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Description
INTRODUCTION
Lubrication is crucial to reliability and high performance in refrigeration compressors. For large open-type compressors, delivery of lubrication oil to various sliding surfaces is usually carried out by a separate oil pump attached to the compressor, but use of oil pumps is uncommon in small hermetic refrigeration compressors, such as those for household refrigerators. This is due to poor cost effectiveness and a requirement of compactness. Most small hermetic refrigeration compressors use the rotating motion of a crankshaft as a power source for oil pumping. Therefore, it is not easy to fine control the oil supply, particularly when compressor speed is variable. For variable-speed compressors, shortage or overflow of the oil supply can occur, depending on the compressor speed. If the oil supply is significantly low, heavy metal contact at the sliding surfaces of the moving elements will result. If the amount of oil supply is excessively large, the discharge of the oil from the compressor housing will increase and the oil level in the compressor housing will be reduced, particularly in rolling piston-type rotary compressors.
Several studies of hermetic refrigeration compressor oil supply systems have been made. For a rolling piston-type rotary compressor, Asanuma et al. (1984) carried out an experimental and theoretical investigation of the shaft oil-feed mechanism. Under loadless pneumatic conditions, the oil-feed pressure of the shaft rotation and the flow rate through an oil-feeding hole on the shaft were measured. Also, measurements of oil flow through the main bearing oil slot were compared to theoretical results for different cross-sectional areas of the slot. Similar but more inclusive work for the same type of compressor was conducted by Itoh et al. (1992). In evaluating a pumping head created by shaft rotation, head drop due to the oil cap hole was taken into account, and an attempt was made to clarify the relationship between pressure loss and volumetric flow rate in oil-feeding holes. A computer program was developed by integrating characteristics of individual lubricating elements to predict the oil flow rate to each lubrication part. A comparison was made between the calculated total oil supply and experimental results at different compressor speeds. Similar work was also carried out for a rotary compressor by Kim and Lancey (2003).
For horizontal compressors, a rotating shaft produces no centrifugal pumping effect at all, and special care needs to be taken to secure the oil-pumping power source. A vane pumping mechanism is one way of producing pumping power. The reciprocating motion of a vane, the primary usage of which is to separate a compression chamber from a suction chamber inside the cylinder, is used to pump oil in the oil sump into the shaft inlet via an oil-conveying pipe. Kim (2005) analyzed the oil supply mechanism in such a horizontal rotary compressor equipped with a vane pumping system and evaluated the effects of several main system design factors on the oil supply rate.
For scroll compressors, a theoretical model of the oil flow rates in the compressor lubrication system was (Drost and Quesada 1992) and computational fluid dynamics (CFD) simulation of an oil-pumping system was performed (Bernardi 2000). However, there are few studies concerning oil supply systems for hermetic reciprocating compressors (Kim et al. 2002).
This paper presents a mathematical model of the oil supply system for a hermetic reciprocating compressor used in household refrigerators and includes predictions of oil flow distributions inside the lubrication system.
MODELING OF OIL SUPPLY SYSTEM
Structure of Oil Supply System
A reciprocating compressor and its main lubrication paths to be investigated are shown in Figures 1 and 2, respectively. The compressor is of 1/3 hp class and is for household refrigerators. An oil cap (Figure 2a[1]) is inserted into the lower open end of the crankshaft, and a substantial portion of the... |
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