Twin-screw Expander Research Articles

Performance Research on a Twin Screw Expander Applied to a Power Generation System for Recovering Waste Pressure Energy of Natural Gas

The twin screw expander, applied to a power generation system for recovering waste pressure energy of natural gas (NG), is reliable. It has lower capital costs and maintenance performance compared to the others. In this paper, a computational procedure based on the Benedict-Webb-Rubin (BWR) equation is established. Meanwhile, the experiment is carried out to study the performance of a twin screw expander prototype. The results shows that the twin screw expander has an optimum rotational speed where the specific shaft work, efficiency reach their peaks when delivered a certain gas, and the optimum rotational speed reduces with the decrease of the inlet pressure.

Development and application of screw expander in natural gas pressure energy recovery at city gas station

There is a tremendous energy loss in the process of natural gas delivery by pipes. The energy lost in the pressure reduction process can be first transformed into mechanical energy and then further converted into electricity by a generator. In this study, an oil free twin screw expander was designed and applied for the nature gas energy recovery at a city gas station for the first time. And the critical speed of the rotors, the seals and bearings were discussed and presented in the design, which was proved that the system designed with the twin screw expander is much simpler without challenges of the sealing and bearing selection facing in the traditional turbine expander used in the CGS. Finally, the feasibility and simplicity of using the screw expander in natural gas energy recovery and power generation were proved through its stable running performance at site. The twin screw expander, therefore, could make up for the defects of the traditional turbine expander at a high rotation speed applied in the samilar area, and turned out to be an effective way to recover the pressure energy from the natural gas.

Modeling and performance analysis of twin-screw steam expander under fluctuating operating conditions in steam pipeline pressure energy recovery applications

In steam pipeline pressure energy recovery application, twin-screw expander is a promising approach to recover waste pressure energy dissipated in the throttling process. Since the expander operates at off-design condition in most circumstances, regulating expander capacity under fluctuating operating conditions appears to be an urgent problem. In this paper, a thermodynamic mathematical model is developed to study the performance of twin-screw steam expander under fluctuating operating conditions. In the modeling, suction pressure loss is taken into account, as well as leakage and heat transfer during the two-phase expansion process. Simulation results show that the filling factor varies 0.82–0.88 and isentropic efficiency varies 0.73–0.83, which proves twin-screw expander an efficient technology in steam pipeline pressure recovery application. Compared to suction pressure and back pressure, rotational speed affects the suction pressure loss more significantly. Increasing rotational speed has a negative effect on filling factor and larger expansion ratio leads to higher isentropic efficiency. In addition, over-expansion results in more energy loss than under-expansion. Therefore, the built-in volume ratio should be elaborately designed to ensure the expander operating at an under-expansion condition. Furthermore, rotational speed adjustment is recommended as the currently most convenient and efficient way for expander capacity regulation.

Two-phase chamber modeling of a twin-screw expander for Trilateral Flash Cycle applications

Low temperature (<100°C) streams have the largest share of waste heat recovery potential and may represent an attractive opportunity for a sustainable economy. Among the bottoming thermodynamic approaches that have been proposed to convert this waste heat into electricity, the Trilateral Flash Cycle (TFC) proved to be theoretically capable to recover more heat from a low-temperature single-phase heat source than any simple Rankine cycle. However, the commercialization of TFC recovery units has been so far prevented by the lack of an expander technology that should efficiently operate with high mass flow rates of a two-phase flashing flow. A promising candidate to tackle these challenges is the twin-screw technology thanks to its positive displacement nature and the capability to run at high revolution speeds without remarkable efficiency drops.In the current research work, a twin-screw expander has been modeled in the commercial software GT-SUITE™. The modeling activity resulted in a two-phase chamber model based on the coupling of the conservation equations and the REFPROP library to calculate the thermophysical properties of the liquid-vapor mixture of the R245fa working fluid at each time step. Using pre-processed geometrical data, the model includes a detailed breakdown of the leakage paths and allows to retrieve key information for a future optimization of the machine such as the indicator diagram and the quality-angle diagram. Parametric analyses were eventually carried out to assess the expander behavior at different operating conditions, namely manometric expansion ratio, revolution speed and inlet quality.

