Compressed air for cement plants

Where is compressed air used in a cement plant?

An integrated cement works with a quarry, raw mill, kiln and cement mill uses air differently from a stand-alone grinding and packing unit. The starting point is a register of air users by area, with what each does with the air and what happens if it stops.

Typical users are shown below. Pressures are typical working values; actual figures come from the equipment maker’s data and site measurement.

Dust-extraction fan volume, combustion air, kiln cooling air and low-pressure aeration are not compressed-air loads. They are handled by fans and blowers. Keeping those boundaries clear avoids sizing compressors for work they will never do.

Bag-filter pulse cleaning: the biggest and most misunderstood user

A cement plant can have dozens of bag filters, each with rows of diaphragm valves firing short pulses into the bags. The average air use is often moderate, but each pulse draws a very high flow for a fraction of a second. If the header pressure falls during the pulse, the bag is not cleaned, filter differential rises and the fan loses draught.

1. Take the filter maker’s air per pulse, the number of valves and the cleaning interval for each filter.
2. Add the filters that may clean at the same time, especially on-demand (differential-pressure) cleaning during process upsets.
3. Size the header receiver and feed line so header pressure recovers between pulses.
4. Fit a local receiver close to large filters on long routes rather than raising the station pressure.
5. Supply dry air: water in the header damages diaphragms, wets bags and in winter or monsoon causes blinding.
6. Measure header pressure during cleaning to confirm the design.

Typical system architecture

Most cement plants run a central compressor station near the kiln or cement mill, sometimes with satellite stations at the packing plant or a remote crusher. A sound arrangement includes:

• Several rotary screw compressors with a sequencer, with fixed-speed machines on base load and a variable-speed unit for trim.
• A wet receiver at the station and dry receivers at large pulse and conveying users.
• Refrigerated dryers for general plant air and a desiccant dryer on the instrument-air branch.
• Separate branches for instrument air, pulse cleaning and conveying, with isolation valves.
• Pressure gauges or transmitters at the far ends of the system so the control room sees delivered pressure.

Separate stations for conveying are common where the transfer distance and pressure differ greatly from plant air. Running a conveying system from the 7 bar main through a reducing valve wastes energy; on an oil-injected screw compressor, every extra bar takes roughly 7% more power.

Air treatment in a dusty plant

Cement dust is fine, abrasive and hygroscopic. It enters compressors through intakes, coats coolers and, once it meets moisture, sets hard in drains and pipes.

• Intake: draw air from a clean, shaded point away from packers, loading bays and clinker handling; use heavy-duty or pre-filtered intake elements and track their differential pressure.
• Cooling: clean oil and air coolers on a schedule based on the dust load, not the calendar. Normal oil-injected screw discharge temperature is about 75–95 °C; a steady rise usually means fouled coolers.
• Drying: refrigerated dryers at about +3 °C pressure dew point suit most plant air; instrument air and conveying air are often better with a desiccant dryer at about −20 °C or −40 °C PDP.
• Filtration: general and coalescing filters after the dryer; point-of-use filters at instrument panels.
• Room: keep the compressor room under slight positive pressure with filtered ventilation air, and duct hot cooling air out.

Reliability, criticality and standby cover

A kiln stop costs far more than any compressor. Loss of instrument air can upset kiln and cooler control within minutes; loss of pulse air lets filters choke, reducing draught and output, and can affect stack emissions.

• Keep N+1 capacity for instrument and pulse-cleaning air, with duplicate or bypassable dryers.
• Alarm on low header pressure and high dew point in the control room.
• Shed non-essential users, such as blow-off and general tools, automatically when pressure falls.
• Fit a valved rental connection near the station so a diesel or electric hire compressor can be tied in quickly during a breakdown or overhaul.
• Plan rental air before annual kiln shutdowns, when refractory removal, cleaning, chipping and blasting need far more air than normal.

Energy in cement-plant air systems

A measured audit, logging power, flow and pressure through normal production and a cleaning-heavy period, gives the specific power of the station and the evidence needed before any purchase.

Cement in Telangana, Andhra Pradesh and India

Telangana and Andhra Pradesh hold one of India’s important cement belts, built on limestone deposits around Nalgonda and Suryapet in Telangana and the Rayalaseema districts and the Krishna valley in Andhra Pradesh, alongside grinding units and packing terminals close to markets and ports. Similar conditions apply in the larger cement regions of Rajasthan, Madhya Pradesh, Chhattisgarh, Karnataka and Tamil Nadu.

