Oil Drag Research Articles

Experimental validation of the screw compressor oil drag model for various rotor profiles

Injecting oil inside the compressor chambers of the oil-flooded, twin-screw compressors has several advantages. Oil cools the compressing fluid upon mixing with it and hence the compression process is brought nearer to the ideal isothermal compression process. The oil also serves as a lubricant between the meshing rotors and other clearance gaps in the compressor. The thin film of oil formed in the clearance gaps prevents internal leakages too; enhancing the volumetric efficiency of the compressor. Among these desirable effects of injecting oil in screw compressors, there is an undesirable effect too. The interaction (friction) of oil films formed in various clearance gaps with the rotors leads to a drag power loss. Recent studies such as Abdan et al. have proposed more detailed and accurate methods to estimate the oil drag losses in screw compressors. These methods enable the modelling of the effect of even minor changes in rotor profile on the drag loss power. Predictions of this model were hence used to tweak the screw rotor profiles with an objective to reduce the oil drag losses. Such profiles were then retrofitted in the existing machines and tested. Comparing the differences in power consumption of these machines, the component of oil drag loss was deduced. The experimental results show close agreement with the oil drag loss prediction model. The reduction of oil drag losses through profile modifications led to an improvement in the specific power of oil-flooded screw compressor.

Oil drag loss in oil-flooded, twin-screw compressors

Rotary twin screw compressors are widely used because of their high efficiency and reliability. Their most common mode of operation is as oil-flooded machines when delivering air and gases at moderate pressures and flow rates. In order to achieve the best performance, it is essential to be able to predict the optimum amount of oil, required for the oil injection process, accurately. Analytical procedures for the design and performance estimation of twin screw compressors are well developed and widely available, but the determination of oil drag losses, in oil-flooded machines is only guesstimated. This paper describes a more detailed and accurate procedure for estimating oil drag loss, using a combined Couette-Poiseuille flow model and gives the results of studies on three sizes of machines operating over a range of pressure ratios and speeds. To this end, a parametric analysis has been developed based on a combined Couette-Poiseuille flow model and has been used to estimate the individual effects of pressure ratio, the various clearances and the oil viscosity on the total drag loss, for different sizes of the compressor. It can be seen from the results that at pressure ratios of up to 8.5, the drag loss due to the discharge axial clearance gap is nearly 2/3rd of the total, while nearly 1/3rd is due to the radial clearance. At normal operating speeds, the loss due to the interlobe clearance is insignificant, but as the pressure ratio increases, this rises more rapidly than that due to the axial and radial losses. The gain in the drag loss due to greater oil viscosity becomes more significant as the compressor size is increased. In larger machines, when clearance values are increased, the radial and axial elements of the drag loss are reduced more rapidly than that due to the interlobe loss.

Experimental Study on Heavy Oil Drag Reduction in Horizontal Pipelines by Water Annular Conveying

Transportation of heavy oil by the so-called water-ring technique is a very promising method by which pressure drop and pollution can be significantly reduced. Dedicated experiments have been carried out by changing the phase’s density, viscosity, velocity and interfacial tension to systematically analyze the characteristics of the water ring. On the basis of such experimental data, a mathematical model for pressure drop prediction has been introduced. This research shows that as long as the density of oil and water remains the same, a concentric water ring can effectively be formed. In such conditions, the oil-water viscosity difference has little effect on the shape of water ring, and it only affects the pressure drop. The greater the viscosity of heavy oil, the smaller the pressure drop of the oil-water ring transportation system. The influence of phases’ interfacial tension on the characteristics and pressure drop of the heavy oil-water ring can be considered negligible. The pressure drop prediction model introduced on the basis of the Buckingham’s principle provides values in good agreement (95%) with the experimental data.

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.

Design, Testing and Feasibility Analysis of an Oil-Free Twin Screw Compressor with In-Chamber Flash Cooling

With the current aim of reducing CO2 emissions worldwide compressor efficiency is more in focus than ever before. An estimated 20% of all electricity generated worldwide is used for compressing air. Oil injected twin screw compressors form a large percentage of the total. The injected oil is used for cooling, as well as for sealing and lubrication of the rotors. Typical pressure ratios lie between 8:1 and 16:1. There are many investigations being conducted into oil-free screw technology for specific applications requiring no oil carry over. In this presentation we will take a close look into the possibility of producing a single stage oil-free twin-screw compressor operating at a pressure ratio of 8:1 and using evaporated water for cooling. The main objective in this case is to improve compressor efficiency. Additionally, we will compare efficiency results with a state-of-the-art single stage oil injected twin screw compressor. Previous results from an earlier paper by the author and team from City University of London on CFD analysis of a similar system are taken as the basis. Further simulated calculations using SCORG for the actual case in question allow preliminary performance analysis using the multi-domain chamber model. From simulation we can see that the oil drag force acting on the rotors forms a significant part of the total power requirement. It is therefore suggested that an oil-free solution will achieve improved specific power. To confirm this concept, verify the simulated calculations and understand the heat transfer process in the compression chamber more fully, experimental analysis was undertaken.

