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Top tips on how to specify and buy a stationary air compressor

Do you want to buy a new compressor or replace your old one but don't know where to start? Find out more about our step by step process on how to specify and purchase an air compressor. Do you have further questions? Speak to one of our specialists for advice.

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There are now a myriad of ways to purchase a stationary air compressor, with tried and trusted sales channels, such as manufacturers’ authorised distributors, competing with newer e-commerce initiatives for buyers’ attention. However, although to buy air compressors may be easier than ever - with companies having the flexibility to choose between outright purchase, finance or rental deals; not to mention the availability of government funding via the Carbon Trust’s Green Business Fund – ensuring the chosen machine is a perfect fit for its intended application still needs to be a carefully considered process.

On this page, Stef Lievens outlines the key steps to follow when specifying a compressor, in order to ensure it is optimised in terms of performance, energy-efficiency and Total Cost of Ownership (TCO); before moving on to review both new, existing and emerging methods of purchase.

Compressor sizing: pressure+flow=energy=cost

One of the biggest challenges is to correctly size a new air compressor. The latest compressor designs are more efficient and employ better technology than ever before, so when comparing like for like it is necessary to have at hand all the application and usage information in order to size the machine correctly.

graphic with different types of compressors suitable for different applications
When choosing an air compressor, understanding the flow and pressure requirements for any installation is vital. The laws of physics state that an increase in pressure creates a decrease in flow. This correlation is important to bear in mind when determining the required size of a compressor based solely on its rated motor power. For example, a basic 7 bar compressor driven by a 15 kW motor will deliver air at a maximum volume of approximately 45 l/s. However, the free air delivery from a 10 bar version will be only 37 l/s from the same 15 kW unit. An increase in working pressure by 1 bar increases the power requirement by approximately 6 per cent if the flow stays the same.

While it is perfectly natural for buyers to seek best performance for the least outlay, it should be recognised that right from entry level to premium specification compressor equipment, competitive suppliers’ kW ranges are not always directly comparable. Keep in mind also that rotary screw air compressors tend to output greater flow per kW than piston compressors.

Undersizing a compressor is likely to result in pressure drops and an inability to complete important tasks, while oversizing a machine can lead to future mechanical problems and excessive energy demands from within the system.

What type of air compressor do I need?

Once the appropriate size of the air compressor is determined, the next decision concerns the type of compressor needed for the application; with the most commonly available options discussed below.

Piston Compressors - how they work, benefits and ideal applications

graphic of typical piston compressor
Of the most popular options available, the most economical and relatively inexpensive type of air compressor is the piston or reciprocating machine. It is a mistake to think of small piston compressors as tired, noisy relics clattering away in a high-street tyre bay. Nothing could be further away from this reality than the contribution today’s modern reciprocating compressor technology is making to industrial productivity.

The majority of small-to-medium piston compressors, which commonly range from 0.55 to 15 kW and can be either oil-lubricated or oil-free machines, are available in a number of product variants to match the exact needs of a wide range of end users. They can be supplied either as standard base units, trolley or receiver mounted, or as a complete compressed air station that includes a refrigerant air dryer as well as pre-wired starting and regulation equipment.

Oil-lubricated piston compressors

Compact single-stage oil-lubricated, v-configuration, reciprocating compressors in the 10-bar 1.5 to 15kW range are best suited for applications with limited space requirements. In these designs, a lightweight compressor block, directly coupled to the motor provides excellent cooling characteristics and smooth integration in small spaces.

Oil-free piston compressors

Oil-injected compression, even with filtration, cannot guarantee that all oil vapours and aerosols are removed from the air. For this reason, many leading compressor manufacturers offer oil-free air technology, employing sealed for life bearings and PTFE or carbon piston rings to remove any risk of contamination, which should ideally be designed to meet the certified ISO 8573-1 Class 0 standard in air quality.

Rotary screw compressors - how they work, benefits and ideal applications

In rotary screw compressors the length and pitch of the twin rotor screw element, as well as the form of the discharge port, collectively determine the pressure ratio. Beyond that, there are no valves or other mechanical forces that can cause unbalance, which allows a screw compressor to operate at high speeds while combining a large flow rate with small exterior dimensions.

Rotary screw compressors typically are used for continuous, workplace and industrial applications and are available in two primary versions, oil-free and oil-injected with options of fixed speed or variable speed drive operation.

In oil-free variants, external gears synchronise the position of the counter-rotating screw elements, and, because the rotors do not come in contact and create friction, no lubrication is needed within the compression chamber. As a result, the compressed air is oil-free. Precision engineering within the housing keeps pressure leakage from the pressure side to the inlet at a minimum.

graphic depicting the working principle of the typical rotary screw compressor, available as oil-injected or oil-free
Meanwhile, in oil-lubricated rotary screw compressors, oil is injected into the compression chamber to cool and lubricate the compressor element, remove the heat from the compression process, and help minimise leakage in the compression chamber.

In addition to overall energy efficiency, rotary screw air compressors offer multiple benefits compared to other compressor technologies. These include low noise output, light weight and small footprint for point-of-use installations, continuous operation at temperatures of up to 46 degrees C, extremely low oil-carryover (as little as 3 ppm), simplified maintenance procedures and zero loss of capacity over time.

Furthermore, smaller rotary screw compressors can be tank mounted to allow for additional storage capacity. Full-feature variants are also available with an integrated dryer and filters that are designed to protect the compressed air network and end-user output.

Within the all-encompassing 2.2-500 kW range of workplace and industrial rotary screw compressors there are a wide variety of options available, with more recent introductions offering advanced designs such as a vertical, close-coupled configuration and interior permanent magnet motor drive plus inverter systems capable of achieving average energy savings of up to 50 per cent over conventional fixed speed designs.

