WEST LAFAYETTE, Ind. - CO2 as a refrigerant and scroll technology as an important compressor approach were reaffirmed at the most recent combined conferences on refrigeration/air conditioning and compressor engineering at Purdue University.

Each topic was addressed in close to 30 papers, meaning nearly 60 of the 254 paper presentations were devoted to CO2 and scrolls.

At times the two topics crossed, as with a paper titled “Development of Large Capacity CO2 Scroll Compressor” by a team of engineers from Mitsubishi Electric. The team noted, “CO2 refrigerant, with a high operating pressure, has a large difference between the suction and discharge pressures in compressors. Because of a load increase, the high performance and high reliability of compressors are required.

“Compared to the scroll compressor employing the conventional refrigerant, the thrust load increases in the CO2 scroll compressor. Therefore the design of a thrust bearing needs to be altered.”

So the engineers developed such a bearing, “whose thrust load is very high.” The findings:

• “We developed a new thrust bearing configuration with flexible structure.”

• “The new thrust bearing structure functioned effectively. The friction loss of the thrust bearing was decreased.”

• “Efficiency of the compressor is increased 2 percent with the new thrust structure.”

• “As a result of long-term testing, the reliability of a new thrust structure was confirmed.”

CO2 ISSUES

CO2’s applications extend through a wide range of compressor types, as shown in a number of papers.

In a paper titled “Developing a Two-Stage Rotary Compressor for CO2 Heat Pump Systems with Refrigerant Injection,” also from Mitsubishi, researchers said they had to deal with “intermediate pressure pulsation loss” and then “clarified the merits (of two-stage) compared to single type.”

Among the findings:

• “The two-stage type is superior to the single type with respect to compressor efficiency in the condition of low rotational speed or high pressure ratio because it has characteristics more unlikely to leak compression gas.”

• “The two-stage type is superior to the single type with respect to compressor efficiency and heating capacity because it has the ability to improve these performances with refrigerant injection in the condition of the high pressure ratio.”

Researchers from Danfoss CC, Advanced Technology in Trevoux, France, presented a paper titled “High Efficient CO2 Trans-Critical Reciprocating Compressors.”

They said the main characteristics of the compressor “are a vertical shaft, with two opposed cylinders with direct suction, and a 25-kW input power.” In testing, “the compressor reached a volumetric efficiency of 74 percent and is the best available global efficiency of the market.”

Matsushita Electric engineers looked at CO2 with rotary compressors in a paper called, “Development of High-Efficiency Technology of Two-Stage Rotary Expander for CO2 Refrigerant.”

They said, “In order to improve the coefficient of performance (COP) of a heat pump cycle by using carbon dioxide (CO2), an expander that recovers rotation power from the large pressure difference in the cycle to use the power to partially drive the cycle was developed.”

In the cycle, a two-stage rotary expander was adopted because the suction control mechanism is not necessary in it. In the development process of pursuing higher efficiency, a performance analysis simulation model was developed. This was achieved by combining the dynamic mechanical analysis and the refrigerant pressure analysis in the expander by taking refrigerant leakage into consideration.

“After examining the impact of each design parameter on the performance by using the simulation, an optimized design has been produced and the expander efficiency of 60 percent along with the improvement of cycle COP by 6 percent have been achieved in a short term.”

SCROLLS

Setting the stage for a discussion of scroll technology was a paper from Emerson Climate Technology under the title “An Overview of Past, Present, and Future Developments.”

In that last category, the report said, “Many future compressor technology opportunities will evolve from the need to compress new refrigerants that are environmentally friendly. To date, scroll has demonstrated excellent flexibility over a broad range of applications and should be expected to be a key technology for future needs associated with both high efficiency and refrigerant choice for low environmental impact. Scroll compressors are currently in production for subcritical CO2 and are being developed by several manufacturers for transcritical applications. Existing HFC refrigerants have been successfully applied to scroll compressors and future low global warming refrigerants are not expected to present an unusual technical challenge for scroll products.”

A paper titled, “Development of the New Capacity Control Technique for the High Efficiency Scroll Compressor” came from Daikin.

In the abstract of the paper, the researchers said, “From the viewpoint of global warming, we need to concentrate our effort on the improvement of compressor efficiency.” The company said it had “developed a new capacity control technique, which is a combination of inverter drive and conventional mechanical capacity control technique.

“We have achieved 10 percent lower energy consumption compared with our conventional scroll compressor by improving the efficiency for 15 percent when in the use of part load.”

