Pravin Mehta discusses the importance of compressors in pneumatic systems, and elaborates on vital design and operational parameters The recent advancements in civilisation have been driven by developments in industrial operations—largely with the help of machines. Machines, however, require a continuous energy input to achieve a driving force. This is primarily obtained through non-renewable energy resources such as coal, diesel and gasoline, and most often used in the form of electricity. However, nonrenewable energy resources are limited, which adds to their cost. They also release greenhouse gases on combustion.  Thus, energy efficiency is critical in determining a machine’s suitability for an operation in terms of costs and performance. The energy efficiency of a machine depends on its design attributes, the optimal use of driving media (such as compressed air in pneumatics) and other operating parameters such as the selection of appropriate machinery and operating speeds.Pneumatic systems such as air compressors have found widespread application in a range of industrial operations. This is due to the ready availability of atmospheric air and the smaller size of pneumatic systems as compared to hydraulic systems. Achieving high energy efficiency is one of the key challenges in pneumatically-operated machines. This is because the production costs associated with compressed air are quite high, which provides a further impetus to improve the air compression efficiency, design attributes and operating parameters of pneumatic systems. According to Bhupendra Khatri, Proprietor, Technic Marketing, a pneumatics services provider,“One of the major challenges associated with a pneumatics system is reducing the costs associated with compressed air,and, hence, the cost of the services used by the consumer. To achieve this, it is essential to optimise the production costs of compressed air while improving the work efficiency of the pneumatic system.” Compressors—The Dos and The Don’ts Though compressors are used across industry verticals, realising their optimal performance and efficiency depends upon several parameters such as their size,usage and operating conditions. Among the most important aspects of operating a compressor are its running conditions, particularly the compressor speed and load. For example, running a compressor at zero or minimum load increases its operating costs considerably. This is because while power is being fed into the compressor to do work (generation of compressed air), there is virtually no work (pneumatically-driven movement) outside, resulting in wastage of power.
Another critical aspect of running a compressor is its operating pressure. The higher the pressure of the air that has already entered the compression cylinder, the greater the compressor needs to work to suck in more air. Thus, a compressor’s efficiency decreases at higher pressures, resulting in increased operating costs. KS Natarajan, MD, Trident Pneumatics, says, “The costs associated with pneumatic equipment such as a compressor can be minimised by controlling the running conditions. For example, running or operating the compressor at zero load conditions or at high pressures results in increased costs. It is noteworthy that every kg/cm2 increase in the operating pressure of compressed air increases the operating costs in direct proportion.” Another factor that affects the costs associated with pneumatic machinery is the selection of the compressor. Nowadays, a variety of compressors with different compression capacities are employed for different gases used in pneumatics applications. The cost of compressed gas is directly proportional to the horsepower (HP) of the compressor motor, which should be selected according to the density of the gas and its flow rate. For example, one of the standard compression pressure values for compressed air under standard industrial operations is 6 kg/cm2. According to industry experts, ‘compressor mismatch’ can result in a drastic rise in the operating costs associated with a compressor. Says Mr Khatri, “A mismatched compressor adds significantly to the cost of running pneumatic components. Often, higher-capacity compressors are installed in which moderately compressed air is needed at a not-so-high flow rate, which results in wastage of the excessive power used in high compression.”  Along with compressor sizing, selection and operating conditions, the appropriateness of auxiliary components and accessories used in a pneumatic system is equally important. The selection of an appropriate compressor and accessories and their compatibility with components such as tubing, driers and filters, is also significant from the point of view of cost and efficiency optimisation. “Selecting inappropriate compressor tubing, filter, regulator and lubricator may cause severe drop in system efficiency, leading to higher operating costs. For example, in the case of accessories such as driers and filters in compressed air piping, considerable energy can be conserved by changing filter elements with increase in the pressure drop, and setting the dew point of the driers as per the requirements,”opines Mr Natarajan. Apart from the selection of the appropriate air compression units, another important cost optimisation aspect is to lower the operations cost in cases where the use of high-pressure compressors is indispensable, though all the machines have varying requirements. In such cases, individual machines and their specific requirements must be identified with, and suitable compressors should be installed accordingly. According to Abhishek Gambhir, Director, SWAM Pneumatics Pvt Ltd, “If a machine requires higherpressure air,then there is no other way to achieve this than by generating higher pressure. But if only one machine requires higher pressure, instead of running the entire factory at a higher pressure, a smaller high-pressure