Single Stage Centrifugal Compressor Research Articles

Upgrading the Compressor Stage of the CAT250TJ Micro Gas Turbine Engine

AbstractDue to their simplicity and relative ease of manufacture, single-stage centrifugal and mixed flow micro gas turbine (MGT) engines are preferred in thrust-based remotely piloted aerial vehicles. A single-stage mixed-flow compressor upgrade for the 200 N CAT250TJ MGT engine is numerically evaluated and presented. An in-house developed mean line application and commercial CFD software is used for the design and performance evaluation of the proposed upgrade configurations. The CAT250TJ – Gen1 engine features a single-stage centrifugal compressor, annular combustor, and single stage axial turbine. Apart from an upgraded impeller, a new crossover diffuser configuration is introduced to replace the wedge-type, straight outlet diffuser configuration of the Gen1 engine. The new single vane crossover diffuser configuration provides a design point total-to-total efficiency and pressure ratio increase of 8.3% and 12.1%, respectively. A disadvantage of a single-vaned crossover diffuser compared to legacy diffusers is a narrower operating range. To alleviate this issue, various combinations of tandem and splitter vane crossover diffuser configurations are proposed. These provide an enhanced operating range, comparable with the operating range displayed by the Gen1 configuration. A turbine power matching analysis is additionally completed to ensure proper compressor integration. Gas turbine cycle software is used to evaluate the on-engine performance of the upgraded compressor configurations. It is shown that the new baseline, single vane crossover diffuser configuration provides a 10.74% increase in design point static thrust.

Study on Flow Field Analysis and Structure Optimization in Impeller of Single-Stage Centrifugal Compressor

Fluent's built-in Latin hypercube sampling is used to generate a sample space, a total of 32 design points, a high-precision calculation model needs to be generated by CFD, the design parameters and their value ranges are determined, the response surface is used to establish a surrogate model, and the particle swarm optimization algorithm is used to obtain the optimal design parameters of the impeller with the pressure ratio and efficiency of the single-stage centrifugal compressor as the optimization goal, so as to achieve better performance of the impeller of the single-stage centrifugal compressor.

Highly efficient centrifugal compressors series development for the turbo-cooling machines

BACKGROUND: Currently, there is a growing demand for single-stage centrifugal compressors with a high pressure ratio. Such requirements encourage the use of high-pressure stages with high circumferential speeds u2 300 m/s, which leads to increased levels of Mach number and, as a consequence, pressure losses. Work to improve the efficiency of such steps leads to a reduction in energy consumption and an increase in the cooling capacity of refrigerating machines.
 AIMS: To develop a series of ten highly efficient high-head model centrifugal compressors with the pressure ratio of c = 2.0.
 MATERIALS AND METHODS: a complex calculation technique developed by the authors is used to increase the efficiency of the stages of centrifugal compressors. The method consists of sequential execution of 4 stages: gas dynamic calculation, profiling, three-dimensional viscous calculation, multiparametric optimization.
 RESULTS: a series of the model centrifugal compressors has been designed for the following range of design mode parameters: conditional flow coefficient 0.035 Fr0.12; theoretical head coefficient t.p. =0.74; conditional Mach number 0.93Mu0.96. The estimated isentropic efficiency of the developed centrifugal compressors ranges from 77% to 84%, depending on the flow rate of the stage.
 CONCLUSIONS: a series of ten highly efficient flow parts of centrifugal compressors have been developed, which can be used in gas-dynamic projects when designing modern centrifugal compressors.

Centrifugal Compressors Gas-Dynamic Characteristics Influence on the Refrigerating Machines Efficiency

The article is devoted to the study R134a refrigerating machine efficiency and consisting of a centrifugal compressor, a condenser, a temperature-regulating valve and an evaporator. The main purpose of the work is to analyze the centrifugal compressor gas dynamic characteristics effect on the refrigeration machine vapor-compression cycle efficiency. This goal is achieved through the study by actual working process numerical experiment in the refrigeration machine centrifugal compressor with an idealized process for other elements. The object of the study are the refrigeration machine characteristics, expressed by the theoretical refrigeration coefficient COPRt. Single-stage centrifugal compressors with the design conditional flow coefficient in the range from 0.035 to 0.12 are considered. The design of centrifugal compressors was carried out according to a new calculation method to the flow part efficiency increase. The method comprehensively combines the inviscid and viscous flow calculations with the use of the single-criteria and multiparametric optimization. Previously, the method was tested and compared with experimental data. The most important result is the results of the refrigeration cycle efficiency evaluating through the centrifugal compressors highly efficient flow parts design methodology application. An increase in COPRt was obtained taking into account the centrifugal compressor actual process in the range from 2.6% to 7.2%. The significance of the results obtained lies in the possibility of using high-efficiency centrifugal compressors gas dynamic characteristics for the chillers refrigeration cycles analysis and calculation. The level of the compressors isentropic efficiency ranges from 0.80 to 0.85, depending on the design conditional flow coefficient.