Vane compressors - how they work, benefits and ideal applications

graphic depicting the working principle behind vane compressors
The operating principle for a vane compressor is the same as for many compressed air expansion motors. The vanes are usually manufactured of special cast alloys and most vane compressors are oil-lubricated. A rotor with radial, movable blade-shaped vanes is eccentrically mounted in a stator housing. When it rotates, the vanes are pressed against the stator walls by centrifugal force. Air is drawn in when the distance between the rotor and stator increases. The air is captured in the pockets between the vanes, which decrease in volume with rotation. The air is discharged when the vanes pass the outlet port.

Assessing an air compressor system

Over the past few years, the cost of energy has been increasing drastically. This causes many companies to question the cost effectiveness of their compressed air system and look for ways to reduce consumption and increase energy efficiency within their plant or facility. Conducting energy audits is seen as the definitive approach to identifying trends, establishing how well a system is performing and installing energy-efficient solutions where needed.

graphic depicting assessments available for compressors and compressed air installations
Walk-the-line compressed air assessment is the simplest and most cost-effective way to gain a better understanding of a compressed air system’s status. It does not involve any hardware or specialised loggers and can be conducted by a compressed air professional who can perform load/unload time tests to quickly identify efficiency and utilisation of the unit. Such walk-through procedures consist of visual checks and are carried out on the supply side or the compressor room as well as the demand side or the production floor. Visual checks include inspection of the compressor, dryer, filtration and condensate drains, as well as the pipework system and its configuration.

Audit your air compressor system with data logging

graphic depicting data logging audits which provide more in-depth information about the running status of a compressor and the installation it is part of
Walkthrough assessments may also reveal the need for a full air audit using data loggers to measure operating current and provide an insight of the compressed air utilisation within a given system. Data loggers are attached to the compressor for a minimum of 7-14 days in order to observe and record any trends and demand fluctuation throughout working week days as well as the weekend. Results of such audits can provide a good understanding of the actual compressed air usage over a period of time, identify peaks and troughs in the air demand pattern and identify ways to improve the overall efficiency of the system. Other than helping to properly size an air compressor, air audits can also identify leaks in the installation, especially during night or weekend plant shutdown periods.

Purchasing a stationary air compressor

As with most capital equipment items, leading manufacturers and suppliers offer various ways in which air compressors, blowers, industrial gas generators and quality air accessories can be procured or rented, ranging from outright purchase, finance deals, contract rental, pay-as-you-go contracts and initiatives such as the Carbon Trust’s Green Business Fund. There are even try-before-you-buy schemes available for certain products.

graphic depicting money - buying a compressor nowadays is available through multiple methods such as outright purchase through traditional means or buying online
For many years the traditional source of supply for smaller workshop and industrial compressors has remained in the hands of tried and trusted authorised distributors, with the supply of larger capacity units and systems often handled directly by the manufacturer.

However, more recent e-commerce initiatives also mean it is now possible to buy smaller compressors online; directly from the manufacturer. This is particularly helpful for companies seeking 2-7kW ex-stock units for use in garages, bodyshops, workshops and small manufacturing and light engineering applications, backed by warranty and full after sales service.
Atlas Copco Compressors is a Carbon Trust Accredited supplier and our products are compatible with Enhanced Capital Allowance tax breaks
It is worth noting that an incentive for purchasers of energy-efficient compressor equipment is the publication of the Energy Technology List (ETL), which is managed by the Carbon Trust on behalf of the UK government. It provides information on products and components that qualify for the government's Enhanced Capital Allowance (ECA) scheme. The ECA scheme allows businesses to write off the entire cost of any product or a component included on the list against taxable profits in the year of purchase.

AIRPlan: a flexible scheme for contract air

While this article deals with matters related to compressed air equipment ownership, there is an alternative school of thought that asks, in specific circumstances, why buy a compressor when all you need is air?

Get the air you need in the quality that suits you without having to break the bank with contract air plan options
Recently introduced, there is now a pay-as-you-go compressed air scheme, which is designed specifically for customers who may experience periodic extra usage demands or have limited capital reserves at their disposal.

AIRPlan's contract air scheme embraces the latest energy-efficient air compressors and planned maintenance , courtesy of a full after-sales service support, to provide end users with a guaranteed and reliable compressed air supply under a flexible agreement. Customers only pay for what they use. A monthly fixed service charge and a variable compressed air usage fee, measured by a smart technology remote monitoring system, enable users to plan and manage their operational costs according to current needs and budgets.
Contract Air allows you to pay only for the compressed air that you are using
A contract air agreement covers the installation of all the equipment needed for a complete compressed air solution based upon a VSD rotary screw compressor that is capable of delivering average energy savings of 50 per cent compared with fixed-speed alternatives. The scheme includes ancillaries such as dryers, filters, air receivers, condensate drains, oil-water separators and a modular alloy pipework system.

Parts, service and breakdown cover during working hours are included within the fixed monthly fee. This eliminates any unexpected or hidden costs for unforeseen repairs. On-time preventive maintenance is provided by expert engineers who only use genuine OEM parts to assure uninterrupted performance at optimum efficiency. In addition, equipment can be upsized or downsized where necessary, with the option to exchange for alternative capacity units; or even return them at the end of the contract period.

About the author
Stef Lievens is a Belgian citizen, with a Master of Science in Mechanical Engineering and Business Administration from the University of Leuven, Belgium. He and his sales team have a customer-centric focus on offering reliable and cost-effective compressed air and industrial gas solutions that have a short payback. They seek to raise awareness of complementary technologies that offer improved productivity; such as quality air ancillaries, energy recovery systems, remote monitoring programs and real-time control and management software.

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