Theoretical investigation of flash vaporisation in a screw expander

In the present study flash vaporisation of liquid injection in a twin screw expander for a Trilateral Flash Cycle (TFC) is examined theoretically. The TFC process comprises a pressure increase in the working fluid, followed by heating the liquid close to boiling point. The hot liquid is injected into the working chamber of a screw expander. During this process the pressure of the liquid drops below the saturation pressure, while the temperature of the liquid remains virtually constant. Hence the liquid is superheated and in a metastable state. The liquid jet seeks to achieve a stable state in thermodynamic equilibrium and is therefore partially vaporised. This effect is referred to as flash vaporisation. Accordingly, a two-phase mixture, consisting of vapour and liquid, exists in the working chamber. Thermodynamic simulations were carried out using water as the working fluid for representative screw expander geometry. The simulations presented are performed from two different aspects during the filling process of a screw expander. The first case is the vaporisation of the injected liquid in a state of thermodynamic equilibrium, whereby the two-phase mixture is treated entirely as a compressible and homogeneous gas. The second case considers flashing efficiency. It describes the quantity of flashed vapour and consists of a liquid and vapour domain. Both models are compared and analysed with respect to the operational behaviour of a screw expander.

Investigating a Small Oil-Flooded Twin-Screw Expander for Waste-Heat Utilisation in Organic Rankine Cycle Systems

Screw-type expanders offer excellent prospects for energy conversion in lower and medium power ranges, for instance as expansion engines in Rankine cycles with regard to either waste or geothermal heat recovery. With the aim of identifying the potential in organic Rankine cycle (ORC) power systems, an oil-flooded twin-screw expander without timing gears was designed and experimentally investigated in an ORC with R245fa as working fluid. Here, the scope for the experimental determination of the expander characteristic map was limited by the test rig specifications. Based on the experimental results, a multi-chamber model of the test twin-screw expander was calibrated and theoretical approaches according to mechanical and hydraulic loss calculation were applied. Consequently, the expander’s complete characteristic map could be calculated. Furthermore, relevant mechanisms influencing the operational behaviour of oil-flooded twin-screw expanders were identified and analysed in-depth.

Experimental Investigation on the Performance of a Twin-screw Expander Used in an ORC System

Abstract Organic Rankine Cycle (ORC) is an effective and promising technology to convert low-grade thermal energy to electricity. As a key component in an ORC system, the expander plays a significant role in energy conversion efficiency in the ORC system. In this paper, an ORC experimental system is developed to investigate the performance of a twin-screw expander under various working conditions. Pressure sensors with high sensitivity and accuracy are installed at appropriate locations in the expander casing to monitor the p-V (pressure-volume) indicator diagrams which indicates the performance of the expander. The effect of the key parameters such as expander rotating speed, the suction pressure on the expander performance is studied using the experimental data. The results show that high expander rotating speed leads to large suction pressure loss, low volumetric and indicated efficiencies. As the rotating speed increases from 900 rpm to 1900 rpm, the volumetric efficiency decreases by up to 17.2% from 1.004 to 0.831and the indicated efficiency reduces by up to 27.1% from 0.846 to 0.617. Furthermore, as the suction pressure increases from 550 kPa to 750 kPa the indicated efficiency rises up to a peak value of 0.815 and then falls. The volumetric efficiency does not show significant change as the suction pressure varies.

Experimental Investigation on the Characteristics of ORC System with Twin Screw Expander

Experimental Investigation on the Characteristics of ORC System with Twin Screw Expander

Thermodynamic Simulation on the Performance of Twin Screw Expander Applied in Geothermal Power Generation

A three-dimensional (3D) geometry model of twin screw expander has been developed in this paper to measure and analyze geometric parameters such as groove volume, suction port area, and leakage area, which can be described as functions of rotation angle of male rotor. Taking the suction loss, leakage loss, and real gas effect into consideration, a thermodynamic model is developed using continuity and energy conservation equation. The developed model is verified by comparing predicted results of power output and internal efficiency with experimental data. Based on the model, the relationship between mass flow rate through inlet port and leakage path with rotation angle of male rotor as well as effects of the inlet parameter and operating parameter on the performance of the expander are analyzed.

