Modeling and evaluation of advanced compression techniques for vapor compression equipment

Margaret M Mathison, Purdue University

Abstract

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.

Degree

Ph.D.

Advisors

Braun, Purdue University.

Subject Area

Mechanical engineering

COinS