International Journal of Advanced JOURNAL OF ADVANCED RESEARCH ISSN 0976 –         INTERNATIONAL Research in Engineering a...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –        6480(Print), ISSN 09...
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Improving performance and development of two stage reciprocating compressors

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  1. 1. International Journal of Advanced JOURNAL OF ADVANCED RESEARCH ISSN 0976 – INTERNATIONAL Research in Engineering and Technology (IJARET), IN 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 3, Issue 2, July-December (2012), pp. 119-136 IJARET© IAEME: www.iaeme.com/ijaret.htmlJournal Impact Factor (2012): 2.7078 (Calculated by GISI) ©IAEMEwww.jifactor.com IMPROVING PERFORMANCE AND DEVELOPMENT OF TWO-STAGE RECIPROCATING COMPRESSORS Ashraf Elfasakhany* Department of Mechanical Engineering, Faculty of Engineering, Taif University, Box 888, Al-Haweiah, Taif, Saudi Arabia * Corresponding author Tel.: +966 (02) 7272020; Fax: +966 (02)7274299 E-mail address: ashr12000@yahoo.com ABSTRACT The most troublesome part in the development of a compressor technology depends strongly on improvement of its performance. For this purpose, a performance characteristic evaluation of a two- stage reciprocating compressor is carried out in this paper. The aims were to improve compressor performance by illustrating the effects of various parameters: primary air tank, compressor running time, background working condition, and air leakage. The effect of each parameter was compared with the normal performance condition and, in turn, it was demonstrated the most/least important parameters on the performance. The parameters were measured using three techniques: the digital display unit, instruments fixed on system layout, and a PC-data acquisition system. The experiment addressed some factors that led to the inefficient performance of the compressed air system and cause energy losses. The results advocate the optimal time for starting each stage of the two-stage compressors. This work, in addition, may give the insight for the development of the design of multi- stage compressors and presents some key design parameters. Keywords: Reciprocating compressor, Two-stage, Performance, Development, Experimental. I. BACKGROUND A compressor is a mechanical device that takes an ambient air and increases its pressure [1]. In the early time, the compressor was bellow that used by blacksmiths to intensify the heat in their furnaces. The first industrial compressor was simple, a reciprocating piston-driven machine powered by a water wheel. In the early 1960s, modern engineering was first applied to air compressor, and hereafter its design was enhanced significantly. A current industrial compressor is a system composed of several sub-systems and many components. Subsystems include prime mover, controllers and accessories, treatment equipment, and distribution unit. Controllers serve to regulate the amount of compressed air being produced, and accessories keep properly operated system. The prime mover powers the compressor and the treatment 119
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME equipment removes contaminants from the compressed air. Distribution unit transports compressed air to where it is needed. Generally, the sub-systems and equipment of compressed air systems are organized as air compressor followed by coolers, separator, air dryers, and air storage tank. Such organization is common for both single stage and multistage compressors. In a multiple stage compressor, the final discharge pressure is generated over several individual steps/stages. The individual stages are mounted in tandem with the second stage driven directly from the rear of the first stage. Alternatively, the stages may be mounted side by side either in separate stators or within a common, multi bore housing stator. Multistage compressors save energy by cooling the air between stages, reducing the volume and work required to compress the air. After final stage, same as in a single stage compressor, compressed air passes though coolers, separator, air dryers, and air storage tank. Storage tank has many functions; it represents the available air that can be released or replenished at any time as required. It can be used to control demand events (peak demand periods) in a compressed air system by reducing both the amounts of pressure drop and the rate of decay. Storage can be used to protect critical pressure applications from other events in the system. Storage can also be used to control the rate of pressure drop to end uses. Due air or gas compression generates heat. Heat must be removed to maintain the compressor equipment tolerances and clearances, and the compressed air is cooled to make it suitable for the intended use. Compressor equipment units are cooled with air, water, and/or lubricant. Liquid-cooled compressors have jacketed cylinders, heads and heat exchangers through which liquid coolant is circulated to dissipate the heat of compression. Lubricating oil is used for cooling as well as lubricating of mechanical parts at contact moving. Air-cooled versions have external fins fixed on the compressed cylinder and cylinder head. Air is blown by a fan across the fins for heat dissipation. Separators are devices that remove suspended water droplets from streams of air or gas. A separator is generally installed following each cooler to remove the condensed moisture by the cooler. When air leaves the cooler and moisture separator, it is typically saturated. Any further radiant cooling, as air passes through the distribution piping and exposed to colder temperatures, will cause further condensation of moisture. Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems when such compressed air is used. These problems include freezing of outdoor airlines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments, and fouling of processes and products. These problems can be avoided by a proper using of compressed air dryers. Air dryer is a device used for removing water vapor, formed after the separators, from compressed air by increasing air temperature. The higher the air temperature, the more moisture the air is capable of holding. However, drying the compressed air beyond the required pressure dew point will result in unnecessary energy and costs. Air dryers vary in types, and their performance characteristics are typically defined by the dew point. It is worth to clear that the separators and dryers could be classified as parts of the treatment equipment system or as individual components and, in this case, treatment equipment is considered only as air filters used to clean the coming air. Subsystems and components of industrial compressor are common for all compressor types although these types vary significantly. The compressor types could be classified as two main types: positive-displacement and dynamic. Each type has a completely different working principle than the other. In dynamic compressor type, impellers rotate at very high speeds and impart velocity energy to flow air or gas. The velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers. This process uses the speed or velocity of the air to increase the air pressure. Positive-displacement type, on the other hand, trap a given quantity of air or gas in a compression chamber and the volume which it occupies is mechanically reduced, causing a corresponding rise in pressure before discharge. At constant speed, the air flow remains essentially constant with variations in discharge pressure. Dynamic compressors are available in two main types: centrifugal and axial. Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the 120
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME impeller, and, in turn, increasing the velocity of the gas. A diffuser section converts the velocity energy to pressure energy. The centrifugal air compressors rotate much faster and generate much more energy than the other types of dynamic air compressors due to their extremely fast moving blade. Since it produces an immense amount of energy, these compressors are used for applications that require a large amount of energy. They are used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Many large snowmaking operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Out of some other types of dynamic air compressors, the commonest is the centrifugal compressors. Their application can be from 100 horsepower (HP) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures that reach to 10,000 psi (69 MPa). Axial-flow compressors are dynamic rotating compressors types that use arrays of fanlike airfoils to progressively compress the air or gas. The arrays of airfoils are set in rows, usually as pairs: rotating and stationary. The rotating airfoils, also known as blades or rotors, accelerate the air/gas. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the air/gas, preparing it for the rotor blades of the next stage. Axial compressors are usually multistage, with the cross-sectional area of the air/gas passage diminishing along the compressor to maintain optimum axial Mach number. Axial-flow compressors are used where there is a need for a high flow rate or compact design. Such compressor types are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Mainly, they can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants. Dynamic air compressors (in both their main types, centrifugal and axial) are very useful and widely used in many applications, but not as common as the positive displacement compressors. The positive displacement compressors are the most universally used compressors; not only are they common in the industry but also they are popular at home and they are widely used by the mechanics and woodworkers. The positive displacement compressors are available in two types: reciprocating and rotary. Reciprocating compressors work by pumping air into an air chamber then reducing this chambers volume. The manner in which they work is very similar to that of an internal combustion engine but more or less in a reverse manner (they were classified, in many classifications, in a separate category of piston type compressors). They have pistons, valves, cylinders, housing blocks and crankshafts. The piston is used to compress energy by moving up and down, and the air stored inside the compressor becomes compressed and converted into energy. Based on this simple mechanism, these compressors are capable of producing a large amount of energy which can be used for many purposes; consequently, reciprocating compressors are the most common types available in the market today. They are generally found in wide ranges that vary from fractional to very high horsepower. Small reciprocating compressors from 5 to 30 HP are commonly seen in automotive applications, and they are typically for intermittent duty. Larger reciprocating compressors, over 1000 HP, are commonly found in large industries and petroleum applications. Reciprocating compressors can be either stationary or portable, can be single or multistage, and can be single acting, double acting or diaphragm. The reciprocating air compressor is single acting when the compressor is accomplished using only one side of the piston. A compressor using both sides of the piston is considered as double acting. In diaphragm compressor type (also known as a membrane compressor) the compression of gas occurs by the movement of a flexible membrane instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in contact with the gas/air being compressed. Diaphragm compressors are a less common type and are used for compressing hydrogen and natural gas as well as in a number of other applications. Generally, multistage double- acting reciprocating compressors are said to be one of the very efficient and the largest compressors available, and more costly than comparable rotary types. Rotary compressors have gained popularity and are now the “workhorse” of American industry. Generally, the efficiency of the rotary compressor is higher than that of the single stage reciprocating 121