Comparison of Organic Rankine Cycle Systems under Varying Conditions Using Turbine and Twin-Screw Expanders

A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications using both turbine and twin-screw expanders when account is taken of performance variation due to changes in ambient conditions. The cycle simulation contains thermodynamic models of both types of expander. In the case of the twin-screw machine, the methods used to match the operation of the expander to the requirements of the cycle are described. The performance of turbine expanders in a superheated ORC has been modelled using correlations derived from operational data for single stage reaction turbines to predict the turbine efficiency at “off-design” conditions. Several turbine configurations have been considered including variable nozzle area and variable speed. The capability of the cycle model has been demonstrated for the case of heat recovery from a steady source of pressurized hot water at 120 °C. The system parameters are optimised for a typical operating condition, which determines the required size of heat exchangers and the expander characteristics. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on the system performance, and the seasonal variation in net power output for the turbine and twin-screw ORC systems. A case study is presented for a low temperature heat recovery application with system electrical power output of around 100 kWe at design conditions. The results indicate that similar overall performance can be achieved for ORC systems using either type of expander.

Development of a thermodynamic low order model for a twin screw expander with emphasis on pulsations in the inlet pipe

A twin screw expander is a positive displacement machine used in various applications of waste heat recovery. The performance of this machine is influenced by internal leakages, gas pulsations formed in the inlet pipe and the properties of the refrigerant. In this paper a multi-chamber mathematical model of a twin screw expander is presented to predict its performance. From the mass and energy conservation laws, differential equations are derived which are then solved together with the appropriate Equation of State (EoS) in the instantaneous control volumes. In order to calculate the mass flow rates through leakage paths more accurately, flow coefficients used in the converging nozzle model were derived from 3D Computational Fluid Dynamic (CFD) calculation. Due to high gas pulsation levels at the inlet port, a coupling with a 3D CFD inlet pipe model is introduced in order to better predict throttling losses. The maximal deviation between predictions by the developed model and 3D CFD calculations of the complete machine is around 5% for the mass flow rate and the power output.

New insights in twin screw expander performance for small scale ORC systems from 3D CFD analysis

Twin screw expanders are widely used for power generation in small scale Organic Rankine Cycles (ORC). In order to increase the efficiency and maximize the generated power, an optimized design and appropriate operating conditions should be used. The use of Computational Fluid Dynamics (CFD) allows the analysis of the flow inside these machines which is impossible to investigate experimentally but which is influencing the performance of the expander. Some of the challenges when performing CFD analysis in these machines are the complexity of the rotors’ motion and the properties of the refrigerant. In this paper a 3D CFD analysis of a twin screw expander is presented. The 3D block-structured grid for the twin screw expander is constructed from the solution of Laplace problems in two-dimensional sections on an unstructured grid of the same geometry. During the transient calculations, grid nodes are moved while keeping the mesh topology. The properties of the refrigerant R245fa have been evaluated using the ideal gas Equation of State (EoS), the Aungier Redlich–Kwong EoS and the CoolProp. 3D CFD analysis of the screw expander showed that the difference in power output between the ideal gas EoS and the Aungier Redlich–Kwong EoS is 8% and between Aungier Redlich–Kwong EoS and CoolProp is negligible for operating conditions of interest. To investigate the performance of the expander, different pressure ratios and rotational speeds were studied for two different designs of the twin screw expander. The flow analysis inside the clearances that are forming leakage paths gives more insight in the performance of the expander. It is concluded that the biggest pressure drop is caused by a throttling loss at the inlet port and therefore an optimized design of the inlet port is necessary.