Investigating the effects of shroud and blade thickness profiles on aeromechanical behavior and fatigue-life of 17-4PH impellers

In the present study, effects of blade and shroud thickness profiles on the stress distribution and fatigue life of centrifugal compressor impellers are investigated. Impeller geometry is generated based on an available data of a single-stage centrifugal compressor and is simulated in actual operating conditions. Pressure distribution on the impeller blades is obtained using Computational Fluid Dynamics (CFD) simulation which is then incorporated into Finite Element (FE) structural analysis. Equivalent stress and total deformation contours on the impeller are employed to pinpoint critical areas and maximum stress values. Simulations are performed for multiple cases with various shroud and blade thickness values to capture the role of these important parameters in aeromechanical behavior of the structure. Furthermore, fatigue stress amplitude limit and fatigue-life are evaluated based on the results of the FE analyses. It will be demonstrated that semi-open configuration is the most desirable design from mechanical stress and fatigue-life viewpoints. For shrouded impellers, it is shown that the mechanical stress increases and fatigue life decreases with increasing the blade thickness while for the shroud thickness, there exist an optimal point with minimum stress level and maximum fatigue life. The methodology and results of this paper can be employed for other impellers with different geometries and operating conditions to find the optimal design points from the structural mechanics perspective.

Experimental Study on a Supercritical CO2 Centrifugal Compressor Used in a MWe Scale Power Cycle

The centrifugal compressor is the core component of supercritical CO2 power cycle, and its performance and operation stability are research hotspots. However, there are few experimental studies, especially for compressors used in Mwe-scale power cycles. In this paper, based on a 1 MWe supercritical CO2 power cycle, a single-stage centrifugal supercritical CO2 compressor is designed with speed of 40,000 RPM, a pressure ratio of 2.5 and a mass flow of 16.3 kg/s. In order to carry out the compressor test, a general experimental platform for MWe sCO2 compressors is built. In the test, the mass flow range is 13.5~18 kg/s and the maximum experimental pressure ratio is close to 2.0. The performance curve of the compressor of 31,000 ± 1000 RPM is obtained, and the historical curve of the experiment is given. Then, the experimental curve is compared with the design curve using a dimensionless method. The isentropic head coefficient of the experimental curve is lower than the design value, and the experimental curves shift towards the boundary of small flow coefficient. Finally, the influence of compressor inlet condensation on compressor performance and the change of operating boundary is preliminarily explained.

Investigation of Surge in a Transonic Centrifugal Compressor With Vaned Diffuser—Part I: Surge Signature

Abstract The identification of stall inception mechanisms and stability-limiting components in a centrifugal compressor is required for the development of effective surge suppression approaches. Part I of this two-part paper investigates the surge signature of a centrifugal compressor at subsonic, transonic, and supersonic inlet tip conditions, and Part II considers the relationship of the surge signature with the compressor static pressure characteristics, as well as the impeller leading edge relative tip Mach number. Experiments were performed in the single stage centrifugal compressor (SSCC) facility at Purdue University with both steady performance and dynamic pressure data recorded. Results show the presence of both mild and deep surges on the compressor map. Deep surge occurs at subsonic and supersonic impeller tip relative Mach numbers while the mild surge is observed as the tip relative Mach number nears unity. Long length-scale modal oscillations can be detected only at the transonic operating condition while spike disturbances are identified as the precursor to instability at subsonic and supersonic tips relative Mach numbers. Finally, the impeller is determined to be the origin of instability throughout most of the compressor map. The observation of impeller-induced instability refreshes the conventional understanding that the diffuser is typically the stability-limiting component for centrifugal compressors with vaned diffusers and suggests various surge suppression approaches may be necessary to achieve range extension throughout a compressor map.

