Oil Injected Screw Compressor Research Articles

Thermodynamic properties of oil droplets in oil-injected screw compressors and their influencing factors

As a typical positive displacement compressor, screw compressors are widely used in various industrial fields with a fast-growing trend, especially with the boost of the application of oil injection technology. The oil injection process inside the screw compressor is complicated and difficult to be captured by direct observation experiment, so is the accompanying heat transfer process. On the basis of single particle dynamics, the Lagrangian method is used to establish a spatial kinematic model of oil droplets in the simplified chamber of an oil-injected screw compressor, the droplet trajectory is analyzed. The relationship between the oil droplet diameter, injection velocity and temperature and the free flight time of oil droplets in the compression chamber is obtained. In order to investigate the liquid-gas two-phase flow after oil injection, the Level Set Method is adopted to simulate the oil distribution in the compression chamber. The impacting process of oil droplet on a hot wall is simulated, the influence of oil injection parameters such as the injection speed, temperature difference, droplet diameter, and the initial velocity on the heat transfer between oil droplets and hot gas and the wall is analyzed theoretically. For the simulated cases studied, the oil droplet diameter has the greatest effect on the gas-liquid heat transfer, while the temperature difference plays a greater role in the heat transfer between the oil droplet and the hot wall than other factors. The research in this paper helps to further understand and accurately predict the heat transfer process in oil-injected screw compressors, which is important for improving the energy efficiency of screw compressor.

A newly developed dilution system for optical particle size and concentration measurements in oil-injected screw compressors

Oil mist coalescence separators are widely used downstream of screw compressors. Unfortunately, due to high concentration most of the available literature reporting particle sizes in raw or clean gas of screw compressors is on model systems at atmospheric pressure and room temperature. To study oil mist coalescence separators under real-world conditions, the pressurized aerosol needs to be diluted.In this work, two newly developed isobaric and isothermal dilution systems for particle size measurements using an optical particle counter (200 nm–10 µm) in the application scenario are presented. For both systems, one a single-stage and the other a triple-stage system, transfer functions were determined in the laboratory using a pressurized oil atomizer.As a proof of concept, the triple-stage dilution system was used on a compressor test bench to study the influence of operating conditions (varying pressure and volume flow) on the particle size distribution and concentration in raw and clean gas. Both raw and clean gas particle size distributions showed most particles in the size range below 500 nm. The achievable dilution factor of 1000 was sufficient for measuring the raw gas concentration.On a second compressor test bench, the single-stage dilution system was used to study the influence of different coalescence separators on clean gas particle size distribution and concentration. Three separators were used for experiments: one industry-typical reference separator, one with less layers, and one with a weakened entrainment separation fleece. As expected, the particle size distributions showed clear differences, i.e., higher penetration or increased entrainment.Copyright © 2023 American Association for Aerosol Research

On performance optimisation for oil-injected screw compressors using different evolutionary algorithms

Energy consumed for pressurizing air makes a significant proportion of total electrical energy consumption worldwide. To reduce the carbon footprint, it is necessary to have air compressors, which can operate efficiently over a large range of pressures and flow including full load and part load conditions. Several studies have been performed in this area including some which monitor the performance of a large number of compressors to develop strategies for their designs.This paper focuses on the design optimisation of geometrical and oil parameters of oil-injected screw compressors using different evolutionary algorithms such as genetic algorithm (GA), covariance matrix adaptation evolution strategy (CMA-ES), and so on. A comparison of the performance of these algorithms is presented. SCORG and GT-SUITE (commercial software tools for screw compressor thermodynamic simulations and optimisation) are used in the integrated model producing promising results. Feasibility of the optimum outcomes generated by these algorithms is critically evaluated from machine and system design point of view. Finally, in the context of optimisation presented here, the simplex converges fastest as compared to other algorithms. In the future study, the system design limitations are to be incorporated as constraints for the optimisation along with the objective to improve energy efficiency.

Experimental research on influence of interstage oil injection in a dual-motor-driven two-stage screw air compressor

Energy saving for compressed air production is of great importance all over the world due to its great energy cost and the indispensable utility of compressed air in the industrial sector. In recent years, oil-injected two-stage screw air compressors, especially driven by dual motors, are becoming a promising alternative to other compression equipment for their extraordinary efficiency and high reliability in a wide range of applications. However, two-stage screw air compressors have a different and complicated oil injection system compared with single-stage compressors. And an appropriate setting of oil injection parameters might result in a considerable performance improvement of the compressor. In this paper, a dual-motor-driven two-stage oil-injected screw compressor was tested under the actual service conditions and control modes experimentally. The effect of interstage oil injection on the compressor performance was studied in detail by varying the interstage oil injection flow rate and rotating speed. The performance parameters such as specific power, volumetric efficiency and adiabatic efficiency were calculated from the measured data. The results indicate that an excess amount of oil exists in the second stage compressor and reducing the oil moderately could decrease the power consumption of the two-stage air compressor unit.

