For decades, the world of bulk material handling operated under a clear assumption: vacuum systems could only move materials in dilute phase mode. Dense phase conveying was reserved for pressure-driven systems. 

That belief shaped how industries handled everything from fragile food powders to sensitive pharmaceutical blends — until AZO challenged it. By proving vacuum dense phase conveying possible, AZO redefined what pneumatic conveying systems can do.

Why Vacuum Conveying Was Once Seen as Limited

Vacuum conveying has long been valued for its cleanliness and ability to pull materials from multiple pick-up points. But it came with constraints. Conventional wisdom held that vacuum could only move materials quickly and in relatively small quantities. Distances over 60–90 meters, or throughputs above a few tons per hour, were believed to require positive pressure systems.

The reason? Dense phase conveying, where materials move in slow, protective slugs, demands precise control and a high pressure differential. With vacuum, the maximum differential is about 1 bar, which is far less than a pressure pot system. Many engineers assumed slugs would collapse or pipelines would plug if attempted under suction.

Dense Phase vs. Dilute Phase Pneumatic Conveying: The Major Differences

In dilute phase conveying, material particles are suspended in a fast-moving air stream. It’s the most widely used conveying mode that’s both economical and effective for a wide range of bulk solid materials where delicate particle structure and/or critical blends are not a concern. However, dilute phase requires high gas velocity, which can increase product degradation, segregation, and equipment wear.

By contrast, dense phase conveying moves materials in managed slugs (or dunes) at low velocity. This method protects friable, abrasive, or blended products with less breakage. The conveying density in dense phase systems can be 20 to 400 times higher than dilute phase, which also results in lower energy demand and reduced wear on pipelines. Because dense phase often employs smaller diameter pipes, it can also navigate tight plant layouts more flexibly and with less installation cost. 

The divide was once clear: vacuum meant dilute, pressure meant dense,. but had always required pressure systems. That gap created an opportunity — and a challenge— that AZO set out to solve.

AZO’s Breakthrough Innovation

AZO has been innovating in pneumatic conveying since the 1960s. Our engineers began asking: could dense phase conveying be achieved under vacuum? It would mean combining the gentleness of dense phase with the cleanliness of vacuum, a solution the industry had never seen. Over years of research and experimentation, that question evolved into a pursuit.

By the late 1980s, our engineers had successfully applied dense phase vacuum conveying for handling milk powders, proving early on that gentle, high-quality transfer was possible. AZO continued to refine and scale the technology designing and installing vacuum dense phase conveying systems demonstrating that it was possible to move fragile or sticky bulk materials efficiently while maintaining product integrity, minimizing residue, and ensuring sanitary operation. This decade of field experience laid the technical foundation for a new generation of conveying innovation.

Building on that foundation, intensive R&D led to a patented airflow control method that advanced what was previously not thought possible under vacuum. By regulating how and where air enters the system, AZO achieved stable, controllable slug flow — a level of process reliability that redefined vacuum conveying performance.

In 2012, AZO launched VacuumPlus, the first commercial advanced vacuum dense phase conveying system and industry media like Food Engineering magazine began to recognize the innovation. This wasn’t just incremental improvement. It changed the industry’s perception of what vacuum systems could achieve.

How Vacuum Dense Phase Conveying Works

Unlike dilute phase conveying, instead of racing through the pipeline, materials move in slow-moving plugs separated by pockets of air. Because the flow velocity is low — often under 4 m/s — there’s minimal particle impact or abrasion. Blends remain uniform and fragile granules arrive intact.

Vacuum dense phase conveying typically uses a vacuum pump in lieu of a positive displacement blower which allows greater depths of vacuum to be achieved. Material enters the convey line at the pick-up point through a feeder and fresh air is regulated by means of an impulse valve that works in concert with the automation system according to the overall conveying parameters, whether in a batch or continuous conveying mode.

3 Real-World Benefits of Vacuum Dense Phase Conveying

1. Protecting Product Quality

From delicate infant formula powders to coated pharmaceutical pellets, product quality is preserved during transfer. The gentle slugs prevent breakage, reduce fines, and maintain uniform blends. These are challenges that dilute phase conveying often fails to address. Dense phase is particularly effective for abrasive, friable, or mixed-batch materials, minimizing segregation and downstream processing issues.

2. Clean and Safe Operations

Because the system operates under negative pressure, any leaks draw air inward rather than expelling dust outward. This helps contain dust and reduces the risk of combustible dust explosions. For food and beverage products or fine pharmaceutical powders, this dust-tight design ensures both safety and compliance with strict hygiene standards.

3. Energy Efficiency and Sustainability

Vacuum dense phase systems use significantly less conveying air than dilute systems. This not only lowers energy consumption but also reduces the cost of conditioning air when cooling or dehumidification is required. The lower gas volume translates to meaningful operational cost savings while supporting sustainability goals. VacuumPlus has proven to be highly energy-efficient, an advantage that resonates with manufacturers seeking sustainable operations.

Another advantage: reduced wear on equipment. Because products move slowly with a high material-to-air ratio, pipelines and components experience less abrasion. This extends equipment life and lowers maintenance requirements.

About the VacuumPlus System

VacuumPlus intelligently combines the advantages of pressure and vacuum systems. It uses a strong vacuum pump to pull material, while automated valves modulate airflow to maintain gentle, low-velocity slugs. The result is a conveying process that protects delicate materials, maintains blends, and keeps environments clean. By minimizing segregation and controlling dust at the source, VacuumPlus is especially valuable in industries where hygiene and safety are non-negotiable.

The heart of the system is its self-adjusting airflow regulation. Sensors monitor line pressure, and valves automatically admit air when resistance rises. This feedback loop ensures slugs stay stable without collapsing or blocking the line. It’s what allows a vacuum system to perform like a pressure dense phase unit. In addition, some designs incorporate targeted air injection to prevent compacting or plugging when materials resist fluidization.

VacuumPlus is available in both batch and continuous modes. In batch setups, it conveys slugs of material into a receiver vessel for timed discharge. In continuous systems, tandem receivers or specialized controls allow uninterrupted transfer. Both approaches integrate easily into sensitive processes like tablet presses, spray dryers, or packaging lines.

From Skepticism to Adoption

When AZO introduced VacuumPlus, skepticism was high. But validation trials told the story. Pharmaceutical clients confirmed that vacuum dense phase conveying did not affect tablet quality. Food producers saw baby formula powders conveyed 60 meters without segregation via the AZO®MULTIAIR. Real-world performance silenced doubts and opened doors to wider adoption.

Today, engineers designing bulk handling systems have a new option. Vacuum dense phase conveying fills the gap between conventional dilute vacuum systems and pressure dense phase systems. It offers gentle, efficient, and hygienic transfer where both product quality and process cleanliness are non-negotiable. 

Key factors like bulk density, abrasiveness, and moisture content remain essential considerations when designing the system, but with proper calculations for pipe size, length, and pressure, dense phase can reliably handle everything from medium-sized granules to fragile powders.

By proving that “impossible” was possible, AZO advanced the entire field of pneumatic conveying. As manufacturers face increasing demands for product protection, energy efficiency, and cleanliness, vacuum dense phase conveying offers a solution designed for the future.