• Heat: summer ambient of 45 °C and above, plus radiant heat near the kiln and cooler, reduces compressor margin; choose packages rated for high ambient.
• Dust: service intervals for intake filters, coolers and drains must follow site conditions.
• Monsoon humidity: raises the water load on dryers sharply; a dryer that copes in April may not in August.
• Power quality: voltage dips and feeder trips can stop compressors and dryers; review auto-restart, phase protection and backup for critical air.
• Remote sites: plants far from cities need a spares plan for filters, separators, oils and drain parts.

Common problems and fixes

Sizing a cement plant air system: a worked example

The method is a user register: list every consumer, convert each to average free air in litres per second, then add allowances. The figures below describe an illustrative grinding and packing unit. The per-valve and per-machine values are assumptions for the example; take your own from the filter, packer and valve data sheets. Pneumatic conveying is left out on purpose, because it belongs on its own supply (see the next section).

The air-cannon line needs one step of arithmetic. Each shot empties a 150 litre tank from 7 bar(g) to about 1 bar(g), so the free air to refill it is 150 × (8.013 − 2.013) ÷ 1.013 = 888 litres. Twenty-four cannons every 900 seconds use 24 × 888 ÷ 900 = 23.7 l/s.

Allowances. Add 10% for leaks: 271.2 × 1.10 = 298.3 l/s. Then add 10% for growth: 298.3 × 1.10 = 328.1 l/s. That is 19.7 m³/min, or about 695 cfm.

Station layout. Two duty machines plus one standby, each able to deliver at least 328.1 ÷ 2 = 164 l/s (9.8 m³/min, about 348 cfm). In practice that means three screw machines of about 10–11 m³/min, typically in the 55–75 kW class at 7.5 bar(g), with one fitted with a VSD to trim. Losing any one machine still leaves full design capacity, which is the N+1 cover that instrument and pulse air need.

Pressure. If the filter data sheet asks for 6 bar(g) at the header, allow about 0.5 bar for the dryer, filters and mains, and 0.5 bar for the control band. The machines then unload at about 7 bar(g) (101.5 psig). Setting them higher to cover one distant filter costs about 7% more power per extra bar on every machine.

Local storage at big filters. One 80 litre pulse drawn from a local receiver with an allowed dip of 0.5 bar needs 80 × 1.013 ÷ 0.5 = 162 litres. A receiver of about 200 litres beside each large filter keeps pulses sharp without raising the whole plant’s pressure.

Dense phase or lean phase conveying: how much air, and blower or compressor?

Conveying is usually the largest single air duty in a cement plant, and the choice between dense and lean phase decides the type of machine. Dense phase moves material slowly as slugs or a moving bed at high pressure, using little air. Lean (dilute) phase blows material along at high velocity in a large volume of low-pressure air.

Worked example (illustrative): 30 t/h of fly ash over the same route.

• Dense phase at a loading of 40 kg/kg: air = 30,000 ÷ 40 = 750 kg/h = 0.208 kg/s. At 1.204 kg/m³ that is 0.173 m³/s, or 10.4 m³/min (about 367 cfm), at about 3 bar(g).
• Lean phase at a loading of 10 kg/kg: air = 30,000 ÷ 10 = 3,000 kg/h = 0.833 kg/s, or 41.5 m³/min (about 1,466 cfm), at about 0.8 bar(g).
• Indicative shaft power, from ideal adiabatic compression of 35 °C intake air with assumed package efficiencies of 65% for the compressor and 55% for the blower: about 48 kW for dense phase against about 85 kW for lean phase.

The lean-phase system moves four times the air at a quarter of the pressure. Which one wins on energy depends on the material, distance and lift, so take the final air figure from the conveying supplier’s test data. One rule always holds: do not feed blower duties from the 7 bar(g) main through a reducing valve. Ideal compression work per kilogram of air to 7 bar(g) is about 4.5 times that to 0.8 bar(g), and the throttle throws the difference away. Dense-phase air must be dry, typically about −20 °C pressure dew point or lower, or cement and fly ash cake in the vessel and line. For supply design, see dense-phase conveying air supply.