compressor or a pressure booster must be used for the purpose.” Since the beginning of the pneumatics era, gases of varying physical and chemical properties have been used to affect motion, depending upon the type of applications. Spatially fixed applications (such as those in industrial workshops and shop floors, where high compression is required) generally use dry atmospheric air due to its ready availability and neutrality. It is very important that the air fed into the compressor is absolutely dry. Lubricants should be used to protect the compressor from corrosion. According to Mr Natarajan, “Using a suitable lubricator in a pneumatic filter-regulator-lubricator (FRL) plays an important role in maximising machine uptime as it makes the machine corrosion-resistant. However, to ensure optimal consumption, too much oil should not be dumped into the machine.” Remote pneumatic facilities (in which the hazard of asphyxiation is virtually absent) generally use gases such as nitrogen as the compressed media. This is later stored in compressed-gas cylinders. It is noteworthy that gases such as oxygen, though they do not pose an asphyxiation threat, are a fire hazard. So the use of oxygen as the driving medium in pneumatically-powered equipment must be strictly prohibited. The Design In pneumatic systems such as air compressors and other pneumatic equipment, the flow rate of air and the compression pressure are among the critical parameters determining design specifications. In turn, the cost of compressed air and the operational efficiency of a pneumatic system depend upon the design of the compressor. An air compressor basically comprises three components—the compressor unit, the treatment unit, including air driers and filters, and the control unit, which comprises solenoid valves, FRL, actuators and pneumatic cylinders.
According to Mr Khatri, “Because of its physical impact on the system, the air-flow rate for a compressor, measured in cubic feet per minute (CFPM/CFM), defines the design of a compressor. Compression pressure also determines the compressor design attributes because of its direct impact on vanes and piston.” Unfortunately, industry in general is ignorant of several intricacies associated with compressor design, which largely affect operational costs. Compressor air suction is an important design consideration for the equipment because the suction of air by a compressor is inversely proportional to the CFM. This fact can be used to maximise the energy efficiency of compressors as it helps to determine optimal design parameters in terms of CFM and the desired power needed for a compressor. Mr Natarajan shares his views, “From the technological aspect, one should take into account the specific power consumption of each compressor—that is, power per CFM hourly—to design and select a particular model. This can improve the energy efficiency of the system.”  FRL: One component of a compressor which requires considerable design input is the FRL unit, as it greatly helps in terms of compressor pressure correction. In this regard, Mr Gambhir says,“There are basically two problems with high-pressure compressors that can be eliminated with a good FRL. FRL is essential for air pressure correction of the machine, as higher pressure implies faster wearing of machine components along with greater energy consumption. Also, the pressure of an ideal FRL should not fluctuate much. Based on the dimensional factors, a smaller FRL would lead to a higher pressure drop across it.So,pressure fluctuation is one of the key factors while deciding the sizing of an ideal FRL.”Actuator: The actuator is another important component of the control unit, which must be designed optimally, particularly in terms of its dimensions. In pneumatic systems, an actuator is the component that actuates motion using the force obtained from compressed air. As force is represented by the product of pressure and cross-sectional area, the design of the actuator is critical in realising the force, knowing that pressure remains constant. Size: Currently, a common issue with all types of actuators or pneumatic cylinders is their size: they are usually bigger than required. Due to this, they consume more compressed air and produce more force than required, resulting in greater power consumption. According to Mr Khatri, “An oversized actuator does no apparent harm, but it will consume more compressor air, and, hence, more power. So, actuator sizing and selection is very important to minimise the operational costs associated with pneumatic systems.” Leakage of air: Another major reason behind efficiency loss and increased costs in a pneumatic system is the leakage of air. It has often been observed that bigger compressors, even after consuming several watts of power, do not give satisfactory results. Sometimes, smaller compressors, though suitable for an operation, have to be replaced with bigger machines as a result of air leakage. Industry experts have cited improper inspection and maintenance as the major reasons behind the issue.“At the maintenance end, there are usually several leakages in compressed air lines. Unlike a hydraulic fluid which drips all over, nobody notices air, which simply leaks and lowers the compressor efficiency. The problem has been difficult to appreciate so far due to the associated costs from the running standpoint,” says Mr Natarajan. Getting the Most From Compressors Due to the costs associated with the production of compressed air, it is important to ensure leak-proof operation of pneumatic systems while deriving optimal performance efficiency. This depends upon a range of parameters such as compressor design, selection of a compressor for a particular application, operating conditions and the maintenance regime. Optimal compressor performance also depends upon the level of individual awareness and initiatives—the first step towards resolving the issues. |
