Low-pressure rotary screw compressors

What is a low-pressure screw compressor?

Most packaged screw compressors in Indian plants are built for general plant air at about 7–10 bar (101.5–145 psig). A low-pressure screw compressor uses an airend, rotor profile and built-in pressure ratio designed for a lower discharge pressure, typically in the 2.5–5 bar range. It sits between blowers, which serve pressures below about 1–2 bar, and standard compressors.

Because the pressure ratio is lower, the same motor power moves much more air. That is the whole point: for a process that only needs 3 or 4 bar, a dedicated low-pressure machine can deliver the required flow with far less energy than a standard compressor working against a regulator.

Where low-pressure air is genuinely used

Low-pressure compressors make sense where one user or one group of users needs a large, steady flow at a modest pressure. Common examples in Indian industry include:

• Pneumatic conveying of cement, fly ash, lime, flour, sugar and plastic granules, especially dense-phase systems.
• Glass container manufacture, where forming machines use large volumes of low-pressure air.
• Textile processes such as air-jet looms and texturising, depending on machine design.
• Fermentation and bioprocess air in pharmaceutical and food plants, usually with oil-free compression and sterile filtration.
• Wastewater treatment and aeration where depth or diffuser design needs more than blower pressure.
• Agitation, flotation and process air in chemical and mineral plants.

In each case the pressure the process needs should come from the process designer or the equipment manufacturer, measured at the point of use, not assumed from habit.

Pressure is not free: the energy case

As a working rule, every extra bar of compressor discharge pressure needs about 7% more power for the same flow. The effect is larger when the gap is big. A process that needs 3.5 bar but is fed from a 7.5 bar compressor through a regulator is paying to compress air to nearly twice the absolute pressure it uses (about 8.5 against 4.5 bar absolute), and the extra energy becomes heat.

The comparison should be made in kWh per year, using measured flow and running hours:

1. Establish the flow the low-pressure process uses in m³/min or cfm FAD, and its operating hours.
2. Find the specific power of the existing supply (kW per m³/min at the pressure actually generated).
3. Obtain the specific power of candidate low-pressure machines at the required pressure, from tested data.
4. Multiply the difference in kW by annual hours to get the kWh difference.
5. Weigh that against purchase, installation, separate piping and the maintenance of an extra machine.

The answer varies widely. For a large, continuous low-pressure user the case is usually strong. For a small or intermittent user it may not justify a separate compressor, and a well-set regulator on the main supply is the practical choice.

Low-pressure compressor or a low-pressure problem?

Many plants that ask about low pressure actually have a different problem: the main system is not delivering its normal pressure. These are two separate situations and must not be confused.

Low system pressure does not automatically mean the compressor is too small. Typical causes include leakage, blocked filters and separators, undersized pipes, long hoses and quick couplings, a dryer with high pressure drop, a failing inlet valve, or compressors that are not loading because of a control fault. Buying more capacity without diagnosis often moves the problem rather than solving it. Measure first, diagnose second, size third.

One special user should not set the whole plant pressure

The reverse problem is common too. One machine needs 8 bar, so the whole plant runs at 8.5 bar while most users need only 5 or 6. Every leak and every open blow-off then consumes more air, and every compressor uses more power.

• Separate a large low-pressure user onto its own compressor and network where flow and hours justify it.
• Serve one small high-pressure user with a local booster rather than raising the entire system.
• Check whether the “high-pressure” user really needs that pressure, or whether its supply line is restricting it.
• Reduce main header pressure step by step while watching critical users, once the outliers are dealt with.

Airflow still decides the size

A pressure label alone does not define a compressor. The machine must deliver the required free air delivery (FAD) at the required pressure, under real site conditions. FAD should be quoted to a recognised test standard such as ISO 1217 and compared at the same pressure.

Lower pressure does not justify a large capacity margin. Oversized low-pressure machines unload or run at reduced speed for long periods, losing much of the energy advantage that made them attractive. Sizing should follow the logged demand profile:

• Minimum demand, to see how far the machine must turn down.
• Average demand, where it should run at its best specific power.
• Peak demand and its duration, which may be met by a receiver rather than extra compressor capacity.

Conveying systems in particular have strongly cyclic demand. Receiver volume and control strategy are as important as compressor size.

