Modeling and Investigation of Refrigeration System Performance with Two-Phase Fluid Injection in a Scroll Compressor
Introduction
1.1 Background
In 2005, the 111.1 million households in the United States consumed 3.1 trillion kWh of energy, accounting for 22% of the nation’s total energy consumption. The use of air-conditioning equipment in 91.4 million, or 82%, of these households contributes significantly to the total energy consumption, accounting for 258.0 billion kWh of energy use annually. In addition, household refrigerators, which use the same vapor compression cycle as air-conditioning equipment under different operating conditions, consume 149.5 billion kWh of energy annually. Combining these two applications, vapor compression equipment accounts for 13% of the total residential energy use in the United States [5].
The commercial building sector, responsible for 19% of the total national energy use, also uses vapor compression based refrigeration and air-conditioning equipment, and large refrigeration systems can be found in industrial applications as well, which account for 31% of total energy use. The transportation sector, where vapor compression cycles are used for vehicle air- conditioning and refrigerated transport containers, accounts for the remaining
28% of the national energy use. Therefore, the utilization of vapor compression equipment in all sectors of the U.S. market is responsible for a significant portion of the national energy consumption [5].
1.2 Problem Statement
Vapor compression cycles are widely used in heating, refrigerating and air- conditioning. A slight performance improvement in the components of a vapor compression cycle, such as the compressor, can play a significant role in saving energy use. However, the complexity and cost of these improvements can block their application in the market. Modifying the conventional cycle configuration can offer a less complex and less costly alternative approach. Economizing is a common modification for improving the performance of the refrigeration cycle, and provides a cooling effect that decreases the work required to compress the gas per unit mass. Traditionally, economizing requires multi-stage compressors, the cost of which has restrained the scope for practical implementation.
Compressors with ports, which can be used to inject economized refrigerant during the compression process, introduce new possibilities for economization with less cost.
Injecting liquid or low quality refrigerant is effective for reducing the compressor exit temperature, while injecting refrigerant vapor improves the cooling or heating capacity of the system. However, very little information is available for cycles operating with injection states between these limits of liquid and vapor injection.
Theoretical work suggests that cycle performance with two-phase refrigerant injection can provide greater improvements in COP than vapor injection. Experimental work has also shown that the performance in an economized cycle driven by multi-stage compressor can be improved by increasing the number of stages. Meanwhile, it has been proved theoretically that increasing the number of injection ports would have a similar effect.
Therefore, this work focuses on computationally investigating a refrigeration system performance with two-phase injection, developing a better understanding of the impact of injected refrigerant quality on refrigeration system performance as well as evaluating the potential COP improvement that injection provides based on compressor information provided by Copeland.
1.3 Objective
First, a scroll compressor will be selected for studying the impact of two- phase injection in this work, because scroll compressor has no poppet valves and thus has a high tolerance for liquid compared to other compressors. In addition, scroll compressor has a successful history in HVAC applications. Acceptance has been quick, creating a demand for millions of units over the past 20 years. Scroll compressors have proved their reliability in that time to be as good as or better than other technologies. Since their introduction, millions of scroll compressors have seen successful service world-wide in food and grocery refrigeration, truck transportation, marine containers, and residential and light commercial air- conditioning.
To begin with, a model of conventional vapor refrigeration cycle will developed to analyze the system performance based on a Copeland scroll compressor performance data. In order to understand the basic cycle well, the correlations of mass flow rate vs. evaporating temperature and compressor efficiency vs. pressure ratio will be detailedly developed. In addition, model results will be compared with two-phase injection cases to investigate if two- phase injection has the potential COP improvement.
Then, a model of a refrigeration system with controlled injection pressure will be developed for directly studying the impact of two-phase injection on the refrigeration system at different operating conditions that data sheet provides.
Model results will show at which conditions in the data sheet two-phase injection has the potential to improve COP. Meanwhile the results will give the best system performance numerically it can achieve at what injected mass flow rate and what injected pressure for each case that has potential COP improvement.
Further, a model of a refrigeration system with controlled injection fluid state will be developed in order to prevent the compressor from slugging. The model is intended to find the best system performance numerically it can reach at what injected mass flow rate, pressure and quality, taking the constraint into account. This model will give a better understanding of the effect of injected refrigerant quality on refrigeration system performance as well as evaluate the potential COP improvement that injection can reasonably provide. Besides, a differential analysis on COP of the refrigeration system with injection will be conducted at last.