Reconstruction and operation of the helium purification system in the cryogenic system for EAST Tokamak

EAST (Experimental Advanced Superconducting Tokamak) cryogenic system has participated in the physical experimental campaigns over 15 years since the first cool-down experiment in 2006. Some fault of the heat exchanger blocking due to the purification of gas helium has occurred several times during the cryogenic system’s operation since 2017, which caused the system warmed up to deal with the malfunction. Therefore, the helium gas purification system including the oil-removal system after the oil-injected screw compressors, the adsorbers inside the cold-box and two sets of external purifiers as well as the control system has been redesigned and reconstructed from 2019 to 2020, and put into operation in the 18th cool-down experiment of EAST cryogenic system at the beginning of 2021. The process flow of new helium purification system and the equipment parameters together with the reconstruction process will be presented in detail in this paper. Meanwhile, the commissioning operation results will be illustrated to show the purification performance good and the reconstruction successful.

Reducing rotors clearance - a way to increase the performance of a screw compressor

The screw compressors due to their constructive simplicity and the known advantages in operation is increasingly demanded on the market. Many companies are producing this type of compressor. The influence of leakages – conditioned by rotors clearances – on compressor performance is well known. The literature addresses the problem of coatings, but for dry compressors. Our intention is to apply this technology in the case of oil injected screw compressors (when the profile deviation is large or in the case of rotors with slight wear). Tests were performed to evaluate the performance of a screw compressor with uncoated rotors and performance of compressor assembled with coated rotors (both pairs of rotors have the same profile deviations – the profile accuracy measured is -8 ÷ -10 μm). The test aim to highlight only the influence of the coating layer – by reducing clearances – on the performance of the compressor, without performing further quality test of the coated layer. The quality of the coating process is the subject of additional research. We intend to use this technology for oil injected screw compressors both for new manufactured rotors (with large profile tolerances) and rotors with slight.

Investigation of Screw Compressors for Low Pressure Ratio Applications

Screw compressors have been widely investigated for many applications including air and process gas compression, and refrigeration systems. There is however a surprising lack of literature for low pressure ratio application of these machines, defined here as application requiring volume ratios in the range of 1-1.5. The aim of this paper is to characterise the loss mechanisms for oil injected screw compressors with low volume index (defined as the ratio of maximum to minimum volumes during the internal compression process), V i. This knowledge will be applied in the future to identify appropriate optimisation of rotor profiles and porting geometry for a range of low-pressure ratio applications. The current study involves the use of chamber models to investigate the influence of wrap angles and porting. This initial analysis will be developed in future work to allow a detailed parametric study of the factors that limit the performance of low V i oil-injected screw compressors.

Estimation of radial shaft seal, oil drag and windage loss in twin screw oil injected compressor

The objective of this paper is to study the mechanical power loss arising from radial shaft seal, oil drag, and windage within the twin-screw oil-injected screw compressor. The previous papers in the series presented the evaluation of power loss due to the gears and bearings where it was found that the calculated total power loss was underestimated when compared to the experimental results for different sizes of compressors and different operating conditions. This indicates that few additional contributing elements were missing and needed due consideration to enhance the prediction. Literature review of the basic fluid dynamics and transmission gears confirms that oil drag loss and windage loss can also contribute to the power loss considerably.This paper presents a review of a few methods for the prediction of the radial shaft seal, oil drag, and windage loss. Predictions with the use of these methods and their comparison with the experimental results are analysed in this paper. Experimental results of three different sizes of the compressors tested in laboratory conditions were taken as reference for the comparison at different operating speeds and pressure ratios. The analysis helps to understand the contribution of each element of the power loss which can help a designer to optimally design a screw compressor.

Numerical investigation of oil injection in screw compressors

Oil injected screw compressors are commonly used in the industry. The position and amount of oil injection are determined experimentally or by heat balances with an assumption of uniform oil distribution in the compression chamber, which is not validated. This paper presents a study on the oil distribution within a screw compressor chamber using Computational Fluid Dynamics (CFD) with Volume of Fluid (VOF) multiphase model. Prerequisite for using CFD in screw compressors is a body fitted numerical mesh produced by software SCORG. The analysis was carried out on a newly designed screw compressor with a 4–5 lobe combination. It was found out that injecting the oil through two separate ports , one on each rotor, has advantages compared to the injection through a single port. The injection ports are conveniently positioned to ensure an even oil distribution within the compression chamber. If the same amount of oil is injected through the two separate ports instead of the originally designed single port , the maximum gas temperature within the chamber could be reduced by 30°–35 °C leading to reduction in specific power by 1.8%. Techniques used in this study can improve the performance and efficiency of a wide range of screw compressors.

