Medium-pressure rotary screw compressors

What does medium pressure mean for a screw compressor?

Manufacturers commonly offer packaged screw compressors in pressure variants such as 7.5, 8.5, 10 and 13 bar. Standard plant air in India is usually generated at about 7–8 bar (101.5–116 psig). Medium pressure, in common usage, refers to the 10–13 bar variants (about 145–190 psig). Above that, two-stage screws, piston compressors or boosters are normally used.

These bands are typical, not formal standards. What matters is the pressure your process needs and the FAD the compressor delivers at that pressure.

Start with point-of-use pressure

A request for “a medium-pressure compressor” should be turned into a precise statement: this process needs this flow at this pressure at this point of use. Compressor discharge pressure is never the same as the pressure the user receives.

The required compressor pressure is a stack of genuine needs:

1. Pressure required at the user, taken from the equipment manufacturer’s data, not habit.
2. Plus pressure drop through dryer and filters at full flow, including allowance for filter loading.
3. Plus distribution loss through headers, drops, hoses, regulators and quick couplings.
4. Plus a control band for load/unload or VSD control.

If that stack comes to 9.5 bar, a 10 bar variant is reasonable. If it comes to 7.5 bar but the plant runs at 10 bar “to be safe”, the extra pressure is wasted energy for every running hour.

More discharge pressure is not automatically better

Higher pressure has real costs beyond electricity:

• About 7% more power for each extra bar at the same flow.
• Lower FAD for the same motor size, because the airend works against a higher ratio.
• More air lost through every leak, since leakage rises with pressure.
• Unregulated users, such as blow guns and open blow-offs, consume more air.
• Higher discharge temperatures and more stress on seals, hoses and components.

When users report low pressure, raising the compressor setpoint is the reflex, but it treats the symptom. Often the better fix is to preserve pressure that is already being generated: clear a blocked filter, enlarge a restricted drop, shorten a hose run, or replace undersized couplings.

Medium-pressure duty or a pressure fault?

Two very different situations lead plants to consider a medium-pressure compressor.

In the second case, a higher-pressure compressor might seem to help, but it only pushes more air through the same restrictions and leaks. Measure pressure at the compressor outlet, after the dryer and filters, and at the user. The differences show where pressure is being lost.

FAD and pressure must be read together

cfm tells you how much air; pressure tells you its condition. A compressor is fully defined only by its FAD at a stated pressure, measured to a recognised standard such as ISO 1217. Motor kW alone does not define capacity, because the same motor delivers less air at higher pressure.

This matters when comparing variants. The 10 or 13 bar version of a model generally delivers less FAD than the 7.5 bar version with the same motor. If you need both higher pressure and the same flow, the compressor may have to be one size larger, which affects price, power and cooling.

• Check each offer’s FAD at your required pressure, not at the model’s best rating.
• Compare package specific power in kW per m³/min at that pressure.
• Confirm the maximum working pressure, the unload pressure and the safety valve setting.

One user or the whole plant?

Often only one machine or one area needs medium pressure. Raising the whole plant to serve it penalises every other user. Compare the options:

Where medium-pressure air is used in Indian plants

Genuine medium-pressure duty exists, but it is narrower than many plants assume. Typical cases include:

• Fibre laser cutting machines using compressed air as assist gas, where the machine maker specifies a pressure and dryness at the cutting head.
• Certain pneumatic presses, clamps and riveting tools sized for higher pressure to achieve force in a compact cylinder.
• Long or extended networks, for example in large campuses or quarries, where designers accept a higher generation pressure after reviewing pipe losses.
• Some textile, packaging and moulding machines whose data sheets specify 8–10 bar at the machine inlet.
• Pneumatic test benches and leak-testing equipment that need a regulated pressure above standard plant air.

In each case, confirm the requirement from the equipment data sheet and measure what actually reaches the machine. A machine that “needs 10 bar” sometimes needs 6 bar at its cylinder and is losing the rest in its own filter-regulator, hoses and valves.

A practical method to decide

1. List every user that is said to need more than standard plant pressure, with its flow and hours.
2. Measure pressure at the compressor outlet, after treatment and at each of those users under load.
3. Fix obvious losses first: blocked filters, undersized drops, long hoses and leaking couplings.
4. Recalculate the pressure stack and decide whether the whole system, one area or one machine needs medium pressure.
5. Compare options in kWh per year, including the lower FAD of higher-pressure variants.

