Dilute-phase conveying air supply

How dilute-phase conveying works

In dilute-phase conveying, the air velocity in the pipe is high enough to keep every particle suspended. Material enters the air stream at a feeder, usually a rotary valve, venturi or screw, is accelerated to conveying speed, travels along the route and is separated from the air at a receiving hopper, cyclone or filter. It suits free-flowing, non-abrasive materials such as flour, grain, plastic pellets, sugar, starch, many chemicals and pharmaceutical powders.

There are two main arrangements:

• Positive-pressure systems push material from one source to one or several destinations. A blower supplies air above atmospheric pressure, and a rotary valve feeds material into the pressurised line.
• Vacuum (negative-pressure) systems pull material from one or several sources to one destination. A vacuum pump or exhauster at the receiving end draws air through the line, which suits picking up from bag dump stations, silos or several points.

Combined pull-push systems also exist. In every case the air mover follows the conveying duty: the type of machine is chosen once the required flow and pressure are known.

Pressure and flow together: the conveying duty point

An air mover for dilute-phase conveying is specified by two numbers that belong together: the volume flow of air and the pressure (or vacuum) at a defined point. Flow without pressure cannot prove the machine will convey; pressure without flow cannot prove capacity. Both come from the conveying designer, ideally from tests or proven operating data for that material and route.

The pressure the blower must overcome includes:

• air-only friction in the straight pipe and bends;
• the extra pressure needed to accelerate the material at pickup and after each bend;
• the pressure to lift material in vertical sections;
• losses across the feeder, diverter valves and flexible sections;
• backpressure from the receiving filter or cyclone, which rises as filter bags load;
• a control margin for variation in material and throughput.

Straight-pipe compressed-air friction alone is not the conveying pressure. The solids-loading and acceleration terms often dominate, and they come from the conveying design, not from an air-only formula.

Velocity, flow basis and why the reference condition matters

Conveying velocity is the air velocity at the actual pressure and temperature in the pipe: v = Q ÷ A, where A = πD² ÷ 4 and Q is the actual volume flow at that point. Blower and compressor flows are usually quoted as free air delivered (FAD) or at a standard reference condition, so they must be converted before use in velocity calculations. Air expands as pressure falls along the line, so velocity rises towards the discharge end of a positive-pressure system.

There is no universal dilute-phase velocity. Typical industry ranges are often quoted as roughly 15–30 m/s depending on the material, but the correct pickup velocity for a given product comes from specialist design, the material supplier or test work. Too low and the line can block; too high and pipe wear, bend erosion, product breakage and power all increase.

Choosing the air mover: blower, compressor or vacuum pump

Positive-displacement blowers deliver nearly constant volume as pressure changes, which suits conveying because flow, and therefore velocity, stays steady as line resistance varies. Centrifugal blowers do the opposite, so they need careful control on conveying duty.

Using 7 bar plant air throttled down to conveying pressure wastes most of the compression energy. For anything beyond a small transfer, a dedicated blower is usually the efficient choice.

Feeder leakage and other losses

A rotary valve feeding a positive-pressure line leaks air back through its clearances and through the pockets returning empty to the inlet. This leakage does no conveying work but must be supplied by the blower. It grows as the valve wears and as line pressure rises. The conveying designer’s air flow should state whether feeder leakage is included; if not, it must be added from the valve maker’s data.

Leaks at diverter valves, flexible connections and inspection hatches also reduce the air reaching the pickup. In vacuum systems, any leak upstream of the material pickup reduces the air flow available at the pickup point, while leaks near the pump simply load the pump.

Air quality and temperature

Blower air is usually not dried or filtered to compressed-air standards, but it still needs attention:

• Intake filtration protects the blower and keeps dust out of the product, which matters in dusty plants and in dry Telangana and Andhra Pradesh summers.
• Discharge temperature from a Roots blower rises with pressure ratio and intake temperature. Heat-sensitive products such as sugar, some plastics and powders may need an aftercooler.
• Moisture can cause hygroscopic materials to stick. Where the product demands it, air can be cooled and dried, or a dryer fitted on a compressor supply.
• Oil: Roots and screw blowers are oil-free in the conveying chamber, with oil confined to gears and bearings, which suits food and pharma duties, provided seals and vents are maintained.

