Screw compressors - how they work and how to choose the right one
A screw compressor is today the standard source of compressed air in production plants, metalworking workshops, paint shops, food plants and anywhere air is needed throughout the whole shift. A rotary screw compressor differs from a piston machine in the very principle of generating pressure, and that difference shows up in parameter stability, noise level, energy use and how the machine is serviced.
This guide describes step by step how a screw compressor is built, how its variants differ, how to calculate a plant's demand and what to check before signing an order. We write from the perspective of a company that selects, commissions, services and overhauls screw compressors, so we also point out things that only become visible after several thousand running hours.
How a screw compressor works
The heart of the machine is the airend, a pair of meshing rotors housed in a precision-machined casing. The male rotor usually has fewer lobes than the female rotor, and their profiles are matched so that, as they turn, they form closing chambers. Air drawn in through the filter and inlet valve enters a chamber of large volume, and as the rotors turn, that volume shrinks along the axis of the stage. Compression is therefore a continuous process: air is carried from the suction side to the discharge side without reciprocating motion and without valves working in time with rotation.
In oil-injected machines, which make up the majority of the industrial fleet, oil is fed into the compression chamber. It performs three functions at once: it seals the clearances between the rotors and the casing, it absorbs a large part of the heat generated by compression, and it lubricates the bearings and mating parts. Thanks to oil injection, the temperature at the stage outlet stays within a predictable range, and the stage itself can run at high efficiency without metal-to-metal contact on the rotor profiles.
The air-oil mixture then goes to the separator vessel. There the oil first drops out by gravity and on the walls, and the separator element catches the remaining oil mist. The separated oil returns through the oil cooler and filter to the airend, closing the loop. Compressed air passes through the minimum pressure valve to the aftercooler, where it cools before entering the installation. A properly working separation system keeps residual oil in the air at levels measured in milligrams per cubic metre, and a worn separator element is one of the most common causes of an oily network.
Cooling determines the durability of the whole machine. In air-cooled versions the oil cooler and aftercooler are ventilated by a fan, so the compressor room must have proper air intake and hot air discharge. In water-cooled versions, heat is carried away by a water circuit, which makes it easier to reuse the waste heat. Too high an ambient temperature raises the oil temperature, speeds up its ageing and shortens bearing life, so when sizing a machine we always ask about the room's volume and ventilation.
Compared with a piston compressor, a screw machine has several advantages that are noticeable from the first week in multi-shift operation. Outlet pressure is stable because air flows in a continuous stream rather than in pulses, so processes sensitive to fluctuations, such as powder coating or pneumatic controls, run more evenly. Noise level is markedly lower because there are no mechanical impacts, and the sound-absorbing enclosure damps what remains. A screw machine is also designed for continuous duty: it needs no cooling-off breaks and tolerates a high share of running time under load, while a piston compressor wears out quickly under such conditions. Finally, vibration is lower, which simplifies foundations and pipework connection.
Types of screw compressors
One name covers several design solutions that differ in efficiency, price and how they respond to changing demand. Choosing the variant affects running cost more than the brand itself.
Single-stage
Air is compressed in a single screw stage to the target pressure, most often 7.5, 10 or 13 bar. This is the simplest, cheapest to buy and entirely sufficient solution for most plants with demand up to a dozen or so cubic metres per minute. Fewer components also mean simpler service and cheaper overhauls.
Two-stage
Compression is split into two stages with intercooling. Each stage works with a smaller pressure rise, so losses from internal leakage and overheating are lower, and specific energy use per cubic metre of air produced drops. Two-stage design is typical for higher outputs and for plants running three shifts, where the efficiency difference translates into real money on the electricity bill. Purchase cost is higher, so payback improves with more operating hours per year.
With a permanent magnet (PM) motor
A PM motor has a rotor with permanent magnets, so it does not lose energy generating a field in the rotor. It keeps high efficiency across a wide speed range, including at part load, and allows more compact and lighter drives to be built. In machines with a high share of running time at variable demand, a PM motor makes the biggest difference, because a classic asynchronous motor operates far from its optimum point there.
With a variable speed drive (VSD)
A variable speed drive changes the motor's supply frequency, and therefore the airend's rotational speed, so the machine's output is regulated smoothly and adapts to current air demand. Network pressure stays within a narrow band instead of oscillating between load and unload thresholds. Energy wasted in idle running during load and no-load cycles disappears, and starts are gentle, which saves belts, bearings and the electrical installation. Smooth regulation makes sense wherever air demand clearly changes during the shift.
How to choose a screw compressor
Sizing is a simple calculation based on a few figures, but they need to be gathered reliably. The most common mistake is buying a machine based on motor power and the previous compressor's nameplate, without checking how much air the plant actually consumes.
Air demand, i.e. FAD
Compressor output is stated as FAD, meaning free air delivery referred to intake conditions, most often in cubic metres per minute. Add up the demand of all consumers from their documentation, note which run at the same time, and add a margin for plant growth and for the installation's natural leaks. If machine data is missing, output can be calculated from measuring the current compressor's running time under load, or from flow logging. When comparing offers, check whether the stated FAD values refer to the same pressure, because otherwise the figures are not comparable.
Working pressure
Pressure should be matched to the most demanding consumer, not to the highest value in the catalogue. Every extra bar raises energy use by a few percent and increases leakage losses, so raising the setpoint for a margin is costly. If one outlet needs clearly higher pressure than the rest of the plant, it is usually cheaper to serve it locally than to raise pressure across the whole network. It is also worth checking pressure drops across filters, the dryer and the pipework, because the problem often turns out to be the installation, not the compressor.