Experimental and Theoretical Study of Surge Behavior in a Boil-Off Gas Centrifugal Compressor on an LNG Carrier

In this study, we conducted experiments and numerical analysis on a 270 kW industrial-scale centrifugal compressor used in the fuel supply system of an LNG carrier in order to improve the lumped-parameter surge model by considering the viscosity in the pipeline and to confirm whether the improved model would be applicable. The steady and unsteady compressor performance curves were constructed using measurements and predictions, respectively. The flow through the pipeline was assumed to be both steady and unsteady, and each governing equation under the assumptions was derived in accordance with the lumped-parameter model. In the steady flow case, the surge behavior of the modified model was in a good agreement with the lumped-parameter model at surge parameter B = 4.8716. In the unsteady flow case, however, the modified model showed a deviation from the lumped-parameter model, and the simulation from the modified model described the surge behavior 5% more accurately than the lumped-parameter model. Through experiments and numerical analysis, this study showed that the present models are useful and applicable for describing the surge behavior of an industrial-scale single-stage centrifugal compressor.

Experimental Investigation on the Mechanism of Impeller Synchronous and Nonsynchronous Vibrations in an Industrial Centrifugal Compressor

Abstract Understanding of the fluid–structure interaction phenomena in centrifugal compressors is essential for the impeller structural integrity design. To enhance the understanding of blade vibration in a realistic flow environment, a single-stage centrifugal compressor representative of industrial architecture has been investigated experimentally. For deterministic synchronous vibrations, the response amplitudes at different operating lines are measured and compared. The fundamental relations between flow excitation and rotor modal resonance are established by pairwise strain gages from experimental perspective. Compared with the third mode, the impeller first bending mode shows traveling wave response characteristic only within the frequency band where two adjacent blades have the same resonant frequency. Besides, the impeller encounters unexpected nonsynchronous vibrations when operating near the flow instability boundary. Speed ramp tests show that the stall cell propagating speed increases along with the rotor rotational speed without changing the cell count. Response amplification is further measured when throttling the compressor into stall. Experimental findings point toward vaned diffuser rotating stall and the corresponding propagating pressure waves in circumference leads to the impeller vibration. The aerodynamic asymmetry of stall cells increases the possibility of aeroelastic coupling with the blade modes. These results contribute to an in-depth understanding of the aeroelastic phenomena in industrial centrifugal compressors. The observed nonsynchronous vibration is important for the aeroelastic design and also of great interest for numerical predictions near stall.

Feasibility of Hybrid Desalination Plants Coupled with Small Gas Turbine CHP Systems

Nowadays, several technologies for desalination processes are available and widely employed. However, they consume a considerable amount of energy and involve high capital and operating costs. Therefore, the techno-economic analysis of a system coupling different energy sources with the desalination processes is of value. The possibility of coupling a small gas turbine combined heat and power system (GT CHP) with hybrid desalination plants (HDPs) has been assessed in this study. The proposed gas turbine power generation system, based on a single-stage centrifugal compressor and an uncooled centripetal turbine, provides design simplicity and reasonable installation costs for the power generating plant. The hybrid desalination technique, based on the use of two different desalination technologies, i.e., Reverse Osmosis (RO) and a thermal desalination process, has been chosen to better exploit the electrical and thermal energy produced by the mini CHP plant. The proposed solution is numerically investigated from both thermodynamic and economic points of view, and the results of the thermodynamic analysis of the cycle are used as input for the evaluation of the amount of freshwater produced and of costs. The economic assessment of standalone desalination systems is also shown for the comparison with the hybrid solutions here proposed. Results show that the total cost of the water produced by MED + RO was less than the total cost of the water obtained by MSF + RO, and that the energy cost of MED + RO hybrid desalination system was about 15% less than that for stand-alone RO desalination technology. Thus, the MED + RO hybrid desalination system can be considered a promising solution for the coupling with the proposed mini GT CHP plant, which, due to the small size and cost, as well as the easy installation, can be easily applied in off-grid or remote areas.

