Ahmed Kovacevic

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%.

Optimisation of screw compressors

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.

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

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

Abstract Gas pulsations in suction and discharge chambers are a significant source of noise in screw compressors. This paper shows how such effects in the discharge chamber are influenced both by the compressor operating conditions and its geometric characteristics. An area function is identified for the discharge port as an important parameter influencing the gas pulsations and it is shown how their amplitude can be reduced by optimization of the port shape.

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

SUMMARY This paper describes the design of a high-efficiency twin-screw compressor with new rotor profiles. A well-proven mathematical model of the compression process within positive displacement machines was used to determine the optimum rotor size and speed, the volume ratio, and the oil injection position and jet diameter. In addition, modern design concepts such as an open suction port and early exposure of the discharge port were included, together with improved bearing and seal specification, to maximize the compressor efficiency. The prototype was tested and compared with the best compressors currently on the market. Its minimum measured specific power input at a delivery pressure of 8 bar (abs) was 5.6 kW m-3 min-1. This is less than the published value for any other equivalent compressor currently manufactured. Both the predicted advantages of the new rotor profile and the superiority of the design procedure were thereby confirmed.

Steam as the Working Fluid for Power Recovery from Exhaust Gases by Means of Screw Expanders

Rankine cycle systems, using steam as a working fluid, are not well suited to the recovery of power from heat sources in the 300–450 °C temperature range, such as internal combustion engine exhaust gases, mainly due to the relatively large enthalpy of vaporization of water. Admitting the steam to the expander as vapour approximately 50 per cent dry, would be preferable but turbines cannot be used to expand vapours from this state. However, screw expanders can operate well in this mode. It is shown that, apart from being environmentally benign and free from flammability risks, a screw-driven wet steam cycle system can recover power from engine exhaust gases, with comparable efficiencies to turbine-driven systems using organic fluids at a significantly lower cost per unit output.

Rotor interference as a criterion for screw compressor design

Rotor clearances are mainly responsible for screw compressor leakage. They must therefore be minimized in order to obtain high volumetric and adiabatic efficiencies of these machines. Continuing improvement in the rotor manufacturing equipment has enabled tighter production tolerances to be achieved than was previously possible. Consequently, tolerances of other components, such as the rotor and bearing housings, and the bearings themselves, have to be included in the design process, in order to obtain the best possible overall performance. A mathematical procedure to evaluate rotor interference in screw compressors due to bearing clearances and imperfections in the compressor housing manufacture is presented in this paper and has been used to evaluate how these affect screw compressor behaviour. It is shown that rotor interference resulting from the build-up of these tolerances may affect the compressor when stationary as well as when rotating, and may reduce the compressor efficiency, safety and reliability and also increase compressor noise. This simple but effective method of quantification of the rotor interference can now be applied as a useful criterion in design of screw compressors.

Opportunities for innovation with screw compressors

Abstract It is falsely considered, but widely accepted, that little can now be done to improve the screw compressor process and design. Screw compressors are indeed a mature product today and they form a highly competitive market, especially the oil-flooded air compressor. However, the simultaneous efforts of a large number of manufacturers have resulted in a compact and efficient oil-flooded screw compressor machine. Even small design details now tell the achievement of better compressor efficiency, and there is still some place left for implementation of new methods and procedures that will undoubtedly result in a better final product. New rotor generation procedures, rotor and compressor design optimization for a specified compressor duty and specialized compressor design are innovation elements that are continually improving and developing screw compressors and opening up hitherto unknown ways to produce compressors with better features. Also, the development of a combined compression and expansion machine is reported which could further increase the useful range of application of such machines, especially in refrigeration systems. A review of recent results of computational fluid dynamics (CFD) in screw machines is also given in the paper.

CFD Integrated Design of Screw Compressors

Abstract Positive displacement screw machines are used in variety of applications such as compressors, expanders, blowers, vacuum pumps, liquid and multiphase pumps. To improve their appearance, efficiency and robustness they are designed with the aid of analytical tools, based often on one-dimensional flow models solved by numerical methods that are confirmed by experiment. Continuing demand for further improvements has led to the need for improved assessment of fluid flow losses in the inlet and outlet openings and how these are affected by the shape of the ports, the deformation of machine components due to the effects of pressure and temperature gradients and their effect on performance, the behaviour of multiphase flows and many other effects. These require more advanced analytical procedures, based on three dimensional numerical flow analysis and fluid-structure interaction. The way to estimate these phenomena is to use CFD analysis and to integrate the results with three dimensional CAD systems. As computers become cheaper and faster and advances are made in numerical methods, such techniques are becoming available for everyday use by design engineers. This paper describes how CFD is merged with other design software by means of an integral management system to obtain interactive control of the entire design process of screw compressors. The methods described are of considerable scope and can be applied, not only to screw compressors but also to any other type of twin rotor rotary machines with parallel axes, such as gear pumps, multiphase pumps, vacuum pumps and roots blowers.

A numerical study of fluid solid interaction in screw compressors

Efforts are continually being made to produce screw compressors with smaller clearances in order to reduce internal leakage. However, since the compression process induces large pressure differences across the rotors and temperature rise, they deform. A reliable method of estimating the interaction between fluid flow parameters and rotor deflection is thus needed in order to minimise clearances while avoiding contact between the rotors and the casing. A 3-D mathematical procedure is presented here to generate a numerical grid comprising both solid and fluid domains. This can be used to calculate the fluid flow and compressor structural deformation simultaneously by means of a suitable commercial numerical solver. Simulation results demonstrate the effects of change in working clearances, caused by rotor deformation, on compressor performance.