what constitutes degradation because how you measure a material’s degradation level varies depending on the material being handled. Materials such as coffee beans, sugar granules, or microspheres would have differing measurements for what constitutes as degradation. It can be said, though, that the impact velocity from the material’s momentum has the strongest correlation with the resulting particle size reduction. In nearly all dilute-phase conveying cases studied, an increase or decrease in conveying velocity produced a proportional increase or decrease in degradation.
Buildup. The ability of particles to attach themselves to pipe walls is a difficult material attribute to study or quantify. However, certain materials over others have been known to demonstrate adhesive and cohesive qualities by sticking to a pipe’s internal surface while additional particles attach to those stuck particles until a layer of material is formed. This material layer can continue to build, eventually partially blocking the line and affecting the conveying line’s ability to process material. While the underlying cause of buildup is often not fully understood, one reason could be because fatty materials release encapsulated oils that can attach themselves to hard surfaces. Another reason could be because the water in high-moisture materials can make particles cohesive, which cements them in place when they dry. Extremely small particles sometimes attach to surfaces due to static force. Whatever the reason, the material’s ability to build up is often noted after the process already happens, so this information isn’t well-defined. When studied, the material’s treatment, including exposure to air velocities, correlates with the degree of buildup. In other words, the rate or amount of conveying line buildup can be decreased by reducing the effective air velocity.
Power consumption. The nature of air compression dictates that the energy to drive a compression device is proportional to both the volume being compressed and the differential pressure it achieves. If you can reduce either the volume or the pressure, then a reduction of energy power is accomplished. However, if you can reduce both the volume and the pressure simultaneously, then the two factors will have an additive effect and attain an even greater power savings. Dilute-phase conveying systems will naturally have lower pressure and volume when the conveying velocity is reduced. The equation in Figure 2 describes adiabatic compression where η is the compression efficiency, Q is the mass airflow rate, P1 and P2 are the pressures before and after compression, respectively, and γ is the adiabatic constant for air. This equation establishes the power needed to drive a rotating compression device. Let’s apply the information in the table in Figure 2 to the adiabatic compression equation. If a 10 percent reduction in airflow simultaneously caused a 5 percent reduction in the system’s pressure, then the additive effect on the compression device would be a 14 percent reduction in power. In this way, velocity control can significantly affect the dilute-phase system’s power requirements. The compression volume is inherently less with reduced air velocity, but the pressure is also decreased from less resistance in the conveying line, as shown in the Zenz diagram in Figure 3.
Putting velocity control to work
If the benefits of velocity control in a dilute-phase system are accepted as significant, then examining the methods of accomplishing this task is a worthwhile activity. An obvious feature to add to a blower is a VFD, which can be directed to change the system’s blower speed. The system can be designed in the traditional way with conservatism and safety factors (extra velocity or motor power) employed. Then, at the point of implementation, the speed can be reduced to accomplish some intended goal related to the benefits. Speed reduction can be a manual process or automated in the form of an algorithm.
Manual speed changes can normally have a noticeable impact on the system’s performance without adding significant, undue risk. Considering the system’s design safety factors related to determining effective air velocity, a 5 percent reduction in speed is a reasonable change that can be tested and proven in a production environment. The assumed risk is that the speed reduction will place the effective air velocity close to but not below the material’s minimum entrainment velocity. Process variations such as rate, ambient temperature, and material properties can marginally increase or decrease the effective air velocity. If the system is operating too close to the material’s minimum entrainment velocity and the effective air velocity dips, then a cycle of reduction begins. Figure 3’s Zenz plot shows the relationship between the velocity and the resulting pressure drop per distance (mbar per meter) of the system. On the right-hand or “correct” side of each rate line, the pressure reduces to the minimum entrainment velocity; this is where the system should be. On the left-hand or “wrong” side of the minimization curve, lower velocity will cause a rise in pressure. Additional leakage and air compression cause yet more uncontrolled velocity reduction, and the cycle continues until the system is in distress. A system in distress will be unstable with large swings in pressure and effective air velocity. In the best-case scenario, the system will hit a high-pressure setpoint, and the process controls will stop the material feed so the system can recover. In a less-beneficial scenario, the pipe will become plugged with material due to the lack of effective air velocity and manual intervention will be required. Because this instability is detrimental to the performance of the system and process, manual speed reductions require that some amount of residual excess velocity remain.
A more productive and beneficial method of velocity control uses an algorithm to direct the blower speed; this process is called dynamic velocity control. By taking into account real-time, real-world inputs about the system, such as pipe diameter, blower characteristics, and airlock characteristics, speed corrections have a tangible influence over the effective air velocity. The algorithm also has performance input, such as the system’s operating pressure, which helps the pneumatic conveying system adapt to changing process conditions. Dynamic velocity control is different from a proportional-integral-derivative (PID) control loop, which uses a process condition setpoint to modulate a device. The pressure minimum curve excludes typical control mechanisms because the system could be operating on either side (left or right) of the minimum; in this case, the blower might slow down when it should be speeding up or vice versa. An algorithm will relate the blower speed and performance to the effect on velocity as the pressure changes. When a velocity is operating below the minimum, the blower will speed up, and when a velocity is greater than the minimum, the blower will slow down. The dynamic velocity control method allows the system to operate at or just below the material’s minimum entrainment velocity instead of above it. However, this doesn’t mean that the system is more susceptible to distress. At any sign of distress due to low velocity, the algorithm will signal the blower to speed up until the condition has passed. As a result, the dynamic velocity control method is more efficient and robust compared to statically operated systems.
