Compressor Selection

There are two main types of compressed air systems: oil-free and lubricated. One design will be chosen over the other, depending on purification and industry requirements. Oil-Free Systems: Applications that cannot tolerate a lubricant require an oil-free system. It is critical to remove unwanted oil aerosols and vapours from compressed air, not just moisture. These aerosols and vapours are found in ambient air and can be generated by the compressor. This oil ends up degraded and oxidized by the heat of compression. Once heated, the oil can carbonize and form a solid, varnish-like substance on downstream equipment, causing valves and air equipment to malfunction. If the oil is mixed with water, it forms a sludge that can gum up components of the air line. Downstream from the compressor, an air receiver stabilizes system pressure, serves as a demand reservoir, and holds some moisture. Downstream from the receiver, an air dryer, which will provide the correct pressure dew point, traps the remaining moisture. If either of these fails, there is still a coalescing filter after the dryer to provide protection. A dry receiver can also be installed after the coalescing filter to stabilize pressure and serve as a reservoir for times of high demand.

Lubricated Systems: These types of systems use a lubricant to alleviate friction between moving parts. In rotary screw compressors, the lubricant also seals clearances and removes heat of compression. The viscosity of the lubricant used depends largely on the operating ambient temperature range. It must offer adequate lubrication for bearings and rotors at operating temperature. In addition, it must have a pour point low enough to provide fluidity at low starting temperature. A modern, lubricated rotary screw compressor and high-efficiency purification system can produce compressed air with very high purity. These systems are very similar to the oil-free system, consisting of a wet receiver, an air dryer, and a coalescing filter. There is, however, a charcoal filter between the coalescing filter and the dry receiver that removes any leftover oil vapours.

There are eight basic elements that must be considered in designing your compressed air system: demand, compressed air quality, supply, storage, distribution, installation, maintenance, and condensate management.

One of the most important and most difficult things you can do when designing your compressed air system is to determine the true demand in your system. Air demand will fluctuate beyond the predetermined average demand. If the actual demand is known, storage and distribution systems can be designed to meet demand without the installation of additional compressors.

The most precise way to determine demand in the system is to monitor the air flow using a flow meter, which would normally be positioned in the main headers. For small, simple systems, the ratio between loaded and unloaded compressor running time can be indicative of average demand over a long periods of time.

Often, leakage and artificial demand represent a substantial portion of the overall demand. There are various methods to stop leaks. Excess volume of compressed air created for unregulated users is called artificial demand. It occurs when greater line pressure than necessary was supplied. It includes the following:

  • all unregulated consumption, including appropriate and inappropriate production usage
  • open blowing
  • leaks
  • point of use with regulators adjusted to their maximum setting
  • tooling


These applications track the supply pressure as though no regulators were being used. The artificial demand challenge can be resolved by positioning a regulator at the point of use or at the beginning of the distribution network. Operating pressure requirements, compressed air requirements, and the duty cycle of individual equipment must all be considered when establishing demand for your system.

Air quality: Different applications demand different levels of compressed air quality. With each level, the cost to produce the compressed air increases. Therefore, it is essential to meet, but not exceed the level required by your particular application. If different levels are required for different applications within the plant, it is more cost-effective to treat smaller amounts of compressed air for the application with the highest level of quality requirements, than to treat the whole air supply.

The compressed air supply must always meet the compressed air demand by utilizing sufficient storage and correct distribution. Properly sized compressors and purification equipment will aid in meeting demand with supply. If the supply, storage, and distribution are not in sync, excessive pressure fluctuation will occur. Most compressors are controlled by line pressure. A drop in pressure normally signifies a demand increase. This is corrected by increased compressor output. A rise in pressure usually indicates a decrease in demand, which causes a reduction in compressor output. To accommodate the fluctuating demand, a load/no load or constant speed control can be used to run the compressor at full load or idle. Either a single compressor or a multiple compressor installation, which can be centralized or decentralized, can provide the entire plant supply. There are three other types of compressor control systems:

  • Auto-dual control: Most traditional modulating controls throttle the capacity 30%-50% before fully unloading the compressor. This type of modulation is known as auto-dual control. It combines start/stop and constant speed control into a single control system. Auto-dual control automatically selects the most desirable control method and runs the compressor in constant speed control. When the compressor unloads, an unloaded run timer energizes, which usually has a time range of 5 to 60 minutes. If the compressor does not reload, the timer will shut the compressor off. The compressor will restart and reload when the pressure switch senses low pressure.
  • Sequencing: Sequencing is also known as a central controller. This has the advantage of little cost per compressor and is usually available for systems with up to 10 compressors. A sequencer should have a single pressure transducer in the air header. Logic should maintain a target pressure within +/- 1 Bar. The sequencer should automatically start and stop compressors, as well as load and unload them. The control should be set to rotate the order of loading and unloading to optimize compressor combinations for different demand conditions.
  • Lead/Lag: Lead/lag controls are typically found on reciprocating compressors. When there are two compressors in the system, one compressor can be set as the lead compressor, and the other as the lag compressor. When the pressure drops to a certain point on the lead compressor, the lag compressor will then take over. These can also be switched so that the other compressor is the lead compressor.



All devices containing compressed air make up the storage system. Adequate storage is essential. It represents available energy that can be released or replenished at any time it is needed. The air receiver tank normally makes up the majority of the total storage capacity of the system. If this tank is properly sized, excessive cycling will be prevented, and adequate storage capacity for any peaks in demand will be provided. In the distribution system, there will periodically be large volume demands, which will rapidly drain the air from surrounding areas, and cause pressure levels to fall for surrounding users. However, strategically located receivers in the system can supply these abrupt demands and still provide a consistent air flow and pressure to the affected areas. The total storage capacity needed is dependant upon the amount of excess demand in cubic feet, the available pressure differential between the flow controller, the system and compressor start-up time, and the time available to replenish stored compressed air.

The distribution system is the link between supply, storage, and demand. Ideally, the distribution system will allow the required air to flow with minimum pressure drop. It will supply an adequate amount of compressed air at the required pressure to all of the locations where compressed air is needed. The compressed air travels through a network of pipelines, but the flow creates friction and results in pressure drop. The pressure drop should never exceed 1-2 psi (0.07 – 0.14 bar). The longer and smaller diameter the pipe is, the higher the friction loss. To reduce pressure drop effectively, a loop system with two-way flow can be used. Pressure drop caused by corrosion and the system components themselves are important issues. These typically range from 5-25 psid (0.34 – 1.7 bar) and their control is essential for the efficiency of the system.



A variable speed compressor is not recommended to stand alone. To start off with, you need a constant speed compressor which will supply you 65% of your demand.  In addition to that you need a  VSD compressor making up 55% of total demand.  The reason for that is you have peaks whereby you will exceed 100% for fractions at a time.  Refrigeration dryers  are not recommended depending on your RH and PDP.  To obtain the required 0,01ppm you need the following; a heatless absorption dryer with a PDP of -20°C and a purch value of not more than 4%.  To obtain the 0,01 you need the following inline filters; Pre-filter  1µ,  After the dryer  1 Coalescing filter 0,1µ, a Fine filter 0,003µ, and Oil absorber of 0,005µ this will ensure that a constant pressure and flow is maintained at all times and fraction peaks will not cause a dip in production or pressure.