Experimental and Numerical Investigation of Clearance Effects in a Transonic Centrifugal Compressor

Extensive research studying tip clearance effects on compressor performance has been conducted during the past few decades with significant progress achieved. However, most of the previous investigations focus on stage-level analysis at the design condition, with very limited studies at the component level or part speed conditions. To fill this gap, this paper investigates the effects of impeller tip clearance on the performance of a high-speed centrifugal compressor, at both stage and component levels, covering a variety of operating speeds resulting in subsonic to transonic inlet conditions. Experiments were conducted on the Single Stage Centrifugal Compressor facility at two different running tip clearances. With increased clearance, the stage and impeller pressure ratio, isentropic efficiency, and work factor drop, but the static pressure recovery coefficient in the diffuser increases. As the compressor transitions from subsonic to transonic operation from 60 to 85% corrected speed, the sensitivities of stage and impeller pressure ratio and efficiency reach their peak values. Lastly, numerical simulations at design speed showed that the computational fluid dynamics (CFD) model sufficiently predicts the sensitivities due to tip clearance for most of the parameters at the stage and component levels, indicating the potential of using CFD to investigate the flow physics.

Compressor-Surge Behaviors Distinguished Between Near-resonant Type and Convective Type

In deep surges in compressors, the behaviors tend to show either a near-resonant one or a convective one, the latter performing emptying and filling actions. However, detailed information on the differences between them have not been obtained so far. In order to understand the situations, surge simulations by a one-dimensional simulation code were conducted with respect to a single-stage centrifugal compressor, paying attention to the effects of changes in the compressor tip speed and in the geometry of the flowpaths. As the results, it has been found that near-resonant surges appear at very low tip speeds and for relatively narrow sectional areas of the delivery flowpath, and they transform to convective surges of emptying and filling actions toward higher speeds and wider sectional areas. The features of the behaviors are generalized in terms of several non-dimensional surge frequency parameters. The results are visualized by several global framework maps showing the relations among the parameters. The obtained results will give deeper understandings on the surge phenomena.

Aerodynamic design and multi-dimensional performance optimization of supercritical CO2 centrifugal compressor

This paper dedicates to develop a systematic approach of design and multi-dimensional performance optimization of the Supercritical carbon dioxide (SCO2) compressor. A 5 MW SCO2 simple regenerative cycle used in small scale lead-cooled fast reactor (SLFR) was firstly proposed and simulated to define the operation condition of SCO2 compressor. Under the inlet and outlet condition of 305.15 K/7.56 MPa and 22 MPa, isentropic efficiency and mass flow rate of 80% and 69.88 kg·s−1 respectively was obtained. Secondly, a single stage centrifugal compressor was applied as the SCO2 compressor and one-dimensional (1D) design based on self-developed code was conducted to determine the basic geometry of the compressor, the code was verified with experiments and shows satisfactory accuracy. Additionally, CFD analysis was carried out to validate the results of 1D design and also shows good agreement. Then, three one-dimensional optimizations, two single objective optimizations using Particle Swarm Optimization (PSO) and one multi-objective optimization using Genetic Algorithm (GA), on aerodynamic efficiency of the SCO2 centrifugal compressor were performed based on 1D design code, five crucial parameters with ± 5% variation range were optimized. The 1D optimization results show that the isentropic efficiency received an improvement of about 5 percentage points compared to that of 1D design. Lastly, three-dimensional optimization concentrated on the blade profile of impeller were conducted on a fully automatic optimization platform to further improve the isentropic efficiency of SCO2 centrifugal compressor. Wrap angle and the leading and trailing edge of the main blade were optimized. Two optimal impellers with wrap angle in the range of + 5% and −5% were obtained respectively, the former impeller got the highest pressure ratio, while the latter one gained a higher isentropic efficiency but the lowest pressure ratio over the whole operation range.

Effects of number of inlet guide vanes on the aerodynamic performance of a centrifugal compressor

Variable inlet guide vanes (VIGVs) are widely used for flow throttling and also extending the operating range of centrifugal compressors. Although there are several studies on the effects of adding IGVs on the performance curve of the compressors, none of them have focused on the number of vanes. In the current study, high-fidelity three-dimensional numerical simulations are carried out to analyze the effects of adding VIGVs with different number of vanes on the aerodynamic performance of a single-stage centrifugal compressor. The selected compressor prototype is a high flowrate single-stage compressor equipped with a vaned diffuser, designed and fabricated by Siemens. Computational fluid dynamic simulations are performed for three different number of guide vanes at three IGV inclination angles of 0, −30 and +45 degrees. The numerical results are validated by comparing the pressure-rise curves with the available experimental data of the compressor data sheet, where a good agreement was achieved. Results show that at the fully-open condition, the number of vanes does not have considerable effect on the performance curve of the compressor. However, as the IGV inclination angle increases, the number of inlet vanes plays a considerable role in the compressor efficiency. For example, at IGV inclination angle of +45 degree, increasing the number of vanes from 7 to 11 can increase the compressor maximum efficiency up to 5 points. Numerical results showed that increasing the number of inlet guide vanes imposes a higher pressure drop in the inlet passage of the compressor while generating a more uniform velocity distribution at the suction surface of the impeller. Due to the existence of several counteracting effects, an optimum number of inlet guide vanes can be found.

