Delivering adequate supplies of the right resin material to production equipment is what vacuum conveying, feeding, and loading systems are all about.
But while the goal of these systems is straightforward, their design and implementation require considerable thought.
So, what are the best-practices for designing vacuum conveying systems? Where do you begin and how do you proceed? Here are 12 do’s and don’ts for your consideration:
1. Do: Identify all sources.
- Bulk Bags
2. Do: Identify all material consumers.
- Injection/Extrusion machines
- Blender component bins
- Drying hoppers
- Intermediate surge bins or day bins
3. Do: Identify throughputs (lbs/hr) required for all material consumers.
For applications where materials and throughput may change from day to day — like a custom mold shop that changes molds frequently or when plant production varies day to day — a system design engineer will typically plan throughputs based on a “worst case scenario.”
4. Do: Envision where your pump will be placed and where your conveying lines will be run.
Minimize vacuum line “equivalent distance” as much as possible for each of your pump and receiver relationships. Based on the pump technology used, there is only so much vacuum capacity available. By minimizing the vacuum line distance, you maximize the possible material line equivalent distance with respect to the limitation of pump vacuum capability.
Keep all conveying lines as straight as possible, minimizing any changes in elevation or direction? In particular:
- Minimize the use of 90° bends in conveying lines, either vertically or horizontally, because these increase system backpressure and consume some of the vacuum potential of your pump, which reduces its available capacity to lift and move material.
- Minimize or eliminate the need for inclined sections of conveying line, as these also increase backpressure and consume the pump’s vacuum capacity. Inclined sections also increase the likelihood that material “plugs” could occur in the system at the base of the inclined sections.
5. Don’t: Use mitered elbows on material lines.
In tight spots where other equipment may be present, it’s tempting to substitute a sharp, mitered bend for a gentle, sweeping “centerline radius” elbow. But don’t do it.
Instead, try to cut back the straights that are entering/exiting the elbow location so that there’s room for a CLR elbow or re-route the material line with a different exit point so that proper CLR elbows can be used throughout the run.
Once again, this is all about minimizing “equivalent distance” and pressure drop to maximize pump capacity. Sweeping CLR elbows limit pressure drop and ease material flow, while a mitered corner is like hitting a wall, causing a large pressure drop and speed loss and having to re-accelerate the resin all over again.
The same holds true on the vacuum line — every mitered bend has a greater pressure drop then a sweeping CLR elbow — even for air-only calculations. So use mitered elbows only if absolutely necessary on vacuum lines.
6. Do: Calculate the “equivalent distance” of conveying system lines to all destinations.
Horizontal, vertical, and bend degree all factor into a calculated “equivalent distance” that you must design against.
By minimizing the system logistics in #4, this will be vital to maximizing your system’s potential.
7. Do: Choose a line size and accessories that are matched to pump capacity and throughput requirements.
Line size selection is critical to ensuring the throughput requirements are met. In relative terms – the larger the line size, the larger the volume of material that can be conveyed.
Line size dictates the size of the pump, while the physics of airflow and speed are determined by pump CFM and line size.
Note that different materials require different “pickup velocities” (the velocity of air entering the inlet probe where material is drawn into a material line).
8. Don’t: Overlook the relationship between pump sizing and line size.
This is the #1 error that inexperienced people make when designing or retrofitting pneumatic conveying systems.
Motivated by a desire to save money, they try to use the “old pump from the previous system” with a brand-new or retrofitted conveying line. However, this rarely works as hoped.
You must check the ICFM (input cubic ft/min) rating of any potential pump to ensure that it matches up with your line size.
9. Do: Choose a conveying method—and equipment—that minimizes velocity to limit resin attrition and system erosion.
Because material represents a major element of production cost, conveying systems must not only deliver resins in the proper quantity, but with minimal attrition due to physical damage during conveying.
Attrition occurs when high conveying speeds (> 5,500 feet per minute) cause softer materials (like polyethylene) to heat up and smear against the walls of the conveying lines, creating “angel hair,” or when brittle materials (like polystyrene or polycarbonate) strike line walls and bends and break up, creating dust and fines.
Much of this wasted resin is captured in filters, which must be cleaned regularly, or else it can clog filters and rob vacuum pumps of airflow, reducing their capacity and causing them to overheat.
Conveying system erosion occurs when abrasive materials, such as glass-filled resins, are conveyed at high speeds, causing wear on the inside surfaces of conveying lines that result in leaks that reduce conveying efficiency and require repairs.
Both resin attrition and system erosion increase exponentially as material conveying velocity increases, so the best way to reduce both is to choose conveying methods and equipment that can convey adequate material throughputs at the lowest velocity.
For more than 50 years, there wasn’t any choice about how to convey resin, because dilute phase conveying was the only practical technology. However, dilute phase conveying works only at very high speeds, typically 5,000 feet per minute and greater.
In recent years, Conair has developed a slower-speed, dense-phase resin-conveying technology called Wave Conveying™. It too is vacuum-driven, but has major differences from dilute phase:
- It relies on more powerful vacuum pumps with greater lifting power.
- It uses more advanced controls that regulate air and material inputs to minimize the pressure drops that cause uncontrolled changes in conveying speeds.
- It moves large material throughputs at relatively low and controlled speeds, so it virtually eliminates the problems of resin attrition and system erosion.
Dense-phase Wave Conveying picks up resin at essentially the same speed as older dilute-phase systems – between 3,000-4,000 feet per minute. However, what happens after the resin enters the conveying line is very different.
In dilute phase systems, the large pressure drop from source to receiver means that high pickup velocity is used to get the pellets moving, and air and material conveying speeds continue to accelerate all the way, often reaching extremely high speeds (>5,500 ft/min and higher) that can cause resin attrition and system erosion.
In Wave Conveying systems, changes in air flow and pressure are carefully controlled: The resin is picked up at a very low velocity in a deep regulated vacuum and subsequent airflow controls prevent the material speed from accelerating much at all. Wave Conveying speeds range from 300 ft/min to 2,800 ft/minute in semi-dense phase, so attrition and erosion are virtually eliminated.
10. Do: Select a control system that easily accommodates future growth.
Just as you don’t want to force-fit an old pump onto a new conveying system, you won’t want to invest in a new system without having a conveying system control that can handle current needs and accommodate future growth.
11. Do: Choose a partner who offers deep experience in material handling system design and the support of expert installation, service, and support staff.
Systems and technology represent a major investment, so you’ll want to work with a partner that offers experienced personnel to plan, build, and support your system over the long haul.
12. Don’t: Get overwhelmed!
Designing, building, or retrofitting a vacuum-driven resin conveying system isn’t necessarily easy. Nor is it something that you should do yourself. But if you turn to an experienced system designer and builder, with access to innovative technology and affordable, expandable solutions, you’re likely to find all the answers and all the help that you need.