Energy improvement of an oil injected screw compressor skid

Eficientizarea energetică a unui skid compresor cu şurub, după 80.000 de ore de funcţionare, prin introducerea unei pompe de ungere destinate circulaţiei uleiului în instalaţia de ungere și injecție, eliminând necesitatea de a mări presiunea de refulare prin strangularea refulării, pentru ca uleiul să poată circula prin instalaţie datorită presiunii din vasul separator. Astfel, motorul […]

Analysis of Oil-Injected Twin-Screw Compressor with Multiphase Flow Models

Growing demands for energy are motivating researchers to conduct in-depth analysis of positive displacement machines such as oil-injected screw compressors which are frequently used in industrial applications like refrigeration, oil and gas and air compression. The performance of these machines is strongly dependent on the oil injection. Optimisation of oil has a great energy saving potential by both increasing efficiency and reducing other impacts on the environment. Therefore, a three-dimensional, transient computational fluid dynamics study of oil injection in a twin-screw compressor is conducted in this research. This study explores pseudo single-fluid multiphase (SFM) models of VOF (Volume of Fluid) and a mixture for their capability to predict the performance of the oil-injected twin screw compressor and compare this with the experimental values. SCORGTM (Screw Compressor Rotor Grid Generator) is used to generate numerical grids for unstructured solver Fluent with the special interface developed to facilitate user defined nodal displacement (UDND). The performance predictions with both VOF and mixture models provide accurate values for power consumption and flow rates with low deviation between computational fluid dynamics (CFD) and the experiment at 6000 RPM and 7.0 bar discharge pressure. In addition, the study reflects on differences in predicting oil distribution with VOF, mixture and Eulerian-Eulerian two-fluid models. Overall, this study provides an insight into multiphase flow-modelling techniques available for oil-injected twin-screw compressors comprehensively accounting for the details of oil distribution in the compression chamber and integral compressor performance.

Characterization of oil injected screw compressors and air/oil separators at realistic operating pressures

AbstractAn experimental setup to determine the particle size distribution (PSD) of a pressurized oil aerosol (p<10 bar) was validated. The combination of optical particle counter (OPC) and scannin...

Use of CFD to Investigate Flow Characteristics and Oil Distribution Inside an Oil-injected Screw Compressor

The well-developed CAE tools are used to investigate flow characteristics and oil distribution inside an oil-injected screw compressor. The flow field of gas mixed with oil is calculated by ANSYS CFX. TwinMesh generates high quality hexahedral grids for the compression chambers. There are two cases studied in this study. In the first case, the compressor models under four loading conditions are analyzed. The pressure curve is used to see if the designed built-in volume index is appropriate. In the second case, the compressor models under oil-free and oil-injected conditions are analyzed. The pressure gradients on rotor surfaces and velocity vector fields are used to describe the effects of lubrication and sealing in the sealing line gap, the tip gap, and the outlet end gap of rotors. The designer could optimize the slider and know if the oil is properly distributed in each gap.

Waste-Energy (Heat) Recovery System from the Gases Compressed by an Oil-Free Screw Compressor

During compression, a gas heats up, almost in all cases this heat being wasted, either by cooling the gas because it is too hot for the application, or by storing the gas and letting the compressed gas cool naturally in the storage tank. This paper presents a waste-energy (heat) recovery system from the gases compressed by an oil-free screw compressor. The gases compressed by this compressor have a very high temperature compared to an oil injected screw compressor, due to the fact that the oil used to lubricate the rotors also acts as a heat sink, the oil free variant which is used when you want a very high purity of the gas, has higher tolerances and more friction between the rotors which result in a higher gas temperature. The recovery system uses a heat exchanger to extract the waste energy from the gas and at the same time it will cool it for immediate use. Depending on the requirements, the energy recovered may be used immediately to produce useful work or stored for a later use. It may be used for heating a building, to produce steam for a turbine driving electrical generator, or in other forms.

Identification and analysis of screw compressor mechanical losses

Screw compressors are compact machines, used for a wide range of applications where gases or vapours are required to be delivered at moderate pressures with high efficiency and reliability. They are most effective when the compressed medium requires power inputs, approximately in the 10 kW - 1-2 MW range. At lower inputs alternatives such as reciprocating and scroll compressors are preferable and at higher inputs turbo-compressors are more suitable.In industrialised countries, compressors absorb 15-20% of the total electrical power generated. Hence there is a continuing demand to improve their efficiency. This is normally expressed as the specific power consumption, which is the power required to compress unit mass of gas delivered.There already exist mathematical models to assist in the design of such machines and to estimate their performance, which include the estimation of the dynamic loads acting on the rotors and bearings and these loads determine their mechanical efficiency. However, these models do not estimate the magnitude of the mechanical losses, which are only guesstimated as an additional increment to the power required to compress the gas. Such an approximation does not enable the optimum selection of bearings and lubricating oil to minimise the frictional power losses.The aim of the study, described in this paper, was to estimate the effect of the individual parameters responsible for mechanical power loss in oil injected screw compressors and is focussed on the losses incurred in the gear box, bearings and shaft seals.It was found that in the gearbox, meshing, bearing and seal losses all increase both with speed and gear ratio. In the main rotors, it was found that sliding friction losses in the bearings are not significantly affected by the radial load, nor are rolling friction losses significantly dependent on the axial load. However, both axial and radial loads have a significant effect on the total frictional power loss. Lubricant viscosity affects the frictional power losses but the oil level does not.

