Performance Evaluation Of Vacuum System General Electric Air Conditioner AS_10 Researchpaper\Performance Pump Down Time
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International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 ISSN 2229-5518 1 Performance Evaluation of Vacuum System: Pump-down Time Vishal D. Chaudhari, Avinash D. Desai Abstract— Vacuum system finds applications in number of industries like process industry, pharmaceutical industry, petroleum industry, material handling industry, etc. In most of its applications it is used as vital part of the system. A suboptimal performance in vacuum system may result in inferior overall system performance. The performance of vacuum system is practically gauged in terms of time required to achieve the requisite low pressure. This time is referred as pump-down time. Among the number of factors affecting pump-down time important are pressure to be achieved and configuration of the system (length and diameter of tubing used to connect vacuum tank and pump). In this paper theoretical procedure for calculating pump-down time is explained for the given pressure and configuration of the system. These theoretical calculations are compared with the actual observations taken from the system. Graph of evacuation pressure vs time is drawn for theoretical and actual observations and it is analyzed. Index Terms— Vacuum, vacuum pump, pump-down time, performance, conductance, pumping speed. —————————— —————————— 1 INTRODUCTION Vacuum pump finds application in many number of industries as a vital part of the system. But very few things are known by the engineers regarding vacuum pump and vacuum system performance Vacuum is defined as the space devoid of mater. For general sense it is considered as pressure below atmospheric pressure. Vacuum is broadly divided in the following categories Pressure Range Pressure in mbar Low vacuum 103 - 100 Medium vacuum 100 – 10-3 High vacuum 10-3 - 10-7 Ultra high vacuum 10-7 – 10-12 The different types of vacuum pumps are available for different pressure ranges of vacuum. Noramally the manufacturere provides the characteristics curve showing relation of pumping speed versus pressure for each vacuum pump. This characteristic curve helps to determine the pump-down time in ideal situation. Pump-down time is the time required to to achieve the required pressure. But the characteristics curve has limitations when actual system consisting of tank to be evacuated, piping of differeent dimensions with bends, is there. In this paper theoretical analysis for finding pump-down time of a simple system is explained followed by the experimental results for the same system. 2 Theoretical Calculation for pump-down time The theoretical analysis for punp-down time calculation for a vacuum system consists of following procedural steps: Step 1: calculation of Knudsen Number to determine whether the flow is viscous or molecular.Knudsen number is the ratio of mean free path of molecule to the diameter of pipe. For Kn < 0.01 the flow is regarded as viscous and for Kn > 0.03 the flow is regarded as molecular. Step 2: Conductance calculation: the pumping speed of a vacuum pumping station is reduced upto the recipient through intermediate line, components such as valves and bellows. The longer the lines and smaller the cross section, the grater are the losses. For determination of losses, in practical applicatons, conductance C is used. In addition to length and diameter, conductance also depends upon the type of flow of pumped down material. For vacuum technology mainly viscous and molecular flow are suitable. In the range of viscous flow the conductance is dependent on pressure while in molecular flow range it is independent of pressure. The conductance for round pipes is calculated universally for all pressure ranges and for all gas types as: C ducas: ———————————————— Vishal D. Chaudhari, currently pursuing masters degree program in mechanical engineering in Pune UniversityIndia, E-mail: vishaldchaudhari@gmail.com Prof. Avinash D. Desai, Vice-Principal and Head of Mechanical Engineering Department, Modern College of Engineering, Pune IJSER © 2011 http://www.ijser.org 3.6 * r 3 l C r. p m T 0.039 30 M r. p 3.6 * r 3 0.039 m 95 l For air the contance is calculated International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 ISSN 2229-5518 For air in the laminar flow range, the second term in the paranthesis can be omitted, yielding a simplifies formula, C 7750 r 4 . pm l m3 / hr In the molecular flow range the first term in the paranthesis can be omitted, yielding the formula for air as; 340 . r 3 C l 2 The value of assumed pressure should match with the pressure calculated afterwards.For this purpose iteration method is to be used to find out the proper value of effective pumping speed. Step 5: Using the value of effective pumping speed from step 4 the value of pump-down time is calculated using the formula; t 3 m / hr For a system having number of valves, joints and bends, for calculating conductance equivalent length is to be considered instead of pure length of pipe. P V ln 1 hr Seff P2 Using above procedures the pump-down time for a particular pump is calculated theoretically to achieve a specific pressure. The experimental set-up for the system studied is as follows: Step 3: Assuming mean pressure the value of effective pumping speed is calculated by the following formula: Seff 1 1 1 C S P C.S CS As shown in experimental set-up, a system is studied in PEff . S eff S Step 4: The effective pressure can be calculated using equation; which it is required to evacuate a tank of 98 litres using