The world market for systems for power recovery from low-grade heat sources is of the order of £1 billion per annum. Many of these sources are hot liquids or gases from which conventional power systems convert less than 2.5 per cent of the available heat into useful power when the fluid is initially at a temperature of 100° C rising to 8–9 per cent at an initial temperature of 200°C. Consideration of the maximum work recoverable from such single-phase heat sources leads to the concept of an ideal trilateral cycle as the optimum means of power recovery. The trilateral flash cycle (TFC) system is one means of approaching this ideal which involves liquid heating only and two-phase expansion of vapour. Previous work related to this is reviewed and details of analytical studies are given which compare such a system with various types of simple Rankine cycle. It is shown that provided two-phase expanders can be made to attain adiabatic efficiencies of more than 75 per cent, the TFC system can produce outputs of up to 80 per cent more than simple Rankine cycle systems in the recovery of power from hot liquid streams in the 100–200°C temperature range. The estimated cost per unit net output is approximately equal to that of Rankine cycle systems. The preferred working fluids for TFC power plants are light hydrocarbons.

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 diyerent working fluids, varying rotor profiles and sizes and power outputs of 5350 kW. Good agreement was obtained between predicted and measured pegormance 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 eficiencies 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 coeficient of performance of large refrigeration, air conditioning and heat pump systems may thereby be raised by up to approximately 8 per cent. A Adis.rnax A S U C Asuc.max C d e E G h H m M P Q Re

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.

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTGas pulsations in suction and discharge chambers are widely accepted to be a significant source of noise in screw compressors. This paper analyses the influence of both the compressor operating conditions and its geometric characteristics on the level of gas pulsations generated in its discharge chamber. The transfer function of the discharge port area is identified here as an important parameter influencing gas pulsations. It is shown how the gas pulsation amplitude in the discharge chamber can be reduced by optimization of its port shape.

This paper presents a method of general geometrical definition of screw machine rotors and their manufacturing tools. It describes the details of lobe shape specification, and focuses on a new lobe profile, which yields a larger cross-sectional area and shorter sealing lines resulting in higher delivery rates for the same tip speed. A well proven mathematical model was used to determine the optimum profile, compressor housing size and compressor ports to achieve the superior compressors.