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME compressor [2]. Rotary compressors are most commonly used in sizes from about 30 to 200 HP. The commonest type of rotary compressor is the helical-twin screw-type (also known as rotary screw or helical-lobe). This type works on the principle of air filling in a void that is present between two helical mated screws. As these screws turned, the volume is reduced volume and, in turn, air pressure increased. This compressor type is mostly used to inject oil into the compression area and bearing for a function of lubrication, cooling and even creating a kind of seal to reduce any leakage. The rotary screw is considered to be the commonest rotary types because it has low initial cost, compact size, low weight, and easy to maintain. However, sliding-vane, liquid-ring, lobe, and scroll-types are considered among less common rotary types. 2. INTRODUCTION Currently, compressors are multipurpose tools used widely in industry for a variety of purposes. Compressors are used to operate various machines, tools and hydraulic devices; and, in many cases, compressors are so vital that the facility cannot operate without them. Most industrial plants, from a small workshop machine to an enormous power plant, pulp, and paper mill, have some type of compressed air system wherein the energy generated from these compressors is essential to operate the mechanical equipment and power tools. In view of that, plant air compressor can vary in size from a small unit of 5 horsepower (HP) to huge systems with more than 50,000 HP. Unfortunately, running air compressors often uses more energy than any other equipment in industrial facilities [3]. Energy savings by means of system improvements of air compressors can range from 30 to 50 percent or more of the electricity consumption [4]. For many facilities this is equivalent to thousands, or even hundreds of thousands of dollars of potential annual savings, depending on use. Since compressing air is one of the most expensive sources of mechanical energy in the industrial setting [3], it is often financially beneficial and more energy efficient to use all possible methods to reduce the energy consumption. The energy consumption of any compressed air system depends on several factors: the compressor type, model and size, the motor power rating, control mechanisms, system design, and performance. Minimizing the energy consumption of air compressors and thereby improving compressors efficiency and performance has always been the researchers goal. Hamilton et al. [5] Summarized different ideas that were presented by other researchers, as a reviewing study, to improve compressors performance. The study presents improvements in electric motor efficiency, internal losses, system effects, speed variation, valve stresses, accelerated life tests, and interaction of valve stress and compressor performance. Hayano et al. [6] deserves attention on friction losses in scroll compressors and compared with other designs such as the rotary compressors; different frictional losses of different parts of the compressors were predicted mathematically with identifying the location of the maximum occurred frictional losses. Duggan et al. [7] evaluated the performance of compressors using two different measuring methods: calorimeter and flow measuring techniques. Keribar and Morel [8] improved the heat transfer in reciprocating compressors using the finite element analysis. Futakawa [9] reported improvements in compressors with special emphasis on events in Japan. Etemad and Neuter [10] discussed the optimum design of scroll compressors using a parametric study analysis. Hirano et al. [11] reported a study of the leakage problems on performance of scroll compressors. While there are many papers, as presented early, discussed the improvement of scroll compressors, there are very few papers discussed the improvement of reciprocating compressors although reciprocating compressors are one of the most popular machines in use in industry [12,13]. For this purpose, a performance characteristic evaluation of a two-stage reciprocating compressor is carried out in this paper. The aims were to improve compressor performance by illustrating the effects of various parameters. Specific attention is devoted to valve leakage, primary air tank, compressor running time, and background working conditions. Even though some parametric studies of reciprocating compressors have been presented by other researchers, see e.g. [14–20], such studies are usually based on the global thermodynamic models other than experimental. The technique applied in the current study is experimental. Besides, until today there are no theoretical methods currently 122