pump for which the rated pumping speed is 10 m3/hr upto a pressure of 0.1 mbar. The tank is connected to the vacuum pump with standard pipe for which internal diameter is 30mm. The characteristic curve for the pump is as shown below: IJSER © 2011 http://www.ijser.org International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 ISSN 2229-5518 3.6 * 1.53 2150 *1.5 * 0.096 95 m3 / hr C 500 C 9.83178 m3 / hr The abov e char acter istics curve for the pump selected indicates that pump speed of 10 m3/hr is fairly constant for pressure upto 0.1 mbar. For pressure lower than the pressure from the curve deviates from linearity, the volume rate curve is divided in several partial pressure ranges of small volume flow rate. From this characteristics curve it is possible to calculate the pump-down time using standard pump-down time formula but in that case limitations of practical system such as configuaration of actual system are not considered. Using the conductance formula for air, conductance is calculated. For calculating conductance the initial guess for mean pressure is taken as 0.096 mbar. 3 Hence the time required to evacuate the tank of 98 litres consisting of tubing of length 5 m using the given vacuum pump is 12.1 minutes. The calculations are repeated for pressures ranging from atmospheric pressure to the final low pressure of 0.1 mbar and graph of pressure vs pump-down time is plotted on semilogarithmic scale. 3. Results and Discussion: From this value of conductance , effective pumping speed is caluculated. Theoretical pumping speed is taken as 10 m 3/hr. C*S CS 9.83178 *10 Seff 9.83178 10 Seff 4.95759 m 3 / hr Seff The following observations are recorded from the above graphical analysis: 1. The pumping speed varies with pressure to be achieved and system configuaration. Hence it is not advisable to calculate the evacuation time from the characteristics curve of vacuum pump alone. 2. As pressure decresres, the time required for evacuating the tank increases and this increase is more for very low pressures. 3. The variation of actual evacuation time from theoretical evacuation time may be attributed to leakages through the various joints in the sytem. Then pressure is calculated as: P Peff * S eff S 0.1* 4.95759 P 10 P 0.04958 mbar This pressure is compared with the assumed mean pressure and the error is found out. Iteration method is used to reach maximum correct value. Microsoft excel tool can be used to acieve the maximum correct value of pumping speed. For the above given conditions using bisection method of iteration the value of effective pumping speed comes to be 4.4645 m3/hr with an error of 1.5 %. Using this pumping speed value, the time required to evacuate tank of the given size is calculated as given below. 0.098 1013 ln * 60 min 4.4645 0.1 t 12.1 min t 4 CONCLUSION: When a vacuum system is to be designed to get a particular pressure in predecided time, pressure should not be the only criteria for selection of vacuum pump. Evacuation time also depends on length and diameter of tubing. So before installing any vacuum system proper study of overall configuaration of system with pump is necessary so that pump-down time can be optimized using proper length and diameter of tubings in the system. 5 ABBREVIATIONS: C-Conductance in m3/hr S- Pumping speed for pump in m3/hr IJSER © 2011 http://www.ijser.org International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 ISSN 2229-5518 Seff – Pumping speed at tank in m3/hr Pm – mean pressure in mbar r – Radius of tubing in cm l – Length of tubing in cm T – Absolute temperature in K M – Molecular mass of gas η – Viscosity of gases in Pa-s t – Pump-down time in min V – Volume of tank in m3 P1, P2 – Pressures at respective ends in mbar ACKNOWLEDGMENT The authors express their sincere thanks to all the staff of Modern College of Engineering for their kind support. REFERENCES [1] Yasuhiko Senda, ―Theoretical analysis of vacuum evacuation in viscous flow and ita applications‖, SEi Technical Review (October 2010) [2] Robert E. Pearson and gary M. Atkinson,‖Teaching vacuum technology using spreadsheet calculations‖, IEEE (September 2003) [3] GE Xiaohong, Huang Hongwu, Li Hui and Li Yadan, ― Design and verification of an auxiliary systemfor high vacuum die casting‖, Chinese Journal of Mechanical Engineering, China (August 2010) [4] Douglas J. Reinemann,‖the history of vacuum regulaton technology‖ MNC Annual Meetings Proceedings.(2005) [5] David M. Hata,‖ Vacuum system laboratory development‖, CERN [6] John F. O’Hanlon, A user’s guide to vacuum technology, John Wiley and sons, Inc [7] Vacuum pump manual, Pfeiffer Vacuum Gmbh. [8] D.J. Hucknall and A. Morris, Vacuum Technology- Calculations in chemistry, RSC publishing house, Lonon. [9] Dorothy Hoffamn, Bawa Singh, John H. Thomas III, Handbook mof Vacuum Science and Technology, Academic Press, London. [10] Dr. Walter Umrath, Fundamentals of Vacuum Technology, Wiley Publication. IJSER © 2011 http://www.ijser.org 4
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File Type : PDF File Type Extension : pdf MIME Type : application/pdf PDF Version : 1.5 Linearized : No Page Count : 4 Language : en-US Tagged PDF : Yes Title : Performance Evaluation of Vacuum System: Pump-down Time Author : Vishal D. Chaudhari, Avinash D. Desai Subject : Research paper publishing, Volume 2, Issue 11, November-2011 Keywords : Vacuum, vacuum pump, pump-down time, performance, conductance, pumping speed. Creator : Microsoft® Office Word 2007 Create Date : 2011:11:25 18:21:05 Modify Date : 2011:11:25 18:21:05 Producer : Microsoft® Office Word 2007EXIF Metadata provided by EXIF.tools