OVERALL EFFICIENCY CONSIDERATION OF PNEUMATIC SYSTEMS INCLUDING COMPRESSOR, DRYER, PIPE AND ACTUATOR Toshiharu KAGAWA*, Maolin CAI*, Hirotaka KAMEYA** *P recision and Intelligence Laboratory, Tokyo Institute of Technology 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503 Japan (kagawa@pi.titech.ac.jp)** Mechanical Engineering Research Laboratory, Hitachi, Ltd. 502, Kandatsu, Tsuchiura, Ibaraki, 300-0013 Japan (kameya@merl.hitachi.co.jp) ABSTRACT This paper shows how to analyze the energy efficiencies of typical pneumatic equipment. Firstly, a proposed new item, air power, is introduced as the quantitative standard of energy in compressed air. Then, a pneumatic system is divided to the four parts: air production, cleaning, transmission and consumption for discussion. In air production, compressor including cooler is discussed. After discussion on the overall adiabatic and isothermal efficiencies, the overall efficiency of compressor is defined with air power and proposed as the final energy- conversion efficiency. In air cleaning, energy efficiencies of air dryer and filter are investigated and their general data are given. In air transmission, the energy losses due to pressure loss in pipe and air leaks are discussed, and the energy saving effect of a regulator is shown. In air consumption, the energy distribution in a cylinder actuation is clarified. The efficiency analysis methods discussed in this study will be greatly helpful to an energy-saving equipment selection. KEY WORDS Overall efficiency, Energy saving, Pneumatic system, Air power, Compressor NOMENCLATURE E: available energy of compressed air [J] E: air power[w] P: absolute pressure of compressed air [Pa] Q: volumetric flow rate of compressed air [m3/s]v : volume of compressed air[m3] : energy efficiency N [] p: air density [kg/m3] Subscripts a: the atmospheric state 1: the input of air power 2: the output of air power INTRODUCTION Pneumatic systems are widely used in factory automation because of their low-cost equipment and easy maintenance, Fluid Power. Fifth JFPS International Symposium (c)2002 JFPS. ISBN4-931070-05-3
etc. However, today, the users have been becoming aware of the fact that a pneumatic system is not low-cost to operate. In Japan, the cost of producing compressed air with the pressure of 0.6-0.9MPa is 2-3yen/Nm3, and compressor electricity is responsible for around 80-90% of the cost [11 Pneumatic systems in Japan are consuming approximately 5% of the national total electricity [2]. This electricity amounts to about 1.5 trillion yen. Furthermore, a large amount of energy is being wasted in pneumatic systems due to unreasonable compressor setting and operation, air leaks, over-selection of equipment, etc. It is even pointed out that savings of 30% are quite normal for the present condition [3]. Therefore, energy saving has become an urgent project for the users who operate quantities of pneumatic equipment in their factories. In the recent years, it was reported that some users were trying to optimize compressor operation and to cut down air consumption in terminal equipment. However, energy losses in the whole system were not investigated. There was no data to illustrate energy flow and distribution in the whole system. There was also no general method to quantify the amount of energy losses in all kinds of pneumatic equipments. All of these are because there was no standard to determine energy in compressed air. In our previous study, the available energy of compressed air is discussed, and air power is proposed to represent the power transmitted by flowing compressed air [4]. Based on this proposal, it becomes possible to discuss energy efficiencies for pneumatic equipment. This study shows the methods to determine the energy losses and efficiencies of typical pneumatic equipment. These methods will be greatly helpful to an energy-saving equipment selection. Simultaneously, they are necessary to the clarification of the overall efficiency of a pneumatic system. It can be seen that these will improve energy saving activities in the future. A typical pneumatic system, as shown in Fig.1, can be divided to the four parts: air production, cleaning, transmission and consumption. In the following sections, after air power is introduced, the efficiencies in these four parts are discussed respectively. Meanwhile, the methods to analyze the energy losses and efficiencies of typical pneumatic equipment are introduced. Fig.1 A typical pneumatic system AIR POWER In the previous study, the available energy of flowing compressed air, which presents the energy that can be theoretically converted to mechanical work, is discussed and expressed as at the atmospheric temperature. Air power is defined as the flux of the available energy. Given Q and Qa as the volumetric flow rate under the compressed state and the atmospheric state respectively, the calculation equation of air power can be given as follows by differentiating Equation 1 [4]. With air power, it is possible to determine the power of flowing compressed air just like electricity. This makes it easy to discuss the energy supply and its distribution in a pneumatic equipment. According to thermodynamics, the available energy will be conserved only under a reversible process. Therefore, air power loss will happen if air undergoes an irreversible process such as friction with external object or among internal flow layers, air mixture, heat