Technology choices and how to decide

Low-pressure screw compressors are available in oil-injected and oil-free designs, with fixed-speed or variable-speed drives. The choice depends on air quality, demand pattern and duty.

For pneumatic conveying of food, pharmaceutical or sensitive powders, oil-free air is often specified. For cement or fly ash, oil-injected compression with appropriate filtration is common, provided the condensate and oil carry-over are controlled. Air drying is still needed for most conveying duties, because moisture causes material to cake and lines to block, particularly in monsoon humidity.

Installation and maintenance points

• Keep the low-pressure network separate from the main plant network, with clear labelling, to avoid cross-connection.
• Size pipes for the large volumes involved; pressure drop is a bigger proportion of a low working pressure.
• Fit a correctly sized receiver to stabilise control and support conveying cycles.
• Provide ducted ventilation; Indian summer ambients of 40–45 °C, and above 45 °C on open sites, affect any compressor.
• Follow the manufacturer’s service intervals for filters, oil, separators and, on oil-free machines, element inspections.
• Log pressure, temperature and load so changes in performance are seen early.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad since 2002, is an independent multi-brand specialist. We sell new and used/refurbished compressors and blowers, and we rent, service and supply spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands. We are not an authorised dealer of any brand, which lets us compare options neutrally. We have tie-ups with all major and reputed brands.

• Pressure and flow logging to confirm what the process really needs and whether the main system is the real problem.
• Comparison of low-pressure compressor, blower and regulated-supply options in kWh per year.
• Supply of compressors, blowers (Roots, screw, centrifugal, side-channel), dryers, filters and receivers.
• Service, AMC and spares across Telangana and Andhra Pradesh by more than 20 service engineers and technicians, with equipment and spares supplied across India.
• Rental compressors from our own fleet, 50–2,000 cfm and 2–30 bar, to trial a pressure change or cover a breakdown.

What to send us

• The process, the pressure required at the point of use and who specified it.
• Flow required or the equipment data sheet, and operating hours per day.
• Air quality requirement: oil, water and particle class, or whether air contacts product.
• Details of the present supply: compressor make, model, setpoint and nameplate photo.
• Any pressure readings at the compressor and at the user.

Frequently asked questions

What pressure is considered low pressure for a screw compressor?

For screw compressors, low pressure usually means a discharge pressure of roughly 2.5–5 bar, about 36–73 psig. Below about 1–2 bar, blowers are normally the more suitable machine. Standard plant-air screw compressors work at about 7–10 bar. The exact band depends on the manufacturer’s design, so always compare FAD at your actual required pressure.

Can I just reduce the pressure setting on my normal compressor?

Lowering the setpoint of a standard compressor does save some energy, roughly 7% per bar, and is worth doing where users allow it. However, a standard airend is designed for its rated pressure ratio, so running it far below that point is not as efficient as a machine built for low pressure. Check the manufacturer’s minimum pressure limits first.

Is a low-pressure compressor better than a blower?

It depends on the pressure required. Blowers are efficient up to about 1–2 bar and are widely used for aeration and dilute-phase conveying. Above that, a low-pressure screw compressor is normally the better choice. For pressures near the boundary, compare tested specific power for each machine at the exact pressure and flow needed.

Why is my compressed air pressure low at the machine?

Low pressure at a user is often caused by leakage, blocked filters, undersized pipes, long hoses and couplings, high dryer pressure drop or compressors not loading properly. It does not always mean the compressor is too small. Measure pressure at the compressor outlet, after treatment and at the user to find where the loss occurs before buying capacity.

Do low-pressure conveying systems need dry air?

In most cases, yes. Moisture in conveying air makes powders such as cement, fly ash, flour and sugar cake and stick, which blocks lines and vessels. This is worse during the Indian monsoon. A refrigerated dryer giving a pressure dew point of about +3 °C is common, with desiccant dryers used where lines run outdoors or the material is very sensitive.

Should a low-pressure compressor be oil-free?

Oil-free compression is usually specified where air contacts food, pharmaceutical or sensitive products, or where the required ISO 8573-1 oil class is very strict. For many industrial conveying and process duties, an oil-injected machine with suitable filtration is acceptable. The decision should come from the product risk and the required air-quality class, not from habit.

Related pages

• Rotary screw compressors hub
• Medium-pressure rotary screw compressors
• Blowers vs compressors
• Oil-free rotary screw compressors
• Pressure-drop audit