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Article Excerpt
INTRODUCTION
The high environmental impact associated wit the refrigeration sector favors, from the point of view of energy efficiency, the use of more efficient refrigeration methods. Two-stage compression refrigeration systems, first developed in the 19th century, are among these methods and are now an efficient and readily available solution, since they allow cold to be produced with coefficient of performance (COP) and cooling capacities that are higher than those resulting from single-stage compression systems.
The first analyses of these systems, most of which were conducted from a theoretical point of view, focused on the energetic optimization of the interstage level and its relation to the evaporating and condensing temperatures. Examples of such studies include the works of Rasi (1955), Czaplinksy (1959), Arora and Dhar (1971), Domanski (1995), Zubair et al. (1996), and Ouadha et al. (2005). Research on two-stage compression cycles, especially with compound compressors, currently focuses on their application in air-conditioning systems with [CO.sub.2] (Celik 2004; Cavallini et al. 2005), in commercial refrigeration with hydrofluorocarbons (HFCs) (Torrella et al. 2009), and in high-efficiency heat pumps with HFCs (Zehnder 2004) and [CO.sub.2] (Agrawal and Bhattacharyya 2007; Agrawal et al. 2007), where the improvement in energy efficiency in relation to single-stage systems can be as much as 20% (Zehnder 2004).
Continuing with a series of publications in HVAC&R Research (Llopis et al. 2007) devoted to two-stage compression refrigeration plants and their interstage configurations, this work focuses on the analysis of one of the most used two-stage cycles--that with subcooler. In this configuration, part of the liquid refrigerant leaving the condenser is expanded to the intermediate pressure and then evaporated to achieve a degree of subcooling in the rest of refrigerant flowing to the evaporator and, thus, increasing the specific cooling capacity. Furthermore, this cycle can be used with zeotropic refrigerant mixtures, since it avoids problems associated with the distillation of the refrigerant and with retention of the lubricant oil in intermediate tanks. The experimental plant used in this work is the same as that used by Llopis et al. (2007) to analyze the direct liquid-injection system, where the effect of the desuperheating between the compression stages was examined. R-404A, one of the most widely-used refrigerants in Europe for low-temperature applications, is used to carry out the analysis.
The aim of this work is to complement the experimental research on two-stage refrigerating cycles and to present and analyze from experimental data the effects of the subcooler system on the main parameters of a real facility: cooling capacity, compressor power consumption, and COP. Furthermore, with this work we delve deeply into the knowledge of one of the most efficient and used two-stage cycles in medium capacity refrigerating applications.
EXPERIMENTAL PLANT DESCRIPTION AND TEST PROCEDURE
The experimental plant used in this work, which is presented in Figure 1, consists of three fluid loops in which the main loop is the refrigerant loop and the others are auxiliary systems that allow the behavior of the system to be studied under different operating conditions.
[FIGURE 1 OMITTED]
The refrigerant circuit (Figure 2), whose working fluid is the refrigerant R-404A, corresponds to a two-stage compression cycle with a liquid subcooling system. It consists of an isolated brazed-plate exchanger with a heat-transfer surface area of 0.29 [m.sup.2] and a thermostatic expansion valve whose bulb is placed at the refrigerant vapor outlet of the subcooler (P11, Figure 2). 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 which correspond to the low compression stage and the rest to the high one. The refrigerant from the high compression stage (P1, Figure 2) is condensed in an isolated brazed-plate heat exchanger with a heat-transfer surface area of 0.62 [m.sup.2], and it feeds the liquid receiver of the facility (P3, Figure 2). The liquid refrigerant that leaves the receiver (P4, Figure 2) is divided into two currents: the main one, which flows to the evaporator through the subcooler (P5, Figure 2) and the secondary one, which is expanded by a thermostatic expansion valve to the interstage pressure (P10, Figure 2) and used to subcool the main refrigerant current (by evaporating the refrigerant in...
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