Air compressors are used in a variety of industrial applications from filling vehicle tyres to manufacturing semi-conductors. They all work from essentially the same principle – when you force air into a smaller space pressure increases and this pressure can be used to drive motion – but the mechanical methods of achieving this vary in complexity and cost. This means that choosing the right industrial air compressor for the job is an essential part of maximising efficiency while minimising cost.
Types of Industrial Air Compressor
The following are all types of compressor found in common use today. They are listed in order of increasing cost:
- Reciprocating (or Piston Driven) Compressors
These use a mechanical piston driven pump to compress air in a cylinder. They come in two types – direct drive compressors which can achieve up to 15cfm at pressures of up to 10 bar, or the harder wearing belt drive compressors which can achieve up to 40cfm at 10 bar. You might also be offered a choice between single phase and 3 phase motors, the latter providing greater efficiency (and therefore lower electricity consumption). The motor you choose will depend on the output required and your available power supply (typical domestic supply is single phase). These compressors are usually portable and therefore commonly used in the trades.
- Rotary Screw Compressors
The engineering involved in rotary screw compressors is more sophisticated: air is compressed by a pair of interlocking helical screws (or rotors). These compressors are mostly fixed and housed in a dedicated cabinet for use in factories, hospitals or universities. They can achieve upwards of 1,000cfm at pressures of up to 13 bar. The air is generally cooler and has fewer pulsations than a reciprocating compressor.
- Variable Speed Compressors
These are rotary screw compressors which incorporate variable speed drives for greater control over airflow and pressure. They can achieve similar output to rotary screw compressors but due to greater efficiency and control they invariably offer huge energy savings to facilities where the air consumption is variable such as machine shops and factories.
Assessing Your Power Requirements
Of course the above power ratings are all very well if you know exactly what outputs you’re looking for, but what if you don’t? How do you establish exactly what your requirements are from an industrial air compressor in order to pick the right one?
Tools and devices that are powered by compressed air will state either on the tool itself or somewhere in its documentation, what its specific power requirements are. These power requirements will come in the form of a pressure and an airflow rating.
- Pressure rating: Depending on the country of origin, the pressure rating will be given in either psi (pounds per square inch) or bar (meaning number of atmospheres). These figures denote the recommended pressure that the air is delivered from your compressor to the tool you’re using for optimal performance.
- Airflow rating: As well as the pressure, you need to calculate the airflow. A rating will be given in either cfm or ltr/min, standing for cubic feet per minute or litres per minute respectively. This measurement will inform you of how quickly the compressed air needs to be delivered to ensure continuous operation.
Having collected the power requirements for all the tools and machinery you will need to use, determine which tools you will be using concurrently and then add together all their respective cfm or ltr/min ratings. The resulting figure is the bare minimum airflow capacity that your compressor will need to run these tools together. Next you should examine the pressure requirements for the tools; you will not need to add them up but you do need to ensure that the tool which requires highest operational pressure will be satisfied (i.e. if your highest rated tool requires 8 bar, as a bare minimum your air compressor should be capable of delivering this).
Most pneumatic tools, especially if well used, will start to leak some air. As such, if your air compressor is at capacity when delivering the minimum operational requirements for your tools, you are likely to have problems in the future, as your system becomes less-efficient over time. In light of this then, it is wise to have some spare capacity; especially if you purchase more equipment in the future. However, you shouldn’t overdo it; as you will pay for too much spare capacity in needlessly high running-costs. Most compressor companies can offer an ultrasound leak detection service to identify and log your wastage and potential savings to allow you to maintain your system efficiently. They can usually offer a data log service whereby your existing compressor arrangement is logged for a period of time and energy saving recommendations are made. The payback is generally very fast on larger systems.
Wear and Tear
Most air compressors should have an operational life of around 10 years if properly maintained and correctly sized. Like most machinery an air compressor's lifespan will ultimately be determined by how heavily it’s used and how well it’s maintained. As such, when choosing your compressor ensure it can comfortably cope with its workload, even if that means spending more on your initial purchase. An overworked compressor will need to be replaced or repaired sooner and thus, could prove to be more expensive in the long-run.
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Assessing Your Quality Requirements
Pressure and airflow are not the only factors to consider. Depending on the application it can be very important to ensure that the quality of the compressed air delivered meets certain requirements. Filling car tyres, filling scuba tanks or cleaning micro-processors are processes which have differing levels of acceptable air quality.
Like most machinery, air compressors need oil to lubricate their moving parts. Given that the act of compression requires the air to come into contact with at least some of these moving parts; it is not uncommon for the compressed air produced to have small traces of oil present in it. If you are simply using pneumatic tools then this needn’t be a problem, as they are likely to be tolerant of some oil in their air supply. However, in medical applications for instance, this would be unacceptable and perhaps even dangerous. Given your use then, you have the choice of applying a filter to a industrial air compressor or purchasing what is known as an oil-free compressor. The former will clean (to a certain extent) the air produced but when absolutely clean air is required, it is unlikely to suffice. Oil-free or oil-less compressors on the other hand, although more expensive, make absolutely sure that your compressed air is free of all oil traces by separating the air and oil throughout the compression process.
