Dense-phase conveying air supply

What dense-phase conveying is and why its air demand is different

In dense-phase conveying, bulk material moves through the pipe at low velocity as slugs, plugs or a dense moving bed, pushed by compressed air at relatively high pressure. It suits abrasive, friable or blend-sensitive materials such as fly ash, cement, alumina, sand, sugar, salt, detergents and some pharmaceutical and food powders, because low velocity means less pipe wear and less product damage.

Most dense-phase systems in Indian power, cement and process plants use a blow tank, also called a pressure vessel or transporter. The cycle repeats: the vessel fills with material, closes, is pressurised with air, discharges material into the conveying line, the line clears, the vessel vents and the cycle starts again. The air demand therefore rises and falls with every batch, unlike a dilute-phase blower line that runs at steady flow.

Whether a material can be conveyed in dense phase, and at what pressure and air flow, is decided by the conveying system designer, usually from test data on that material. The compressed-air system’s job is to deliver that duty reliably.

The blow-tank cycle, stage by stage

1. Fill. The inlet valve opens and material drops into the vessel while displaced air vents. Conveying air demand is low or nil, but the vent and dust-collector function still matter.
2. Seal. Inlet and vent valves close, usually actuated by small pneumatic cylinders on instrument air.
3. Pressurise. Air enters the vessel to raise it to conveying pressure. This is often the sharpest peak in the cycle, because a large volume has to be filled quickly.
4. Convey. The discharge valve opens and air pushes material along the line. Air flow is sustained at the designer’s pressure-flow point, often with fluidising air in the vessel and booster air injected along the line.
5. Clear. When the vessel empties, pressure falls and air sweeps the remaining material out of the line. Flow can be high for a short time.
6. Vent and reset. Remaining air in the vessel is released to the destination or a filter, and the cycle restarts.

Recording air pressure and flow against each stage is the foundation of a good air supply design. A single average flow hides the pressurisation and line-clearing peaks that actually size the receiver and the controls.

Working out the air supply: throughput, cycles and air per batch

Hourly throughput must reconcile with vessel capacity and cycle time. A scheduling check is simple: batches per hour equals 3,600 divided by the cycle time in seconds, and theoretical throughput equals usable batch mass multiplied by batches per hour. This only checks the schedule; it does not prove the material can be conveyed in that time.

Air per batch has two main parts:

• Pressurisation air: the free-air volume to raise the empty space in the vessel (vessel volume less material volume) from atmospheric to conveying pressure, approximately the gas volume multiplied by the gauge pressure divided by atmospheric pressure.
• Conveying and clearing air: the designer’s air flow multiplied by the conveying and clearing time, plus any fluidising and booster air.

The following illustrative example shows the method only.

In this example the average is about 6.4 m³/min, but during conveying the line needs 8 m³/min, and pressurisation wants 4 m³ in perhaps 30 seconds, a rate above 8 m³/min. A compressor sized for the average would starve the line; one sized only for the peak would cycle heavily. A receiver between them is the usual answer.

Compressor, receiver and controls for dense-phase duty

Oil-injected or oil-free rotary screw compressors are the most common sources for dense-phase systems, with reciprocating compressors on smaller or older plants. Key design points:

• Size the compressor for the conveying flow at the pressure needed, with margin for leakage, booster air and future throughput.
• Use a receiver dedicated to conveying, sized so that pressurisation and line clearing can draw from storage without the pressure falling below the vessel’s minimum. A receiver needs a pressure differential to deliver stored air, so the compressor runs a little above the conveying pressure.
• Separate conveying air from instrument and plant air where possible, or protect instrument air with a check valve and its own receiver so a blow-tank cycle cannot starve valve actuators.
• Regulate air to the vessel and booster points with flow-control valves or orifices as the designer specifies, so the conveying air is set by the system, not by whatever the compressor happens to deliver.
• Coordinate compressor control with the cycle: variable-speed or modulating control suits cyclic demand better than frequent load-unload cycling.

Every extra bar of compressor pressure costs about 7% more power, so the conveying pressure should not be set higher than the designer’s figure plus a sensible control margin.

Air quality for dense-phase conveying

Dense-phase air contacts the material directly. Moisture can make hygroscopic materials such as cement, fly ash, sugar and salt cake on the vessel walls, bridge in valves and build up in the pipe, causing blockages. Oil can contaminate food and pharma products and coat sticky powders.

Typical treatment is a refrigerated dryer giving a pressure dew point of about +3 °C for most mineral duties, with coalescing filtration, or a desiccant dryer at about −20 °C to −40 °C PDP where the material is very hygroscopic or the line is exposed to low temperatures. Food and pharma conveying usually needs oil-free compressed air with appropriate filtration to the class the product specification requires under ISO 8573-1.

