An Indicated Loss Analysis of a Light-Commercial Spool Compressor using High-Speed Pressure Measurements

An analysis of the indicated and frictional losses is presented for a light-commercial prototype spool compressor. The spool compressor prototype was instrumented with four high-speed pressure sensors, three in the compression process and one in the discharge valve plenum. These sensors were triggered with a high-fidelity rotary encoder attached to the compressor motor shaft. This coupling of rotational position and pressure measurements allowed the development of an indicator (pressure v. volume) diagram for the compression process. Additionally, the added sensor in the discharge valve plenum allowed for a de-coupling of discharge valve losses and flow losses within the discharge plenum itself. The analysis shows that the suction and compression losses for this prototype compressor are relatively small compared with the discharge/valve losses. The total losses during the discharge process are generated by pressure drop and backflow through the discharge valve ports as well as when gas flows from the discharge plenum to out of the compressor body. The compressor was tested at three shaft speeds (900, 1300, 1620 rpm) at a condensing and evaporating temperatures ranging from 100 – 120 °F (37.8 – 48.9 °C) and 25 -60 °F (-3.8 – 15.6 °C), respectively at a fixed suction superheat of 30 °R (16.7 K). It was found that the total losses during the discharge process were the dominant indicated losses in the compressor and the discharge plenum losses accounted for between 32 and 66% of the total losses during the discharge process. As a result of this study, a small modification to the discharge plenum of the compressor was incorporated which resulted in a 1-3% increase in overall isentropic efficiency without additional modifications to the compressor mechanism or valves

Empirical Indicated Loss Analysis of a Semi-hermetic Light-Commercial Spool Compressor

An analysis of the indicated losses is presented for a semi-hermetic, light-commercial, prototype, spool compressor. The spool compressor prototype was instrumented with five high-speed pressure sensors, three in the process chamber, one in the discharge valve plenum, and one in the motor cavity. These sensors were triggered with a proximity sensor actuated by means of a custom rotary fixture attached to the compressor motor shaft. This coupling of rotational position and pressure measurements allowed for the development of an indicator (pressure v. volume) diagram for the compression process. Additionally, the added sensor in the discharge valve plenum allowed for a decoupling of discharge valve losses and flow losses within the discharge plenum itself. The sensor in the motor cavity allowed for an analysis of the flow losses leaving the compressor shell. The compressor was tested at five motor speeds (1100, 1300, 1500, 1700 rpm and line voltage) at saturated condensing (SDT) and evaporating (SST) temperatures ranging from 37.8 – 48.9 °C (90 – 130 °F) and -3.8 – 15.6 °C(30 -60 °F), respectively at a fixed suction superheat of 11.1 K (20 °R) . Quantitative analysis shows that the suction and compression losses for this prototype compressor are relatively small compared with the discharge/valve losses. The total losses during the discharge process are generated by pressure drop and backflow through the discharge valve ports as well as when gas flows from the discharge plenum across the motor through the compressor body. It was found that a 5-6% improvement in compressor efficiency can be accomplished by redesigning the discharge plenum and motor cavity to reduce over pressurization. Further investigation into the valve dynamics need to be performed to improve the 11-12% loss in the valves. The valve losses were found to be sensitive to operating speed and SDT with maximum variations of 5% and 3%, respectively

Design Methodology Improvements of a Rotating Spool Compressor using a Comprehensive Model

An improvement to the design process of the rotating spool compressor is presented. This improvement utilizes a comprehensive model to explore two working fluids (R410A and R134a), various displaced volumes, at a variety of geometric parameters. The geometric parameters explored consists of eccentricity ratio and length-to-diameter ratio. The eccentricity ratio is varied between 0.81 and 0.92 and the length-to-diameter ratio is varied between 0.4 and 3. The key tradeoffs are evaluated and the results show that there is an optimum eccentricity and length-to-diameter ratio, which will maximize the model predicted performance, that is unique to a particular fluid and displaced volume. For R410A the modeling tool predicts that the overall isentropic efficiency will optimize at a length-to-diameter ratio that is lower than for R134a. Additionally, the tool predicts that as the displaced volume increases the overall isentropic efficiency will increase and the ideal length-to-diameter ratio will shift. The result from this study are utilized to develop a basic design for a 141 kW (40 tonsR) capacity prototype spool compressor for light-commercial air-conditioning applications

Updated Performance and Operating Characteristics of a Novel Rotating Spool Compressor

The basic mechanism of the novel rotary spool compressor has been described previously by Kemp et al. (2008, 2010). The device combines various aspects of rotary and reciprocating devices that are currently well understood.  Due to increasing pressure in the global market for refrigerants with very low GWP levels extensive modeling was conducted to explore a spool compressor design for operation on medium pressure low GWP refrigerants in sizes applicable to the commercial air conditioning marker, specifically for application on air and water cooled chillers. The basis for a compressor design in this space is operation on R134a realizing that most low-GWP medium pressure gases are similar in nature to R134a from a compressor design point of view. A new compressor design with specific aspect ratio optimized for operation on R134a has been constructed and tested. The compressor was tested at a range of conditions suitable for evaluation in this application. The current compressor performance is comparable with today’s screw compressors operating in this size range

Dynamic Modeling of a Poppet Valve for use in a Rotating Spool Compressor

The operational efficiency of the rotating spool compressor, first introduced by Kemp et al. (2008), has been improved in recent years through the modeling of various elements in the compression cycle. In order to continue the advancements of the design, a model for predicting the discharge valve dynamics is introduced. The model is created using high-pressure measurements both upstream and downstream of the valve. The model is able to predict the initial valve displacement with a mean absolute percent error of 6.91%. The model is also able to predict the amount of time the valve is open in one valve oscillation with a mean absolute error of 9.51%

Spool Seal Design and Testing for the Spool Compressor

Several seals, designed to minimize leakage between the rotating spool assembly’s endplates and the stationary compressor housing, are introduced, constructed and tested. Some designs are deemed impractical due to either high leakage or high torque. A novel one piece hybrid design that blends the function of a face seal and piston ring is tested and shown to achieve excellent results. A test apparatus is introduced to isolate and measure the spool seal’s performance independent of the compressor. It is concluded that the hybrid design is highly effective and well suited for many applications of the spool compressor

Performance and Operating Characteristics of a Novel Rotating Spool Compressor

The basic mechanism of the novel rotary spool compressor has been described previously by Kemp et al. (2008, 2010). The device combines various aspects of rotary and reciprocating devices currently well understood to achieve high efficiency at a low manufacturing cost. A dimensionless variable, the Zsoro number, is developed which represents the ratio of the geometric configuration of the compressor relative to the potential friction components of the compressor. This number allows for rapid evaluation of the geometric features of the device. Four prototype spool compressors have been tested using R410A and R134A at standard air conditioning conditions with various Zsoro numbers. Experimental data collected have shown a strong correlation between the overall isentropic efficiency and Zsoro number. These results have allowed for rapid design iteration of the rotating spool compressor. The most current prototype compressor has operated with a 5% higher overall isentropic efficiency than a typical commercial rolling piston compressor and within 5% of a commercial scroll compressor