Despite the climate there is always water vapour present in the atmosphere. As ambient air is compressed, this vapour often then condenses into water droplets. With most equipment prolonged exposure to moisture from your air compressor can cause severe damage. The most effective solution to this problem is the use of an additional dryer. Driers can be purchased separately and fitted to most compressors but also come as standard with many industrial air compressors. Varying levels of dryness can be achieved with different types of dryer depending on your needs.
- Moisture Disposal
The authorities do not usually allow discharges containing more than 20ppm of oil to be introduced to the drains. Just 250ml of oil can cover acres of water surface, preventing oxygenation; reducing the effectiveness of treatment plants and killing wildlife. A compressor of only 18kW can generate close to ten thousand litres of condensate each year, often containing up to 1000ppm of oil and other waste at the point of discharge. It makes sense to treat this on site because the 99.9% of compressor condensate that is only water can then be legally discharged to the sewer, while the disposal of a few litres of captured oil is easy and affordable. Condensate separators should be installed that collect all the removed condensate and they generally use a carbon filtration system to separate oil from water, allowing safe disposal of both.
Anyone familiar with basic physics will be aware that when a gas expands, it cools. The inverse is also true; when a gas is compressed it becomes hotter. As such, the use of compressed air will always be susceptible to temperature fluctuations. The most common temperature related problem is that the air delivered from the compressor, having been rapidly compressed, is too hot for its application. This problem may be solved with a cooler, which works much in the same way as an air-conditioning unit. Coolers can be bought separately or, again, may come as standard on the more heavy-duty types of air compressor.
It is usually acceptable for a mechanic's garage to be a noisy place. However, the same level of noise would be absolutely unacceptable in a dentists’ surgery. As such, it is important to consider the noise that your air compressor is likely to make and how, if necessary, to minimise it. Due to their design some air compressors, especially rotary or screw compressors, are much quieter. Most piston manufacturers’ offer silenced versions which have much lower noise levels. When noise is a serious consideration then, it is wise to consider a screw compressor but you could also get around this problem by housing your compressor in another room or building, should you have the available space.
All compressors require good ventilation to work correctly and reliably. Consideration to where you site a compressor is often just as important as which compressor you choose. The following conditions should be considered; ambient temperature, localised air contamination (dust/swarf) and removal of heat generated by the compressor either to outside or a more useful area (for heating purposes). Most compressor companies can advise on this, and nowadays offer heat reclamation solutions to increase your energy efficiency and reduce heating costs on site.
- Air Storage
In order to ensure your compressor is not running unnecessarily and reacting to site usage fluctuations, storage of air at the pressure you require is always recommended for efficiency and reliability. Piston compressors and smaller rotary screw compressors generally have a storage receiver underneath them to save space or it is an option. You can however fit storage receivers at any point in either your plant room or pipework ring main. If you have areas where a large volume (flow) of air is used for a short period, then a receiver would be useful nearby. They cost nothing to run as they are purely high pressure storage vessels, but as such will usually require annual insurance inspections to comply with your policy and H&S.
- Air Distribution Pipework
There are many types of pipework systems available nowadays. However, consideration should be given to the environment and the efficiency required of a system. Steel pipework has historically been commonplace, although rising raw material costs and the large labour cost involved in installing it are making it less attractive. Aluminium extruded systems are becoming more popular as not only are they visually pleasing as they generally come colour coded correctly for Compressed Air use and marked up for pressure and diameter, but they offer rust resistance for cleaner air delivery, leakage resistance for less wastage, and flow properties far superior to steel systems for improved efficiency. This usually allows a smaller diameter pipe to be used. The installation times are much quicker than steel due to the ease of connection and the weight reduction, and future alterations/additions are much simpler and cost effective.
- Multiple Compressor Control Systems
If you have multiple compressors, to ensure you use the most efficient combination at any given time, intelligent controllers can be added to a system. These are programmed to assess your usage and choose the correct size or type of compressor(s) to run in a power and cost efficient manner. This is especially useful if you have a Variable Speed machine in the system. Compressor running hours can be shared equally using these panels, start and stop times can be setup, and even different pressures can be set for known daily requirement changes. They can often be linked to a building management system for better on site control and some can even go as far as requesting an engineer by SMS to attend site in the event of a breakdown.
Industrial Air Compressor Suppliers
Although it’s important to correctly assess your needs, you needn’t worry if you are not absolutely sure which air compressor is right for you. There are plenty of specialist air compressor suppliers out there and, with a reasonable understanding of your needs, they should be able to give you some sound advice and perhaps raise some points not discussed in this article.
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