Demand profile and control

Sizing should follow logged demand, not connected load. Record pressure and compressor load across all shifts for about a week to find minimum, average and peak demand. Steady demand suits a fixed-speed machine running loaded; variable demand suits a VSD unit, which also holds a tighter pressure band. A tighter band can itself allow a lower average setpoint, which saves energy at medium pressure just as at standard pressure.

A receiver sized for the duty reduces load/unload cycling and supplies short peaks. For a process with a sharp momentary demand, local storage near the user is often more effective than adding compressor capacity.

Treatment, temperature and components at medium pressure

• Dryers and filters must be rated for the maximum working pressure, with margin.
• At higher pressure, refrigerated dryers handle less water per m³ of free air, but they must still be sized at real inlet temperature and ambient.
• Refrigerated dryers typically deliver a PDP of about +3 °C; desiccant dryers −20 °C or −40 °C where needed.
• Receivers, hoses, valves and fittings must be rated for the pressure, with correctly set safety valves.
• Normal oil-injected screw discharge temperature is about 75–95 °C; higher pressure brings it closer to the trip point in hot rooms, so ventilation matters.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka in Secunderabad since 2002, is an independent multi-brand specialist and not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We sell new and used/refurbished compressors and we rent and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. More than 20 service engineers and technicians cover Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India.

• Pressure surveys from compressor to user to separate genuine duty from system losses.
• Neutral comparison of compressor variants, dedicated machines and boosters.
• Supply of medium-pressure screw compressors, dryers, filters and receivers rated for the duty.
• Service, AMC and spares identified from make, model, serial number and nameplate.
• Rental compressors from our own fleet, 50–2,000 cfm and 2–30 bar, for trials, breakdowns and projects.

What to send us

• The machine or process that needs higher pressure, with its data sheet if available.
• Required pressure and flow at the point of use.
• Present compressor make, model, setpoint and nameplate photo.
• Pressure readings at the compressor and at the user, if taken.
• Working hours and any air quality requirement.

Frequently asked questions

What pressure is a medium-pressure screw compressor?

In common usage, medium-pressure screw compressors are the 10–13 bar variants, about 145–190 psig, compared with standard plant air at about 7–8 bar. These are typical industry bands rather than formal standards. The right choice depends on the pressure your process needs at the point of use, plus genuine losses through treatment and pipework.

Will a 10 bar compressor solve my low-pressure problem?

Not necessarily. If a 7.5 bar compressor gives only 5.5 bar at the user, the cause is usually leakage, blocked filters, undersized pipes, long hoses or a capacity shortfall. A higher-pressure compressor pushes more air through the same losses and uses more energy. Measure pressure along the line first to locate the loss.

Does a higher-pressure compressor give less air?

Yes, generally. With the same motor, a compressor variant built for higher pressure delivers less FAD because the airend works against a higher pressure ratio. If you need higher pressure and the same flow, you may need a larger machine. Always compare tested FAD at your actual working pressure.

How much extra power does higher pressure use?

A widely used rule of thumb is about 7% more power for each additional bar of discharge pressure at the same flow. Over a year of long running hours this becomes a large number of kWh. That is why pressure should be set from genuine point-of-use requirements, not raised as a safety margin.

Should I use a booster instead of a medium-pressure compressor?

A booster suits a small flow needed at higher pressure, supplied from the existing plant-air network. It avoids raising the whole system’s pressure. For a large user running long hours, a dedicated compressor may be more efficient. Compare flow, hours, energy in kWh and maintenance for both options.

Can I raise the pressure setting on my existing compressor?

Only within the manufacturer’s rated maximum working pressure, and with the safety valve, motor load and discharge temperature checked. Raising the setpoint increases power, reduces FAD and increases leakage. If the compressor is a 7.5 bar variant, operating it near 10 bar is usually not possible without changing the machine.

Related pages

• Rotary screw compressors hub
• Low-pressure rotary screw compressors
• High-pressure rotary screw compressors
• Booster compressors
• Pressure profiling