A POWER RECOVERY TYPE PRESSURE REDUCER IN THE HIGH PRESSURE PNEUMATIC SYSTEM

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1 Proceedings of the ASME/BATH 14 Symposium on Fluid Power & Motion Control FPMC14 September 10-1, Bath, United Kingdom FPMC A POWER RECOVERY TYPE PRESSURE REDUCER IN THE HIGH PRESSURE PNEUMATIC SYSTEM Qiyue Xu, Junpeng Sun, Maolin Cai, Yan Shi, Xiangheng Fu School of Automation Science and Electrical Engineering, Beihang University Beijing , China ABSTRACT Pressure reducing devices are the essential and critical components of a pneumatic system. But the energy lose due to the pressure reducing process is considerably huge when there is a high pressure difference. Aiming at this issue, firstly, the mechanism and quantification method of the energy loss in the throttling reducing process is analyzed based on pneumatic power theory. Then, a power recovery type (PRT) pressure reducer which can output shaft power during the reducing operation is proposed. The basic mechanical structure and the woring principle of the PRT reducer are described. For evaluating the feasibility of the design, a thermodynamic model of the PRT reducer is established. And under specific parameter settings, simulations on the PRT reducer s power output characteristics and the pressure regulating ability are carried out. On this basis, the principle structure of an energy recycling high pressure pneumatic system is proposed. This research is the theoretical foundation of the prototype design of a PRT high pressure reducing device. KEYWORDS Energy lose, Power recovery, Pressure reducer, Pneumatic system, Modeling and simulation 1. INTRODUCTION As a ind of power system, the high pressure pneumatic system (HPPS) has characteristics of large energy density, high explosive force and pollution-free [1-4], therefore it plays an important role in special fields such as Aerospace industry, weapon equipments and deep ocean operations [6-8]. Generally, the woring pressure of terminal equipments is relatively lower than the supply pressure. Thus, the pressure reducing device is the most basic and most critical lin in a HPPS. However, traditional pressure reducers wor on the throttling effect, the pressure of compressed air decreases when it is forced through the reducer s orifice. In this process, the pneumatic energy has a substantial loss due to the pressure difference and the temperature drop of the throttling effect. And to prevent the ice-blocing problem, additional power is needed to heat up the system. So the pressure reducing process is a high energy consumption part in the HPPS. In order to improve the efficiency of the HPPS, some researchers focused on the studies of energy-efficient reducers. Jia Guangzheng put forward a concept of the pressure reduction with expander, and illustrated its advantages compared with traditional reducers [9]. Zhou Jie modified the above-mentioned reducing device by adding a heat exchanger in the system [10]. Chen Yize designed a multiple stage pressure reducing method, and proved that the power output with two stages is bigger than one stage by simulation [11]. In these wors the expander is just set as a buffer to compensate the energy by restoring the gas s temperature, but no energy has been used during the expansion process, therefore the energy recovery effect is not obvious. In this paper, a power recovery type (PRT) pressure reducer based on utilization of the expansion energy is proposed. A thermodynamic model of the PRT reducer is established. Then its power output characteristics and pressure regulating ability are simulated. Results of this paper will be a theoretical guidance for the design of the PRT reducer s prototype. 1 Copyright 14 by ASME

2 . ENERGY LOSS IN THE PRESSUR REDUCING PROCESS For a compressible fluid, its power is comprised of transmission power and expansion power, while expansion power is usually the larger one [1]. Specially, the compressed air has wor-producing potential when its pressure is higher than the atmosphere pressure so that it can expanse from a specified state to the atmosphere state. The pneumatic power of flowing compressed air P a is defined by the following expression: dm p P = a RT g ln dt p (1) a here m, T and p are the mass, temperature and pressure of the flowing air respectively, R g is the gas constant and p a is the atmosphere pressure. As can be seen, compressed air s pressure is the sign of how much available energy that can be extracted from the air. The bigger the pressure difference between p and p a is, the more pneumatic power the flowing air has. As a result, regardless of any pressure reduction principle, the available energy of compressed air is reduced as its pressure decreases. In a typical HPPS, specifically the air powered engine system, mentioned in a former research [8], the upstream pressure is 30MPa, the downstream pressure is 5MPa and the average air flows through the high pressure reducer is 0.045g/s. Calculated by Equation (1), the pneumatic power loses due to the pressure reduction is approximately 6.3W which is considerably high. Without considering the temperature changes, this part of energy loss accounts for more than 30% of the total pneumatic power in the air source. In addition, the air temperature can drop instantly after the throttling reducing