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Description
INTRODUCTION
One-stage vapor-compression systems are usually used when the compression rate between condensing and evaporating pressures is moderate, but when this rate increases due to a low evaporating temperature necessity, some factors reduce the refrigerating capacity and the coefficient of performance (COP) of the cycle. According to Gosney (1966), an appropriate limit for one-stage vapor-compression systems is a 40 K difference between condensing and evaporating temperatures. For temperature differences higher than 40 K, one-stage systems are not recommended and multi-stage systems (both staged compression and cascade systems) should be considered and analyzed.
The research on this type of refrigerating cycle, most of it from a theoretical point of view, has been focused on energetic optimization of the interstage level between compression stages and its relation to the condensing and evaporating pressures and temperatures. Starting with the work of Behringer (1928) (described by Gosney [1966, 1982]) for a two-stage ammonia cycle, different refrigerant fluids (R-12, R-717, and R-40) were analyzed theoretically by Rasi (1955), and Czaplinsky (1959) worked on different configurations based on ideal cycles. Other researchers, Arora and Dhar (1971), De Lepleire (1973), Domanski (1995), Zubair et al. (1996), and Ouadha et al. (2005), studied the optimum interstage pressure level, and Threlkeld (1966) proved the inadequacy of the geometric mean pressures as a criterion to define the interstage pressure level in two-stage refrigerating cycles.
Since there is a lack of research on the experimental behavior of two-stage refrigeration cycles, the purpose of this work is to present an analysis, based on experimental data, of a two-stage compression cycle known as a two-stage liquid injection system, where part of the liquid refrigerant leaving the condenser is expanded to the intermediate pressure between compression stages to produce desuperheating in the vapors leaving the low compression stage. The aim of the analysis is to understand the effects of the liquid injection system on the main parameters of the refrigerating cycle: the cooling capacity, the power consumption, and the COP.
EXPERIMENTAL PLANT AND TEST PROCEDURES
The experimental plant used in this work consists of three fluid loops in which the main loop is the refrigerant and the rest are auxiliary systems that allow study of the behavior of the system under different operating conditions.
The refrigerant circuit (Figure 1), whose working fluid is the refrigerant R-404A, corresponds to a two-stage compression cycle with a liquid injection system to control the degree of desuperheating between the compression stages. The refrigerant is driven by a 4 kW semi-hermetic compound compressor with six cylinders (bore: 50.8 mm; stroke: 31.8 mm); four of the cylinders correspond to the low compression stage and the rest to the high one. The refrigerant from the high compression stage is condensed in a brazed-plate heat exchanger with a heat transfer surface area of 0.6195 [m.sup.2], and it feeds the liquid receiver of the facility. The liquid refrigerant that exits from the receiver is divided into two currents: the main one, which flows to the evaporator, and the second one, which is expanded by a thermostatic expansion valve and used for desuperheating between the compression stages. The evaporator corresponds to a brazed plate heat exchanger with 1.239 [m.sup.2] of heat transfer surface area, and it is controlled by a thermostatic expansion valve with external equalization. The facility has an accumulation tank to avoid problems related to liquid suction in the compressor at the evaporator... |
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