Booster compressors for high-pressure air

What is a booster compressor and how does it work?

A conventional compressor draws air at atmospheric pressure. A booster draws air that has already been compressed by a primary compressor, usually a rotary screw or portable diesel unit, and compresses it further. Most air boosters are reciprocating machines: one or more piston stages, intercooled between stages, driven by an electric motor or diesel engine.

Because the booster starts from an elevated inlet pressure, each cubic metre it swallows already contains several times the mass of air at atmospheric conditions. That is why a relatively compact booster cylinder can handle a large free air flow. It is also why the booster’s output depends completely on the primary compressor: if the inlet pressure or flow falls, the booster’s delivered air falls with it.

The pressure ratio across a stage drives the discharge temperature. Keeping the ratio per stage moderate and cooling between stages keeps temperatures within the design limits of valves, rings and lubricant. High final pressures therefore need more stages, not just a stronger motor.

When is a booster the right answer?

A booster makes sense when the required pressure is beyond what the existing or chosen primary compressor can deliver, and when buying a single high-pressure machine for the full flow would be impractical or wasteful. Typical cases include:

• Deep borewell and DTH drilling, where a rig’s compressor supplies the bulk of the air and a booster lifts it to the higher pressure needed to overcome water head at depth.
• PET bottle blowing, where low-pressure air of about 7–10 bar (101.5–145 psig) is boosted to the high-pressure blowing stage, often in the range of 25–40 bar (362.6–580.2 psig) depending on the machine.
• Pipeline and vessel pressure testing, where only part of the system needs high pressure, under strict test procedures.
• Raising pressure for one user, so that a whole plant does not have to run at a higher pressure for one machine.

The last point matters for energy. Running an entire plant one bar higher costs about 7% more compressor power. Boosting only the small flow that needs high pressure is often far cheaper than raising the setpoint of every compressor.

A booster is not a fix for pressure that is lost through leaks, blocked filters, undersized pipework or a wrongly set regulator. Find and fix those causes first.

How do you size a booster?

1. Define the final pressure needed at the point of use, not the highest pressure available. Add the losses between the booster and the user.
2. Define the flow required at that pressure, expressed as free air delivery (FAD) in cfm or m³/min. Pressure and flow must both be met; one cannot make up for the other.
3. Confirm the booster’s inlet pressure and the flow the primary compressor can supply at that pressure, after its dryer and filters.
4. Check the duty. Continuous drilling or production is a very different load from short test cycles.
5. Select the booster from the manufacturer’s performance data for that inlet pressure, final pressure and speed. Booster capacity is always quoted against a stated inlet pressure.
6. Check the primary compressor’s margin. The primary must feed the booster and any other users at the same time without its pressure collapsing.

In principle, the booster’s FAD cannot exceed the free air the primary compressor delivers to it. If the primary supplies 1,000 cfm and the booster needs 1,000 cfm, nothing is left for anything else on that line.

Booster vs a single high-pressure compressor

Boosters for borewell and DTH drilling

On borewell rigs, compressors typically supply 600–1,500 cfm at 200–400 psig (13.8–27.6 bar), with special units up to about 35 bar (507.6 psig). As holes go deeper and the water column rises, the air must overcome more hydrostatic head to lift cuttings and keep the hammer running. Where the primary machine cannot reach the needed pressure, a booster can be paired with it.

For drilling, the booster and compressor must be matched as a set: inlet pressure to the booster, the flow both machines can sustain, hose ratings, and the hammer manufacturer’s pressure and flow needs. The booster also adds heat and needs its own cooling, lubrication checks and safety devices. Our booster-assisted drilling page goes into the drilling side in more depth.

What safety controls must a booster system have?

Higher pressure means more stored energy in every hose, pipe and vessel. A booster system should include, as a minimum:

• A correctly sized safety valve on each stage and on the final discharge, with nothing that can isolate a stage from its relief.
• High discharge temperature protection on each stage, and low oil pressure protection where the booster has pressure lubrication.
• Low inlet pressure protection, so the booster does not run on a starved inlet with an excessive pressure ratio.
• Intercoolers and an aftercooler with moisture separation and drains between stages.
• Hoses, couplings, valves and pipe rated above the final working pressure, with whip restraints on hose connections.
• A controlled shutdown and depressurisation sequence, and a clear lock-out procedure before maintenance.