Structural Response of a Single-Stage Centrifugal Compressor to Fluid-Induced Excitations at Low-Flow Operating Condition: Experimental and Numerical Study

The article describes an assessment of possible changes in constant fatigue life of a medium flow-coefficient centrifugal compressor impeller subject to operation at close-to-surge point. Some aspects of duct acoustics are additionally analyzed. The experimental measurements at partial load are presented and are primarily used for validation of unidirectionally coupled fluid-structural numerical model. The model is based on unsteady finite-volume fluid-flow simulations and on finite-element transient structural analysis. The validation is followed by the model implementation to replicate the industry-scale loads with reasonably higher rotational speed and suction pressure. The approach demonstrates satisfactory accuracy in prediction of stage performance and unsteady flow field in vaneless diffuser. The latter is deduced from signal analysis relying on continuous wavelet transformations. On the other hand, it is found that the aerodynamic incidence losses at close-to-surge point are underpredicted. The structural simulation generates considerable amounts of numerical noise, which has to be separated prior to evaluation of fluid-induced dynamic strain. The main source of disturbance is defined as a stationary region of static pressure drop caused by flow contraction at volute tongue and leading to first engine-order excitation in rotating frame of reference. Eventually, it is concluded that the amplitude of excitation is too low to lead to any additional fatigue.

Blade vibration measurement and numerical analysis of a mistuned industrial impeller in a single-stage centrifugal compressor

Understanding and predicting the dynamical behavior of the impeller structure under realistic condition is vital for resonance-free and reliable design. This paper presents a novel method for high fidelity measurement and efficient modeling of impeller blade vibration considering realistic operating environment and mistuning effect. To clarify the blade forced vibration, strain gauge and blade tip timing (BTT) measurement have been conducted. Measurement introduced mistuning and flow disturbance from strain gauges are further excluded with tip-timing. The blade mistuning is then identified and quantified with the help of parameter estimation method and updates the numerical mistuned forced response model. Aerodynamic forcing function due to downstream diffuser vanes is also numerically evaluated based on a decoupled two row configuration and time transformation method. Results shown that BTT system is well applied to capture the blade resonance response and shows a promising way for monitoring individual blade component. The aerodynamic forcing is also found to be well captured and presents a wave decay trend along the flow path towards the upstream. Finally, based on the proposed BTT parameter driven vibration prediction method, the blade mistuned forced response shows a good agreement with measurement results. Improvement also implies variations of parameters and mistuning pattern should be considered as a critical part and uncertainty source in the numerical prediction. This work is of scientific significance to enrich the knowledge of blade vibration in centrifugal compressor using tip-timing and of industrial application value to consider mistuning problems with efficient reduce-order modeling method.

Analysis of a 10 MW recompression supercritical carbon dioxide cycle for tropical climatic conditions

Recompression supercritical CO2 (sCO2) power cycles offer higher efficiencies over other sCO2 cycle configurations. This paper addresses the design of a 10 MW sCO2 recompression cycle suitable for tropical climates with a compressor inlet temperature of 45 °C and a maximum turbine inlet temperature of 565 °C to facilitate the use of conventional steel alloys. A design space analysis is carried out by incorporating individual component performance under various operating conditions. The proposed cycle uses a single two-stage axial turbine and two single-stage centrifugal compressors. The results are presented on an efficiency-net-specific work diagram showing the Pareto optimal curves for a range of recompression fractions. The optimum cycle parameters, pressure ratio, and recompression fraction are found to be 2.0 and 0.25, respectively, yielding a design point efficiency of 35%. The paper brings forth the effect of turbomachinery and heat exchangers sized for 45 °C compressor inlet temperature and 565 °C turbine inlet temperature on off-design cycle performance. The analysis is applied to develop strategies for optimally operating the recompression cycle at part load and off-design conditions.