Air demand profile
How demand is distributed over time decides whether a VSD and a PM motor are worth paying for. A plant with steady, high demand throughout the shift is well served by a machine with step regulation. A plant where demand jumps between shifts, production batches or individual machines gains from smooth regulation. A common and effective setup uses two machines: one base-load unit with smooth regulation covers day-to-day fluctuations, and a second, fixed-output unit comes in at peaks and acts as a reserve.
Conditions in the compressor room
A compressor releases almost all the power it draws into the room as heat, so cool air intake and hot air discharge, ideally through an outdoor duct, are needed. Also important are the cleanliness of the intake air, since dust quickly clogs the filter and coolers, ambient temperature, room height, access to the machine for servicing, and condensate drainage. On top of that there is an air receiver sized to the output and demand pattern, a dryer suited to the required air quality class, and filtration downstream of the dryer.
Life-cycle cost, i.e. TCO
Over the whole ownership cost of a compressor, electricity usually accounts for 70 to 80 percent of spending, with purchase and service sharing the rest. That means a difference in purchase price can be won or lost within a few months of operation. When comparing offers, calculate energy use for your own operating profile, not just for the nominal point, then add the cost of servicing, oil, filters and separator elements. Looking at the installation side often helps: lowering the pressure setpoint, fixing leaks and a sensible buffer often deliver savings faster than replacing the machine.
Sizing also includes an air receiver buffer. We cover the rule of 6 to 10 litres of capacity per l/s of flow, positioning and accessories on the compressed air receivers page.
What determines the price of a screw compressor
We do not publish prices in a table, because the same output can cost very different amounts depending on equipment and control method. Below are the main factors that move the price up or down and that are worth comparing between offers.
Power and output
The basic factor: the higher the motor power and FAD output, the larger the airend, coolers, motor and enclosure, and so the higher the price. Stepping up to a higher working pressure at the same output also raises the cost.
Number of compression stages
Two-stage design costs more than single-stage because it has two stages, intercooling and a more extensive oil system. You are paying for efficiency, so it makes sense with a high number of operating hours per year.
Variable speed drive
Smooth regulation means an extra frequency converter, filters and more extensive control. The cost pays back through energy savings, but only with variable air demand.
Permanent magnet (PM) motor
A PM drive raises the purchase price compared with a classic asynchronous motor, in exchange for higher efficiency across a wide speed range, especially at part load.
Dryer and receiver in the set
A compact version with a receiver and an integrated refrigerant dryer costs more than the compressor alone, but usually less than the same components bought separately and connected on site. It is worth costing both variants together with installation and space in the compressor room.
Serviceability and parts availability
A factor often overlooked at purchase and most felt in the following years. Check access to filters, separator and coolers, whether consumable parts are available quickly and at what price, how long you wait for components after a failure, and whether the controller lets you read the running history. A machine with easily available parts is cheaper to maintain than a design where every service requires ordering from abroad.
Hertz Kompressoren machines in our range
We are an authorised distributor of Hertz Kompressoren. We focus our sales on this brand because it lets us take responsibility for the whole chain: machine sizing, commissioning, servicing and parts supply. The range covers single-stage and two-stage screw compressors, VSD and permanent-magnet-motor versions, plus dryers and air treatment components, so a compressor room can be ordered as one coherent set.
Sizing starts with a conversation about demand and the operating profile, not with a catalogue. If data is missing, we suggest an on-site measurement. On delivery we handle commissioning the machine, checking parameters under load and handing operation over to your operators, then follow up with servicing at agreed intervals.
See also: Hertz Kompressoren compressors and Compressor sales.
Service and overhauls of screw compressors
We service more than 30 brands of screw compressors, regardless of where the machine was bought. Servicing and repairs are carried out at the customer's site, in their compressor room, with tools and typical consumable parts carried in the service van. This approach shortens downtime and lets us assess the conditions the compressor actually works in: room temperature, dust levels, the state of filtration and drying, and pressure drops in the installation.
When an on-site repair is not enough, we remove the airend and overhaul it at our own workshop in Radom. The overhaul includes measuring clearances, reconditioning or replacing bearings and seals, checking rotor profiles and a test bench run, then re-measuring parameters under load once the machine is rebuilt. Reconditioning a stage is usually clearly cheaper than buying a new machine and restores output without replacing the whole installation.
See also: Screw compressor service and Screw compressor overhaul and reconditioning.
Renting a compressor during a repair or a production peak
If a machine is down and production cannot wait, we supply a stand-in compressor from 4 to 75 kW. Rental is also used during a compressor room overhaul, during a seasonal rise in demand, and when a plant wants to check in practice what output is really needed before deciding to buy. We deliver the machine, connect it to the installation, commission it, and collect it once the rental period ends.
See also: Compressor rental.
Screw compressors - frequently asked questions
The questions we get most often on sizing, buying and operating screw compressors.
Need service or spare parts?
Our team will recommend the best solution and send a quotation the same day.
We'll help you choose the right screw compressor for your plant
Tell us how much air you need, at what pressure, and how demand changes during the shift. We'll prepare a sizing proposal including drying, filtration and a receiver, and if data is missing we'll suggest an on-site measurement.
Need consumable parts for your compressor? Compressor spare parts
Contact
Need an inspection, overhaul or rental? Fill out the form - we'll respond within 24 hours.
Send inquiry
Location
ul. Słowackiego 201 26-600 Radom Mazovian Voivodeship, Poland
Find us on the map
ul. Słowackiego 201, 26-600 Radom