STUDI EKSPERIMENTAL PENGARUH KECEPATAN ALIRAN MASUK, SPLIT – RATIO DAN DIAMETER VORTEX FINDER TERHADAP UNJUK KERJA LIQUIDLIQUID CYLINDRICAL CYCLONE (LLCC) SEPARATOR

The large number of ports in Indonesia generate a lot of waste oil that contaminate the ocean around the port. However, the pollution prevention is not supported by a good facility. Hydrocyclone separator can be used as an alternative to supporting conventional separataor technologies such as vessel type separator or gravitational based separator to prevent the waste oil pollution. The purpuse of this study is to determine the effect of split ratio, inlet mixture velocity, and vortex finder diameter on the performance of the liquid-liquid cylindrical cyclone (LLCC). Oil (Kerosene) and water were selected as the working fluid and a red color (dye) was given to the oil to enable visualization . Performance of LLCC determined from watercut that produced from separation process. The examined inlet mixture velocity are 0.7 m/s, 0.9 m/s and 1.1 m/s with split-ratio 5%, 15%, 30%, 45%, 60%, 75% and 90%. . All of them are tested with vortex finders that have diameters 18 mm, 22 mm and 27 mm with 0 mm depth. The oil volume fraction inlet is kept constant at 25%. The result showed that LLCC was capable to separating oil-water mixture and produced watercut on underflow reaches 100%. The increasing of split-ratio will increase the watercut until reached the maximum value for all diameter of vortex finder. The condition when the oil content begin to move to the underflow on certain split-ratio is called oil drag. The split-ratio that used before oil drag occurs is called Optimum Split Ratio (OSR). LLCC should operated on OSR to obtain the best performance. The OSR is shifted to the higher split-ratio for higher inlet mixture velocity. The increasing of vortex finder diameter did not change LLCC performance significantly.

Planetary gear sets power loss modeling: Application to wind turbines

Power losses in any geared-transmission can be attributed to gear-mesh and bearing-contact frictional effects and the interaction of rolling element bearings and gears with the lubricating medium. In this paper, a composite power loss model combining a non-uniform load distribution model with a local friction coefficient at any point of contact and oil drag formulations has been developed. A design sensitivity study has been carried out that examines how the design parameters of planetary gear sets and bearings and lubricant properties influence performance. The accuracy of these models was assessed by comparing predicted results to published measured values taken from planetary gear sets and parallel axis gear set experiments.

An Improved Quasi-Dynamic Analytical Method to Predict Skidding in Roller Bearings under Conditions of Extremely Light Loads and Whirling

Slip often occurs in high-speed and light-load roller bearings (HSLLRBs) when the frictional drive force is inadequate to overcome the drag forces between the rolling elements and the raceway. Formerly, skidding analysis of HSLLRBs considering bearing whirling based on a simplified method using the Dowson-Higginson empirical model, although the analytical results of the cage slip fraction show significant discrepancies with the experimental data, as for extremely light radial loads. One of the main reasons is the inaccuracy of the evaluation of oil drag forces using the empirical equations. In this study, the elastohydrodynamic lubrication (EHL) method was adopted to calculate the oil film thickness and pressure distribution of HSLLRBs, so as to obtain more accurate oil drag forces. The cage speed and cage slip fraction were obtained by combining the whirl orbits, drag forces, load, kinematic equations and other related equations and then solved using the Newton-Raphson method. The skidding mechanism was investigated in terms of various operating parameters such as whirl orbit radii, radial load and viscosity. The results showed that the cage slip fraction and cage speed oscillate over time because of the whirl, which leads to an increase in the risk of bearing skidding damage. Under the extremely light load and high speed, the slip and influence of the whirl on bearing skidding increases as the whirl radius and radial load increases, while the viscosity shows a reverse trend. Therefore, in order to reduce slip and skidding damage, the whirl radius and radial load should be decreased suitably, while the viscosity should be increased moderately. A comparison between the calculated and experimental results shows that the proposed method is both feasible and valid.