What drives the cost of a cement plant compressor installation

A quotation for “a compressor” is rarely what a cement plant pays for. The table lists the cost elements of a complete installation and what pushes each one up. Use it to make supplier bids comparable before looking at the total.

Capital is the smaller number. As an illustration, a station drawing an average of 130 kW for 8,000 hours a year uses 1,040,000 kWh. At an assumed tariff of ₹8.50/kWh, that is about ₹88 lakh a year in electricity. Spending at the design stage on the right pressure, a VSD trim machine and the right dryer type usually returns more than cutting the purchase price. Compare bids over their life with the lifecycle cost comparison calculator, write the specification with procurement and bid evaluation, and see the screw compressor price guide for how machine prices are built up.

Compressor house design for dust and heat

Almost all the electrical power a compressor draws ends up as heat. Two 75 kW machines on duty reject about 150 kW into whatever space they stand in.

Worked example (illustrative). If that heat goes into the room and the room may run only 5 °C above a 45 °C ambient, the ventilation air needed is 150 ÷ (1.11 kg/m³ × 1.005 kJ/kg·K × 5 K) = 26.9 m³/s, or about 96,800 m³/h, all of it through dust filters. If each machine’s cooler exhaust is ducted straight outside, about 90% of the heat (135 kW) leaves through the ducts. The coolers heat their air by much more than 5 K, say 15 K, so their cooling air is 135 ÷ (1.11 × 1.005 × 15) = 8.1 m³/s. The 15 kW left in the room needs 2.7 m³/s. The filtered louvres then pass about 8.1 + 2.7 = 10.8 m³/s (about 38,700 m³/h), around 40% of the undusted figure. Take the real cooling-air flow from each machine’s data sheet. Ducting the hot air out is the single most effective design decision for a cement compressor house.

Intake temperature matters too. Raising intake air from 35 °C to 50 °C cuts the mass of air each machine delivers by about 4.6% (308.15 ÷ 323.15 = 0.954), and pushes oil temperature towards the trip point.

• Location. Upwind of the prevailing wind, and away from packing plants, clinker coolers, coal mills and truck loading.
• Intake filtration. Louvres sized for a low face velocity, about 2 m/s or less, behind panel pre-filters that can be changed without a shutdown. Keep the room under slight positive pressure.
• Hot-air ducting. A separate duct for each machine, with a damper, and duct losses within the cooler fan’s allowance or an extract fan added. Recirculation of hot air must be impossible.
• Sealing. Cable trenches, pipe entries and doors sealed so that dust cannot bypass the filters.
• Monsoon. Raised plinths, drain trenches to an oil-water separator and no rainwater path into the intake louvres.
• Maintenance access. At least 1 m around each machine, room to withdraw coolers, and a lifting beam over the airend and motor.
• Cooler cleaning. Blow coolers from the clean side outward at low pressure, on a fixed schedule tied to the pressure-drop or temperature trend.

How Reddy Compressors helps

Reddy Compressors has served industry since 2002 from Tarnaka, Secunderabad. As an independent multi-brand specialist, we sell, rent and service compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes; we are not an authorised dealer for any brand. We have tie-ups with all major and reputed brands.

• Sales: new and used or refurbished rotary screw compressors, dryers, filters and receivers for plant, instrument and pulse air; brand-new machines for long-term project sites.
• Rental: diesel and electric machines from 50 to 2,000 cfm and 2 to 30 bar from our own fleet, for breakdowns, kiln shutdowns and project work.
• Service and AMC: over 20 service engineers and technicians covering all of Telangana and Andhra Pradesh, for preventive maintenance, breakdown repair and overhaul.
• Spares: identified from nameplate details and the old part, with fit confirmed before quoting, and supplied across India.
• Audits: flow, pressure, dew-point and leak measurement to support decisions.

Our track record. We have supplied compressors to leading companies in cement across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a plant’s own compressor fails – backed by our own service team.

What to send us

• Plant type: integrated works, grinding unit or packing terminal.
• Compressors installed: make, model, nameplate photo, running hours, set pressure.
• Number of bag filters and any conveying systems with their air data.
• Dryers and filters fitted and any water or dew-point problems.
• For hire: cfm, pressure, duration, diesel or electric, operators needed.
• Plant location and site access.

Frequently asked questions

How much compressed air does a cement plant need?

