流れ 2005年9月号 目次
― 現場で用いられる流体力学 ～どのように使われているか～ ―
｜ リンク一覧にもどる ｜
Md. Anwar Hossain
Fluid mechanics is one of the oldest and richest branches of mechanics and applied physics. Fluid mechanics has been studied since the beginning of recorded history. We are literally immersed in fluids, our bodies are primarily water, and we simply cannot live without air and water. The fluids are subdivided into the perfect fluid, viscous fluid, compressible fluid, and incompressible fluid etc. Not only are fluids and their complex physics of general interest, but it is widely recognized that fluid mechanics is an essential part of the comprehensive design and manufacture of nearly all modern machinery, structures, and devices. And, yes, even the design of your computer requires some form of cooling and the manufacture of its chips requires a proper understanding of fluid flow.
The physical phenomenon related to fluid mechanics are extensively studied in recent years for its application in pump, hydraulic turbines and compressors which are called fluid machinery. We would like to discuss here about the compressor which is one of the fluid machinery and is used to compress any form of compressible fluids such as air, refrigerant etc. A clear understand of fluid properties is very essential before designing a compressor for practical use. It is because the thermodynamic and fluid dynamic properties changes when it compresses from the atmospheric pressure. The development of compressor is actually not a new invention. Most of we are familiar with reciprocating compressor that use pistons and valves to compress and move air. Screw type and tooth type compressors are also very old invention. The latest development in the compressor field which is now available in the market is scroll type compressor. Scroll Compressors use advanced engineering and flow dynamics to more efficiently and smoothly compress air.
2. Scroll Compressor:
The concept of compressing a gas by turning one involute against another around a common axis is nearly a century old. It wasn’t until the late 1990s, however Anest Iwata is the only company who first turned theory into practical reality for oil free scroll compressor, using sophisticated, computer-assisted design and manufacturing methods to achieve the critical tolerances required. Presently Anest Iwata has become one of the leaders in oil free scroll compressor applications.
A Scroll compressor consists of two spiral scroll elements. One element is stationary and the other rotates in an orbiting motion around the motor shaft center. The two elements are identical spirals that are assembled at 1800 phase difference. The orbital motion of the rotating scroll is circular and at the same amplitude as the motor and is maintained at 1800 phase difference by use of an anti-rotation device, typically crank or Oldham coupling. During operation, the two scroll elements make contact at several points forming an independent series of pockets at each orbital motion position. These pockets progressively decrease in size in toward the center. The compression process of a scroll compressor is defined as a positive displacement process. This type of process increases the pressure of the air by reducing the internal volume of the compression chamber through mechanical force. During each successive revolution of the motor, the initial air mass is moved inward by the orbiting and its volume is significantly reduced as it moves from pocket to pocket. The compression process is finally completed when the air is compressed to its maximum discharge pressure and is vented through a discharge port. This is a common port located at the innermost pocket formed by the paired elements. Since it take several revolutions to complete this process there is a continuous compression process during operation (Fig.1).
|Fig.1. As the orbiting scroll orbits from No. 1 position to No.4 position, a crescent-shaped compressor room at the point symmetry position is gradually reduced, compressed and exhausted through the exhaust port at the center.|
The experimental result of a scroll air compressor is shown in Fig.2. The free air delivery rate of the scroll compressor is measured by using orifice meter. The relation of the free air delivery rate and revolution with discharge pressures of a 5HP scroll compressor are shown in the graph. The decrease of free air delivery rate and revolution speed with the increase of discharge pressure is clearly observed. In comparison to reciprocating or other rotary compressors, the free air delivery of scroll compressor is relatively high for the same discharge pressure. For example, the free air delivery rate of a reciprocating compressor at a discharge pressure of 0.8 MPa is 370 L/min. However, as shown in figure 2, the free air delivery rate of 5HP scroll compressor at a discharge pressure of 0.8 MPa is nearly 410 L/min.
|Fig. 2 The relation of free air delivery and speed with discharge pressure of a 5HP scroll compressor|
Scroll compressors, like other rotary technologies, require fewer moving parts. Scroll compressors are inherently low noise and low vibration machines. Though fluid mechanics are not widely used in our company, however, the knowledge of fluid mechanics is essential in designing more efficient compressors.
McCullough J. E., “Design Manual Scroll Compressor Development Program,” Arthur D. Little, Inc.
|2.||Lee, G. H., “Performance Simulation of Scroll Compressors,” Int. Conference of Compressors and Their Systems, City University, London, (2001), 123-132.|
|3.||Li, L. and Wang, G., “Modification of the spiral profile of a scroll compressor and its effect on performance,” Int. Conference of Compressors and Their Systems, City University, London, (2001), 145-154|