For pneumatic pressure testing, the client’s or engineer’s procedure governs. Stored energy in a pneumatic test is far greater than in a hydrostatic test, and exclusion zones and step-wise pressurisation are standard practice.

Common booster problems and their causes

Booster sizing worked example: 2 m³/min at 35 bar(g) from 7 bar(g) plant air

An illustrative PET line needs 2.0 m³/min (70.6 cfm) of free air at 35 bar(g) for final blowing. The plant already runs screw compressors at 7 bar(g). This is how the booster is sized, step by step. For simplicity, free air is referred to 1.013 bar(a) and the booster inlet is taken at 35 °C after the dryer.

1. Convert to absolute pressure. Inlet 7 + 1.013 = 8.013 bar(a). Outlet 35 + 1.013 = 36.013 bar(a).
2. Overall pressure ratio. 36.013 ÷ 8.013 = 4.49. This is the number that sets stage count and temperature, not the 28 bar difference.
3. Stage count. In one stage, the ideal discharge temperature from 35 °C would be 308.15 K × 4.49^0.286 = 473 K, about 200 °C, before any inefficiency. That is too hot for valves and lubricant. Two intercooled stages share the ratio: √4.49 = 2.12 per stage, giving about 109 °C ideal and roughly 125–130 °C in practice. So the answer is two stages.
4. Flow at the booster inlet. The booster sees compressed air, not free air: 2.0 × 1.013 ÷ 8.013 = 0.253 m³/min (about 253 litres per minute, 8.9 cfm actual). This is why booster cylinders look small for the duty. Never compare a booster’s swept volume with the FAD of a low-pressure compressor.
5. Power. The ideal adiabatic power for two stages with full intercooling is 2 × 3.5 × (101,325 Pa × 2.0 ÷ 60 m³/s) × (2.12^0.286 − 1) = 5.66 kW. At an assumed 70% combined adiabatic and mechanical efficiency, the shaft power is 8.1 kW, so an 11 kW motor is the practical selection.
6. Primary compressor check. The primary must supply the full 2.0 m³/min of FAD on top of the plant’s existing demand at its 7 bar(g) peak, plus the booster’s drain and cooling losses. If the primary is 10 m³/min and the plant peak is already 7.5 m³/min, the booster leaves only 0.5 m³/min spare. That is acceptable for trials but thin for a production line with no standby.

The same method gives a quick stage-count guide. The temperatures assume a 35 °C inlet to each stage, full intercooling and an 80% stage efficiency.

Makers set their own limits, and a booster with tight clearances or poor cooling runs hotter than this table. Use it to sense-check a quotation, not to replace the maker’s data.

Energy: booster route vs a dedicated high-pressure compressor

For the same 2.0 m³/min at 35 bar(g), thermodynamics alone puts the two routes almost level. The table uses the efficiency assumptions above, a 92% motor efficiency and a primary screw compressor specific power of 6.8 kW per m³/min at 7 bar(g).

The real difference lies in how the primary is loaded. Take an illustrative 10 m³/min fixed-speed primary drawing 68 kW loaded and 28% of that (19.0 kW) unloaded, and averaging 7.5 m³/min. Its power is 19.0 + 49.0 × 0.75 = 55.8 kW. With the booster’s 2.0 m³/min added, it becomes 19.0 + 49.0 × 0.95 = 65.6 kW. The extra 9.8 kW buys 2.0 m³/min, or 4.9 kW per m³/min.

The booster route then totals 9.8 + 8.8 = 18.6 kW against 22.3 kW, a saving of 3.7 kW (17%). Over 6,000 hours a year that is about 22,300 kWh, or about ₹1.9 lakh at an illustrative ₹8.50/kWh tariff. The saving disappears if the primary has to run at a higher set pressure to feed the booster, because every extra bar adds about 7% to its power across the whole plant. It also disappears if the extra load pushes the primary into a second machine.