transfer, etc. EFFICIENCY IN AIR PRODUCTION Compressed air is produced by air compressor. The usually used compressor types are screw, piston, vane and centrifugal type. Generally, compressors are driven by an electric motor, whose energy source is electricity. If high-pressure compressed air is required, several stages of compression may be employed. One or two intercooler is usually used for cooling compressed air to decrease the power requirement. When selecting a compressor installation, several factors should be considered. These include efficiency, size (flow rate output and power), pressure range, the quality of air delivered, the cooling system, future adaptability, noise levels and ease of maintenance [5]. In this study, it is discussed how to properly assess the efficiency of compressor. Energy Losses in Compressor The efficiency of electric motors generally varies between 80 and 96%. Smaller motors (<10kW) are generally less efficient. Efficiencies can be raised to as high as 98% with highefficiency motors and variable speed drives [6]. The power output of motor is usually transmitted by belt. Belt drives have typical efficiencies of above 95% [5]. For the power arriving at the compression mechanism, the energy losses due to mechanical friction and mixture, air leak and insufficient cooling will happen. The total amount of these losses is the greatest part of the overall loss. Generally, these losses account for 20-40% and vary with compressor type, size and cooling condition. Except for the electricity to motor, electricity is also required (1) (2)
by intercooler, after-cooler and air dryer. This part of electricity consumption is around 10% of the motor. Air dryer will be discussed in the next section. For example, Fig.2 shows the efficiencies detail in a moderate screw compressor. Fig.3 Comparison between the overall adiabatic efficiency and the overall isothermal efficiency Fig.2 Energy distribution in a moderate screw compressor Overall Adiabatic Efficiency and Overall Isothermal Efficiency It is known that less power will be required if air is compressed near an isothermal process. However, it is a fact that air compression is actually rapid and this makes it near an adiabatic process. This may be the reason why the overall adiabatic efficiency is usually used for assessing the efficiency of compressor. Furthermore, the overall adiabatic efficiency is always higher than the overall isothermal efficiency. The two efficiencies are defined as follows. where E tad is theoretical adiabatic power, E as is theoretical isothermal power, E shaft is measured shaft power. Although the overall adiabatic efficiency is a good performance index to indicate the losses resulting from mechanical loss and air leaks for an adiabatic compression, the more required power due to an adiabatic compression is not considered as the loss. This is different from the overall isothermal efficiency. It is clear that the air with high temperature after an adiabatic compression should be cooled before supplied to pneumatic equipment. Therefore, the theoretical adiabatic power cannot indicate the minimum power requirement for compression. Furthermore, the overall adiabatic efficiency depends on the number of internal compression stages, not just the input of shaft power and the output of compressed air. Fig.3 shows the difference between (3) (4) the two efficiencies under the same input and output. As shown in Fig.3, the overall adiabatic efficiency is 37% higher than the overall isothermal efficiency when the output pressure is 0.8MPa under one stage compression. Proposal of the overall efficiency of compressor According to Equation 4, the overall isothermal efficiency only considers the losses in compression mechanism shown in Fig.2 because its input is motor shaft power. The losses in motor and transmission, energy consumption of intercooler are not involved. Therefore, an overall efficiency is desirable for the assessment of the energy-conversion in compressor. In this study, the overall efficiency of compressor is proposed as follows. (5) where the energy input E e is the total electricity supplied to compressor including intercooler, the energy output E 2 is the output of air power, which can be easily calculated by Equation 2 with the output of air pressure and flow rate. The overall efficiency defined above involves all the losses in compressor. It is an easy and practical index for a user to evaluate compressor performance. Fig.4 shows the data of some typical compressors on the present market [4]. As shown in Fig.4, the overall efficiency of compressor has a range of 35-50% when motor power is less thanlokw, 40-60% when motor power is10-100kw and 50-70% when motor power is larger than 100kW. In ISO 1217-Displacement Compressors Acceptance Tests, specific energy, defined as the ratio of required motor power and air output volume, is prescribed as an energy performance index. The minimum specific energy for compressing air to 0.8MPa is 0.06kWh/m3. However, in practice, only very large piston compressors are able to approach this figure. The energy consumption of a good installation is more likely to be 0.08-0.12kWh/m3. Table 1 shows the actual values of some typical
