Empirical indicated loss analysis of a semi-hermetic light-commercial spool compressor

Abstract

• Indicated losses of a semi-hermetic spool compressor are quantified. • Shaft speed variation is quantified and found to be reasonably approximated as fixed. • Valve losses represent up to a 12% loss and are a function of shaft speed and SDT. • Valve plenum and motor cavity represent up to a 6% loss in the compressor. An analysis of the indicated losses is presented for a semi-hermetic, commercial, prototype, spool compressor. The spool compressor prototype was instrumented with five high-speed pressure sensors, that were trigged based on shaft position to generate an indicator (pressure v. volume) diagram. 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). The total losses during the discharge process were found to represent a 11-12% total loss relative to ideal compression. These losses 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. A potential for up to a 5-6% improvement in compressor efficiency is possible by redesigning the discharge plenum and motor cavity to reduce over pressurization. Further investigation into the valve dynamics is necessary to address the remaining losses in the discharge process. The valve losses were additionally found to be sensitive to operating speed and SDT with maximum variations of 5% and 3%, respectively. It was also found that heat rejection plays a significant role in the deviation of the compression process from isentropic which suggests the spool compressor may be well suited to modifications to take advantage of this, such as flooded compression.