Experiments have shown that injecting liquid or low quality refrigerant is effective for reducing the compressor exit temperature and improving system reliability. Cho and Kim (2000) experimentally investigated the impact of liquid injection on a scroll compressor and concluded that liquid injection reduces the compressor discharge temperature [6]. Liu et al. (2008) performed experiments employing a rotary compressor with a liquid injection port, the discharge temperature dropping significantly because of the injected liquid refrigerant [7].
While liquid injection reduces the compressor discharge temperature, previous studies have demonstrated that injecting refrigerant vapor improves the cooling or heating capacity of the system. Wang et al. (2008 and 2009) conducted an experiment using vapor-injected compressor to test system performance improvement provided by both flash tank (FT) and internal heat exchanger (IHX) economization as shown in Figure 1.1. They gave similar performance improvements, increasing the capacity by up to 15% in cooling mode and 33% in heating mode as well as increasing the COP by 4% and 23% respectively, as compared to the conventional compression system with a scroll compressor [8] [9].
Vapor and liquid injection have been studied not merely experimentally but also computationally. Yamazaki et al. (2002) created a calculation program to predict the performance of the scroll compressor with liquid refrigerant injection and the modeled discharge temperature agreed very well with experimental
FT vapor injection cycle schematic (b) IHX vapor injection cycle schematic
Figure 1.1: Vapor Injection Patterns
results [10]. Winkler et al. (2008) conducted a simulation on a two-stage vapor compression system with and without a flash tank and performed experimental validation for the baseline cycle and flash tank cycle with R410A [11]. Siddharth et al. (2004) quantified the potential benefits from employing a scroll compressor with IHX vapor injection. The modeled results showed large advantages will be offered by vapor injection when the temperature lift is high; relatively smaller benefits are observed in very low temperature lift situations such as residential air conditioners [12].
Despite the many studies on cycles operating with liquid or vapor injection, very little information so far is available for cycles operating with injection states between these limits. Liu et al. (1994, 1995) studied the compression of two-phase refrigerant by developing a mathematical model and analyzed the factors causing slugging problem and the effect of compressor kinematics on slugging [13] [14]. Dutta et al. (1996) studied a two-phase refrigerant injection compression process through experiments and simulations.
Three mathematical models, droplet model, homogeneous model and slugging model were proposed. The droplet model assumed that the gaseous and liquid refrigerant exist in the control volume dividedly with different temperatures. The homogeneous model assumed that each phase of the two-phase refrigerant has the same temperature at any time instead. The slugging model assumed that the liquid and vapor refrigerant have the same temperature and the gas is always saturated vapor during the compression process. They found the homogenous model had a good agreement with the experimental results.
Theoretical work suggests that cycle performance with two-phase refrigerant injection can provide greater improvements in COP than vapor injection. Mathison et al. (2014) developed a model of an economized cycle with three injection ports compressor. The model predicts injecting saturated vapor will provide a 12% improvement in COP , which is approximately 67% of the maximum benefit provided by economizing with continuous injection of two- phase refrigerant, for an air-conditioner using R-410A with an evaporating temperature of 5◦C and a condensing temperature of 40◦C [15].
In addition, experimental work has showed that increasing the number of stages in an economized cycle with a multi-stage compressor improves the cycle performance and theoretical work suggests that increasing the number of injection ports would have a similar effect. Mathison et al. (2011) stimulated a vapor compression cycle with multi-port injection and flash-tank economization. The modeled results indicated the addition of the injection ports can improve COP, approaching the limit when continuously injected refrigerant kept a saturated vapor state in the compression [16].
Therefore, there is a need for further work investigating the performance of cycles with two-phase economized refrigerant injection through multiple injection ports. However, continuously injecting refrigerant is not only beyond the capabilities of current compressors, but also requires the development of equipment to continuously supply refrigerant to the compressor at the desired pressure and quality. In addition, injecting a two-phase mixture introduces the possibility for damage to the compressor if the evaporation process is not well- understood.
The current study demonstrates that injecting two-phase mixture using a finite number of injection ports provides a practical means for approaching the limiting cycle performance. Therefore, a model of a refrigeration system with one injection will be developed for investigating a refrigeration system performance with two-phase injection, developing a better understanding of the impact of injected refrigerant quality on refrigeration system performance as well as evaluating the potential COP improvement that injection provides based on compressor information provided by Copeland.
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