Thermal expansion in liquid-injected screw compressors

In screw compressors gaps between rotors and casing have to be small, in the range of 30 to 300 μm. These gaps are influenced by thermal expansion when the screw compressor is in operation. The amount of thermal expansion is about the same size as the gaps, even in oil-injected screw compressors. Especially when starting a cold compressor, there are temperature differences. These temperature differences were measured in a 22 kW oil-injected screw compressor for industrial compressed air as an easily manageable example. Measuring the temperatures in the rotor teeth was done with Pt1000-sensors and transmission with Wireless-LAN. The results showed lower temperature differences than expected. The influence of the oil-injection-temperature is dominant. In bigger screw compressors the temperature differences tend to be larger because of dimensional reasons: A simple numerical model for the temperature differences in a screw compressor with fluid injection allows an extrapolation for larger rotors.

Mathematical modeling of working processes of variable frequency screw compressor with differentiated oil supply into the working chamber

Currently, a method of screw compressors regulation by smooth change of rotors spinning frequency, or so-called frequency regulation, is widely used. The operation process of oil-injected screw compressors at frequency regulation modes were examined in this paper in terms of the relationship of a compressor operation modes and its oil system. The calculations based on the operation process mathematical model of oil-injected screw compressor were aimed at defining its integral characteristics when changing the rotor spinning frequency, the quantity of the oil supplied and the discharge pressure.

Dynamic modelling and PID loop control of an oil-injected screw compressor package

A significant amount of time is spent tuning the PID (Proportional, Integral and Derivative) control loops of a screw compressor package due to the unique characteristics of the system. Common mistakes incurred during the tuning of a PID control loop include improper PID algorithm selection and unsuitable tuning parameters of the system resulting in erratic and inefficient operation. This paper details the design and development of software that aims to dynamically model the operation of a single stage oil injected screw compressor package deployed in upstream oil and gas applications. The developed software will be used to assess and accurately tune PID control loops present on the screw compressor package employed in controlling the oil pressures, temperatures and gas pressures, in a bid to improve control of the operation of the screw compressor package. Other applications of the modelling software will include its use as an evaluation tool that can estimate compressor package performance during start up, shutdown and emergency shutdown processes. The paper first details the study into the fundamental operational characteristics of each of the components present on the API 619 screw compressor package and then discusses the creation of a dynamic screw compressor model within the MATLAB/Simulink software suite. The paper concludes by verifying and assessing the accuracy of the created compressor model using data collected from physical screw compressor packages.

Design and evaluation of rotor clearances for oil-injected screw compressors

Designing twin-screw compressors to safely operate at higher than normal temperatures poses a challenge as the compressor must accommodate larger peak thermal distortions while maintaining efficiency at nominal operating conditions. This paper will present a case study of an oil-injected compressor tested at elevated discharge temperatures with original and revised clearances. A procedure is presented to use boundary conditions derived from a chamber model to approximate component temperature distributions that are then used to predict possible thermal distortions and the resulting effect on clearance gaps. The original and revised clearance designs are evaluated and performance penalties incurred due to the modifications are discussed.

Performance Testing of Jefferson Lab 12 GeV Helium Screw Compressors

Oil injected screw compressors have essentially superseded all other types of compressors in modern helium refrigeration systems due to their large displacement capacity, reliability, minimal vibration, and capability of handling helium's high heat of compression. At the present state of compressor system designs for helium refrigeration systems, typically two-thirds of the lost input power is due to the compression system. It is important to understand the isothermal and volumetric efficiencies of these machines to help properly design the compression system to match the refrigeration process. It is also important to identify those primary compressor skid exergetic loss mechanisms which may be reduced, thereby offering the possibility of significantly reducing the input power to helium refrigeration processes which are extremely energy intensive. This paper summarizes the results collected during the commissioning of the new compressor system for Jefferson Lab's (JLab's) 12 GeV upgrade. The compressor skid packages were designed by JLab and built to print by industry. They incorporate a number of modifications not typical of helium screw compressor packages and most importantly allow a very wide range of operation so that JLab's patented Floating Pressure Process can be fully utilized. This paper also summarizes key features of the skid design that allow this process and facilitate the maintenance and reliability of these helium compressor systems.