Compressed Air for Pneumatic Conveying Systems

Flour mills sending flour from the grinder to the packaging line, chemical plants moving powdered catalyst from silos to reactors. The pipeline is sealed, no dust escapes. Pipes can turn corners, climb, punch through walls, cross between floors. Very flexible routing. Inside the pipe there are no chains, scrapers, screws, none of those moving parts. Maintenance is far less hassle than mechanical conveying.

Conveying

Pneumatic Material Transport

Airflow is the first parameter that gets underestimated

Air inside the pipe has to maintain enough velocity or the material settles out. Dilute phase conveying typically runs at pipe velocities between 18 and 25 meters per second. Lightweight stuff like flour sits toward the low end, cement and silica sand toward the high end. Convert that to a 6-inch pipe at 20 meters per second, and the corresponding free air volume is roughly 750 CFM. A plant running three or four lines simultaneously needs a total of 2,500 to 3,000 CFM, while all the air tools, cylinders, and blow-off nozzles in the same plant might only add up to a couple hundred CFM.

Pressure level is directly tied to equipment type

Positive-pressure dilute phase conveying operates at 5 to 12 psi. A Roots blower or low-pressure screw blower handles that just fine. The Aerzen GM series Roots blower and the Atlas Copco ZS series low-pressure screw blower are both commonly seen in this kind of application.

Negative-pressure conveying relies on a vacuum pump at the pipe end to generate suction. Typical vacuum is negative 4 to negative 7 psi, suitable for scenarios where material is drawn simultaneously from multiple pickup points and collected at one receiving point.

Dense phase conveying is different. The material does not fly suspended in the airstream at high speed. Instead it moves through the pipe as slow-traveling plugs at only 3 to 8 meters per second. Material wear and breakage rates are much lower. The driving pressure required is higher, generally 30 to 60 psi, and for long distances or heavy materials it can reach 75 to 87 psi, which puts it squarely in screw compressor territory.

Some projects, wanting to keep things simple, hook up a single 110 psi screw compressor with a pressure regulator knocked down to 7 psi to feed a dilute phase line. Compressing to 110 psi and then throttling back to 7 psi turns all the intermediate compression work into waste heat. For the same airflow, a Roots blower producing 7 psi directly draws only about 40% to 50% of the shaft power of that screw compressor. On a conveying line running 6,000 hours a year, that gap adds up to a six-figure dollar amount in annual electricity costs.

Oil content is a hard threshold in food and pharmaceutical industries

Roots blowers have no lubricating oil in the compression chamber. Inherently oil-free. Oil-free screw compressors and oil-free screw blowers work the same way. These two types of equipment dominate food and pharmaceutical pneumatic conveying.

If oil mist gets into a pipeline conveying flour, milk powder, or pharmaceutical powder, the entire batch is scrapped. The food industry typically requires Class 0 or Class 1 per ISO 8573-1, total oil content below 0.01 mg/m³.

Oil-Free

Blower Technology

Treatment

Air Filtration

Where a plant already has oil-injected screw compressors, addressing the oil issue through downstream treatment is also doable: a combination of coalescing filters and activated carbon adsorbers can bring oil content below 0.01 mg/m³. Once the filter elements and activated carbon saturate, their removal capability drops off a cliff. Skip the replacement cycles, skip the periodic oil content testing, and risk is accumulating. During FDA and GMP audits, equipment that is oil-free at the source passes review much more smoothly. Auditors scrutinize downstream oil-removal schemes far more rigorously.

Moisture problems concentrate at elbows and risers

Flour clumps when it hits moisture. PVC powder sticks to the pipe wall. Calcium carbonate absorbs moisture and cakes onto the inner wall of elbows. Let that buildup keep going and you get a blockage. Clearing it means half a day to a full day of downtime. In winter, on pipe sections running outdoors, condensate freezes inside the pipe and can block it completely.

A refrigerated dryer brings the pressure dew point down to 37 to 45°F, generally adequate for short-distance conveying inside a temperature-controlled plant. When pipes cross outdoor cold zones, or when conveying highly hygroscopic materials like starch or citric acid, a desiccant dryer is needed to bring the pressure dew point down to negative 4°F or even negative 40°F. There is one hard rule for dew point selection: it must be lower than the lowest temperature the pipe will encounter along its entire route, with an additional 18°F margin. If the pipe passes through an outdoor section where winter lows hit 23°F, the pressure dew point must be at least 5°F.

The regeneration process in desiccant dryers consumes energy. Heatless regeneration types use about 15% to 18% of the treated airflow as purge air. On a high-volume pneumatic conveying system, that loss is the equivalent of running an extra small blower for nothing. Blower-heated regeneration types bring purge air consumption down to 2% to 3%. The equipment costs more upfront, but the long-term math works out.

Loss of air causing pipe blockage

Once material in the pipe loses its driving airflow, it settles and piles up. In a dilute phase line where material was flying at high speed in suspension, the moment airflow stops, everything drops to the bottom instantly. Elbows and risers clog first. In dense phase conveying, the material plug simply jams in the pipe once pressure is gone. Restarting requires higher pressure than normal operation to blast through. If you cannot clear it, the pipe has to come apart.

On the controls side, when system pressure drops below the safe lower limit, the right sequence is to stop material infeed first, let the material already in the pipe finish traveling on residual airflow, then shut off the air. This avoids leaving material sitting in the pipe. This interlock logic exists on many projects. Projects that lack it see noticeably higher blockage frequency.

Choosing between a dedicated air source and a unified supply

Pneumatic conveying draws a lot of air, and its pressure level often does not match the plant's general-purpose air. A lot of plants set up a separate air source system for conveying. The big flow swings when conveying lines start and stop do not pull down the plant header pressure and do not disturb precision pneumatic equipment. Two separate systems, each sized for its own pressure level, drying spec, and filtration grade.

Unified supply also has its place. A small dense phase line needs 45 psi, the plant already has a 95 psi screw compressor bank, and the conveying line accounts for only 10% to 15% of total air consumption. Running a branch off the main header is cheaper than installing dedicated equipment.

Dedicated

Separate Supply

Unified

Shared Header

Startup conditions and sizing margin

The starting point for sizing calculations is the conveying system's own design parameters: pipe diameter, total length, number of elbows, vertical lift, material type, throughput. When starting on an empty pipe, you first have to fill the pipe with air up to working pressure before feeding material. The instantaneous air demand during this pressurization phase is higher than during steady-state operation. If receiver capacity is not large enough, the pressurization phase pulls the header down and other equipment suffers. As pipes age, material builds up in elbows, wall roughness increases, and conveying resistance gradually climbs. Leaving 10% to 15% airflow margin during sizing is reasonable.