A Study on a Suction Tube of a Compressor

I, Bedok South Road, Singapore 1646 This study attempts to qualitatively quanti:(y the thermal design aspects of the suction tube of a hermetic refrigeration compressor with high shell pressure configuration. The effects on heat transfer and fluid flow which affect the design dimensions and material selection of the suction tube will be shown and discussed. Results obtained suggested that the tube material plays a minimum part in restricting heat transfer. The reason for this being that a higher heat transfer resistance comes from the convective heat transfer inside the tube, which depends on the flow situation of the tube. rather than the tube materials. A good design should incorporate effects ofreducing the flow velocity of the gas in the tube. INTRODUCTION In vapour compression refrigeration systems. the outlet of the evaporator is connected to the inlet of the compressor by a tube, which is generally called suction tube, see Fig. 1. The performance of the compressor depends greatly on the refrigerant conditions upstream of the suction port and the latter depends on. among other parameters, the design of the suction tube. The suction tube design is commonly known to have effects on the noise and volumetric efficiency of the compressor I L2]. The design criteria of the suction tube are always concentrated on reducing the following three criteria: noise. suction heating and pressure drop. To overcome the noise at the suction side. it is common to place a suction muffler at the end of the suction tube. The existence of the suction muffler also reduces pressure drop upstream of the suction valve during the suction process. Suction heating effects reduce the volumetric efficiency of the compressor. To reduce the suction heating effects. the thermal design aspect on the suction tube must be considered_ This paper presents a simplified study on the thennal aspects of the suction tube design" It focused on the suction heating effects in relations to various design variables. The following are taken into consideration: convective heat transfer along the tube. conduction heat transfer across the tube and convective heat transfer outside the tube. To simplify the analysis. the investigation begins by postulating the suction tube as a simple horizontal tube that is placed in a hot chamber. The latter representing the shell of the compressor. This arrangement is commonly found in hermetic compressors with high shell pressure configuration. where the refrigerant gas is first discharged into the shell before it is led to the condenser_ The definition of the suction tube from compressor point of view is the section that extends from the shell of the compressor to the inlet port of the pump unit. Fig. 2 shows the schematic view of the simplified suction tube model applied in present analysis The tube is not more than an ordinary tube with refrigerant gas flows inside. Outside the tube is the hot refrigerant gas at temperature T = . The conditions of the refrigerant gas outside the tube is not known, but it is assumed that the flow velocity outside the tube may be simplified to an equivalent cross flow velocity U across the tube. Results on the preliminary analysis show that under the postulating conditions. the tube materials, in generaL has very little effects on the heat transfer to refrigerant flow inside the tube. HEAT TRANSFER ANALYSIS In this analysis, heat transfer correlations commonly found in literature arc employed. The correlations appeared in the literature can be broadly divided into two categories depending on whether the problem considered is under constant wall temperature or constant heat flux. Under the situation described previously. it is assumed that the current situation may be treated as a constant wall temperature problem. It is however noticed that [3 j for the Reynolds number ranging