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME available that guarantee predicting the efficiency and performance optimization [21]; thus, the experimental technique currently is more believed. Not only are the parametric studies of reciprocating compressor in demand to improve the compressor’s performance, but also the energy losses are fundamental in the efficiency and performance. The improvement of energy losses has been highly in demand since energy loss from compressor was up to 80% [4]. For this reason, minimizing the energy consumption of compressor stages for a specific pressure output and thereby improving compressor efficiency is covered in this study. Development on system design of a two-stage reciprocating compressor is discussed as well. 3. EXPERIMENTAL 3.1. Experimental Setup The experiment was carried out in a two-stage reciprocating air/gas compressor mounted in a V- shape with two separated cylinders, as shown in Figs. 1-2. The compressor, manufactured by Kaeser (Model type K 2502 H35), is capable of producing about 35 bar maximum output pressure and 13 bar output working pressure. The compressor was mounted in tandem that the second stage was driven directly from the rear of the first stage. The air is firstly drawn into the intake tank via a measuring nozzle that used to determine the intake volume. The intake tank acts as a calming zone and housing for the measuring sensors of the intake state, i.e., pressure transducer and manometer. Between the first stage and the second stage, there is a small pressure vessel for intermediate cooling. After the second stage, the compressed air is forced into a storage tank via a cooling tube. To achieve a steady operating state, the compressed air is blown off via a bleeder valve with sound absorbing. Safety and pressure regulator valves, which are compulsory components in any compressor, are installed for safety and control. Fig.1. Layout of the two-stage reciprocating compressor connected with PC 123
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME (1) Trolley (2) Drive motor (11) Differential (3) Acoustic pressure transducer attenuator (12) Switch cabinet (4) Pressure vessel with digital 2nd stage displays (5) Pressure (13) Resistance transducer thermometer (6) Manometer (14) Pressure vessel (7) Safety valve and intermediate (8) Regulating cooler for 1st stage valve (15) 1st Compressor (9) Inlet pressure Stage vessel (16) Stage piston (10) Nozzle for compressor volume flow (17) 2nd measurement Compressor Stage Fig. 2. Schematic diagram of a two-stage reciprocating compressor showing different components and locations of various measuring instruments The entire test stand is mounted on a trolley, which is a welded square steel tube supported with four castors for a simple movement and maneuvering of the unit. It also contains two rollers with breaks for secure positioning. The overall dimensions of the unit are 1520 mm length, 800 mm width, and 1500 mm height. Such dimensions are designed to be not so heavy (260 kg weight approx.) for effortlessness moving and accessing through normal doorways. Cushioned unit suspension was added to calm down the experimental operation; a large intake damper was installed to smooth the volume flow and as a support for the measuring sensors. The compressor was fully instrumented with different sensors (pressure transducers, manometers, and thermocouples) for the experimental intention, as shown in Figs. 1-2. Table 1 summarizes the specifications and characteristics of different components of the two-stage reciprocating compressor. All electrical controls and displays are fitted into a switch cabinet, which contains, as shown in Fig. 3, the master switch, emergency stop switch, and digital displays for whole measuring variables. It also displays the electrical output data, and the electric motor switch for compressor. Switch cabinet is connected to a PC (computer) via USB cable for displaying and recording the measured data. 3.2. Experimental Procedure The test stand was placed on a ground level and secured against rolling away by locking the brakes. By switching on the system by pulling the emergency button OFF and turning the master switch ON, the drive motor with a rating of 2,2 kW and speed of 3000 RPM begins to drive the two- stage compressor. The first stage strokes the piston (bore 78 mm and 150 mm of driving rod length) with a supplied intake capacity of 15 m3/h (250 l/min) from the intake tank 1 (20 liters volume and 16 bar maximum pressure). Air is compressed in the first-stage cylinder and forced through the intermediate cooler tank (5 liters volume and 16 bar maximum pressure). Once air cooled down in the intermediate cooler tank, air moved into the smaller second-stage compressor (bore 45 mm, length of driving rod 150 mm, and stroke 72 mm). The second stage compresses the air further, and then air is directed to the after cooler, as shown in Figs. 1, 2 and 4. Finally compressed air is stored in the storage tank (20 liter volume and 16 bar maximum pressure). It is important to clarify that, sometimes the compressor was not able to work after switching on the compressor. This may attribute to the over-current protection switch, which may have cut out 124
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME electricity on the motor. Hence, we need to restart the compressor by turning the button in the switch cabinet ON and turning the button on the pressure switch ON (between the two upper pressure tanks). Normally, the compressor cuts-out with closing a bleeder valve at approximately 13 bar and cuts-in at approximately 10 bar with opening a bleeder valve. The cut-in and cut-out pressure can be adjusted by the pressure switch. Table 1 Characteristics of the two-stage reciprocating compressor Characteristics Values Overall dimensions Length 1520 mm, Width 800 mm, Height 1500 mm, Weight approx. 260 kg Power supply 400 V / 50 Hz / 3 phase Compressor layout: 2 cylinders in V-shape Max. pressure 35 bar Working pressure 13 bar Intake capacity 15 m3/h = 250 l/min Speed 710 rpm Stage 1 details Bore 78 mm, Length of driving rod 150 mm Stage 2 details Bore 45 mm, Length of driving rod 150 mm, Stroke 72 mm Drive motor: Rating 2,2 kW , Speed 3000 rpm Inlet tank: Volume 20 l, Max. pressure 16 bar Intermediate cooler tank: Volume 5 l, Max. pressure 16 bar Outlet Pressure Vessel: Volume 20 l, Max. pressure 16 bar Differential pressure sensor: Measuring range 0 - 10 mbar, Output signal 0 - 10 V DC, Supply 24 V DC Pressure sensor: Measuring range 1x 0-1.6 bar abs. and 2x 0 - 16 bar abs., Output signal 0 - 10 V DC, Supply 24 V DC Resistance thermometer with Type PT 100 , Measuring range 0 - 200 °C, transducer: Output signal 0 - 10 V DC Power transducer: Measuring range 0 - 2500 W, Output signal 0 - 10 V DC 125