Design, Analysis, and Manufacture of an Axial Length-Saving Disk-Oriented Engine

Abstract A disk-oriented engine was designed to reduce the overall length of a gas turbine engine, combining a single-stage centrifugal compressor and radial in-flow turbine (RIT) in a back-to-back configuration. The focus of this research was to understand how this unique flow path impacted the combustion process. Computational analysis was accomplished to determine the feasibility of reducing the axial length of a gas turbine engine utilizing circumferential combustion. The desire was to maintain circumferential swirl from the compressor through a U-bend combustion path. The U-bend reverses the outboard flow from the compressor into an integrated turbine guide vane in preparation for power extraction by the RIT. The computational targets for this design were a turbine inlet temperature of 1300 K, operating with a 3% total pressure drop across the combustor, and a turbine inlet pattern factor (PF) of 0.24 to produce a cycle capable of creating 668 N of thrust. By wrapping the combustion chamber about the circumference of the turbomachinery, the axial length of the entire engine was reduced. Reallocating the combustor volume from the axial to radial orientation reduced the overall length of the system up to 40%, improving the mobility and modularity of gas turbine power in specific applications. This reduction in axial length could be applied to electric power generation for both ground power and airborne distributive electric propulsion. Computational results were further compared to experimental velocity measurements on custom fuel–air swirl injectors at mass flow conditions representative of 668 N of thrust, providing qualitative and quantitative insight into the stability of the flame anchoring system. From this design, a full-scale physical model of the disk-oriented engine was designed for combustion analysis.

Experimental investigation of a novel micro gas turbine with flexible switching function for distributed power system

Micro gas turbine (MGT) is widely used in small-scale distributed power systems because of its low emissions and fuel flexibility. However, the under-utilization of its exhaust heat and the low electric efficiency are the main bottlenecks that restrict its application. Additionally, the flexible switching between the power generated by the MGT and the power grid is also a key factor for keeping the secure operation of a distributed power station. Therefore, this paper conducted some experimental investigations of a 30 kW MGT to provide reference solutions for the above issues. This MGT is located at Shanghai Jiao Tong University (SJTU), which is designed by the Gas Turbine Research Institute of SJTU, and is manufactured by a turbo-machinery factory in Chongqing, China. The demonstration prototype is mainly composed of a single stage centrifugal compressor, a radial turbine, a combustor, a high-speed permanent magnet generator, and a control system. The results show that the MGT can achieve steady operation at a low rotational speed from 10000 r/min to 34000 r/min in the case of using oil lubricated bearings, which can greatly reduce the economic cost compared with the use of air bearings. At the same time, the ignition success rate of combustion chamber (CC) reaches 98% at a low rotational speed, and a wide range of stable combustion area can be obtained, because of the novel design method of combustor by referencing the way applied in an axial flow aero-engine. The MGT generating set can achieve functions, such as starting up, ignition, stable operation, loaded operation, grid-connection and stopping. This system also can realize flexibly switching from the start motor mode to the generator mode, and from grid-connected mode to off-grid mode, because the innovative multi-state switching control system is adopted. The above research work can make our state master independent intellectual property rights of micro gas turbine, rather than continue to be subject to the technological monopoly of the developed states, which can provide theoretical and experimental support for the industrialization of MGT in China.

Scale limitations of gas bearing supported turbocompressors for vapor compression cycles

An analysis of single-stage centrifugal compressors supported on dynamic, gas lubricated bearings for driving vapor compression cycles suggests that the maximum feasible compressor power is limited, primarily by the bearing load capacity, in the range from 80 kW to 250 kW. The analysis further suggests that the maximum feasible compressor power depends significantly on working fluid selection. Low-pressure fluids result in large impellers with good efficiency but heavy machines, the power of which is limited by the bearing load capacity. High-pressure fluids require small machines rotating at high speeds, the power of which is limited by the rotordynamic eigenfrequencies. Consequently, there is a family of fluids that allows maximization of the power range of gas bearing supported turbocompressors. Finally, the analysis suggests that the maximum compressor power can be expressed as a function of the reduced evaporation pressure, regardless of the working fluid choice. Maximum COP is reached at a reduced evaporation pressure decreasing nearly linearly from 0.22 to 0.11 as the temperature lift between the evaporator and the condenser increases from 20 K to 50 K. The feasible maximum compressor power increases with increasing specific compressor speed, bearing tip speed (NDm) and rotordynamic stability safety margin.