Produced in Association with


SERIES 21 / Module 07 Compressed Air


compress the air. The most efficient theoretical


compression cycle is isothermal, during which the temperature is constant. Compressor designers attempt to approach isothermal by such methods as intercooling and oil or water injection. In practice, when optimising air


generation systems engineers should ensure that compressor intake air and cooling stream temperatures are as low as possible, suction filter pressure drop is minimised, and the delivery pressure is set as low as possible to keep the compression ratio down and reduce the work being done. Compressors are normally sized


with some spare capacity to allow for peak demands so pressure drops are avoided and some growth for the future. This means that efficient control is important both of individual and groups of machines. The most popular method of individual


control of fixed speed machines can be by inlet valve opening when air is required or shut when there is no air demand known as two-step or all on-line off line control. Inlet valves can also be modulated over the higher ranges of demand from around 60 to 100%. Inlet valve operation is controlled by the system air pressure that when rising to its top limit the machine will unload and when the pressure falls to its pre-set low limit the machine will load. Two step control of fixed speed


machines is the most frequently seen. Long periods of no-load running should be avoided as the power consumption will be around 20 to 25% of the full load power. Variable speed drive is available for


positive displacement machines; this can be more efficient than two step or modulation control as long as the machine is correctly sized and does not run for long periods above 80% of capacity where invertor and other losses make the machine less efficient. The most common form of dynamic


machine found in industry is the centrifugal flow machine. This type of machine inhales a volume of air at the atmospheric conditions prevailing, then accelerates it in high speed impellors, thus imparting kinetic energy that is transformed into pressure energy by reducing the air speed in the diffusers. This machine has a characteristic


curve as shown above in Fig 3. These machines can be controlled efficiently by inlet valve, inlet guide and diffuser guide vanes over the stable operating range before natural surge pressure becomes close to the design pressure.


Application To arrive at the best machine for a given duty the configuration must be established. Issues that must be considered are:


● The air demand pattern over a typical process period


● The air pressure required ● The air quality required in terms of


18 EIBI | FEBRUARY 2024 Fig 3. Centrifugal flow machine Fig 2. Pressure volume of positive displacement compressors


their best efficiency and provide good energy savings.


Air treatment Following generation, the air is treated to the standard required by the end using process by a variety of methods. The quality of the air used ranges from quite low for tools to extremely high for microelectronics paint finishing, food, and pharmaceuticals where the air can be in contact with the product. When the air leaves the final stage of


the compressor at terminal pressure it is hot and fully saturated with water that has been inhaled from the atmosphere. As the air is cooled water condenses and if there is no treatment it will arrive at the usage points. This cannot be tolerated and


treatment to remove the water following air compression is required. The first stage of treatment is


the aftercooler that can be air or water cooled. This will reduce the air temperature from over 100°C to within 10° of the cooling medium's temperature and will remove around 80% of the water. This good enough for some end users but most require that further treatment will be required. There is usually a wet air receiver sized


pressure dewpoint, hydrocarbon and particulate content and any special requirements such as silica free


● Maintenance requirements ● Standby capacity needed Given this information a detailed


specification should be written from which qualified vendors can produce proposals and detailed bid analysis can be undertaken. This work will result in ensuring that the machinery chosen may not have the lowest cost but will have the best life cycle costs for the duty.


Group control systems Group control systems should always be aimed at ensuring that the most efficient machines in the installation are on line at all times at the minimum sensible generation pressure. Electronic panels are available that the right mix of machines is on line at any one time. Correct sizing will avoid control gaps


Fig 4.


that can occur when running a fixed and variable speed control machine together. The example below (Fig 4) shows a


typical control gap where two equal sized compressors, one fixed speed the other variable speed, are run together to provide air to a plant. This is a very wasteful situation and


can be quite damaging as the fixed speed machine is having to load and unload frequently. One solution to the control gap


problem is to use a smaller fixed speed machine as base load with output below the minimum demand of the control gap and a larger variable speed machine with capacity to cover the rest of the demand as shown below in Fig 5. Another solution that is gaining


popularity is to have the demand met by machines that are all driven by variable speed motors. In this case the group controller will run the all the machines at


Fig 5.


correctly for the delivered volume of the installed compressor capacity. This removes some of the entrained moisture and helps smooth out any pulsations. Selection of the correct dryer for the


duty will depend on the required pressure dewpoint for the process. Condensate removal from the aftercoolers, air receivers, filters, dryers and other drainage points should be by use of automatic drains the best type being of the zero-loss electronic configuration. Other contaminations found in


compressed air are oil and particles. The most popular general purpose


compressors are usually lubricated screw or piston machines that have oil in the compression chambers. Some hydrocarbons are present in the atmosphere in industrial areas and these are inhaled and concentrated by the compressors. For specialist end users, such as


pharmaceutical plants, microelectronic manufacturers, food and beverage plants and motor vehicle paint shops, oil free compressors are normally specified. Oil


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36