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME (1) P1-Inlet pressure (2) T1-Inlet (7) dp-Differential temperature pressure across (3) P2-Pressure after Venturi nozzle 1st compressor stage (8) T4-Temperature (4) T2-Temperature after 2nd after 1st compressor compressor stage stage (9) Emergency stop (5) P4-Pressure after switch 2nd compressor stage (10) Master switch (6) T3-Temperature (11) Electric motor before 2nd switch compressor stage (12) Electrical power Fig. 3. Switch cabinet with digital displays and controls (presented in Fig. 2 No 12) El electricity Pl pressure indicator Tl temperature indicator PD pressure differential M motor Fig. 4. Schematic diagram of a two-stage reciprocating compressor showing process diagram with measuring the locations of different instruments During experiment, the system is allowed to run until constant pressures have built up and stabilized of the measured values. The parameters were measured and recorded using three techniques: the digital display unit (Fig. 3), instruments on the system layout (Fig. 4), and a PC-data acquisition system (Fig. 1). The digital displays indicate the absolute pressure at three locations in bar (P1 at inlet condition, P2 after first stage and P4 after second stage). The manometers on the system layout indicate the pressure at the same three measured locations that presented in the digital displays. Four resistance thermometers (Pt100) with transducers were used to measure the temperature at four locations (T1 at inlet condition, T2 after first stage, T3 after intercourse but before second stage, and T4 after the second stage); temperature values were indicated in the system layout using fine-wire thermocouples 126
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME (K-type). In addition, the same temperatures were indicated on the digital displays. Fig. 4 shows different measuring locations of transient air temperatures and pressures, as well as hydraulic pressures and flow rates. The measuring locations were chosen carefully to monitor the performance of each stage and to evaluate the overall compressor performance. The effective power of the motor for compressor is measured using a transducer and indicated on the digital displays. Hydraulic pressure and flow rate were measured at the inlet condition using a pressure transducer and a flow meter, respectively. The differential pressure in the system inlet is measured via the differential pressure transducer and showed at the digital display in mbar. The differential pressure (DP) is the difference between the ambient pressure and the pressure at the smallest cross-section of the Venturi nozzle, as shown in Fig. 5 and measured based on the following relations. The differential pressure DP in the Venturi nozzle is related to flow rate as: Fig. 5. Venturi nozzle shows pressure differential (∆P or PD) & 2 × DP Eq. (1) V = Ap × ρ Where & V : flowrate (m3 / s ), DP : differential pressure ( Pa), ρ : density (kg / m3 ) and Ap : the smallest cross - section of the Venturi nozzle = 1.131×10−4 m 2 The density ρ of the air depends on the temperature and pressure as: 100 × P0 ρ= Eq. (2) 287 × (T0 + 273) Where ρ in kg / m3 , P0 in mbar , and T0 in 0C The efficiency of the compressed air system is measured and indicated in the PC-data acquisition system. The efficiency is measured based on the following relations. Firstly, the hydraulic power is calculated as: 127
  10. 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME P  Powerhydr = P × V1 × ln 4  1 & P Eq. (3)  1 Where & P in Pa, V in m3 / s, and Powerhydr : hydraulic power ( watt ) The overall efficiency is a relationship between the hydraulic output and the supplied electrical power (measured using an electrical power transducer). Powerhydr η= Eq. (4) PowerElect . The electrical power is indicated in the digital displays and in the PC-data acquisition system. However, the hydraulic power and overall system efficiency are shown only in the PC-data acquisition system. To run the program for PC-data acquisition, the test stand must be connected to the PCs USB port during the experiment, as shown in Fig. 1. The PC allows recording the transient gas temperatures and pressures during the whole working time, and that lends a hand to trace the performance history. In addition to transient gas temperatures and pressures, hydraulic pressures, flow rates, hydraulic and electric powers, and overall compressor efficiency are also obtainable in the recorded PC data acquisition. The measured values (P1, P2, P4, T1, T2, T3, T4, and DP) were indicated, in addition to the PC- data acquisition system and the digital display unit, in instruments placed at the system layout. The response time of various instruments used in this study is significantly smaller than 10 seconds per cycle time of the compressor and 22 seconds of thermal time constant. The measuring range of pressure sensor is about 0-16 bar absolute with output signal of 0-10 VDC and power supply of 24 VDC. The measuring range of resistance thermometer with the transducer of type PT 100 is about 0- 200 °C and the output signal is about 0-10 VDC. The measuring range of power transducer is about 0- 2500 W and the output signal is about 0-10 VDC (see Table 1). 