Bioinspired micro/nanostructured surfaces for oil drag reduction in closed channel flow

Living nature contains amazing structures and materials which provide inspiration for bioinspired solutions that can solve many of mankind's greatest challenges. Two such challenges include reducing fluid drag and promoting self-cleaning. With inspiration from promising flora and fauna, a systematic study has been conducted using a combination of actual and replica samples. These include rice leaf, butterfly wing, fish scale, and shark skin samples, as well as laser etched riblets. In order to mimic properties from actual rice leaves, select replica and riblet samples received a nanostructured coating. Drag is determined by pressure drop measurements from sample lined rectangular closed channels with laminar oil flow. Oleophilic surfaces with appropriate micro/nanostructured surfaces may allow the formation of a thin oil film believed to reduce drag. A conceptual model is presented describing the role of surface structures allowing the formation of such a drag reducing thin oil film.

Paper 26. Synchromesh Mechanisms: Experience of Heavy Truck Gearboxes

The design characteristics of different synchronizers with comparisons of designs, performance, and dependability in operation are discussed. The theory of operation is dealt with in detail. The influence of experience in practice on design from the effects of clash, hard gear changing, wear, clutch and oil drag are included. Suitable materials, machinery processes, and interdependence of dimensions are described. The paper concludes with notes on practical and laboratory tests and future development.

The Thermal Rating of Worm Gearboxes

The paper deals with the factors affecting the temperature rise of totally enclosed self-lubricated gearboxes, with particular reference to worm gearboxes, and is based on observations obtained from a power circulating apparatus through worm gears which has provision for the accurate measurement of efficiency and temperature rise under variable load and speed. The theory underlying the heating and cooling of gearboxes is discussed, for gears running under continuous load and also under a repeated cycle of intermittent load. Temperature rise depends on the heat-dissipating capacity of the gearbox and the power losses within the box; heat-dissipating capacity is dealt with in relation to surface area of the box, speed of the gears, and artificial cooling by air fan; power losses are discussed under the headings of efficiency and oil drag losses. It is shown that gear speed and turbulence in the lubricant contribute considerably to heat-dissipating capacity, and that oil drag losses play an important part, particularly on large gears running at moderate or high speed. Cooling by air or other means is shown to result in an increase in power capacity (for a given allowable temperature rise) much more than in proportion to the increase in heat-dissipating capacity of the box, owing to a higher overall efficiency when transmitting heavier loads. Results of worm gear efficiency tests carried out in the past on the Daimler-Lanchester testing machine at the National Physical Laboratory on the author's design of worm gear, which gave the highest efficiency of any published tests carried out on this machine, are reconsidered in the light of recent work and it is contended that the National Physical Laboratory machine gives efficiency figures which are in general higher than the true efficiency.

On the mechanism of boundary lubrication. I. The action of long-chain polar compounds

The effect of long-chain polar compounds on the coefficient of kinetic friction under boundary conditions has been studied using the Boerlage four-ball friction apparatus in various modifications. With steel balls of the highest grade, coefficients of friction for a great number of lubricants were measured as a function of the relative velocity of the rubbing surfaces. The structure of thin films of these lubricants rubbed on polished mild steel surfaces was investigated by electron diffraction. It was found that lubricants showing little or no surface orientation had a constant coefficient of friction of about 0.1 over the available velocity range from 0 to 1 cm./sec. With oils which showed high surface orientation imparted by addition of long-chain polar compounds, a sudden decrease of the coefficient of the friction was observed at various velocities of the sliding surfaces, depending upon the com pound used. Investigation of a great number of compounds gave a direct correlation of this effect with molecular orientation: those compounds causing the effect to occur at the lowest velocities were found to be most highly oriented with their carbon chains most nearly perpendicular to the surface. Since such a change of the coefficient of friction can only be explained by the wedging of oil under the surface (oil drag), the effect was termed the ‘wedging effect’ leading to a type of lubrication which may be called ‘quasi-hydrodynamic’. By measuring the electrical resistance between the sliding surfaces it was found that the regions of sudden decrease of the coefficient of friction correspond to a change from metallic contact to extremely high resistance. The investigation shows that long-chain polar compounds act primarily by inducing the ‘wedging effect’ and not by giving a direct protection to the surface.