Common dilute-phase air problems

How to brief a dilute-phase air supply

1. Name the material, its bulk density, particle size and any sensitivity to heat, moisture or breakage.
2. State normal, peak and future throughput.
3. Describe the route: pipe bore, horizontal and vertical lengths, number and type of bends, diverters and destinations.
4. Obtain the designer’s air flow and pressure or vacuum at a defined point, including feeder leakage.
5. Define site conditions: altitude, intake temperature range and dust.
6. Agree acceptance: throughput achieved at the stated air flow, pressure and power.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand specialist. We sell new and used or refurbished blowers (Roots, screw, centrifugal and side-channel), rent them, service them and supply their spares, alongside compressors, dryers and air treatment for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands. We are not an authorised dealer of any of these brands. We have tie-ups with all major and reputed brands.

For dilute-phase conveying we can:

• measure blower flow, pressure, temperature and power to compare the air supply with the conveying design;
• recommend and supply the right blower or vacuum source, with intake filters, silencers, relief valves and aftercoolers;
• service and overhaul blowers, and supply bearings, gears, seals, belts and filters after identifying the machine from its nameplate;
• rent blowers or compressors for breakdowns and shutdowns.

The conveying system design stays with your specialist; we look after the air side. Our engineers and technicians cover all of Telangana and Andhra Pradesh, and we supply across India.

What to send us

• Material, throughput and route description
• Required air flow and pressure or vacuum from the conveying design
• Present blower or vacuum pump make, model and nameplate photo
• Motor rating, speed and drive arrangement
• The problem you see and when it started

Frequently asked questions

What pressure is used for dilute-phase pneumatic conveying?

Most positive-pressure dilute-phase systems run below about 1 bar (14.5 psig), typically supplied by a Roots blower, with screw blowers used for somewhat higher pressures on longer routes. Vacuum systems work at moderate vacuum. The exact pressure comes from the conveying design for the material, throughput and route, including feeder, bends, lift and receiving filter losses.

Can I use a screw compressor for dilute-phase conveying?

You can, but throttling 7 bar plant air down to below 1 bar wastes most of the compression energy. For anything beyond a small transfer, a dedicated Roots or screw blower is usually far more efficient. A compressor makes sense mainly where higher pressures are needed or where a small occasional duty does not justify a separate blower.

What air velocity is needed for dilute-phase conveying?

There is no universal figure. Typical quoted ranges are roughly 15–30 m/s depending on the material, but the right pickup velocity comes from the conveying designer, material supplier or test data. Remember that blower ratings are in FAD or standard flow, so convert to actual flow at line pressure and temperature before calculating velocity in the pipe.

Why is my pneumatic conveying blower overheating?

Discharge temperature rises with pressure ratio and intake temperature. Common causes are line pressure above design because of blockage or filter loading, a clogged intake filter, a hot or poorly ventilated plant room in summer, or a relief valve set too high. Check discharge temperature and line pressure together, clean the intake and confirm the relief setting.

Should I choose a positive-pressure or a vacuum conveying system?

Positive-pressure systems suit conveying from one source to one or several destinations, and over longer distances. Vacuum systems suit collecting from several sources to one destination, and they keep dust inside the line if a leak occurs. The conveying designer chooses based on material, layout and throughput, and the blower or vacuum pump is then selected for that duty.

What is the difference between a Roots blower and a screw blower for conveying?

Both are positive-displacement machines that deliver near-constant volume as pressure changes, which suits conveying. A Roots blower is simpler and widely used up to about 1 bar. A screw blower compresses internally, so it is usually more energy-efficient, quieter and able to reach higher pressures, at a higher purchase cost. The choice depends on duty point and running hours.

Related pages

• Pneumatic conveying system requirements
• Dense-phase conveying air supply
• Roots blowers
• Screw blowers
• Blowers vs compressors
• Pneumatic conveying enquiry calculator