There is no single figure per tonne of cement. Demand depends on the number and size of bag filters, conveying systems, packers, blasters and instrument users, and on whether the site is an integrated plant or a grinding unit. Build a register of users with their pressure, flow and duty, add realistic simultaneous peaks, then confirm by logging flow and pressure at the station.

Why are my bag filters not cleaning properly?

The usual cause is low pressure at the filter header during pulses, from an undersized header, a long feed line or too little local storage. Wet air that damages diaphragms or wets the bags, failed solenoids and over-frequent cleaning are other causes. Measure header pressure while the filter pulses, add a local receiver if needed and make sure the air is dry.

What dryer is right for cement plant compressed air?

For general plant air and pulse cleaning, a refrigerated dryer giving about +3 °C pressure dew point is usually suitable, provided it is sized for monsoon humidity and high ambient. For instrument air, outdoor lines and conveying, a desiccant dryer at about −20 °C or −40 °C pressure dew point gives a better margin against condensation and caking.

Can pneumatic conveying run from the plant air system?

It can for small transfers, but large dense-phase systems are often better on their own compressor at the pressure the conveying maker specifies. Feeding them from a 7 bar main through reducing valves wastes energy and can pull down pressure for pulse cleaning and instruments during transfer cycles. Dry air is essential either way to prevent blockages.

How do I protect compressors from cement dust?

Take intake air from a clean, shaded location, use heavy-duty or pre-filtered intake elements and check their differential pressure often. Keep the compressor room under slight positive pressure with filtered air, clean coolers before temperatures rise, and inspect drains frequently. Service intervals should follow the actual dust load on site rather than a fixed calendar.

Can I rent a compressor during a kiln shutdown?

Yes. Kiln and cooler shutdowns need extra air for refractory removal, cleaning, chipping, blasting and air tools, often far from the main station. Diesel portable compressors, or electric units where power is available, can be hired for the shutdown period with operators if required. Plan the hire with the shutdown schedule so machines and hoses are on site before work starts.

What type of compressor is used in a cement plant?

Most cement plants use oil-injected rotary screw compressors for plant air: bag-filter pulse cleaning, packers, air cannons, instruments and maintenance. They work at about 7 bar(g), with refrigerated dryers for general air and desiccant dryers for instrument and conveying air. Dense-phase conveying is often supplied by a separate screw compressor at a lower pressure. Lean-phase conveying, coal firing and silo aeration use rotary-lobe or screw blowers. Very large plants may use centrifugal compressors. A typical station runs two duty machines plus a standby, with one VSD machine to trim.

How much does a compressor installation cost in a cement plant?

Installation cost for a cement plant air compressor depends far more on scope than on the machine. The main elements are the compressors, dryers and filters, receivers, compressor house and ventilation, electrical feeders and drives, the distribution header, condensate treatment, erection and commissioning. Dust-filtered ventilation, long cable runs and a shutdown tie-in add the most. Over a typical service life, the electricity bill is usually several times the cost of the whole installation. Compare bids element by element and over the full life, not on the price of the compressor alone.

Is a blower or a compressor used for cement conveying?

Both, depending on the system. Lean-phase conveying runs at up to about 1 bar(g) with large air volumes, so it uses rotary-lobe or screw blowers. Dense-phase conveying needs about 2–6 bar(g) with much less air, so it uses a compressor, usually with a dryer at about −20 °C pressure dew point. In the illustrative example above, 30 t/h of fly ash needs about 367 cfm at 3 bar(g) in dense phase, against about 1,466 cfm at 0.8 bar(g) in lean phase. Never feed blower duties from the 7 bar(g) main.

Should a cement plant use oil-free or oil-injected compressors?

For most cement plant duties, an oil-injected screw compressor with good filtration is the normal and economical choice. Bag filters, packers, air cannons and tools tolerate the small residual oil left after a coalescing filter. Instrument air needs the right dryer and filters more than oil-free compression. Oil-free machines are worth considering where air touches a product sensitive to oil, or where conveying air must stay free of oil to protect the material or the filters. Choose by the ISO 8573-1 class each user needs, not for the whole plant.

Related pages

• Compressed air across industries
• Air supply for dense-phase conveying
• Refrigerated air dryers
• Compressor rental and temporary air
• Compressed air audits
• Compressor annual maintenance contracts