4. RESULTS AND DISCUSSIONS The results from experimental measurements of a two-stage reciprocating compressor are presented in Figs. 6-15. Firstly, the background working conditions before starting up the compressor are investigated, as shown in Fig. 6. As seen, all measured parameters (pressures and temperatures) have no changes with time since stabilizing of the background conditions. The inlet temperature (T1) and inlet pressure (P1) are about 20 oC and 0.9 bar, respectively. The pressure after first and second stages (P2 and P4, respectively) is identical to P1 (0.9 bar) and, in turn, all curves come over each other in the figure. The temperature after the intercooler (T3) gets slightly higher (about 22 oC); however, the temperatures after first and second stages (T2 and T4, respectively) get in a much higher level (38 oC for both); that may be attributed to the accumulated heat in the compressor material, which leads to the higher temperature of T2, T3 and T4 than background condition (T1). After starting up the compressor, pressures and temperatures performance curves vary with time, as shown in Fig. 7. As seen, at first 3 seconds, all performance curves had no changes with time since the compressor did not start yet, i.e., the same conditions as in Fig. 6. Henceforward, all curves were changed except the T1, P1 and T3. The T1 and P1 (not shown in the figure) have no changes since they are backgrounding conditions. T3 has no changes with time due to that the intercooler cools down the air after the first stage to reach a lower stable condition (28 oC). This stable temperature depends on the intercooler efficiency and the air temperature introduced into the intercooler from the first stage (T2). Accordingly, T3 in all figures shows constant values, which can be high (as shown in Fig. 14) or low (as shown in Figs. 7, 11 and 15) related mainly to T2 values since its efficiency is unvarying. On the other hand, T2 and T4, increase rapidly with time, as shown in Fig. 7. T2 increases 128
  11. 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME even faster than T4 (within the period 3-70 seconds) since the P2 increases faster than the P4 (in the early 20 seconds). After long operating time (about 70 seconds) T4 increases to exceed T2 wherein pressure P4 gets much higher than the P2 (P4 reaches 11 bar but P2 reaches 3 bar). From the period of 20 to 70 seconds, T2 is higher than T4 although P4 goes beyond P2. The reason may be due to that the temperature level is a result of two main reasons, the amount of air pressed and its pressure level. More amounts of air and/or higher air pressure cause a warmer air. In the first stage, more amount of air was compressed with less pressure level since it is larger size; however, second stage presses fewer amounts of air but with a higher pressure level since it is smaller size. From 20-70 seconds, the amount of air dominating and in turn T2 is higher than T4 but after that the pressure level increases to be dominating. 15 150 14 140 13 T1 130 12 T2 120 11 T3 110 T4 10 100 Temp. (C) P1 9 90 P (bar) P2 8 P4 80 7 70 6 60 5 50 4 40 3 30 2 20 1 10 0 0 0 10 20 30 40 50 60 70 80 90 100 Time (seconds) Fig. 6. Temperatures (T) and pressures (P) at background conditions (T1, P1 at inlet condition, T2. P2 after 1st stage, T3 after the intercooler and before the 2nd stage, and T4, P4 after the 2nd stage) 15 150 14 140 13 130 T2 12 120 T3 11 110 T4 10 100 P2 Temp. (C) 9 90 P (bar) P4 8 80 7 70 6 60 5 50 4 40 3 30 2 20 1 10 0 0 0 10 20 30 40 50 60 70 80 90 100 Time (seconds) Fig. 7. Performance in normal working conditions (captions are seen in Fig. 6.) 129
  12. 12. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME It is interested in noting that during the early operating time of the compressor (0-20 seconds), the P2 was higher than P4. This is mainly due to that the first stage delivers air to the second one. Since second stage does not have enough air that was delivered in the early time, this stage works with very little amount of air and, in turn, the pressure on second stage increases slowly. Accordingly, it is not recommended to start both stages at the same time, but the second stage should have a shift time delay by about 20 seconds from the first stage. In case of using a multi stage compressor, the same technique should be applied by delaying each stage 20 seconds from the previous one. This may be fulfilled by adding a controller at each stage, which sets the operation of each stage separately according to a specific starting time. By that way, we may improve energy losses, performance, and efficiency since this working period is ineffective; in addition, as more stages of compressor used as more saving on energy gained in compressor performance. For further verification of this finding, more performance curves are investigated, as shown in Figs. 8-10. As seen, the air pressure of the second stage is lower than the air pressure from the first stage at the first 5 seconds of operation, as shown in Fig. 8. However, at 25 seconds, as shown in Fig. 9, the second stage overcomes this lower pressure and the pressure of second stage increases to exceed the first one. The second stage increases even faster after 35 seconds, as shown in Fig. 10. Fig. 8. Pressure of outlet air from each compressor stage after 5 seconds of starting up the operation Fig. 9. Pressure of outlet air from each compressor stage after 25 seconds of starting up the operation 130