Table 2 K dy of some refrigeration dryer When discussing the efficiency of the whole pneumatic system, this electricity consumption should be added up to the electricity consumption in compressor. It is proposed to use the following coefficient K dy to add up. Fig.4 The overall efficiencies of compressors Table 1 Some typical compressor features (6) (7) where E e is the electricity to dryer, E 1 is the output air power, ri ovr is the overall efficiency of the whole system. Table 2 shows the actual values of some refrigeration dryers on the market. Adsorption dryer uses adsorptive substance such as carbon to stick the water and steam. It always obtains a low dew-point temperature with the maximum value of -70 Ž. However, compressed air is necessary for reusing the adsorptive substance. This air consumption is around 20% of the inputted air. Therefore, from the input and output of air power, the efficiency of an adsorptive dryer can be given by compressors [7]. Given that motor power is only 90% of the total electricity consumption, their overall efficiencies can be calculated and shown in the right of the table. Compared with the specific energy, the overall efficiency is easy to understand, and also necessary for evaluating the efficiency of the whole pneumatic system. EFFICIENCY IN AIR CLEANING After produced, compressed air will be cleaned by air dryer and filter before transmitted to pipe network. Air Dryer Air dryer have a function to remove water and steam in compressed air. There are two types to be usually used: refrigeration type and adsorption type. Refrigeration dryer uses a refrigerator to cool air until 0-10 Ž. It is always installed on the downstream of compressor or after -cooler. Because the inter pressure loss and air leaks are little and negligible, the loss in the air power passing the dryer is also negligible according to Equation 2. However, the electricity is necessary to drive the refrigerator and internal cooling fan. (8) Its value is around 80% and lower than refrigeration dryer. Generally, adsorption dryer has low capacity of air flow rate and is only installed before a terminal device that requires low dew-point, not after compressor like refrigeration dryer. It is clear that the lower the required dew-point temperature, the lower the efficiency of the dryer is. Air Filter In pipe network, the equipment most used to clean air is filter. There are all kinds of filters developed to remove different contaminations such as water, oil, dust, etc. While compressed air flows through a filter, the power loss is determined by the pressure loss there. Generally, as a characteristic, the relation graph between flow rate and pressure loss is given in product catalogue. With this graph, the energy-transfer efficiency of lter can be given by fi (9)
Fig.6 The energy consumption in a regulator and cylinder Fig.5 Pressure loss and energy efficiency of a filter where ƒ Pfl refers the pressure loss. The power loss is expressed as (10) A calculation example is shown in Fig.5. By calculating the efficiencies of a number of filters on the market at the condition of the maximum flow rate, it is concluded that a filter for main pipe is always above 99%, and a filter with a performance under 5 i m for terminal device is always around 95%. EFFICIENCY IN AIR TRANSMISSION In transmission through pipe, pressure loss and air leak are the two factors resulting in air power loss. Pressure Loss through Pipe Pressure loss occurs along pipe and at joints. It can be approximately calculated by (11) However, it is difficult to determine the coefficient K and the average velocity co in an actual investigation. This is because the pipe network in factories is very complicated and air flowing state is not regular. Compared with this theoretical calculation, measurement is recommended. Given zippp as the measured pressure loss, the power loss is (12) Air Leaks The gradual loss of pressure in the system when no air is being used means there are air leaks. These can account for as much as 10-40% of air consumption in factories although permissible air leaks should be 5% or less in industrial installation [3]. Leaks frequently occur at the pipe, hose joints, valves, fittings and terminal equipment. There are several ways to check for air leaks. The total leak of the system can be checked using a flow meter in the supply line while there is no user on the system. Given Qfl, as the flow rate of leaks, the power being lost is (13) The above discussion gives the calculation of power loss in air transmission. It seems impossible to show their general data because these power losses vary largely with the system configuration and operation condition. Regulator Pressure regulation is also required at the pipe network, especially just before the terminal equipment. The function of a regulator is not only making the output pressure stable, but also reducing it to meet the supply requirement as a power limiter. Therefore, regulator can cut down the air consumption in the terminal equipment. However, power loss will inevitably occur due to pressure reduction. If.there is no leak in the regulator, the lost power can be calculated by (14) As a calculation example, a simple cylinder actuating system, where a cylinder of 425mmx250mm is driven by the reduced pressure, is investigated. The supplied energy to cylinder and the lost energy in the regulator per one cycle actuation are calculated and shown in Fig.6. It can be seen that the lower the reduced pressure output is, the less the total energy consumption is. EFFICIENCY IN AIR CONSUMPTION It is known that most of compressed air is being consumed by air nozzles and pneumatic cylinders.
