Elvedin Mujic

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.

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.

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.

Review of Mathematical Models in Performance Calculation of Screw Compressors

The mathematical modelling of screw compressor processes and its implementation in their design began about 30 years ago with the publication of several pioneering papers on this topic, mainly at Purdue Compressor Conferences. This led to the gradual introduction of computer aided design, which, in turn, resulted in huge improvements in these machines, especially in oil-flooded air compressors, where the market is very competitive. A review of progress in such methods is presented in this paper together with their application in successful compressor designs. As a result of their introduction, even small details are now considered significant in efforts to improve performance and reduce costs. Despite this, there are still possibilities to introduce new methods and procedures for improved rotor profiles, design optimisation for each specified duty and specialized compressor design, all of which can lead to a better product and new areas of application. A review of methods and procedures which lead to modern screw compressor practice is presented in this paper. This paper is intended to give a cross section through activities being done in mathematical modelling of screw compressor process through last five decades. It is expected to serve as a basis for further contributions in the area and as a challenge to the forthcoming generations of scientists and engineers to concentrate their efforts in finding future and more extended approaches and submit their contributions.

Experimental investigation on water-injected twin-screw compressor for fuel cell humidification

Water injection in twin-screw compressors was examined in order to develop effective humidification and cooling schemes for fuel cell stacks as well as cooling for compressors. The temperature and the relative humidity of the air at suction and exhaust of the compressor were monitored under constant pressure and water injection rate and at variable compressor operating speeds. The experimental results showed that the relative humidity of the outlet air was increased by the water injection. The injection tends to have more effect on humidity at low operating speeds/mass flow rates. Further humidification can be achieved at higher speeds as a higher evaporation rate becomes available. It was also found that the rate of power produced by the fuel cell stack was higher than the rate used to run the compressor for the same amount of air supplied. The efficiency of the balance of plant was, therefore, higher when more air is delivered to the stack. However, this increase in the air supply needs additional subsystems for further humidification/cooling of the balance-of-plant system.

Noise generation and suppression in twin screw compressors

Screw compressors generate a substantial level of noise during operation. The first stage in reducing it is to identify and minimize its causes. To this end, sources of noise within these machines have been reviewed, the most significant of which are identified, and methods of minimizing their effect are evaluated.

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.

An Evaluation of Encapsulated Oil Flooded Screw Compressor Systems

A study was carried out on oil injected compressors, to assess the value of compressor encapsulation in optimization of the compressor system and eliminating the need for an external oil separator. The analysis was performed using a proprietary software suite for estimating the performance and optimising the design of a screw compressor, when it is subjected to a higher environment temperature, due to its containment in the oil separator. Several features were further investigated by use of 3-D computational fluid dynamics. It was found that full encapsulation seriously worsens compressor performance. Accordingly it should be adopted only when justified by special circumstances. However, the semi-encapsulated compressors behave similarly to bare air ends. Thus, where they are cost effective, their adoption is to be encouraged.Copyright

Noise Control by Suppression of Gas Pulsation in Screw Compressors

The various sources of noise in screw compressors have been determined, the most significant of which are gas pulsations and these have been analysed extensively in this chapter. The parameters most affecting them have been identified and different simulation tools have been used to quantify their effect, together with a brief over‐view of the capabilities of each of them. Resulting from these studies, methods of reducing the pulsations were identified and the improvements resulting from them were predicted. Tests were then carried out on an industrial screw compressor and good agreement was obtained between the predicted and measured levels of noise reduction.

Extending the Role of Computational Fluid Dynamics in Screw Machines

Previous publications show that computational fluid dynamics (CFD) can be readily used for the flow prediction and analysis of screw compressors. Several case studies are presented in this article to show the scope and applicability of such methods. These include solid–fluid interaction in screw compressors, prediction of flow generated noise in screw machines, cavitation modelling in gear pumps, and flow in multiphase pumps for oil and gas industry. Numerical grids for all these cases were generated by the authors using an in-house grid generator, while the CFD calculations were performed with a variety of commercially available CFD codes. In order to validate the accuracy of the CFD calculations, an extended test programme was carried out using laser Doppler velocimetry to measure the mean and fluctuating velocity distribution in screw compressor flow domains. The measurement results are then compared with the CFD simulations. The results confirm the viability of the developed techniques. It is shown in this publication that the flexibility of the developed method creates further opportunities for a broader use of CFD for analysis of twin screw machines in a range of new applications.