Air generation and distribution

The compressed air supply for a pneumatic system should be adequately calculated and made available in the appropriate quality. Air is compressed by the air compressor and delivered to an air distribution system in the factory. To ensure the quality of the air is acceptable, air service equipment is utilized to prepare the air before being applied to the control system.

Malfunctions can be considerably reduced in the system if the compressed air is correctly prepared. A number of aspects must be considered in the preparation of the service air:

• Quantity of air required to meet the demands of the system
• Type of compressor to be used to produce the quantity required
• Pressure requirements
• Storage required
• Requirements for air cleanliness
• Acceptable humidity levels to reduce corrosion and sticky operation
• Lubrication requirements, if necessary
• Temperature of the air and effects on the system
• Line sizes and valve sizes to meet demand
• Material selection to meet environmental and system requirements
• Drainage points and exhaust outlets in the distribution system
• Layout of the distribution system to meet demand.

As a rule pneumatic components are designed for a maximum operating pressure of 800-1000 kPa (8 - 10 bar) but in practice it is recommended to operate at between 500-600 kPa (5 and 6 bar) for economic use. Due to the pressure losses in the distribution system the compressor should deliver between 650-700 kPa (6.5 and 7) bar to attain these figures. A reservoir should be fitted to reduce pressure fluctuations. In some cases, the term ‘receiver’ is also used to describe a reservoir.

The compressor fills the reservoir which is available as a storage tank.

The pipe diameter of the air distribution system should be selected in such a way that the pressure loss from the pressurised reservoir to the consuming device ideally does not exceed approx. 10 kPa (0.1 bar). The selection of the pipe diameter is governed by:

• Flow rate
• Line length
• Permissible pressure loss
• Operating pressure
• Number of flow control points in the line
• Ring

Ring circuits are most frequently used as main lines. This method of installing pressure lines also achieves a constant supply in the case of high air consumption. The pipe lines must be installed in the direction of flow with a gradient of 1 to 2%. This is particularly important in the case of branch lines. Condensate can be removed from the lines at the lowest point. Any branchings of air consumption points where lines run horizontally should always be installed on the upper side of the main line. Branchings for condensate removal are installed on the underside of the main line.

Shut-off valves can be used to block sections of compressed air lines if these are not required or need to be closed down for repair or maintenance purposes.

The air service unit is a combination of the following :

• Compressed air filter (with water separator)
• Compressed air regulator
• Compressed air lubricator

However, the use of a lubricator does not need to be provided for in the power section of a control system unless necessary, since the compressed air in the control section does not necessarily need to be lubricated. The correct combination, size and type of these elements are determined by the application and the control system demand. An air service unit is fitted at each control system in the network to ensure the quality of air for each individual task.

Compressed air filter

The compressed air filter has the job of removing all contaminants from the compressed air flowing through it as well as water which has already condensed. The compressed air enters the filter bowl through guide slots. Liquid particles and larger particles of dirt are separated centrifugally collecting in the lower part of the filter bowl. The collected condensate must be drained before the level exceeds the maximum condensate mark, as it will otherwise be re-entrained in the air stream.

Compressed air regulator

The purpose of the regulator is to keep the operating pressure of the system (secondary pressure) virtually constant regardless of fluctuations in the line pressure (primary pressure) and the air consumption.

Compressed air lubricator

The purpose of the lubricator is to deliver a metered quantity of oil mist into a leg of the air distribution system when necessary for the operation of the pneumatic system.