Modeling and Testing of an R-23/R-134a Mixed Refrigerant System for Low Temperature Refrigeration

Low temperature refrigeration storage equipment in the biotechnology industry typically uses cascade refrigeration to achieve evaporating temperatures of -80 °C (-112 °F) or below. Current systems utilize multiple compressors leading to high energy consumption. Equipment operating costs contribute significantly to the total operating costs of biotechnology companies and therefore motivate the development of more efficient alternatives for low temperature refrigeration. This paper describes a single compressor R-23/R-134a mixed refrigerant cycle that has been designed to extract a load of 0.256 kW (873.5 Btu/hr) from a conditioned space at -80 °C (-112 °F). The designed system compresses a mixture of the gaseous refrigerants to a high pressure and then condenses the R-134a in a water-cooled separator while the R-23 remains in vapor phase. The stream of liquid R-134a is expanded to the suction pressure and is used to condense the R-23 that remains in vapor phase, operating much like an inter-stage heat exchanger in a cascade cycle. The condensed stream of R-23 then expands to the suction pressure and enters a low-temperature evaporator, where it absorbs energy from the load. A model of the cycle is developed based upon first and second law principles of thermodynamics and used to refine the design of a mixed refrigerant test apparatus. Theoretical analysis of the prototype system predicts that it will reach an evaporating temperature of -78.6 °C (-109.5 °F) when it operates with a mixture of 33.4% R-23 and 66.6% R-134a by mass. In experiments conducted using the same condensing temperature and mixture composition the mixed refrigerant apparatus reached an evaporating temperature of -75.0 °C (-103 °F), corresponding closely to the predicted temperature of -78.6 °C (-109.5 °F). To reach the desired evaporating temperature of -80 °C (-112 °F) the refrigerant mixture must be altered to increase the amount of R-23

Design of a Compressor Load Stand Capable of Supplying Two-Phase Refrigerant at Two Intermediate Pressures

The development of compressors with refrigerant injection ports provides a less complex and less costly alternative to implementing multi-stage compressors with economization. The ports can be used to inject economized refrigerant during the compression process, which provides the desired cooling effect and decreases the work required to compress the gas per unit mass. Experiments have shown that injecting liquid or low quality refrigerant is effective for reducing the compressor exit temperature and improving system reliability, 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. In addition, experimental work has shown 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. Therefore, this paper presents the design of a compressor load stand for testing compressors with multiple injection ports. The load stand is based on a traditional hot gas bypass configuration but is capable of supplying refrigerant to injection ports at two different pressures between the compressor suction and discharge pressures. In addition, the state of the injected refrigerant can be controlled such that it is either superheated vapor or a saturated liquid-vapor mixture. To guide the design of the bench and size system components, a model was developed to predict the system performance with a commercially available R-410A compressor that has a single injection port. The model is used to predict the range of injection conditions that can be achieved with the load stand over a range of operating conditions. Finally, preliminary test results for the load stand operating without injection are presented, and the experimentally measured compressor performance is compared to the performance data published by the compressor manufacturer

Aquaponics: A Sustainable Food Production System That Provides Research Projects for Undergraduate Engineering Students

Aquaponics is a closed-loop, recirculating water system in which plants and fish grow together mutualistically. Aquaponics resembles a natural river or lake basin in which fish waste serves as nutrients for the plants, which in turn clean the water for the fish. Tilapia and salad greens or herbs are common fish and plants grown in an aquaponics system. The external inputs to an aquaponics system are fish food, minimal amount of water, and energy for lighting and heating the water for the fish and plants. Aquaponics is a sustainable, efficient system to raise fish protein and vegetables for human consumption. Aquaponics systems can be located anywhere in the world where there is adequate energy with a minimal amount of water. Aquaponics is particularly suited to arid climates because it uses much less water to grow plants than soil-based systems. In fact, the only water that is lost is evaporation and transpiration from the plants. Although the field of aquaponics is growing world-wide, the capital and operational costs of producing the plants and fish have not been quantified intensively in the peer-reviewed literature. The relationship between the amount of external energy (fish food plus energy for light and heat) to the output (weight of fish and plants) has not been measured well for aquaponics units in temperate climates. The lack of quantification of the input-output has suppressed aquaponics progress because it is difficult to compare the cost of fish and salad greens grown with aquaponics and conventional methods, such as aquaculture and soil-based methods. The diverse nature of aquaponics and the need to quantify the relationship between input-output presents opportunities for research projects for undergraduate engineering students in Mechanical, Electrical, and Civil Engineering. The following are examples: Sensors: What type of sensors are ideal to measure air and water temperature, water PH, dissolved O2, and nitrates? Thermodynamics: What type of water heating system is most efficient to maintain desirable water and air temperature? Water Quality: What are the optimal methods to filter out the solid fish waste (feces) and introduce necessary bacteria into the system? Hydraulics: What size of pump and diameter of pipe are needed to maintain optimal flow rate? System Design: What are the optimal ratios between fish tank volume and grow area volume? What is the optimal drop in water level between components to utilize the gravity system? Marquette University College of Engineering is building a laboratory to conduct aquaponics research. The design of the system along with the lessons learned will be presented, along with a detailed list of specific projects for engineering students. Lessons learned from this research will aid the development of aquaponics in temperate climates but also possibly in subtropical and tropical region