A paper called, “Development of Liquid-Flooded Scroll Compressor and Expander Models,” was presented by engineers at Purdue and the University of Liege in Belgium. Detailed flooded scroll compressor and scroll expander models have been developed, they wrote. “The scroll machine model consists of the following submodels:

• “A volume calculation routine, which permits numerical calculations of scroll chamber volumes as a function of the crank angle.

• “A two-phase mass flow model to calculate the instantaneous mass flow rates between scroll chambers.

• “An adjusted heat transfer coefficient model suitable for calculating the heat transfer between gas and scrolls.

• “A differential equation solver that allows the temperature and pressure in the control volumes to be calculated as a function of crank angle.

• “An overall solver function that obtains the temperatures of the lumped masses and calculates the relevant performance parameters.”

OTHER TECHNOLOGIES

A number of other aspects of HVACR were also covered in various papers. Among those was a paper titled “Transmission Error in Screw Compressor Rotors,” in a paper from Trane. “Vibration stemming from interaction of the rotors is one of several factors in overall screw compressor sound and vibration,” the author, Jack Sauls, wrote.

“Rotor mesh transmission error (MTE) has been shown experimentally to affect overall compressor noise levels. Results of analyses and rotor pair measurements explain how geometrical deviation of the rotors and parts that control their alignment give rise to nonuniform rotation of the female rotor - the mesh transmission error. As a result, it is possible to design rotors for quieter operation using relatively simple tools.”

He noted that research showed that “there is an optimum crown for a given rotor geometry and alignment. Characteristics show that crowning more than the optimum has less of a detrimental effect than does undercrowning. A result of this is that overcrowning can be used to reduce variation in MTE from compressor to compressor in production, although on the average, the MTE of the design will be slightly higher.

“Finally, tests showed that going too far in the overcrowning will result in a loss of efficiency. It is, as with so many things in compressor design, a balancing act.”

From researchers at Embraco and the Federal University of Santa Catarina, came a paper called “A Comparative Analysis of Numerical Simulation Approaches for Reciprocating Compressors.”

In it, it is noted, “simulations of small reciprocating refrigeration compressors were carried out by using one-, two- and three-dimensional formulations. In all approaches, the valve dynamics was described through a one degree of freedom model, whereas a finite volume methodology was employed. The valve dynamics and the time-dependent flow field were coupled and solved simultaneously. Experimental data are used for an assessment of the accuracy of each model.”

Using the three-dimensional formulation “allowed the inclusion of actual suction and discharge geometries in the simulation and served as the baseline to verify whether simpler and computationally less expensive models can satisfactorily predict the compressor performance.”

Toshiba Carrier presented a paper titled, “Development of High-Efficiency Two-Cylinder Rotary Compressor for Annual Performance Factor.” The researchers noted, “Residential air conditioner power consumption reduction has been increasingly required in terms of global warming prevention. In Japan, the energy-saving guideline has been changed from the coefficient of performance (COP) measured during rated operation to the annual performance factor (APF), closer to the actual usage condition.”

In response to this, the researchers said, “We have made improvements (to new compressors) with APF efficiency enhanced by 6 percent compared with our conventional two-cylinder rotary compressor.”

In refrigeration technology there was a paper called “Simulation and Validation of a R-404A/CO2 Cascade Refrigeration System” from researchers at the University of Maryland and Ingersoll-Rand Climate Control.

“The interest in natural refrigerants, such as carbon dioxide, has been growing in recent years due to the high direct global warming potential of common HFC refrigerants,” the report said.

“Despite the environment-friendly characteristics of CO2 as a refrigerant, due to high heat rejection temperatures and transcritical operation, CO2 cannot match the high energy efficiency associated with current HFC technology. Thus, additional measures must be taken to achieve high COP when using CO2.

“One approach is to use CO2 as one of the fluids in a cascade system along with a HFC refrigerant as the high-side fluid. Such systems may have roughly 75 percent less HFC refrigerant charge, and the global warming potential is reduced compared to a baseline system using only HFC refrigerant. When used as a second fluid in a cascade system, the CO2 cycle remains in the subcritical region, thus increasing the cycle’s COP.”

In the paper, “an approach to model cascade systems is presented. The model is validated using experimental data for a R-404A/CO2 cascade system and results are discussed. The simulation tool is developed to account for additional components in the system such as multiple condensers and compressors. Some test points in the experimental system were run using a set of parallel compressors, which can be easily addressed by the simulation tool.”

Publication Date: 09/08/2008