process by the Joule-Thomson effect. Especially under a large pressure difference, the temperature drop is drastic. Except leading to the ice-blocing problem, this effect also affects the air s pneumatic power according to Equation (1). In the case of same mass flow and pressure, if the air temperature after reducing process is 10% lower than the room temperature, the air loses 10% available energy due to the temperature drop. Restoring the air temperature can compensate the available energy. However, effective heating methods usually need additional external energy which further affects the system s efficiency. Generally speaing, the energy losses mentioned above result from the loss of expansion power of compressed air. For the HPPS, especially when its source air pressure is higher than MPa, expansion energy is the major part of its source energy. Thus the HPPS is the object of this study for its relatively stronger necessity. 3. WORKING PRINCIPLE OF A PRT PRESSURE REDUCER To solve the above-mentioned problems, a new pressure reducing principle is considered in this paper. Maing use of an expansion power output structure, the compressed air goes through it can achieve pressure reduction. Meanwhile, the available pneumatic power can be recovered during the expansion process. The basic structure of the power recovery unit is a swash plate type axial piston motor as its sectional view shown in Figure 1. As can be seen, the basic structure comprises of an adjustable swash plate, several piston and shoe assemblies, a cylinder-shaft assembly, a valve plate and a valve shaft. For a single piston, at its top dead center, the high pressure air goes into the piston chamber through the inlet passage in the valve plate, expanses to drive the plunger move axially. Because of the swash plate angle, the force between the plate and the piston shoe maes the cylinder-shaft assembly rotate. After the bottom dead center, the cylinder body continues to rotate by the inertia, maing the piston move bac to discharge the low pressure air. The valve plate and the valve shaft achieve flow distribution through relative rotation. When the air passage port of a cylinder chamber in the valve plate rotates to the corresponding position, the intae or exhaust port in the valve shaft connects to it. The intae port opens near the top dead center while the exhaust port starts from the bottom dead center. Time of the intae or exhaust process is determined by the ports connecting duration angle φ l in the rotation process. And open position of the intae or exhaust port relate to the top or bottom dead center is described by the advanced angle φ a. As the valve plate rotates along with the cylinder body, the pistons get pushed sequentially. Then the shaft can output power, at the same time the discharged air has its pressure reduced. By adjusting the swash plate angle, the output torque and air flow rate can be changed swash plate.piston-shoe assembly 3.cylinder-shaft assembly 4.valve plate 5.valve shaft Figure.1 Power output structure of the PRT reducer The complete principle of the PRT reducer is shown in Figure. Due to the discontinuity of the piston, both of the output torque and the air flow are pulsing. Thus, a buffer tan is installed in the downstream to stabilize the pressure. A servo control unit detects the pressure signal in the buffer tan and Copyright 14 by ASME

3 adjusts the swash plate angle, so that the power recovery unit can output matching air flow. As long as the average flow rate of the power output unit is suitable, the PRT reducer can achieve the stable pressure reduction. 1.Source air.power output unit 3.Angle adjusting mechanism 4.Servo control mechanism 5.Pressure signal 6.Buffer tan 7.Low pressure air Figure. Principle of the PRT reducer 4. THERMODYNAMIC MODEL OF THE PRT REDUCER A dynamic model of the PRT reducer is built for the performance analysis. The following assumptions are made in the modeling process. (1)The compressed air is ideal. ()The air in the cylinder is uniform during the thermodynamic process. (3)There is no lea during the woring cycle. For convenience of analysis, the dynamic model of a single piston is established at first. 4.1 Energy equation The Energy equation is expressed in the form of temperature differential equation: where d is the diameter of the piston, R is the radius of the pistons distribution circle, γ is the swash plate angle. 4. Continuity equation The intae and exhaust air flow rate of the piston chamber can be calculated as follows [13]: ì 1 æ ö æ ö ç ç w ï Gi = í é ù 1 1 æ ö æ ö ç ç w ç êè ø è ø ú è ø ïî ë û +1-1 p -1 o A( f) pi RT ç g i +1, p ç i +1 è ø è ø +1 p -1 o p o p æ ö o A( f) p i - > ç -1 RT ç g i p ç i p, ê ú i p i +1 where ω is the angular speed of the cylinder-shaft assembly, is the adiabatic exponent of the air, p i, T i are the upstream pressure and temperature respectively, p o is the downstream pressure, A(φ) is the effective sectional area of the intae or exhaust passage State equation Ideal air meets the equation of the state: g (4) pv= mrt (5) where R g is the gas constant of air [J (g K) -1 ]. 