Modeling of a Hot Gas Bypass Test Block for Centrifugal Compressors

The increasingly competitive building equipment and control industry pushes manufacturers to devote more resources each year to research and development, continually improving the performance and efficiency of their products to develop and maintain a competitive edge. Compressor development is an expensive endeavor because of prototyping and testing costs, but the cost and time required for testing can be minimized by developing a model of the compressor test block to predict its behavior with a given prototype compressor at specified operating conditions. This paper presents a thermodynamic model of a hot gas bypass test block used to evaluate centrifugal compressor performance at a compressor development facility. The test block uses cooling towers to reject the heat of compression to outdoor air, and experience has shown that the range of achievable compressor test conditions can be limited by outdoor air temperature and humidity, which control the heat rejection rate. Therefore, one goal of the model development was to provide a means for evaluating the feasibility of tests at given ambient conditions. By incorporating models of the cooling towers into the test block model, test operators now are able to predict the range of compressor suction and discharge conditions that can be tested under the current outdoor air conditions. A second goal of the model was to assist in selecting the orifice plate used in the orifice flow meter that measures mass flow through the compressor. Operators previously had to make an educated guess as to the best orifice plate size in advance of running the tests, but the model now identifies the orifice diameters that result in pressure drops within the desired range, minimizing the trial and error involved in testing. The model assumes that the system operates at steady-state conditions and uses a compressor map to model expected prototype compressor performance. Therefore, this paper focuses on the condenser and cooling tower models, which are the most important elements for predicting the impact of outdoor conditions on cycle performance. It is shown that the resulting model achieves reasonable agreement with experimental data and provides a useful orifice selection routine

Performance of an R-410A Room Air Conditioner Modified for Use with R-1234ze

This paper presents the results of a senior design project that challenged a team of undergraduate students to reduce the environmental impact of a room air conditioner (RAC) by reducing its energy consumption and/or use of high-GWP refrigerants. Over the course of an academic year, the team was able to investigate, design, model, evaluate, and build a prototype improved RAC. The team began by reviewing literature on approaches that have been proposed to meet or exceed existing energy efficiency and refrigerant selection regulations. Based on these findings and the specified needs of the project sponsor, the team evaluated the appropriateness of different concepts for improving the existing R-410A RAC design and decided to pursue modifications to adapt the unit for R-1234yf. The first step in the redesign process was to develop a thermodynamic model of the existing system. Because very little information was known about the performance of the individual components in the existing RAC, some rough performance estimates were obtained through measurements. The model of the existing system was then modified to provide the same cooling capacity as the original unit but using R-1234yf, and a replacement compressor was selected based on the model results. After the replacement compressor and resized capillary tubes were installed in the RAC, the team was asked to test the prototype unit using R-1234ze instead of R-1234yf. Therefore, the model was modified to predict the cooling capacity of the unit using R-1234ze as the working fluid. The unit was tested using an environmental chamber to simulate the outdoor air conditions and a large room as the indoor environment. Although this setup could not ensure steady-state operation, air temperature measurements indicated that the room temperature did not vary more than 1.5°F over 12 minutes of RAC operation. The cooling capacity calculated based on experimental measurements agreed within 3% of the model predictions. While the team was able to modify the RAC to operate with R-1234ze and was able to predict the unit’s performance with reasonable accuracy, the modifications required a significantly larger compressor and capillary tubes. Therefore, the project clearly illustrated that fitting within the space and weight constraints of window units presents a significant challenge to implementing R-1234ze in RACs

