How to Choose a Vacuum Pump for Resin Conveying

Conveying

How to Choose a Vacuum Pump for Resin Conveying

Today, there are four major types of vacuum pumps available for resin conveying, as well as a number of important considerations involved in selecting, operating, and maintaining them.

Key Vacuum Pump Considerations

Among the most important considerations in pump selection are:

  • Depth of vacuum provided.
  • Length or “equivalent distance” of conveying system.
  • Line size served. Conveying system line sizes typically range from 2.5 to 5 inches.
  • Conveying phase and speed. Today, high-speed dilute phase conveying systems are very common, but Conair offers an alternative, the Wave Conveying™ system, that conveys at lower speeds. Both dilute phase and Wave Conveying systems can deliver high throughputs, but Wave Conveying operates at much lower speeds, as seen in the illustration below.Pneumatic Conveying Phase and Speed Diagram
    • Dilute phase conveying suspends relatively small volumes of resin in large volumes of air moving at speeds of about 5,000 feet per minute, but frequently much faster based on the characteristics of the vacuum pump that is selected. High dilute phase system speeds are associated with resin damage and loss caused when soft pellets rub against conveying pipes, leaving angel hair and streamers, or when hard pellets collide with angles, corners, or receivers and shatter into unusable dust or fines.
    • Conair’s Wave Conveying™ system, uses higher levels of vacuum with much lower air speeds to move resin in a stream of gently rolling “waves” (Wave Pulse) or in a series of very dense “pulses” (Wave Pulse) at even lower speeds. The slow speeds and gentle motion of dilute phase conveying virtually eliminates resin losses due to angel hair, streamers, shattering, dust, and fines. (Note: Wave Conveying can also provide dilute-phase conveying capabilities, but with carefully controlled speeds.) Use wave stream and wave pulse.

Understanding Different Vacuum Pump Types

Any of the four vacuum pumps below can handle dilute phase conveying applications for which they are properly sized.  Two of the pumps can also handle dense-phase conveying when they are used together with other required components of Conair’s patented Wave Conveying™ technology.

Conair vacuum pump

Single- or Dual-Stage Regenerative Pumps

Single- or Dual-Stage Regenerative Pumps are commonly used in small to moderate sized conveying systems – line sizes up to 2-1/2 in. diameter, equivalent distances to 150 ft. (single-stage) or 450 ft. (dual-stage) with low to moderate throughput requirements. These low-cost pumps are relatively simple in design, with motor power driving single- or dual-turbine impellers that draw air, creating maximum vacuum levels of about 12 inches of mercury (inHg) and sustained vacuum of 7-10 inHg down the conveying line.

Single-stage “regen” pumps are very popular for smaller-sized, lower-throughput conveying systems, such as for conveying material to a group of low-tonnage IMMs.  Larger (dual-stage) regenerative pumps are sometimes used in mid-sized central resin distribution systems. However, the relatively low efficiency of these pumps mean that they must draw large volumes of air to build up vacuum suction for conveying or purging.  Thus, they tend to generate much higher air and pellet velocities that can increase the likelihood of pellet damage and losses during conveying or purging.

Conair positive displacement pumps

Positive Displacement Pumps

Positive Displacement Pumps are the workhorses of many relatively large resin-conveying systems, particularly among high-volume, high-throughput processors in North America. “PD” pumps utilize counter-rotating lobe-style impellers that can draw vacuum of up to 14 inHg and sustain vacuum of about 11 or 12 inHg at distances to 400-550 feet (equivalent distance) in line sizes up to 4 inches. Within this distance range, PD pumps can deliver very high system throughputs.  However, the lobe-style impellers used in PD pumps are known to generate a significant level of noise, so If PD pumps are located near personnel or work areas, they are often housed in sound-reducing enclosures.

HRG Series Hybrid Pumps

Hybrid Regenerative Pumps

Hybrid Regenerative Pumps are a very recent innovation.  Unlike ordinary single-stage or dual-stage regenerative pumps, hybrid regenerative pumps use an advanced design with three turbine impellers. This enables the pump to function much like a multi-stage turbocharger on an auto engine, drawing a more powerful vacuum at each successive stage.  As a result, HRG pumps draw a much deeper vacuum (up to 18 inHg maximum) than single- or dual-stage regenerative pumps and positive displacement pumps. And, they can sustain a vacuum of 12-14 inHg over distances to 850 feet or more. Compared to PD pumps of identical horsepower, HRG pumps hold a performance advantage of about 30 percent in throughput or equivalent distance.

Until very recently, processors with conveying systems that demanded high throughputs over distances greater than 500-600 feet had only one real alternative: a long-distance, positive-displacement or LDP pump. However, Conair recognized that not every processor requires both the high throughputs and long distances that this premium pump provides. To fill this price/performance gap in its line of vacuum pumps, Conair evaluated a wide range of products, seeking a vacuum-pump design that combined the competitive cost of regenerative pumps, the high throughputs of PD pumps and the extended range and Wave Conveying capabilities of the LDP pump.