4.4 Torque equation The driving torque of a single piston generated by the expansion of the compressed air is expressed by: M ( f) = F ( f) Rtang sinf (6) i p dt 1 é dv ù = ca t h( f)( Ta - T) + hg hg - p -ug df mc ê v ë df úû () here, F p is the force between the piston shoe and the swash plate: where G 1 =dm 1 /dφ, G =dm /dφ, G=dm/dφ, m, m 1, m is the mass of the air in the cylinder chamber, the mass of the air flow in and out respectively, C v is the constant volume specific heat, c t is the heat transfer coefficient, A h (φ), T a are the total heat transfer area and the temperature of the internal walls respectively, here T a is assumed to be equal to the room temperature, T is the temperature of the air in the cylinder, h 1, h are the specific enthalpies of the air flow in and out of the cylinder respectively, V is the instant volume of the cylinder chamber, u is the specific internal energy and φ is the shaft rotation angle. The change rate of the volume is described by: dv d p drsinf tang f = 4 (3) Fp ( f) = ( p- pa) p d - miw Rtang cosf (7) 4 where, p a is the atmosphere pressure, m i is mass of the piston and shoe. The differential equations of the shaft rotation angle relate the rotation angle with the time: ìdf ï = w ï dt í dw M -M -M ï = ïî dt ICr T n e where t is the time, M T is the total driving torque, M n is a tunable load torque, M e is the torque due to the coulomb friction between relative motion components, and I Cr is the total (8) 3 Copyright 14 by ASME

4 moment of inertia of rotating parts. As N is the number of the pistons, M T is expressed by: M T N = å M ( f) (9) Then, the thermodynamic model of the PRT reducer can be established by connecting the above mentioned models, and all control variables are parameterized. i = 1 i Mp/(N m) single piston total torque 5. PERFORMANCE ANALYSIS OF THE PRT REDUCER To evaluate the feasibility of the PRT reducer, simulations are carried on its pressure reducing and power output performances. 5.1 Running status of the model For a typical high pressure reducer, the upstream pressure p in is 30MPa, the pressure after reduction p out is 5MPa and the average air flow G a is about 0.05g/s. The PRT reducer s structure parameters is designed refer to an axial piston pump whose piston distribution radius is 0.05m and piston diameter is 0.035m. To match the setting reduction pressure p out and flow rate G a, other parameters are adjusted during the operations of the model. Finally, the PRT reducer realizes the pressure reduction function, with its main parameters shown in Table 1. Table.1 Main structure parameters and operation parameters Parameter Value Piston distribution radius R 0.05m Piston diameter d 0.035m Air passage diameter D a 0.01m Chamber s initial volume m³ Piston number N 7 Intae duration angle φ lin 30 Exhaust duration angle φ lout 100 Upstream pressure p in 30MPa Pressure after reduction p out 5MPa Rotate speed n 3r/min Swash plate angle γ 4 Under these specific parameters, the power output characteristics is demonstrated, as the instant torque of one single piston and the total instant torque of the PRT reducer in a period are show in Figure 3. As can be seen, torque of one single piston is pulsing, but the total torque can be relatively smooth with several pistons woring together. The PRT reducer s effective torque and effective output power are 141N m and 3.0W respectively φ/( ) Figure.3 Instant torques against the shaft rotation angle 5. Change of the swash plate angle The swash plate angle γ is under control during the operation. For the downstream equipments air flow is usually stable, as long as the average air flow rate G a of the PRT reducer can be stabilized by adjusting the angle γ, stabilization of the downstream pressure can be obtained. With other parameters unchanged, the average flow ratespeed curves and the power-speed curves at different swash plate angle is show in Figure 4 and Figure 5 respectively. As can be seen, the average flow rate is proportional to the rotate speed. With the angle increasing, the growth of the flow rate is not obvious at the same rotate speed. However the effective power of the PRT reducer also grows, under damping type loads, growth of the power leads to higher rotate speed, resulting in larger flow rate. Thus, the average flow rate can be controlled by the swash plate angle. G a /(g s - 1) n/(r min - 1) Figure.4 Average flow rate-speed characteristic curves (G a -n curves) at different swash plate angle 4 Copyright 14 by ASME

5 P e /W n/(r min - 1) Figure.5 Effective power-speed characteristic curves (P e -n curves) at different swash plate angle 5.3 Upstream pressure drops With the buffer tan in the downstream, the pressure after reduction can be maintained at 5MPa. But in a practical HPPS, the upstream pressure continues to drop in the operation. So a simulation is carried on when the upstream pressure drops from 30MPa to MPa, while the swash plate angle of the PRT reducer adjusts to maintain the average air flow rate unchanged. As shown in Figure 6, to stabilize the flow rate, the angle γ rises when the upstream pressure decreases. This situation results in increasing of the piston s stroe, then, the effective torque grows. Under a damping type load, bigger torques lead to higher rotation speeds, so rotate speed n rises with the angle γ. The effective power P e also grows as the torque and rotate speed both rises. Therefore the simulation proves that through adjustment of the swash plate angle, the PRT reducer is able to achieve stable pressure reduction and enough power output when the upstream pressure drops. 