How the Design of the Suction Return Affects Compressor Efficiency

This pape,r describes a flow-visualizatio n technique that was employed to qualitatively evaluate the gas flow pattern inside a small, low side, hermetically sealed, re ciprocating refrigeration compressor. The applicable compressor des igns are those in which the suction gas from the evaporator is dumped into the compressor shell, and is then drawn through a muffler into the suction plenum of the compressor. The physical separation of the muffler inlet from the suction gas inlet serves to reduce compre ssor noise and also provides an easy and convenient means of separating any liquid (compressor oil or liquid freon) from the freon gas. For the flow visualization studies the compressor housing was replaced by a clear plastic shell. Atmosp heric air, seeded with white smoke was the working fluid. The suction inlet and muffler were from a representative compressor. The flow pulsations were modeled by connecting the muffler outlet to the in put plenum of an auxiliary compressor. The flow patterns in the vic inity of the muffler inlet were recorded with a video camera. The m ixing of the inlet gas with the gas circulating inside the muffler w as studied. The effect of alignment and offset of the muffler inl et relative to the suction inlet, the effect of muffler size, and th e effect o£ a shroud around the muffler were studied. The results we re used to guide a companion study of detailed temperature and press ure measurements inside an actual working compressor (see Ref. 2). INTRODUCTION This paper deals with the operating efficiency of small, hermetically sealed, low side refriger ation compressors. More particularly, it concerns the design of th e refrigerant flow passages between the suction line leaving the evaporator and the intake manifold to the compression cy,linder. Theoretically, a simple, well insulate d, direct connection between the suction line and the intake man,ifold would be the most efficient design. However, other cons iderations make the direct connection undesirable. Those cons iderations include noise propagation, the need to separate the compressor oil from the refrigerant gas, and slugging problems as sociated with start-up. In addition, the suction gas is used to co ol the electric motor and compressor in some designs. One popular design technique that is the b ase line for this study is to dump the suction gas into the compre ssor shell. The gas is then drawn into an acoustic muffler, through flow passages, and into the intake manifold. This design has the d esirable characteristic of damping the compressor noise. The des ign also provides an easy solution to the liquid-gas separation need, and to the slugging problems.