Performance study of a twin-screw expander used in a geothermal organic Rankine cycle power generator

The ORC (organic Rankine cycle) system is an effective technology to generate electricity from low temperature heat sources. The twin-screw expander is a key component that is commonly used in the small-to-medium capacity ORC system to convert thermal energy into work. In this paper, the performance of a twin-screw expander is theoretically and experimentally studied. A mathematical model is developed and subsequently validated using experimental data. The effect of several important factors including expander speed, suction pressure and inlet superheat on the expander performance is investigated. Results indicate that the expander speed and suction pressure have large influences on the expander performance, while the inlet superheat has relatively small effect. The isentropic efficiency of the expander decreases from 0.88 to 0.6 and the expander volumetric efficiency decreases from 0.88 to 0.7 as the expander rotational speed increases from 1250 to 6000 rpm. The results further show that the expander volumetric efficiency decreases from 0.91 to 0.85 as the expander suction pressure increases from 0.33 to 0.47 MPa. Furthermore, the energy conversion efficiency of the studied ORC system using the twin-screw expander is as high as 7.5% under the site conditions.

Numerical simulation of a twin screw expander for performance prediction

With the increasing use of twin screw expanders in waste heat recovery applications, the performance prediction of these machines plays an important role. This paper presents a mathematical model for calculating the performance of a twin screw expander. From the mass and energy conservation laws, differential equations are derived which are then solved together with the appropriate Equation of State in the instantaneous control volumes. Different flow processes that occur inside the screw expander such as filling (accompanied by a substantial pressure loss) and leakage flows through the clearances are accounted for in the model. The mathematical model employs all geometrical parameters such as chamber volume, suction and leakage areas. With R245fa as working fluid, the Aungier Redlich-Kwong Equation of State has been used in order to include real gas effects. To calculate the mass flow rates through the leakage paths formed inside the screw expander, flow coefficients are considered as constant and they are derived from 3D Computational Fluid Dynamic calculations at given working conditions and applied to all other working conditions. The outcome of the mathematical model is the P-V indicator diagram which is compared to CFD results of the same twin screw expander. Since CFD calculations require significant computational time, developed mathematical model can be used for the faster performance prediction.

Influence of liquid in clearances on the operational behaviour of twin screw expanders

A lot of effort has been expended on understanding the influences of an injected auxiliary liquid on a twin screw expander's performance. Sealed clearances improve performance on the one hand, but involve considerable frictional losses on the other hand. This paper contributes to an evaluation of these opposing effects with regard to the efficiency of screw expanders. First, thermodynamic analyses using the multi-chamber model-based simulation tool KaSim, developed at the Chair of Fluidics, are presented for a test screw expander in order to show the maximum potential of clearance sealing. This analysis involves thermodynamic simulations for sealed and unsealed clearances and leads to an order of priorities for different clearance types. Second, hydraulic losses within front and housing clearances are calculated, applying an analytical model of incompressible one-phase clearance flow. Subsequently dry and wet screw expanders are evaluated while both clearance sealing and frictional losses are considered for the simulation of a liquid-injected machine.

Geometry-based Modeling of Single Screw Expander for Organic Rankine Cycle Systems in Low-grade Heat Recovery

The constantly rising energy demand and the depletion of fossil fuel resources currently represent the main concerns for global energy sustainability, alongside environmental issues. Low-grade energy sources have moved into the limelight and Organic Rankine Cycle (ORC) technology is well known for its suitability to recover waste heat.The optimization of the expansion process of an ORC system is crucial in order to maximize the power output, to increase the overall efficiency, and to obtain an economically feasible machine. Scroll, reciprocating, single screw, twin-screw and vane-type expanders are the main technologies. Screw expanders are particularly suited for low-grade heat recovery. The advantages of single screw technology over twin-screw include the balanced loading of the main screw, long working life, high volumetric efficiency, low leakage, low noise, low vibration, and a simplified configuration.In this paper, a detailed geometry-based model of a single screw expander is presented. The model is based on a rotation-dependent function that completely describes the geometry of the screw rotor and the engaging surfaces of the star wheels. A computation of the swept volume at each angular step and of the inlet conditions leads to a solution of the system of differential equations governing the thermodynamics of the expansion process. The model considers heat losses, oil-flooded mixing and leakage paths and permits the investigation of the impact of design modifications on the expander performance.Furthermore, the validation of the proposed model against the available literature is presented.