Modeling and evaluation of advanced compression techniques for vapor compression equipment

Because of the many air-conditioning, refrigeration, and heating applications that utilize vapor compression equipment, the vapor compression cycle has been the focus of significant research. The combination of rising energy costs and increasing environmental awareness motivates the development of more efficient cycle components, including higher performance compressors, heat exchangers, and expansion devices for recovering work. However, modifications to the basic vapor compression cycle also show potential for significantly improving cooling cycle performance through increased cooling capacity and COP. The current study investigates the performance improvements that can be achieved through the use of intercooling and economizing. A basic cycle model that uses a compressor with a fixed isentropic efficiency is developed in EES to study these configurations. The model first considers two-stage compression with intercooling between the stages, which does not improve the cooling capacity but provides an increase in COP as a result of decreased compressor work. The basic cycle model considers two different approaches for economizing with two-stage compression. The first approach uses a flash tank to supply saturated vapor to the compressor between the stages. Drawing off the saturated vapor in the flash tank to mix with the first-stage compressor discharge gas not only cools the compression gas, reducing the compression work, but also results in an increased cooling capacity. Therefore, flash tank economization provides a significantly greater improvement in COP compared to intercooling under the same operating conditions. The second approach to economizing uses an intermediate heat exchanger (IHX) to supply two-phase or vaporized refrigerant to the compressor at the intermediate pressure. The IHX achieves results identical to those for flash tank economization if the IHX has an effectiveness of 100%, but the performance of the system with IHX economizing degrades significantly as the heat exchanger effectiveness decreases. Therefore, the cycle with flash tank economization is selected for further study. The basic cycle model with two-stage compression and flash tank economization is modified to consider an increasing number of injection points and a flash tank that can supply two-phase refrigerant. The decrease in the enthalpy of the injected refrigerant and the increased number of injection ports moves the compression process closer to the liquid-vapor dome and therefore decreases the compression work. The number of injection points is then increased to approach the limiting case of continuous injection, which minimizes the compressor power consumption by maintaining a saturated vapor state in the compressor. To improve the accuracy of these cycle model predictions, two comprehensive compressor models are developed. A thermodynamic model of a two-stage rolling piston compressor is developed with consideration for leakage and heat transfer in the compressor. The compressor model is validated by experimental testing of a prototype two-stage rolling piston, and is then used to study the effect of intercooling on the compressor performance. While the two-stage rolling-piston compressor operates with two separate compression chambers in series, the benefits of staging can also be realized by injecting economized refrigerant through ports in a single-stage rotary compressor. Therefore, a model of a novel rotary spool compressor with refrigerant injection is developed to study the effect of multiple injection ports on compressor and cycle performance. The model without injection is validated through experimental testing of a prototype spool compressor and provides a valuable tool for improving the prototype compressor design. The model is then used to predict the compressor performance with a single injection port while varying the port diameter, port location and injection pressure. For an R-22 cycle operating with an evaporating temperature of –7.2°C, a suction temperature of 7.6°C and a condensing temperature of 48.8°C, the model predicts that a single injection port will increase the COP of the basic vapor compression cycle by up to 12%. Incorporating a second injection port increases the COP of the cycle by 16% over the baseline value without injection

Modeling of a Novel Spool Compressor With Multiple Injection Ports

While models have previously been developed to investigate scroll compressor performance with a single injection port, the model described in this paper explores the effect of multiple injection ports on the performance of a novel rotary spool compressor. The model includes the effects of heat transfer and leakage and is numerically solved to predict the compressor power consumption and mass flow rate. The injection ports are modeled assuming that saturated vapor is injected at a specified pressure and the timing of the injection process can be controlled. Running at a speed of 1907 rpm with R-22 as the working fluid, an evaporating pressure of 391 kPa, an inlet temperature of 7.6°C, and a discharge pressure of 1890 kPa, the model predicts that adding a single injection port will provide a 12% increase in the coefficient of performance (COP) of the cycle. Adding a second injection port increases the COP by an additional 4% compared to the cycle with a single port, or 16% over the baseline performance of the cycle without economization. The compressor model is also used to investigate the effect of injection pressure, injection port location, and injection port diameter on economized cycle performance