Nikola Stosic

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 expansion 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.

Investigation of the influence of oil injection upon the screw compressor working process

Abstract Some results of mathematical modelling and experimental investigation of the influence of oil injection upon the screw compressor working process are presented. Several parameters that characterize oil injection were varied over ranges that were initially determined from a computer model. These include: oil flowrate, inlet temperature, droplet atomization, positions in the casing at which the oil was injected, oil jet speed and angle, and time of oil retention in the working volume. The compressor performances were evaluated from measurements of all important bulk parameters: delivery rate, power consumption, power utilization efficiency, specific power, as well as the instantaneous pressure and temperature at several positions along the working volume, from which the indicator diagram was worked out. In addition to the information about the influence of each oil parameter upon the compressor performances, the collected data served to verify and complement the mathematical model of the influence of oil upon the screw compressor working process developed earlier, which has subsequently been employed for computer-aided design of two different screw-compressor oil systems. The experimental results and the application of the computer simulation helped in modifying the oil injection system, which resulted in a saving in compressor energy consumption up to 7%.

On gearing of helical screw compressor rotors

Abstract Twin-screw compressor rotors are effectively helical gears. When these are formed from a hobbing cutter, the hobbing tool and the rotor together constitute a pair of crossed helical gears. In the present paper, the envelope gearing method is used to derive a meshing condition for crossed helical gears which is then used to create the profile of a hobbing tool. A reverse transformation enables the rotor profile thereby manufactured to be calculated. Simplification of the main gearing condition leads to the meshing expression for helical gears which may be used for the design of screw compressor rotors.

Development of the Trilateral Flash Cycle System: Part 3: The Design of High-Efficiency Two-Phase Screw Expanders

An extensive research and development programme carried out at City University, London, has led to an improved level of understanding of how Lysholm twin screw machines may be used to recover power from two-phase flash expansion processes. The mode of operation of such machines is described together with the various types of rotor shapes used. Details are given of a computer simulation of the expansion process which was used to analyse 636 test results. These were obtained from earlier investigations as well as those of the authors and include three different working fluids, varying rotor profiles and sizes and power outputs of 5–850 kW. Good agreement was obtained between predicted and measured performance parameters and statistical analyses of the results indicate that this is unlikely to be improved without the development of more refined methods of two-phase flow analysis than are currently in use. Included in the tests are a set of measurements of pressure-volume changes within the expander carried out by the authors which confirmed a hitherto unappreciated feature of the expansion process. This is the relatively large pressure drop associated with the initial filling of the volume trapped between the rotors and the casing. The analytical technique thus developed was used both to explain the poor results of earlier studies with water expanders and to estimate optimum design performance. It is shown that, when expanding wet organic fluids, adiabatic efficiencies of over 70 per cent may be obtained at outputs of only 25 kW while multi-megawatt outputs are possible from machines no bigger than large compressors with efficiencies of more than 80 per cent. Two-phase screw expanders may be used not only for large-scale power generation in trilateral flash cycle (TFC) systems, but also in place of throttle valves in vapour compression systems to drive screw compressors in sealed ‘expressor’ units. The coefficient of performance of large refrigeration, air conditioning and heat pump systems may thereby be raised by up to approximately 8 per cent.

COST EFFECTIVE SMALL SCALE ORC SYSTEMS FOR POWER RECOVERY FROM LOW GRADE HEAT SOURCES

The growing need to recover power from low grade heat sources, has led to a review of the possibilities for producing systems for cost effective power production at outputs as little as 20-50kWe. It is shown that by utilizing the full potential of screw expanders instead of turbines, it is possible to produce Organic Rankine Cycle (ORC) systems at these outputs, which can be installed for a cost in the range of

Optimisation of screw compressors

1500 to

Numerical simulation of combined screw compressor–expander machines for use in high pressure refrigeration systems

2000 /kWe of net output. This low capacity cost combined with the ORCs fuel-free specification results in a very favorable value proposition.Copyright

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

Increasing demands for more efficient screw compressors require that compressor designs are tailored upon their duty, capacity and manufacturing capability. A suitable procedure for optimisation of the screw compressor shape, size, dimension and operating parameters is described here, which results in the most appropriate design for a given compressor application and fluid. It is based on a rack generation algorithm for rotor profile combined with a numerical model of the compressor fluid flow and thermodynamic processes. Some optimisation issues of the rotor profile and compressor parts are discussed, using 5/6 screw compressor rotors to present the results. It is shown that the optimum rotor profile, compressor speed, oil flow rate and temperature may significantly differ when compressing different gases or vapours or if working at the oil-free or oil-flooded mode of operation. Compressors thus designed achieve higher delivery rates and better efficiencies than those using traditional approaches, which is illustrated in an example of the 3/5 screw rotors designed for a family of dry air compressors, produced and marketed by a renown British compressor manufacturer.

The influence of port shape on gas pulsations in a screw compressor discharge chamber

Recent interest in natural refrigerants has created a new impetus for studies of CO2 as a working fluid in vapour compression systems for refrigeration and air conditioning. Two major drawbacks to its use are the very high pressure differences required across the compressor and the large efficiency losses associated with the throttling process in the refrigeration cycle. It is shown how these disadvantages can be minimised by the use of a screw machine both to compress the gas and use the expansion process to recover power. Both these functions can be performed simultaneously, using only one pair of rotors, in a configuration that partially balances out the forces induced by the pressure difference and hence, reduces the bearing loads to an acceptable level. A further feature is the use of rotors, which seal on both contacting surfaces so that the same profile may be used for the expander and the compressor sections. This enables the rotors performing both these functions to be machined or ground in the same cutting operation and then separated by machining a parting slot in them. Computational Continuum Mechanics comprising both, fluid flow and structural analysis is used in this paper for the investigation of fluid-solid interaction in such machines.

The Design of a Twin-screw Compressor Based on a New Rotor Profile

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.