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Article Excerpt
INTRODUCTION
The use of refrigeration systems is almost indispensable when it comes to several activities in modern society. Such systems are responsible for everything from providing thermal comfort in office buildings to maintaining low temperatures (-25[degrees]C to 0[degrees]C) in refrigerated chambers so that food products are preserved. For those systems, one of the main components is the compressor.
The main function of the compressor is to aspirate a certain amount of refrigerant vapor produced by the evaporator and increase its pressure and, consequently, its temperature. Afterward, the refrigerant vapor is liquefied in the condenser. After the condenser, the refrigerant flows through the expansion valve and comes out as a two-phase mixture, moving into the evaporator in order to be evaporated again. In this last step, heat is transferred from the controlled-temperature environment to the refrigerant. Therefore, failures and/or poor functioning of such components can result in losses due to the thermal discomfort or deterioration of products (Stoecker 1994). According to Stoecker (1994), the main types of compressors in the refrigeration industry are reciprocating, scroll, vane, and centrifugal types. Among them, the most common types in the plants with capacities up to 1000 kW are the reciprocating and scroll types.
For the reciprocating types, there are variations in their constructions where the open, semi-hermetic, and hermetic compressors should be pointed out. In this study, the focus is on the semihermetic reciprocating compressor. In this compressor, the shell has, in its interior, the compressor itself and an electrical motor. Usually, this type of compressor operates with halogenated fluids that flow in direct contact with the motor parts, promoting the cooling of the electrical motor.
A growing concern to those using the compressor is the efficiency of the refrigeration system, which can be achieved through proper maintenance actions that contribute to energy consumption reduction. Faulty operation and failure of the components causes a high loss of energy (Tassou and Grace 2005; House et al. 2003).
The objective of this paper is to evaluate the reliability of the compressors typically used in refrigeration systems. The analysis was based on a failure database, which presents the failure modes and time-to-failure reports in reciprocating compressors that were provided by a maintenance company. Those compressors were classified into seven types, and the failure modes of each type were carefully analyzed and classified according to electrical and mechanical failures. Based on that classification, a specific probability function was selected to model the compressor reliability.
RELIABILITY
The term reliability has its origin in the failure analysis of electronic equipment for military use during the 1950s in the United States. One of the many definitions found in the literature is from the UK Ministry of Defence:
The ability of an item to execute, or be able to execute, a certain function over specific conditions without failing for a established period of time or of operation expressed in terms of a probability. (UK Ministry of Defence 1979 cited in Carter 1986).
Equipment is designed according to specifications (i.e., equipment is designed according to a basic function that it will perform. Leitch (1995) proposed that the actions that ensure the maintainability of equipment are different from those that ensure reliability: "Maintainability is the probability of an equipment to be restored to its functional state, in a given time and environment" (pp. 47-49). In other words, maintainability is the study of maintenance periods and practices to improve equipment availability.
The concept of reliability is then proposed as the time that equipment, a system, or a plant is available to operate or to be able to produce. One of the tools that allows evaluating actions that will increase the reliability is the failure mode and effect analysis (FMEA). Basically, FMEA is based on the analysis of the function of the system and subsystems and on the analysis of which components are used to achieve those functions. Afterward, the failures that affect each system function and its main failure modes are evaluated. For each failure mode, the defects are analyzed and actions are proposed in order to reduce, eliminate, or simply control the undesirable effects of a possible occurrence of such failure. FMEA is a logical system in which the potential failures are scrutinized. It provides recommendations for preventive actions. It is a formal process that uses dedicated specialists to...
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