  13. 13. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 10. Pressure of outlet air from each compressor stage after 35 seconds of starting up the operation During experiments, results show that the compressor performance curves were not similar at all running times with the same working condition. In the early start time, we have the best performance but after frequent working time, we do not have that good performance. This is because that the compressor systems and components get hotter. Figs. 11-13 investigate the influence of working duration on compressor performance. As shown in Fig. 11, the performance temperatures of T2 and T4 are high at the beginning of operation (42 oC for both temps.). In comparison with the same characteristic curves on cold condition of compressor equipment, as shown in Fig. 7, we note that the temperatures of T2 and T4 at the beginning of operation are 38 oC each for both temps. By the end of running cycle (at 80 seconds of continuing operation) we can see almost the same shift in the final values of T2 and T4 (T2=90 oC, T4=97 oC as presented in Fig. 7, and T2=96 oC, T4=102 oC as presented in Fig. 11). However, we cannot see any differences in pressure characteristic curves in both cases (P2 is similar in Figs. 7&11, and also P4). For further investigation of pressure performance in both cases, Figs. 12-13 present the pressure of the both cases after 45 seconds of operation, and as seen there are no any differences. However, comparing efficiencies, as shown at bottom of Figs. 12- 13, we found that in case of cold starting condition (Fig.12) we have a higher performance efficiency (η=40.6) than in hot starting condition (η=40.1). This means that the running time influences significantly compressor efficiency but not in pressure performance. Hence, in order to stabilize the performance, we may suggest using larger storage tank where compressor works for shorter periods and rest for a long one and, in turn, reducing system and equipment temperature. Investigating of background condition on compressor performance shows that the intake air temperature has a significant impact on compressor efficiency and performance, i.e., working in a colder environment leads to higher compressor efficiency than in hotter case but compressor outlet pressures will be the same. This may attribute to that the energy required to compress the cool air is much less than that required compressing the warmer air. Reducing the intake temperature by moving the compressor intake outside the building and into a shaded area may drastically lower the energy required for compression and, in turn, improving the efficiency. Additionally, recovering the intake air temperature using a heat exchanger can be used to preheat the process (boiler water or space heating) and improving the whole system efficiency. Finally, we may conclude that the background working condition and/or compressor running time can significantly affect on overall system performance and efficiency. Investigating of air leakage on compressor performance is examined as shown in Fig. 14. As seen, when compressor works with air leakage in valves and/or other equipment, the compressor performance is highly affected. The compressor works for a longer period of time to reach the set 131
  14. 14. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME pressure and, in turn, the temperatures of T2 and T4 get to very high value (about 110 oC). Besides, the temperature at the beginning of operation is also very high (about 90 oC), in comparison with no leakage condition (38 oC), as shown in Fig. 7, and that influences significantly the compressor efficiency and performance, as discussed earlier. On the other hand, the outlet pressure reaches about 7 bar after 80 seconds of continuing operation, comparing with 12.5 bar at no leakage condition (in Fig. 7). In case of larger leakage, the compressor may work continuously to reach the set pressure and that may cause failure to some controller and subsystems. The air leakage problem is one of the areas where the most significant energy losses can occur. Fixing the leaks has often been relatively cheap and that have immediate results compared with great impact on energy use. 15 150 14 140 13 T2 130 12 T3 120 11 T4 110 P2 10 100 Temp. (C) P4 9 90 P (bar) 8 80 7 70 6 60 5 50 4 40 3 30 2 20 1 10 0 0 0 10 20 30 40 50 60 70 80 90 100 Time (seconds) Fig. 11. Performance after the definite running time of operation (captions are seen in Fig. 6.) Fig. 12. Pressure of outlet air from each compressor stage and efficiency at hot working condition 132
  15. 15. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Finally, investigating the influence of the primary tank on compressor performance is carried out, as shown in Fig. 15. Comparing performance with and without installing the primary tank (Figs. 7 and 15, respectively) shows that there is no influence on the performance at all, i.e., there is no difference between both cases (with and without primary tank). Although early researchers recommend using the primary tank for stabilization of pressure input, the current study shows an insignificant effect on pressure performance and, in turn, primary tank can be eliminated in the coming designs. Consequently, the new design will be less cost and space. Fig. 13. Pressure of outlet air from each compressor stage and efficiency at cold working condition 15 150 14 T2 140 13 T3 130 12 T4 120 11 P2 110 P4 10 100 Temp. (C) 9 90 P (bar) 8 80 7 70 6 60 5 50 4 40 3 30 2 20 1 10 0 0 0 10 20 30 40 50 60 70 80 90 100 Time (seconds) Fig. 14. Performance with air leakage from system (captions are seen in Fig. 6.) 133
  16. 16. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME 15 150 14 140 13 T2 130 12 T3 120 11 T4 110 P2 10 100 Temp. (C) P4 9 90 P (bar) 8 80 7 70 6 60 5 50 4 40 3 30 2 20 1 10 0 0 0 10 20 30 40 50 60 70 80 90 100 Time (seconds) Fig. 15. Performance with no primary tank installed in the system (captions are seen in Fig. 6.) 