Modeling and Testing a Screw Expander Integrated into a Trilateral Flash Cycle

Trilateral Flash Cycle (TFC) is particularly suitable for recovering energy from low-grade heat source. This paper presented a new mathematical model for calculating the performance of a twin screw expander integrated into a TFC working with organic components. The geometric parameters related to the rotation angle of male rotor e.g. groove volume, suction and discharge port area, leakage area etc were used in the model. The combination effects of internal leakage through five paths, oil injection, gas-oil heat transfer and refrigerant property were taken into account. The sensitivity of single parameter was also analyzed. To verify the model and the calculated p-φ indicator diagram, experimental recording of the diagram of twin screw expander was performed. The results of theoretical calculation were in good agreement with the experimental data, which indicated that the model in the present paper could be used as a powerful tool for performance prediction and product development.

Preliminary experimental study of single screw expander prototype

Abstract Efficient heat conversion of low temperature heat source is a key problem for energy saving, especially in the fields of waste heat recovery and renewable energy utilization. At present, technical bottleneck for low temperature thermal power is lack of suitable prime mover. As the core component of general machinery, single screw has many good features, including balanced loading of the main screw, low leakage, low noise, low vibration and long working life, etc. If single screw technology is applied to the field of expander, more efficient prime mover would be possibly obtained, compared with pistol expander, scroll expander and twin screw expander, and so on. In order to verify the performance of the prototype, the function experiment was made. In this paper, compressed air was used as working fluid and performance test for the prototype was finished at conditions including different intake flow, different humidity, constant torque, and constant rotational speed. From the experimental data, it is shown that the power output is 5 kW, exhaust temperature is −45 °C, difference between the import and export is about 62 °C, in the conditions of inlet pressure at 0.6 MPa and rotational speed 2850 rpm. The test results also show that the single screw expander has good part load characteristics. From the analysis of experimental data, we found that adiabatic efficiency of the prototype is not so high probably because of poor lubrication. The lubrication problem will be considered in the next work.

Development and Optimization of Screw Machines With a Simulation Model—Part I: Profile Generation

This paper presents a method for the design of twin screw compressors and expanders, which is based on a differential algorithm for defining the rotor profiles and an analytical model of the fluid flow and thermodynamic processes within the machine. Part I of the paper describes the algorithm for screw rotor profile generation. It demonstrates the conjugacy condition which, when solved explicitly, enables a variety of primary arcs to be defined either analytically or by discrete point curves. Its use greatly simplifies the design since only primary arcs need to be specified and these can be located on either the main or gate rotor or even on any other rotor including a rack, which is a rotor of infinite radius. Secondary arcs are then generated automatically from this. By such means any profile combination may be considered. The most efficient were obtained from a combined rotor-rack generation procedure. An example of this combination is given which produces a rotor profile with stiff lobes and a higher throughput than any other known type. Part II describes a mathematical model of the compression and expansion processes within positive displacement machines which has been well proven in its use for the design of reciprocating and screw compressors and screw expanders.

Development and Optimization of Screw Machines With a Simulation Model—Part II: Thermodynamic Performance Simulation and Design Optimization

This paper presents a method for the design of twin screw compressors and expanders, which is based on a differential algorithm for defining the rotor profile and an analytical model of the fluid flow and thermodynamic processes within the machine. Part I of the paper presents a method for screw rotor profile generation which simplifies and improves design procedures. An example is given of its use in the development of a new “N” rotor profile, which is shown to be superior to other well-known types. Part II describes a numerical model of the thermodynamic and fluid flow processes within screw machines, which is valid for both the compressor and expander modes of operation. It includes the use of the equations of conservation of mass and energy applied to an instantaneous control volume of trapped fluid within the machine with allowance for fluid leakage, oil or other fluid injection, heat transfer, and the assumption of real fluid properties. By simultaneous solution of these equations, pressure-volume diagrams may be derived of the entire compression or expansions, process within the machine. The procedure has been developed over a period of fifteen years and validated with experimental results obtained from both reciprocating and screw compressors and screw expanders, some of which are included. The rotor profile generation processor, thermofluid solver and optimizer, together with preprocessing facilities for the input data and graphical post-processing and CAD interface, have been incorporated into a design package which provided a suitable tool for analysis and optimization of twin screw machine design. An example of its use is given in the optimization of the gate tip radius of a selected compressor design.