Booster applications at a glance

A pipeline fill shows why flow, not pressure, often governs the hire. Take 2 km of 300 mm bore pipe, holding 141.4 m³ (illustrative). Filling it to 15 bar(g) needs 141.4 × 15 ÷ 1.013 = 2,093 m³ of free air. At 25 m³/min (883 cfm) through a booster, that takes about 84 minutes, before any hold period.

Booster inlet conditions: why dry, cool feed air matters

A booster magnifies whatever is in its feed. Three effects need checking before commissioning.

• Water condenses between stages. Undried air saturated at 7 bar(g) and 35 °C carries 4.4 g of water per kg of air. At 35 bar(g) and the same temperature it can hold only 1.0 g/kg, so 78% of the vapour drops out. If the 2.0 m³/min example were fed undried, that would be about 0.47 kg/h, or 11 litres a day, landing in coolers and cylinders unless it is drained.
• Dew point rises with pressure. Air dried by a refrigerated dryer to a 3 °C pressure dew point at 7 bar(g) has a dew point of about 26 °C once boosted to 35 bar(g). Any high-pressure pipe in an air-conditioned hall, or exposed to monsoon nights, will then run wet. For dry final air, size the dryer for the dew point at the booster outlet. Use a desiccant dryer upstream, or a high-pressure dryer downstream.
• Inlet temperature and pressure move discharge temperature. Each stage’s absolute discharge temperature scales with its absolute inlet temperature, so a 45 °C summer feed runs every stage hotter. A falling inlet pressure is worse. On the example booster, a drop from 7 to 5 bar(g) raises the overall ratio from 4.49 to 5.99. The per-stage ideal discharge temperature then rises from about 109 °C to about 125 °C, while delivered flow falls.

The safety controls listed earlier on this page are the minimum. Add these:

• A low inlet-pressure trip or unload.
• A non-return valve between the primary and the booster, so that stored high-pressure air cannot back-feed.
• A hose, coupling and whip-check rating at or above the relief valve setting.
• Lubricant suited to the stage discharge temperature, because carbon deposits in hot high-pressure lines are a fire risk.

Never use an oil-lubricated booster for oxygen-enriched gas.

Compact booster packages for remote sites

A compact, efficient booster for a remote site is mostly a matter of matching. The booster, primary, power source and site must work as one system.

For drilling packages, see booster-assisted drilling air and DTH hammer air requirement. For temporary duty, see high-pressure air hire and diesel or electric.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand compressor specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. For booster requirements we:

• Work through final pressure, flow at that pressure, primary compressor capability and duty before recommending anything.
• Supply new and used/refurbished compressors and booster arrangements, and high-pressure units from our own rental fleet (rentals range 50–2,000 cfm and 2–30 bar).
• Support borewell and DTH rig owners with sales, rentals, service and spares for all types, brands and models of borewell compressors.
• Service and overhaul reciprocating boosters, including valves, rings, packing, intercoolers and safety devices, with over 20 service engineers and technicians covering Telangana and Andhra Pradesh.
• Identify spare parts from the make, model, serial number, nameplate photo and old part, and confirm fit before quoting. Equipment, rentals and spares are supplied across India.

Our team has supplied high-pressure air for major projects, including a temporary package for a natural-gas development at Kakinada in 2008–09, with compressors rated 1,100 cfm at 300 psig (20.7 bar).

What to send us

• The application and the final pressure needed at the point of use, in bar or psig.
• The flow needed at that pressure, and how long it must be sustained.
• Details of the primary compressor: make, model, rated flow and pressure, and a nameplate photo.
• Site conditions: power or diesel, location, ambient temperature.
• For drilling: hole diameter, depth range and hammer details.
• For an existing booster: make, model, serial number and the fault or symptom.

Frequently asked questions

What does a booster compressor do?

