Ian K. Smith

Developments in Modelling Positive Displacement Screw Machines

It has been estimated that almost 20% of the world’s electricity consumption is used for gas compression and pumping. For example, in developed countries, more than 25% of the electrical power output during the summer months is used for the compression of refrigerants in air-conditioning systems. For most industrial compression and pumping applications, machines of the positive displacement type are used and, due to their technological advantages over other types, approximately 85% of industrial compressors now made, are of the twin screw type. Although these are used for a variety of applications, such as compressors, expanders, blowers, vacuum pumps and liquid and multiphase pumps, the most common use of such machines is for industrial refrigeration, air conditioning and process gas compression. Depending on the application screw compressors may operate flooded by oil or another fluid or without any form of internal rotor cooling or lubrication. Typical examples of a disassembled oil injected screw compressor and an assembled dry compressor are presented in Fig. 1.

Applications for screw expanders

This chapter begins with a more detailed description of problems associated with the lubrication of screw expanders and some solutions to them. It then differentiates between applications of screw expanders, in systems in which the fluid, constituting the source of energy, is expanded to produce power within a larger system, which may have other functions and cycles, where the sole aim of the system is to produce power. In the first case, this includes pressure reduction valve (PRV) replacement in industrial steam plant, refrigeration and air conditioning systems. In the latter case, heat from an external source is supplied to a system operating on a closed thermodynamic cycle and the working fluid is different to the heating and cooling media. A variety of applications and cycle configurations are described suitable for lower, medium and higher temperatures and where it is required to recover power from two heat sources within a single system, such as the jacket coolant and the exhaust gases of an internal combustion engine. The chapter concludes with a description of the factors that need to be taken into account in optimising system design.

Geometry and manufacture of screw expander rotors

This chapter demonstrates a method for screw rotor profile generation that 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. Twin-screw expander 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. 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 calculation of the rotor profile thus manufactured. Simplification of the main gearing condition leads to a meshing expression for helical gears that may be used for the design of screw expander rotors.

Power plant thermodynamics

This chapter begins with the derivation of expressions for ideal power plant efficiencies when heat is received from a finite heat source and emphasises the importance of Carnots principle rather than the Carnot cycle. It is shown that it is impossible to maximise power recovery and cycle efficiency simultaneously and that, as a first approximation, the atmosphere can be treated as an infinite heat sink. The importance of work ratio is emphasised and it is shown that, as a consequence, power plant cycles based on ideal gases as working fluids are unsuitable for systems recovering power from low-grade heat. When generating power from an external heat source, the importance of matching the temperature changes of the heat source and the working fluid in the primary heat exchanger is demonstrated, and it is shown that this limits the value of Carnot or Stirling cycles. Cycles based on pressurising, evaporating, expanding and condensing volatile fluids are shown to be superior both because of their very high work ratios and their superior matching characteristics. The principles of working fluid selection are described for selecting volatile fluids that combine good temperature matching and high work ratio in order to maximise the power recovery from single-phase heat sources.

Expanders for power recovery

This chapter begins with a classification of fluid machinery used for compression and expansion and explains how screw expanders differ from other types of positive displacement machine and why they can be more suitable than turbines for some applications. The mode of screw expander operation is described and details are given of the differences between oil-flooded and oil-free machines and how they are lubricated.