The previous considerations for dynamic velocity control assume that process conditions such as material type, transfer rate, and distance are constant, but when process conditions start to change, a new level of benefit is revealed. As stated earlier, design methods that involve static blower speeds — optimized or not — must take into account the worst-case conditions. Using the dynamic control method, if any of those process conditions are reduced (more free-flowing material, lower transfer rate, shorter distance), then the blower is reduced even more to accommodate. Conversely, in a statically operated system, the less-extreme duty produces less pressure in the system, which results in increased effective air velocity (less compression, less airlock leakage, improved blower performance). The dynamically operated system will reduce its speed to keep the velocity constant, adapting itself to the new process conditions. This means that systems expecting to see wide ranges in material type, material transfer rate, and conveying distance will see the most benefit with a dynamically operated system.
Static versus dynamic velocity control
Experiments comparing a dilute-phase statically operated system to a dilute-phase dynamic velocity controlled system were conducted1 and then presented at the 8th World Congress on Particle Technology in 2018. Fiberglass-filled polyethylene pellets weighing 36 lb/ft3 (576 kg/m3) were conveyed at various distances and rates and used the various control mechanisms discussed.
Distances. The conveying loop had three conveying distances available using 100- to 650-foot-long horizontal pipes, 20-foot-long vertical pipes, and seven to 11 elbows. The three loops had effective equivalent lengths of 200 feet, 460 feet, and 850 feet.
Rates. The maximum rate for the test system was 8 metric t/h (17,500 lb/h), and a decreased rate of 5 metric t/h (11,000 lb/h) was used for comparison. The case of empty line conditions (zero metric lb/h) where the blower runs but no material is fed to the system was also part of the data.
Control philosophy. During standard dilute-phase conveying where there is no velocity control, the static blower speed was set in accordance with typical design practice. For the static velocity control, the static blower speed was set by optimizing the system for the worst-case conditions, and for the dynamic velocity control, the blower speed adjusted itself with a velocity-control algorithm. A sample of the results from the experiments are shown in Figures 4a. and 4b. to clearly demonstrate the relationship between system variables.
In Figure 4a., the conveying rate was held constant while the distance was reduced across three lengths. For the standard dilute phase, the blower speed was set consistent with typical design using 3,997 fpm effective air velocity and then held constant throughout. As the distance gets shorter, representing a different, closer destination, the effective air velocity creeps up as there is less compression on the blower. We see an 18 percent velocity increase at mid-distance and more than a 40 percent increase at the nearest distance. With the static velocity control, the initial blower speed and effective air velocity can be reduced from the standard 3,787 fpm for the farthest destination. However, without additional blower setpoints for each destination, we still see the velocity increase by similar percentages as the standard dilute phase. Finally, with the use of dynamic velocity control, we are able to operate the system at 3,619 fpm for the longest pipe length (farthest distance). Then at the closer destinations, we get nominally the same effective air velocity for all three conditions. This shows that the dynamically controlled system manages the velocity, while the statically controlled system experiences significant velocity increases.
Figure 4a. also shows the impact on power consumption across the various lengths. When conveying material a shorter distance, naturally we expect the power consumption to decrease because less work is being done on the material as the material is of a less-difficult duty. However, with the statically operated systems, the power usage at the shorter distances doesn’t drop significantly. Despite conveying only 25 percent of the distance, the system still uses more than 65 percent of the power. The blower still generates as much, if not more, air volume and only the pressure is reduced. With the dynamic velocity controls, the blower power drops to 40 percent for the shortest distance. The blower volume and pressure are both dramatically reduced, so the conveying is inherently more efficient.
In Figure 4b., the distance is held constant while the conveying rate is varied. A similar pattern emerges where the lower conveying rate produces 10 to 12 percent greater velocities in the underfed statically controlled systems, but with the dynamically controlled system, the velocity can be kept fairly constant. Likewise, the conveying rate reduction only produced modest reductions in energy consumption until the dynamic controls were employed.
Figure 4b. also shows a unique aspect of the dynamically controlled system. If we look at the rate reduction case and continue with it down to no rate, the benefits will become greater unto the endpoint. This clean air condition is normally present when pneumatic conveying systems are starting, stopping, or pausing. The system will idle, and the blower will run at its directed speed with no material feed present. Whenever a process is halted, the decision must be made whether to stop the blower. The blower will be turned off after a period of time if it’s determined that there will be an extended amount of downtime. For a rapid-cycling process, the blower will be left on to save the stop-and-start wear on the motor. The amount of time a dilute-phase system spends in idle varies by application, but it should be noted that major power reductions are possible using the dynamically controlled system. Savings of 70 to 80 percent are typical for these idling periods, and since no work is actually accomplished, it’s really a waste-reduction strategy.
The results of these experiments establish that several performance factors related to dilute-phase pneumatic conveying can be directly tied to the system’s effective air velocity. The level of control over the effective air velocity can have a positive or negative impact on abrasion, degradation, buildup, and power consumption. Lastly, static velocity controls are an improvement over standard dilute-phase operation, but dynamic controls have been shown to provide more benefit across a wider range of process variables.
- Jonathan O. Thorn, et al., “Algorithm to Enhance Performance of Dilute Phase Pneumatic Conveying Systems,” 8th World Congress on Particle Technology, April 2018.
For further reading
Jonathan O. Thorn (800-821-2476) is the executive director, process technology at Schenck Process in Kansas City, MO. He earned a master’s degree in chemical engineering from Pennsylvania-based University of Pittsburgh, focusing on pneumatic conveying-related studies. He has more than 20 years of industry experience and works in many facets of application, design, and R&D as it relates to dry materials handling and processing.
Schenck Process • Kansas City, MO
816-891-9300 • www.schenckprocess.com
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