Conference Agenda

Isentropic compression is typically accepted as the ideal reference process for refrigerant compressors. However, whilst isentropic compression, the discharge temperature is usually higher than the corresponding condensing temperature, depending on the refrigerant’s thermophysical properties, initial state and operating conditions. This discharge superheat is unfavorable in terms of both second law efficiency and practical application limits.

A two-stage isentropic-isothermal compression with heat rejection at the condensing temperature would match the ideal Carnot process and therefore achieve improved cycle efficiency. In this case, the discharge temperature in the cycle would be identical or slightly above the condensing temperature. This would allow for a wider application range of the compressor – especially for refrigerants that typically experience comparably large discharge superheat.

However, due to a comparably high compression frequency, most refrigerant compressors are closer to adiabatic and the partial isothermal compression cannot be achieved. Heat transfer timescales as well as surface area do not match in typical practical applications. For compressors that can tolerate significant amounts of liquid in their working chamber, particularly screw and scroll compressors, effective temperature reduction can be achieved by direct oil or refrigerant injection. For reciprocating compressors, similar approaches are only applicable to a very limited extent due to the risk of liquid slugging.

This study therefore presents a method for high-pressure liquid refrigerant injection for reciprocating compressors, which employs a high-pressure pump and a controllable injection valve in the working chamber. The atomization of the injected refrigerant allows for a rapid evaporation, preventing liquid slugging and achieving significant temperature reduction in the working chamber. By controlling the injection rate through variable injection valve timing, the temperature profile during compression can be adjusted.

The coupled compression-injection is assessed using an energetic chamber model. Various cycle variations, particularly the differences between subcritical and transcritical cycles, are evaluated. Emphasis is put on the cycle components, i.e. components additionally required and the implications for the main components in a standard cycle.

In summary, this study introduces a novel approach for enhancing reciprocating compressor efficiency by employing high-pressure liquid refrigerant injection. The theoretical evaluation of this coupled compression-injection system highlights its potential to achieve significant discharge superheat reduction and – in particular cases – improve the efficiency.

High-temperature heat pumps (HTHP) will be an important technology to replace industrial heating in the temperature range from 100 to 200 °C, which is currently mainly provided by oil and gas burners. The lifetime of compressors and lubricating oil at high suction and discharge temperatures is unclear. For this study, a reciprocating compressor was disassembled after approximately 1,000 hours of operation in a lab-scale HTHP. At least 300 hours of operation showed discharge temperatures of 120 °C and 30 hours of more than 150 °C. The cylinder head seal was damaged, colorization of the valve plate was observed, and small wear was found, possibly because of a low oil viscosity. All other parts of the compressor were unaffected, although the compressor was not specifically designed for operation at such high temperatures. Various oil measurements did not show significant deviations from the fresh oil. The study reveals that the reciprocating compressor and the oil were more resilient to the high temperatures than expected. However, more runtime is needed to gain confidence in operation in a real plant over several years.

Accurate compressor performance prediction is a key tool in heat pump and refrigeration system modeling and design. Correlations applicable to a variety of refrigerant types are rare and would be valuable for multi-refrigerant screenings and mixture development. This work presents correlations for isentropic and volumetric efficiency and heat losses of reciprocating compressors for synthetic and hydrocarbon refrigerants and mixtures. A refrigerant-specific toggle term was included in the isentropic efficiency correlation to distinguish between refrigerant types. Equations were fitted to 365 experimental data points across two compressors, 7 pure fluids and 10 mixtures thereof, with pressure ratios ranging from 2 to 18, suction pressures from 50 to 750 kPa, isentropic efficiencies from 0.30 to 0.70, volumetric efficiencies from 0.35 to 0.90, and heat losses from 0.1 to 0.65 of the compressor power draw. The overall isentropic efficiency (referred to throughout the paper as simply “isentropic efficiency”) correlation has three input parameters and predicts all data with an average deviation of 0.012. The volumetric efficiency correlation has only one input parameter and predicts all data with 0.022 average absolute error. The heat loss correlation has two input parameters and an average deviation of 0.034. All three correlations are valid over the entire experimental range for all fluid/compressor combinations tested.

Liquid slugging is a phenomenon in which liquid refrigerant enters the compression chamber, posing a significant risk to reciprocating compressors by potentially damaging valves and other components, often leading to failure. Reciprocating compressors adopt suction mufflers to attenuate noise resulting from pressure pulsation brought about by piston motion. However, the impact of the suction muffler design on liquid refrigerant volume during liquid slugging events remains unclear due to complex fluid dynamics. This paper reports an experimental investigation on the two-phase flow of refrigerant within a simplified suction muffler model. The aim is to explore how operating conditions influence the volume of liquid refrigerant reaching the compression chamber after a liquid slugging event. A test bench, comprising a calorimeter, liquid injection system, and test section, was developed to measure the volume of R134a in its liquid phase at the muffler outlet following a predefined mass injection. The calorimeter was used to submit the muffler to different operating conditions. The injection system included a refrigerant reservoir in liquid form under controlled pressure, supply line, and a solenoid valve for controlled test section injections. The test section, featuring a sight glass and a transparent acrylic muffler, was integrated into the calorimeter circuit, to replicate hermetic compressor operating conditions. High-speed camera footage facilitated the observation of flow through the muffler. Results indicate that liquid slugging is highly complex, with the volume of liquid refrigerant at the muffler outlet increasing in correlation with the injection pressure.