An air compressor is often one of the most important pieces of equipment in an industrial plant. It takes atmospheric air, compresses it and supplies it at the pressure required for production.

Compressed air is used in almost every industry, from workshops and garages to manufacturing plants, mines and processing facilities. In many operations, it is not simply another utility. It is part of the production process. When the air supply is interrupted, the effect can be immediate — machines can stop, processes can be delayed and production can be lost.

This is why buying a compressor should involve more than comparing quotations.

The purchase price is important, but it is only one part of the cost. Electricity, maintenance, spare parts, repairs, installation and downtime all contribute to what a compressor will ultimately cost a business over its working life.

Start With the Application

Before choosing a compressor, the first question should be simple:

What does the application actually require?

This means looking at the required airflow, operating pressure, hours of operation and, importantly, how the demand changes during production.

Two compressors with the same motor rating can perform very differently. Likewise, two machines using the same compressor technology can have different energy consumption, maintenance requirements and lifecycle costs.

Selecting a compressor simply because it has a larger motor or produces more air is not necessarily the correct approach.

Oversizing equipment can result in unnecessary capital expenditure and operating costs. Under sizing can leave the compressor working excessively hard and may result in pressure problems or interruptions to production.

The compressor should be selected around the application — not the other way around.

Rotary Vane and Rotary Screw

Two established compressor technologies used in industry are rotary vane and rotary screw.

Rotary vane compressors use a rotor fitted with sliding vanes. As the rotor turns, the vanes move within the stator and compress the air.

The design can provide smooth, continuous operation with relatively low vibration. Some rotary vane machines also operate at relatively low rotational speeds, which can be advantageous in continuous-duty applications.

The vanes are, however, wear components. Correct lubrication, clean operating conditions and proper maintenance are important. Incorrect operating conditions or poor maintenance can contribute to damage to the rotor and stator assembly. Depending on the design, specialist knowledge may also be required when carrying out repairs or air-end work.

Rotary screw compressors use male and female screw rotors to compress air. They are widely used throughout industry and are available in fixed-speed and variable-speed configurations.

Variable-speed technology allows the motor speed to change according to air demand. This can be useful where demand varies during production. It should not, however, be assumed that a variable-speed compressor will automatically be efficient under all conditions.

The compressor still needs to be correctly sized and operated within a suitable range.

Screw compressors also contain precision components and require regular maintenance. Oil, filters and separators on oil-injected machines all need attention, while air-end repairs or overhauls can become substantial costs as equipment ages.

Motor Size is Not the Whole Story

The compressor itself is only one part of a compressed-air installation.

The air receiver, dryer, filters, valves, pipework and leaks all influence how effectively the system delivers air to the point of use.

Poorly-sized pipework, restrictions or excessive pressure losses can result in operators increasing compressor pressure to compensate. The plant may then use more electricity without producing any additional useful compressed air.

This is why the complete system needs to be considered when evaluating compressor performance.

Understand the Cost of Running the Machine

Energy consumption is one of the areas that can easily be underestimated.

Consider a compressor or system using an additional 50 kW continuously:

50 kW × 8 760 hours = 438 000 kWh/y

At an electricity cost of R3.00/kWh, this equates to:

R1 314 000/y.

The calculation is simple, but it demonstrates an important point. A relatively small difference in power consumption can become a significant cost when equipment operates continuously.

The cost of owning a compressor can therefore be considered as:

Capital Cost + Installation + Energy + Maintenance + Downtime + Lifecycle/Overhaul

The purchase price is only the starting point.

One Compressor or Several?

The configuration of the compressor room also deserves consideration.

Where demand is relatively stable, a fixed-speed compressor may be suitable. Where demand changes, variable-speed technology may offer advantages when correctly applied.

There is also the question of whether the plant should use one large compressor or several smaller machines.

Multiple compressors can provide staged capacity and operating flexibility. They can also allow production to continue while another machine is being maintained.

There are additional costs, however, including installation, controls, maintenance and equipment. The decision should therefore be based on the plant's actual demand and what happens to production if the compressed-air supply is lost.

After-Sales Support Should Be Considered Before the Purchase

The compressor does not stop costing money when the purchase order has been signed.

It is a working industrial asset and will require maintenance, service, spare parts and technical support throughout its life.

This is where after-sales support becomes important.

Customers should understand who will service the machine, whether the required parts are available, how quickly technical assistance can be provided and whether the people carrying out the work understand the equipment.

A compressor that is technically suitable but difficult to maintain or support can become a very different investment once it is operating in a production environment.

Three Questions Every Buyer Should Ask

When considering a compressor, it is useful to look at three areas:

The Machine — Is the equipment suitable for the application?

The Performance — Will it provide the required airflow and pressure at an acceptable operating cost?

The Support — Who will maintain, repair and support the equipment throughout its working life?

All three questions are important.

A quotation tells you what the equipment costs today. It does not necessarily tell you what that equipment will cost over the next five, ten or fifteen years.

That calculation requires looking at electricity consumption, maintenance, spare parts, repairs, downtime and major lifecycle work.

The right compressor is therefore not simply the machine with the lowest purchase price or the largest motor. It is the machine that is appropriate for the application, economical to operate and properly supported throughout its working life.

For businesses purchasing compressed-air equipment, the questions should start before the quotation is accepted:

What air do we actually need? What will it cost to produce? How will the machine be maintained? And who will be there when something goes wrong?

Those answers can be just as important as the price on the quotation.