The solution was the new HRG, whose competitive price and high performance make it a logical choice for many longer-distance conveying applications.  At the same time, HRG pumps are powerful enough to serve many applications where processors are considering Conair Wave Conveying technology.

Long distance positive displacement pump

Long-distance Positive Displacement Pumps

Long-Distance Positive Displacement (LDP) pumps are a specialized type of positive displacement pump. They utilize a unique, close-tolerance counter-rotating “claw” impeller design that generates vacuum and airflow more efficiently than lobe-style PD pumps. As a result, they can draw a very deep vacuum – a maximum of 20 to 22 inHg – and sustain vacuum levels of 14 to 18 inHg over equivalent distances of 1,000 ft. and more.  These pumps truly offer the ultimate in conveying performance, delivering roughly twice the conveying power (lbs/hour or distance) of lobe-style PD pumps of equal horsepower. However, this all comes at a price:  LDP pumps can cost considerably more than other pump types.

Summary of Key Characteristics:  Vacuum Pumps

Pump Characteristics

Regenerative (RG) Positive displacement (PD) Hybrid Regenerative Long-Distance Positive Displacement (LDP)
Cost  $ to $$ $$ to $$$ $$$ $$$$$
Impeller style Single- or two-stage turbine Rotating lobes Three-stage turbine Rotating “claws”
Noise Moderate High Moderate Low

Pump Performance

Regenerative (RG) Positive displacement (PD) Hybrid Regenerative Long-Distance Positive Displacement (LDP)
Throughput Low to Moderate High High Very High
Effective Distance Single-stage to 150 ft., Dual stage to 450 ft. Moderate, to 550 ft. Longer,  to 850 ft. Longest, 1000 ft. or more.
Line sizes 2.5 inches 4 inches 4 inches 5 inches
Maximum vacuum (inHg, absolute) 12 14 18 20-22
Sustained vacuum for conveying (inHg absolute) 7-10 10-12 12-14 12-17
Applications Multiple short to medium runs, machine-side delivery Central conveying systems to 550 ft. Central conveying systems to 850 ft. Central conveying systems to 1,000 ft. or more.
Dilute-phase Wave Conveying™ capable? No No Yes Yes

 

Sizing Vacuum Pumps

For a given vacuum conveying system, any pump must be properly “sized” so that it has the minimum capabilities required to meet application demands.

For dilute phase: Vacuum pumps used in dilute-phase conveying systems must be able to sustain a vacuum of 10-12 inHg at the pump outlet while providing an air “pickup” velocity of 3,500 to 4,000 feet per minute (2,800 to 3,200 fpm pellet velocity) at the most distant pickup point in the conveying system.  Any of the four vacuum pump types listed here – regenerative, positive displacement, hybrid regenerative, or long-distance positive displacement – can perform in dilute-phase systems.  However, because of the way that they build up vacuum suction, regenerative pumps are most likely to create extremely high and potentially damaging air and pellet speeds and result in increased pellet losses due to angel hair, streamers, shattering, dust, and fines.

For dense-phase Wave Conveying™:  For slower-speed, dense-phase conveying – the type made possible with Conair’s Wave Conveying System – vacuum pumps must provide deeper levels of sustained vacuum to support Wave Stream and Wave Pulse conveying modes. Wave Stream mode operates at 1000 to 2800 feet per minute while Wave Pulse operates at just 300 to 1000 feet per minute. Both of these modes move resin much more slowly and gently to prevent damage, yet with much greater density so that high system throughputs are maintained.

Conclusion:  Consider Carefully

If you need to convey resin, there are a range of vacuum pumps that can be sized to do the job. But understanding the characteristics, capabilities, and sizing of different vacuum pump technologies is just the first step.  An optimal conveying solution ultimately blends the right pump, the right controls, the right lines and hardware with an understanding of the resins you are using, your throughput requirements, and your business needs.

For processors whose operations rely on the use of specialty and engineered resins, or complex ingredient mixes, the issue of resin loss and damage looms increasingly large. Depending on the type of pellets being processed, the air and resin velocities typical of dilute-phase conveying systems (especially those powered by ordinary regenerative pumps can cause significant resin damage and losses. High-velocity conveying of any resin, particularly abrasive resins, is also a cause of component wear, vacuum leaks, and increased maintenance costs.

So, before you select the vacuum pump that will drive your conveying solution, consider carefully whether a typical dilute-phase conveying system will do, or whether the value and volume of your resin usage justifies a variable-speed, dense-phase conveying approach like Conair Wave Conveying. In dozens of internal and customer tests, the Wave Conveying System has been proven to prevent resin damage and loss (dust, snakeskins, angel hair), reduce system wear and maintenance requirements, and meet or exceed system requirements.

For more information about any of the Conair vacuum pumps and conveying technology, don’t hesitate to contact me via e-mail or by calling 412-977-0680.

White Paper on Wave Conveying Technology

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