Optimizing the Back Pressure Port of a Scroll Compressor

This study attempts to search for the combination of the back pressure port size and its location which results in minimum mechanical losses and yet providing sufficient axial sealing. Theoretical study employing a mathematical model coupled with numerical optimisation algorithm predicted that a reduction of 9% in mechanical losses can be achieved. The main cause of gain that resulted in such a significant mechanical loss was found to come from the consequence of optimising the location, and not the dimensions of the back pressure port. INTRODUCTION Scroll compressors are geometrically simple and rugged machines with few moving parts. In its simplest form, it uses two identical spiral plates, the fixed and the orbiting scrolls to achieve compression effects. The self adjusting back pressure mechanism was introduced into the design of the scroll compressor for the purpose of providing axial sealing and hence minimising leakage of working medium. A good design should accomplish this at a minimum mechanical losses. The basic components ofthis mechanism are back pressure chamber and the back pessure port, see Fig. 1. The back pressure chamber is situated behind the base plate of the orbiting scroll while the back pressure port is located on the base plate of the orbiting scroll. During the operation, the back pressure port serves as a communicating channel between the compression chamber and the back pressure chamber. This allows the back pressure chamber to be charged with gas pressure and hence causes the orbiting scroll to axially seal against the base plate ofthe fixed scroll plate. This paper illustrates how the location and the size of the back pressure can be optimised using mathematical modelling and optimisation algorithm. The study may also be used to assess what design modifications may be made in the interest of more cost effective manufacture without undue sacrifice of hard won performance improvements. MATHEMATICAL MODELLING With the advent in the computer technology, comprehensive mathematical models of real physical systems may be formulated with relative ease. But for the case of the optimisation studies, which may involve many repeated model evaluations [1,2] during the search for an optimum, it is advisable to employ a simple model which operates within the required practical tolerance bounds. Geometrical Model The geometrical model describes the variations of the volume of the working chamber of the machine in relation with the rotational angle e, i.e. V(S) = f(r,H,P, t, a) (I) Where r is the radius of generating circle, H is the height of the scroll, P is pitch, t is thickness of the scroll wraps and a is the generating angle. For the purposes of optimisation study, the orbiting and the fixed scroll are assurned to be geometrical identical. In the present model, the number of compression chamber is taken as 3.