Buying an air compressor – a quick buyers guide

 

Do you need help buying an air compressor?

This page lays out some simple steps to consider when buying an air compressor.  Many people would immediately think of a small hand held device which you connect to the tyres on your car and the power socket on the dashboard to inflate your tyres to around 30 psi. 

What is compressed air?

Air compressors start at this size & go up to something which could be as large as a typical room in your house. Recent advances in compressor technologies have reduced the size of these very large machines like the Ultima oil-free models, but the reality is that there is a huge array of different processes which use compressed air from tyre inflation to powering whole factory production lines.

Our dedicated page what is compressed air? air explains why many industries depend on compressed air to power their processes and you can read more about the different compressor applications, the fact is, compressor sizes start at the very small 2kW models and there are models at each kW size all the way up to 320kW and above in some cases as each machine has to cater for a very specific task - to generate the correct volume of air at the correct pressure.

Click here to shop for a CompAir air compressor, select the tabs to see the different types available:

Oil Lubricated Compressors

Oil Free Compressors

Air Treatment & Accessories

Portable Compressors

engineer and air compressor installation

The required process airflow & pressure must be correctly matched & this is the one of the most important considerations to take into account when buying an air compressor. This page provides an overview of what to consider, whether it’s for a 2kW or 250kW model.


What are you using it for?

Select the correct air compressor type for the application you are using it for, you can read more about the different compressed air applications here.

Piston or screw, oil-free or oil-lubricated, regulated speed or not, the latest drive technology or a proven principle? When it comes to purchasing an air compressor or upgrading an existing compressor station plant, there are many options you can choose. Oil-free compressors for example are used in food packaging where the air is in direct contact with the process it serves & so must be completely pure.

 

What CFM (or what airflow requirement) do I have?

To power a typical air tool in a factory workshop would take say 4-5CFM so the compressor output would need to correctly match this application at the correct pressure you need the air tool to operate at, say 5 or 8 bar g. The airflow from the compressor also changes depending on the pressure needed, so correct sizing is very important. Many typical air compressor applications run from 3 to around 15 bar. Above this a high pressure piston compressor would be used, say for example refilling scuba diving air tanks, where pressures up to 250 bar (250 times atmospheric pressure) are required.

Do your prep: good planning

Before purchasing an air compressor, a range of figures and facts need to be established for the entire compressed air network. These include the compressed air demand (current and in the foreseeable future), the required compressed air quality and whether the waste heat from the compressor is used for another operational process, like heating the factory. If the compressor is being added to an existing station, then you also need to consider integration into the control technology.

It is generally advisable to measure the compressed air demand over a period of time to determine a requirement profile for which the compressor or compressors can be selected.

 

Air Audit

Compressed air audits are an effective way to identify your current energy use and to assess your compressed air needs.

 

How do I audit my compressed air?

Air audits can be as simple or as comprehensive as you require. A data-logging unit is attached to each compressor in your network for a fixed period of time. The results from the audit can then be used to analyse your air pressure requirements, power consumption and flow rate.

Our engineers can then provide recommendations that will help you to select the right size and mix of fixed and regulated-speed compressors. The compressed air audit may also identify if you need to switch to a new, more efficient machine, carry out further network improvements or pipework upgrades or if your compressor requires better control. We can then advise on any improvements that you can make to your network to help reduce leaks and save money.

As well as carrying out an audit of your compressed air system, you should also include regular checks for air leaks in your maintenance routines too.

airinsite air audits

A simple leak detection survey can identify any problems quickly and any remedial action you take can have an immediate, positive effect with fast payback periods.

air compressor installation

A simple air compressor network, with compressor (left), tank, air dryer & condensate management on the right.

Leakage measurement and elimination

A comprehensive condition assessment of piping and existing equipment also includes energy consumption and leak measurement. This can result in considerable savings: even in well-maintained compressed air networks, 10 to 20 percent of the generated compressed air is lost due to leaks, in some cases it is up to 40 percent. On average, up to 30% of compressed air generated is lost through leakages.

In some cases it's been know for factories to have a separate compressor just to compensate for leaks. We take this issue very seriously to supply energy efficient sustainable products to use the minimum amount of energy.

Leakage Loss Table (see below)

 
air leakage loss table

Pressure reduction and heat recovery

A precise check of the system pressure can also save energy: is 8 bar really needed or would 7 bar suffice? If it would, then you would save eight to ten percent of the energy costs relating to compressed air – without any investment. Heat recovery can also considerably raise the efficiency of the compressor station, as there are many thermal processes where the waste heat can be used. In the case of oil-injected compressors, 94 percent of the energy used can be recovered as heat.

A focus on compressed air quality

These days, it is no longer possible to decide which compressor technology to use for generating the required compressed air quality without taking new developments and findings into account. High-tech latest generation compressors for generating oil-free compressed air and refined traditional piston compressors both play important roles here. This is particularly true for sensitive production processes, where at least part of the compressed air flow must meet the highest quality requirements, because it is used, for example, as sterile air for ventilating fermenters in the pharmaceutical industry or food production, for filling and packing lines or electronics manufacturing and paint shops. 

In these cases, the process and working medium from the air compressor must be 100% oil-free, which poses the question of whether to generate oil-free air directly or by filtering. Whether to use an oil-lubricated air compressor with air treatment equipment to remove contaminants post compression or a completely oil-free compressor has been a debated at length in the past. While removal of the oil aerosols from the compressed air is a complex process, compression in an oil-lubricated compressor is, to date, more efficient. However in many cases completely 100% pureair is required in food and electronic manufacturing applications. Read more about the different compressed air purity classes here.

CompAir has also launched a new air treatment portfolio designed and manufactured entirely in-house to meet the same quality, performance, and efficiency standards as its world-class compressors, so we supply the entire compressed air package, which is the compressor, piping and any air treatment equipment which may be needed after the compression process to achieve the correct compressed air quality.

 

food manufacturing compressor installation

Impartial planning

It is best to approach planning without preconceived opinions such as “it has to be a screw compressor/piston compressor/oil-lubricated compressor” or “we need a 75 kW machine.” It is better to carefully consider all the options. If you consult specialists, they should also not be tied down to one specific compressor type. A precise analysis of the current situation and calculation of the current compressed air demand and pressure level, along with that expected in the future, should always be used as the basis for your decisions.

If the system components, including air treatment, are coordinated with one another and the maintenance expenses have been determined, then the running costs with their high proportion of energy costs, can be estimated with a great deal of accuracy.

Modernisation pays off

As the running costs of a compressed air system are around 80% of the life cycle costs (as mentioned at the start), even investment in advanced energy-saving compressor technology – results in lower costs than ongoing operation of a system that is not working optimally.

This means that the energy consumption of a compressor station is the decisive factor in terms of costs. A higher initial investment can result in a rapid return on investment. This can be demonstrated with figures: a typical 75 kW compressor, which runs for 4,000 operating hours per year at 70 percent capacity and 20 percent no-load operation, results in energy costs per year of €18,400. This sum is based on an electricity price of 8 ct/kWh.

A diverse range of industries use air compressors to power their processes for a wide variety of applications, you can read more about some of them here.

You can see a screw compressor simulator in action on this page here.

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