Spool, Loss Analysis, Indicator, PV Diagram
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.