TechArchive

Original article date: January 2000

Compressors are essential for pneumatic systems, but they are often incorrectly specified. NICK POOLE of Abac has some advice to help avoid the common pitfalls

No pneumatic system would be complete without a supply of compressed air, but the specification of a new compressor, however, is often carried out urgently in response to either the failure of an existing compressor or a change in the compressed air requirements.

Unfortunately, the brevity of the selection process means that aspects such as compressor control technology and capacity, required air quality and operating conditions may not be fully considered, with the result that most operating problems can be directly attributed to incorrect specification.

The first stage of the specification process should be to establish exactly how the compressor will be used. For example, if the compressor will be responsible for powering a complete air system and compressed air demand varies throughout the plant due to shift patterns, it may be more energy-efficient to install two or more air compressors. It is also important to establish the level of air quality required and to consider the plant environment; for example, the compressor may require corrosion resistance, dust protection or ductwork to improve cooling.

Once the intended use for a new compressor has been established, it is important to determine which compressor type is the most appropriate. Two of the most popular air compressor technologies are rotary screw and reciprocating compressors.

The exact advantages and disadvantages of each technology are complex. However, double-acting reciprocating compressors typically have a simpler internal design and a basic machine will generally have a lower purchase price than a rotary screw compressor. If highly controlled air quality is required, however, for semiconductor manufacturing or to compensate for variable process demands, the cost of additional components, such as after-coolers and operational shutdown switches, must also be taken into account.

Consequently, reciprocating compressors are often used to provide good quality with overall reliability in low horse-power applications, while rotary compressors are used to meet more stringent air quality controls and higher power demands, accounting for the majority of the 100-150psig market above 20hp.

Specifiers must also consider the required size of the compressor. In doing so, it is extremely important to look at the free air delivered (FAD) as this describes the amount of air actually produced by the compressor, as opposed to piston displacement capacity. In practice, the FAD will be 25-30% lower than the piston displacement capacity, so purchasing a compressor with a larger displacement capacity will not necessarily be sufficient to meet an increased or high compressed air demand.

What are the operational benefits? The latest air compressors tend to have a minimal number of moving parts, providing enhanced performance, and simplified operation, installation and maintenance. Maintenance costs will also tend to be lower as components have been designed to achieve a longer operating life. New integrated rotary compressors, incorporating dryers, filters, compressor and all accessories, offer the additional advantage of ensuring the compatibility of all working components. As a result, operators do not need an in-depth knowledge of compressor technology in order to operate the system at its optimal effectiveness.

Specifiers will, understandably, also be considering the purchase costs for alternative compressors. But the most effective way to reduce costs is to maximise the energy-efficiency. This is because, over a typical 10-year operating period, up to 80% of the costs associated with the compressor will relate to power consumption. If a compressor is over-specified, either ‘to be on the safe side’ or to meet the volume requirements of the highest level in the system, power requirements can be increased by 6% for every 1 bar of additional pressure generated. The cost of this extra power can be considerable.

First choice: upgrade or replace?

Before replacing a compressor, it is worth checking whether the existing unit is functioning at its maximum possible efficiency, and, if not, whether it’s operation can be improved. Sources of inefficiency include air leaks, which, in a medium-sized manufacturing plant, can easily account for up to 30% of the total compressed air demand.

Modern compressors will generally have a PLC controller to provide better feedback on system performance and it may be possible to incorporate such a controller into an older compressor. In doing so, operators may also be able to integrate the compressor into a higher level management system, with the associated benefits of enhanced system and process control.

Compressor efficiency and air quality can also be improved through the use of air dryers, which remove water vapour from the compressed air stream, that could degrade end products, increase costs and damage air tools. The most popular technologies in an industrial environment are refrigerated and desiccant air dryers.

• Abac

January 2000