Hot conveying air can also affect heat-sensitive materials. A well-maintained aftercooler and dryer keep air temperature close to ambient, which matters in Telangana and Andhra Pradesh summers when compressor rooms run hot.

Common dense-phase air problems

Blockages are often blamed on the conveying system when the cause is an air supply that cannot hold pressure at the critical moment. Logging pressure and flow at the vessel inlet over several cycles usually settles the question.

Where dense-phase conveying is used in India

Dense-phase conveying is common in thermal power stations for fly-ash handling, in cement plants for cement, raw meal and fly-ash transfer, in alumina and aluminium smelting, in foundries for sand, in glass plants for batch materials, and in chemical, food and pharmaceutical plants for powders. Telangana and Andhra Pradesh have many of these industries, including power stations, cement clusters around Nalgonda, Suryapet and Kurnool, and pharma and food plants around Hyderabad, Visakhapatnam and Nellore.

In these plants the conveying compressor is often one of the most critical machines on site, because ash or cement that stops moving can stop the main process. Standby compressors, spares on hand and an arrangement for rental cover during breakdowns are worth planning in advance.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand compressed-air specialist. We sell new and used or refurbished screw and reciprocating compressors, oil-free compressors, dryers and receivers, rent compressors from our own fleet of diesel and electric machines, and provide service, AMC, overhauls and spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands. We are not an authorised dealer for any brand. We have tie-ups with all major and reputed brands.

For dense-phase conveying plants we can:

• log compressor, receiver and vessel pressure and flow over full batch cycles to show whether the air supply meets the conveying design;
• recommend compressor capacity, receiver size, dryer type and the separation of conveying and instrument air;
• supply compressors, dryers and receivers, and service and overhaul the existing plant;
• supply rental compressors when the conveying compressor fails or during shutdowns.

We do not design the conveying system itself; that stays with your conveying specialist. Our engineers and technicians cover all of Telangana and Andhra Pradesh, and we supply across India.

What to send us

• Material conveyed, throughput and the conveying system maker if known
• Conveying pressure and air flow from the system design or the vessel nameplate
• Vessel size, cycle time and number of vessels
• Compressor, dryer and receiver details with nameplate photos
• The problem you see (blockages, slow cycles, compressor trips, wet material)

Frequently asked questions

What air pressure is used for dense-phase conveying?

Most dense-phase blow-tank systems operate at about 2–6 bar (29–87 psig), with some long or high-resistance routes higher. The correct pressure comes from the conveying designer’s test data for the material and route, not from a general rule. The compressor is then set a little above that pressure so the receiver can supply the pressurisation and line-clearing peaks.

How do I size a compressor for dense-phase conveying?

Start from the designer’s conveying air flow and pressure, then build the batch profile: pressurisation air, conveying air, line clearing, fluidising and booster air, and cycle time. Size the compressor for the sustained conveying flow with margin for leaks and growth, and add a dedicated receiver so the short peaks draw from storage instead of from an oversized compressor.

Why does my dense-phase conveying line keep blocking?

Blockages often trace to the air supply: pressure dipping during conveying, a receiver too small for pressurisation, booster air valves failing, or wet air making the material sticky. Material changes and route alterations also matter. Log pressure and flow at the vessel inlet across several cycles and check dew point before changing the conveying settings.

Does dense-phase conveying need dry compressed air?

Yes, usually. Moisture can make cement, fly ash, sugar, salt and many powders cake in the vessel and line. A refrigerated dryer at about +3 °C pressure dew point suits most mineral duties, while very hygroscopic materials or cold routes may need a desiccant dryer at about −20 °C to −40 °C. Food and pharma conveying generally also requires oil-free air.

Can I use plant air for dense-phase conveying?

It is possible on small systems, but the batch peaks can pull plant pressure down and upset instruments and other users. Larger systems usually have a dedicated conveying compressor and receiver. If plant air is shared, protect instrument air with a check valve and its own receiver so a blow-tank cycle cannot starve valve actuators.

What is the difference between dense-phase and dilute-phase conveying air?

Dense phase pushes material slowly as slugs using compressed air, typically at 2–6 bar, in a cyclic batch pattern. Dilute phase carries material suspended in a high-velocity air stream, usually from a blower below about 1 bar or from a vacuum pump, at a largely steady flow. Dense phase needs a compressor, receiver and dryer; dilute phase usually needs a blower.

Related pages

• Pneumatic conveying system requirements
• Dilute-phase conveying air supply
• Pneumatic conveying enquiry calculator
• Air receiver tank sizing
• Cement plant compressed air engineering
• Compressor rental engineering