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Can Your Pneumatic System Do More for You?

Fri, 04/28/2006 - 10:42am
If you need to handle more capacity, you may be able to retrofit your system with modern equipment.
By Jack D. Hilbert, PE Thanks to new technology, you may be able to upgrade rotary airlocks and tweak associated equipment to handle higher throughput rates. Do you have a dilute phase pneumatic conveying system in your plant that uses a rotary airlock as the line charger and a positive displacement blower as the air supply? Sound familiar? You’re not alone. Pressure pneumatic conveying systems of this type have been supplied for many years and are probably the most common type of pneumatic system in use today. They have been the workhorse of the industry and are characterized by their medium pressure range (8-12 psig) and their relatively high terminal velocity range (6,000-8,000 fpm). If your process requirements have changed and you now need to handle more capacity, you may be able to retrofit your system with modern equipment and technology in lieu of replacing the entire conveying system. The primary factor that has limited this type of conveying system from operating at higher line pressures has been the inability of the rotary airlock to handle a pressure differential in excess of 6-8 psig. Due to the nature of rotary airlock construction, there are clearances that have to exist between the body inside bore and the outside diameter of the rotor. Additionally, there has to be a clearance between the ends of the rotor and the airlock end plates. Clearances are necessary to allow for thermal expansion of the rotor as well as the shaft deflection that occurs as the conveying line pressure beneath the rotary airlock becomes increasingly larger than the pressure on the inlet side of the airlock. Since the clearances are a source of air leakage from the conveying system, they must be kept to a reasonably minimum dimension. Given the level of technology before recent rotor designs, the pressure differentials had to be in the 6-10 psig range. If not, the rotor deflection would be more than the available clearances, causing jamming of the rotors. Naturally, the longer the feeder shaft, the greater the distance between the end-plate supports and, thus, the greater the sensitivity to deflection. The result is the need for lower differential pressures.
Fig. 1: Conveying capacity is directly proportional to conveying line pressure.
It’s also true that single-stage positive displacement blowers, which are typically used on these systems, are limited in the amount of discharge pressure available — previously in the 12 psig range but currently running at 15 psig and slightly higher in certain cases. However, alternative air supply techniques, such as using two-stage blowers or any one of several types of compressors, can provide more conveying pressure. Even with an air supply capable of higher pressure, the rotary airlock is still the limiting factor. However, rotary airlock technology has been improved. Reliable rotary airlocks are available that operate at pressure differentials up to 40-45 psig with special designs going even higher. The design of stronger end plates, improved rotor designs and better blade configurations as well as bearing and seal enhancements have made this possible. In pneumatic conveying, the relationships between key parameters are important for proper design. One such relationship is depicted in Figure 1. This curve shows that for any given installation where the pipe diameter remains fixed and the total equivalent conveying length of the system doesn’t change, the conveying capacity is directly proportional to conveying line pressure. For instance, if you were to operate a system with 25 percent more pressure, you should be able to handle 25 percent more throughput capacity. While that simple statement may sound like the panacea for a lot of existing systems, it unfortunately does not hold true in all cases. Nor does it mean that you don’t need to consider other effects on the system such as getting more air volume from the blower or adding additional filter area at the terminal end of the system. It would be unlikely, for example, for a system operating at 10 psig to triple its capacity if run at 30 psig. However, there is a way to increase the diameter of a portion of the downstream conveying piping to take advantage of the expanding air volume to control velocities while reducing the overall equivalent length of the system. This technique is called line stepping. This article has focused on dilute phase conveying systems where the solids loading ratio is in the range of 5-15 pounds of material per pound of air. You must keep a watchful eye on this relationship, too, or you could find yourself straying into a different mode of conveying — some form of dense phase conveying. There also exists the potential for trying to fit too much material into a fixed diameter pipe. Don’t get the idea that transforming a dilute phase system to dense phase is completely bad. There are cases where it is advantageous. However, those cases have many more concerns to address other than replacing an airlock and tweaking the performance of a blower package. Key Points: • Increase the capacity of the rotary airlock and its ability to handle more pressure differential. • Increase the pressure capability of the air supply by modifying or replacing what exists. • Review the pipe run for ways to reduce the total equivalent length, including line stepping as a viable option. • Consider the equipment’s design margin. It makes a big difference if auxiliary equipment is operating at its maximum or if there is a 10-15 percent margin available to utilize. About the Author: Jack D. Hilbert, PE, is an independent consultant providing engineering, project management, start-up and troubleshooting services in the fields of bulk material handling, storage, blending and environmental control with Pneumatic Conveying Consultants, 529 S. Berks St., Allentown, PA 18104. Questions about this article can be addressed to him at 610-657-5286 or pcchilbert@entermail.net
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