5. CONCLUSIONS A two-stage reciprocating compressor was investigated experimentally in this work in order to develop and improve the compressor performance. Several process parameters were carried out to investigate the performance and efficiency: effect of the primary air tank, compressor background working condition, compressor running time, and air leakage. The parameters were measured using a digital display unit, instruments fixed on system layout, and a PC-data acquisition system. The results show that the primary tank could be eliminated from the compressed air system without influencing on the performance. Although early researchers recommend using the primary tank for stabilization of pressure input, the current study shows an insignificant effect and, in turn, primary tank can be eliminated in the coming designs. Consequently, the new design will be less cost and space Investigating of air leakage on performance shows dominate effect where it is one of the most significant energy losses and, in addition, it causes to reduce the output pressure. Air leakage causes the compressor to work for a longer period of time to overcome the leakage and, in turn, compressor may work continuously in case of a large leakage. Therefore, failure may occur in some system controllers and components. As a recommendation, fixing the leaks has often been relatively cheap and that have immediate results compared with great impact on energy use and failure of the system. Examining of background conditions on compressor performance shows that the temperature of the inlet air has an effect on overall system performance and efficiency. In case of lower background temperature, we have a higher efficiency. This may attribute to that the energy required to compress the cool air is much less than that required compressing the warmer air. Accordingly, reducing the intake temperature by moving the compressor intake to an outside of building and into a shaded area may drastically improve efficiency and, in turn, lower the energy required for compression work. On the other hand, background conditions have no influence on the outlet pressure from the first and second stages. The same findings of background condition are applicable for the compressor running time. More running time means hotter compressor systems and equipments and that leads to less performance and efficiency; however, the output pressure has no changes. Hence, in order to stabilize the performance, we may suggest using a larger storage tank where compressor works for shorter periods and rest for a long one and, in turn, reducing system and equipment temperature. 134
  17. 17. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Finally, this study may conclude that it is not recommended to start both stages of the two-stage compressor at the same time but the second stage should have a shifted delay time by about 20 seconds from the first stage. This is due to that the first stage delivers air to the second one. Since second stage has no enough air received during the early working time, it works with very little amount of air and, in turn, the air pressure of the second stage gets lower than the first one. Setting the starting time of each stage is recommended and that may be occurred via controllers. By that way, we may improve energy losses and efficiency wherever the energy saving may be enlarged in case of using multistage compressors. ACKNOWLEDGMENT The author would like to appreciate the partial financial support from the Faculty of Engineering, Taif University, Saudi Arabia REFERENCES [1] B.N. Royce, Compressors selection and sizing, Library of Congress Cataloging-in-Publication Data Compressors, selection & sizing / Royce N. Brown, 2nd ed. (1997) ISBN 0-88415-164-6. [2] M. Ozu, T. Itami, Efficiency analysis of power consumption in small hermetic refrigerant rotary compressors, International Journal of Refrigeration 4 (1981) 5- 14. [3] Department of Energy: Office of Industrial Technologies, The N.C. Department of Environment and Natural Resources, Division of Pollution Prevention and Environmental Assistance. The State of North Carolina. Energy Efficiency in Air Compressors, Report - 3 January, 2004. [4] M. Yang, Air compressor efficiency in a Vietnamese enterprise, Energy Policy 37 (2009) 2327- 2337. [5] J.F. Hamilton, W. Soedel, R. Cohen, Recent research and development in positive displacement compressors in the USA, Proceedings of the Conference on Design and Operation of Industrial Compressors, The Institution of Mechanical Engineers, London (1978) Paper CI 0/78. [6] M. Hayano, H. Sakata, S. Nagatomo, H. Muraski, An analysis of losses in scroll compressor Proceedings of the International Compressor Conference at Purdue, Purdue University, West Lafayette (1988) 189-197. [7] M.G. Duggan, G.F. Hundy, S. Lawson, Refrigeration compressor performance using calorimeter and flow rater techniques, Proceedings of the International Compressor Conference at Purdue, Purdue University, West Lafayette (1988) 206-215. [8] R. Keribar, T. Morel, Heat transfer and component temperature prediction in reciprocating compressors Proceedings of the International Compressor Conference-at Purdue, Purdue University, West Lafayette (1988) 454-463. [9] A. Futakawa, Improvements in compressors with special emphasis on interesting developments in Japan Proceedings of the International Compressor Conference-at Purdue, Purdue University, West Lafayette (1984) 339-351. [10] S. Etemad, J. Nieter, Computational parametric study of scroll compressor efficiency, design and manufacturing issues Proceedings of the International Compressor Conference at Purdue, Purdue University, West Lafayette (1988) 56-64. [11] T. Hirano, N. Matsumura, K. Takeda, Development of high efficiency scroll compressors for air conditioners Proceedings of the International Compressor Conference at Purdue, Purdue University, West Lafayette (1988) 65-74. [12] M. Elhaj, F. Gu, A.D. Ball, A. Albarbar, M. Al-Qattan, A. Naid, Numerical simulation and experimental study of a two-stage reciprocating compressor for condition monitoring, Mechanical Systems and Signal Processing 22 (2008) 374-389. [13] R. Damle, J. Rigola, C.D. Pe´rezSegarra, J. Castro, A. Oliva, Object-oriented simulation of reciprocating compressors: Numerical verification and experimental comparison, International journal of refrigeration 34 (2011) 1989-1998. 135
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