6. AN ENERGY RECYCLING HIGH PRESSURE PNEUMATIC SYSTEM Output power of the PRT reducer can be used in many ways. As mentioned above, an air powered engine (APE) system is a typical HPPS. Its schematic diagram is shown in Figure 7. More than 30% of the total pneumatic power is lost in the high pressure reduction from 30MPa to 5MPa when the air flow is about 0.045g/s. To prevent the ice-blocing problem, additional power is needed to heat up the system. So the system s energy efficiency can be greatly enhanced by using the PRT reducer to replace the traditional high pressure reducer and to drive the heat transfer system. Based on this theory, a principle structure of an energy recycling HPPS is designed as shown in Figure 8. Some heat transfer medium is stored in a radiator assembly (13) which is composed of a radiator tan and a fan. The medium of low temperature absorbs heat from the atmosphere through the radiator and the temperature rises. Then the liquid is extracted by a pump (5) out of the tan. Through the medium pipelines (3), it flows through the air powered engine (9), heat exchangers (4), (10) and returns to the radiator tan (13). The pump and the heat fan are driven by the PRT reducer. Thus the heat exchange system can be running automatically during operation of the HPPS without additional energy consumption. During the operation of the system when the air pressure in the tans group decreases from 30MPa to MPa, calculated by the simulation, the PRT reducer can produce an average 3.7W power as well as achieving the pressure reduction. Comparing with the original system, 68% of the lost energy can be recovered in the lin of reduction. The heat transfer system driven by this part of energy can greatly enhance the system s performance, by increasing the air temperatures in the pipelines and in the terminal equipment (the APE). 40 γ/ p in /MPa 300 n/(r min - 1) p in /MPa Figure 7. Schematic diagram of a practical APE system 6 P e /W p in /MPa Figure.6 Curves of swash plate angle γ, rotate speed n and effective power P e as upstream pressure pin drops 5 Copyright 14 by ASME

6 1.Tans group.supply pipeline 3.Medium pipeline 4,10.Heat exchanger 5.Pump 6,1.Transmissions 7. Buffer tan 8.Low pressure reducer 9.Air powered engine 11.PRT reducer 13.Radiator assembly Figure.8 Principle structure of an energy recycling high pressure pneumatic system 7.CONCLUSIONS A PRT pressure reducer based on utilization of the expansion energy is proposed. A thermodynamic model of the PRT reducer is established. Then its power output characteristics and pressure regulating ability are simulated. Conclusions are summarized as follows: (1) The average flow rate can be controlled by the swash plate angle of the PRT reducer, then stabilization of the downstream pressure can be obtained by stabilization of the flow rate. () The PRT reducer is able to achieve stable pressure reduction and enough power output when the upstream pressure drops. (3) A principle structure of an energy recycling HPPS is designed based on the PRT reducer. The heat exchange system can be running automatically during operation of the HPPS without additional energy consumption. (4) Although not mentioned in this paper, sealing and lubrication are practical and critical problems in the PRT reducer. But it will be an important consideration in the design of the engineering prototype of the next stage. Results of this paper will be a theoretical guidance for the design of the PRT reducer s prototype. REFERENCES 1. Shearer J.L. Cambridge, Mass. Study of Pneumatic Process in the Continuous Control of Motion with Compressed Air-1. Trans.of ASME 1956: Ibrahim H, Ilinca A, Perron J. Energy storage systemscharacteristics and comparisons. Renewable and Sustainable Energy Reviews 08; 1: Lund H, Salgi G. The role of compressed air energy storage (CAES) in future sustainable energy systems. Energy Conversion and Management 09; 50: Luo YX. A study of the real gas effects on high pressure pneumatics and the basic theoretical and experimental researches of pressure reduction system. PhD Thesis, Zhejiang University, China Motor Development International (MDI) Home Page. Available online: (accessed 8 March 13). 6. Motor Development International (MDI) Web Page. AIRPod. Available online: (accessed 7 August 1). 7. Foley A, Díaz Lobera I. Impacts of compressed air energy storage plant on an electricity maret with a large renewable energy portfolio. Energy 13; 57: Xu QY, Cai ML, Shi Y. Dynamic heat transfer model for temperature drop analysis and heat exchange system design of the air-powered engine system. Energy 14; 68: Guang-zheng J, Xuan-yin W, Gen-mao W. Development of Highpressure Pneumatic Expander Depressurization System. Chinese Hydraulics & Pneumatics 03; 8: Zhou J, Wang JH. The Energy Loss and Compensation of Airpowered Vehicle in the Depressurization. Chinese Hydraulics & Pneumatics 07; Chen YZ, Wang XY. Pilot leaage's influences on the performances of extra high pressure proportional pneumatic valve. Chinese Journal of Mechanical Engineering 05; (). 1. Cai ML, Kawashima K, Kagawa T. Power assessment of flowing compressed air. Journal of Fluids Engineering 06; : Cai ML. Flow Characteristics of Pneumatic Parts. Hydraulics Pneumatics & Seals 07; 7 () : Copyright 14 by ASME