A booster compressor takes air that has already been compressed by another compressor and raises it to a higher pressure. It is used when a process or drilling duty needs more pressure than the main compressor can produce. Its output depends on the flow and pressure the main compressor supplies, so the two machines must be sized together.

Can a booster increase the airflow of my compressor?

No. A booster raises pressure, not free air delivery. The free air it delivers cannot be more than the primary compressor supplies to it. If you are short of airflow, you need more compressor capacity. If you are short of pressure for one user, a booster may be the efficient answer, provided the primary has enough spare flow.

Is a booster cheaper than buying a high-pressure compressor?

It can be, when you already own a suitable primary compressor or need high pressure for only part of the air. A booster then avoids running the whole system at high pressure. Where all the air is needed at high pressure for long hours, a dedicated high-pressure compressor is often simpler. We compare both on flow, pressure, duty and running hours.

Do I need a booster for deep borewell drilling?

You need one when the rig’s compressor cannot supply the pressure the hammer needs against the water head at depth. Borewell compressors typically cover 200–400 psig (13.8–27.6 bar), and special units reach about 35 bar. If your machine is at its limit, we can advise on a booster, a higher-pressure compressor or a rental unit.

What pressure do PET blowing machines need?

PET stretch blow moulding usually needs low-pressure air for pre-blow and controls and high-pressure air for final blowing, often in the range of 25–40 bar depending on the machine and bottle. Many plants use a screw compressor feeding a high-pressure booster. The blowing machine maker’s figures for pressure and flow are the starting point for sizing.

Why does my booster overheat?

Common causes are low inlet pressure, which raises the pressure ratio per stage, fouled intercoolers or aftercoolers, leaking valves, and poor ventilation around the machine. Check the inlet pressure with the system under load first. In hot Indian summers, cooler cleanliness and airflow around the booster are especially important. We can inspect and overhaul the machine.

What is a compressor booster, and is it the same as an air amplifier?

No. A compressor booster is a motor- or engine-driven machine, usually reciprocating, that takes air from a primary compressor and compresses it further in one or more stages. A pneumatic air amplifier, also called a pressure intensifier, is driven by plant air itself. It typically doubles line pressure for a small flow, and it exhausts its own drive air, so its energy efficiency is poor. Amplifiers suit occasional small users such as a clamp or test bench. A booster suits continuous duties such as PET blowing, drilling or nitrogen filling.

How many stages does a booster compressor need?

Use absolute pressures. Divide the outlet bar(a) by the inlet bar(a) to get the overall ratio, then split it into stages of roughly 2–3 each, so that each stage’s discharge stays below the maker’s temperature limit. Boosting 7 to 35 bar(g) is a ratio of 36.013 ÷ 8.013 = 4.49, which needs two stages at about 2.12 each. Boosting 20 to 35 bar(g) is only 1.71, so one stage is enough. Boosting nitrogen from 7 to 200 bar(g) is a ratio of about 25, which needs three to four stages.

Are compact booster compressors efficient for remote sites?

They can be. If the site’s primary compressor has spare capacity that would otherwise run unloaded, a booster turns that idle capacity into high-pressure air at low marginal energy. In the worked example on this page, the booster route uses 18.6 kW against 22.3 kW for a dedicated high-pressure machine. On a remote site, efficiency also depends on a drive suited to the power source, coolers rated for 45 °C ambient, good dust filtration and a primary sized for the booster’s full free-air intake.

Does a booster compressor need dry air at its inlet?

Yes. Compressing wet air from 7 to 35 bar(g) drops about 78% of its water vapour into the booster’s coolers and cylinders. A 3 °C pressure dew point at 7 bar(g) also becomes about 26 °C at 35 bar(g), so the high-pressure line can still run wet. Feed the booster dried, filtered air below about 40 °C, drain every separator, and size the dryer for the dew point needed at the final pressure.

Related pages

• High-pressure reciprocating compressors
• Booster-assisted drilling air
• PET blowing compressors
• Multi-stage reciprocating compressors
• High-pressure temporary air packages
• Borewell and DTH compressor library