It was reported that many measures have been taken to cut down air consumption such as replacing the old nozzles with some efficient types since several years ago. It was also found that heightening the supply pressure while decreasing the owing area is an effective approach to energy saving. fl However, it seems difficult to discuss the energy-conversion efficiency for a nozzle. With a thermodynamics view, the objective of a nozzle is to convert the available energy of compressed air into kinetic energy and thus to produce a force on the front plate. Based on the concept of energy conversion, the energy output should be the kinetic energy. However, the force on the front plate is actually used to describe the performance of a nozzle. Therefore, further discussion is necessary to determine whether the kinetic energy is appropriate to be the effective output energy for a nozzle. In this paper, the efficiency and energy distribution in a cylinder is discussed. For one cycle actuation, the energy input can be given by (15) where Va is the air volume consumption in one cycle actuation, PS is the supply pressure. Here, air leaks and electricity driving valve are relatively little and neglected. Given W,,* as the mechanical work output, the efficiency can be easily calculated as follows. (16) In meter-out and meter-in circuit, speed-controller is used to make the piston velocity convergent. Because speed-controller restricts air discharge or charge to obtain a constant air pressure in cylinder chambers, some energy is inevitably lost at speed-controller. This part of energy can be considered the energy used for velocity control, and represented by Wx in this paper. Besides W, and WW, much of supplied energy is exhausted to the atmosphere with no use. Given W. as this part of energy, the energy distribution in an ideal meter-out cycle actuation, where there is no internal friction in the cylinder, the piston moves in constant velocity and air state changes isothermally, can be given by Fig.7 [8]. In the previous study, it is found that the available energy of compressed air consists of two parts: transmission energy and expansion energy [4]. Fig.7 shows the two parts of supplied energy by shading and not shading respectively. As shown in Fig.7, about half of supplied energy is effectively used for doing mechanical work and controlling piston velocity. Because the energy exhausted with no use is the expansion energy, using and reusing the expansion energy is the key to energy saving for cylinders. For an actual actuation, it is clarified that the energy distributed Fig.7 Energy distribution in an ideal meter-out cycle actuation to internal friction, piston acceleration and heat transfer is very little compared with the above three parts [8]. Fig.7 can approximately describe the energy distribution in an actual actuation. CONCLUSION This paper discussed the efficiency analysis methods for some typical pneumatic equipment at a consideration of the overall efficiency. Based on the discussion of air production, cleaning, transmission and consumption, the overall efficiency of a pneumatic system can be evaluated. It can be seen that the introduced methods will be helpful to an energy-saving equipment selection. REFERENCES [1] N. Oneyama, Air compressor and its operation control, The Journal of Energy Conservation Center, Japan, 1998, 50-2, pp.65-73. [2] Advices of energy conservation for compressors, Japan Society of Industrial Machinery Manufacturers, 2000 [3] Dutch National Team: Compressed air: savings of 30% are quite normal, CADDET Energy Efficiency, Newsletter No.3, 1999 [4] M. Cai, T. Fujita, T. Kagawa, Energy Consumption and Assessment of Pneumatic Actuating Systems, Transactions of the Japan Hydraulics & Pneumatics Society, 2001, 32-5, pp.110-105 [5] CADDET, Saving energy with efficient compressed air systems, CADDET Energy Efficiency, Max Brochure 06, 2000 [6] CADDET, Saving energy with industrial motors and drives, CADDET Energy Efficiency, Max Brochure 02, 2000 [7] Efficient compressed air system in industry, Novem, the Netherlands, 1996 [8] M. Cai, T. Fujita, T. Kagawa, Distribution of Air Available Energy in Air Cylinder Actuation, Transactions of the Japan Fluid Power System Society, 2002, 33-5.