TECHNICAL APPLICATION OF COMPRESSED AIR

Nowadays, compressed air is widely used in machine tools and equipment, however the requirements are not the same for different applications. Proper use and maintenance can ensure optimal production, while incorrect use will create problems.

In the SI Unit (Standard International Unit) system, the basic unit of Pressure is Pa (Pascal; 1Pa = N/m²). However, "bar" is commonly used as it is simpler (1bar = 1.02kg/cm² and 1bar=100kPa). For conversion purpose, the generally accepted equation is: 1bar = 1kg/cm².

In the Imperial System, it is called "psi" (lb/in²). The conversation between Imperial and SI is 1bar=14.5psi.

The highest pressure limit of compressed air is 5-5.5kg/cm² and the cut-off limit is 7-7.5bar or less. The maximum pressure of the accumulator should be 9-10bar (depending on the manufacturers). To ensure safety, each compressor unit is usually equipped with a mechanical automatic pressure relief valve.

Please DO NOT make any changes to the safety device, and be sure to employ an accredited person to inspect the equipment regularly.

Liter [l, L] is the basic unit of volume in the SI Unit system (the capacity of capsule of compressed air unit is also represented by liters). L/min or m³/min represents the quantity of compressed air that a compressor supplies, or the consumption rate of the equipment. Details of consumption rates can be found in the operations manual or the catalogue of the machine or equipment, e.g. 50NL/min. In the figure below, N represents "Normal Pressure or atmospheric pressure".

When choosing an air compressor, users should consider: 1. the supply of compressed air, and 2. the capacity of the accumulator or capsule. If the capsule is too small, the compressor has to work continually or must be switched on and off frequently. The multiply ratio between the capacity of the capsule and the consumption of the equipment is 3-5. For high consumption levels, one of the best solutions is to add an auxiliary capsule.

The atmosphere contains a certain amount of moisture. When the atmospheric pressure and the temperature change, this moisture will be transformed into water. Therefore, the capsule of an air compressor contains water, and some of it will combine with the compressed air. After a capsule is used for a while, the water levels will increase and the space for compressed air will fall. This means the compressor has to work constantly, or the cut-off period can be reduced, and the compressed air that is supplied will contain a lot of water. It is therefore important to drain the water from the capsule regularly to ensure the quality of the compressed air and to maintain the efficiency of the air compressor. This can also prevent rust inside the capsule. As Southern China and Hong Kong have high humidity levels, it is even more important to pay attention to this.

In addition to moisture, the atmosphere also contains a lot of ash and dust, which degrade the air compressor and cause damage. Regular cleaning and replacement of air filters at the intake are the best ways to keep the air compressor in the best possible condition.

The use of appropriate lubricants can improve efficiency. It is also essential to carry out regular checks and to replace worn or faulty parts.

Compressed air for machines is slightly different from compressed air for equipment. In most cases, compressed air that is over-saturated with moisture is not acceptable. It can cause rust and can mix with dust to form dirt on the surface of the parts, hindering operations and jamming the exhaust. Equipment needs pure compressed air and must be oil-free (except when specified). However, machines need a proper lubricant mixed with compressed air for lubrication purposes.

The illustration below shows the general treatment of compressed air before it is delivered to a machine or a piece of equipment.


For general machines:

·	A filter can separate dust and moisture. The effectiveness of the filtration depends on the manufacturer and the requirements of the end user.
·	A regulator can be used to regulate and stabilise pressure.
·	A lubricator can be used to mix a lubricant with the compressed air at a specified ratio.
·	This combination is called an "Air Service Unit".

For general equipment:

·	A filter can separate dust and moisture at the prime stage. The effectiveness of the filtration depends on the manufacturer and the requirements of the end user.
·	A regulator can be used to regulate and stabilise pressure. The effectiveness of the filtration depends on the manufacturer.
·	An oil remover or an oil removal filter can be used to separate oil from compressed air. The effectiveness of the filtration depends on the manufacturer.
·	A mist separator can be used to improve the purities of compressed air. The effectiveness of the filtration depends on the manufacturer.
·	An air dryer can be used to reduce the moisture content of compressed air or to maintain the moisture level. The effectiveness of the filtration depends on the manufacturer.