Oil-free Screw Compressor Research Articles

Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration mitigation in oil free screw compressors

Noise and vibration are problems that are inherent in screw compressors and other Positive Displacement (PD)Machines. This problem is driven partly by the lobe passing frequencies inside these machines that are generated due to the meshing between the gate and main rotors teeth. Because these compressors are operated over a wide speed range and at various loads they usually run at off-design conditions leading to either over or under-compression. In both scenarios, the pressure within the compressor always changes rapidly to match the discharge or process pressure when the discharge opens thus leading to pulsation and noise. To ensure that the compressor is operated without over and or under compression, which also negatively affects the compressor efficiency Hi-Bar Blowers, Inc developed a Shunt Pulsation Trap (SPT) that will contain and reduce the pulsation inside the compressor cavities thus eliminating the use of a silencer at the inlet and the discharge. During the SPT development the CVS Silo King (SKL 1100) compressor was selected for benchmarking this technology. CFD was used extensively to simulate the SKL 1100 without and with the SPT integrated, which allowed the SPT design to be optimized. This paper will present the results obtained to demonstrate that this technology has the potential to eliminate over and under compression while at the same time leading to energy savings and reducing the compressor footprint and the noise and vibration that are commonplace in screw compressors.

Numerical evaluation of an Oil free Screw Compressor’s Suction Port design using Ansys CFX and SCORG

Quasi one-dimensional chamber models and experimental methods are commonly adopted by researchers to predict and validate the performance of the oil free screw compressors. However, to understand the scope for improvement in performance as impacted by compressor ports, one needs to know the details of physical processes happening in compression chamber. This can be done by understanding flow field such as internal temperature and pressure with the help of CFD model.With this CFD setup, comparison of specific power consumption of oil free screw compressor between the axial and radial suction flange is presented in this paper. A hexahedral numerical mesh is used to represent flow field which is generated by SCORGTM. Flow solver ANSYS CFX has been used for study with these single domain deforming rotor grids.This paper presents the methods used for setting up the CFD model and analysis of specific power consumption for two different suction port geometries, an axial and the radial suction flange. At the design operating condition, although axial suction flange seems to be more favourable in terms of lower suction pressure drops at high operating speed, the radial suction flange when simulated using CFD model predicts almost the similar performance. However, radial suction flange gives an added advantage for ready adaptability with the suction elements like the suction filter. The results of the simulation show that the radial suction flange can be adopted for better compatibility with the oil free package and the CFD model used for the analysis can be used for optimization of the air-end at various operating conditions.

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.

Use of CFD to predict trapped gas excitation as source of vibration and noise in screw compressors

This paper investigates the source of noise in oil free screw compressors mounted on highway trucks and driven by a power take-off (PTO) transmission system. Trapped gas at the discharge side is suggested as possible source of the excitation of low frequency torsional resonance in these compressors that can lead to noise and vibration. Measurements and lumped mass torsional models have shown low frequency torsional resonance in the drive train of these compressors when they are mounted on trucks. This results in high torque peak at the compressor input shaft and in part to pulsating noise inside the machine. The severity of the torque peak depends on the amplitude of the input torque fluctuation from the drive (electric motor or truck engine). This in turn depends on the prop-shaft angle. However, the source of the excitation of this low torsional resonance inside the machine is unknown. Using CFD with mesh motion at every 1° rotation of the rotors, it is shown that the absence of a pressure equalizing chamber at the discharge can lead to trapped gas creation, which can lead to over-compression, over-heating of the rotors, and to high pressure pulsations at the discharge. Over-compression can lead to shock wave generation at the discharge plenum and the pulsation in pressure can lead to noise generation. In addition, if the frequency of the pressure pulsation in the low frequency range coincides with the first torsional frequency of the drive train the first torsional resonance mode can be excited.

Performance Characteristics of a 4 × 6 Oil-Free Twin-Screw Compressor

The screw compressor in the early stage of development is generally known as the oil-injection type. However, escalating environmental problems and advances in electronic components have spurred continuous R & D to minimize the oil content in compressed air. The oil-free twin-screw compressor is continuously compressed by inner volumetric change between rotors and casing. For this reason, in order to predict the overall performance of the screw compressor at the early stage of the design process, industry still relies on the empirical method. However, it is difficult using the existing empirical method to gain more information of the inner fluid flow of the twin-screw compressor. Flow simulation techniques using CFD are required. This study presents applications of a recently proposed overset grid method to the solution of the flow around a moving boundary. In order to analyze the performance of a 4 × 6 oil-free screw compressor, the 3-D, unsteady and compressible flow fields were numerically calculated with a shear stress transport (SST) turbulence model, and implemented by the commercial software, Star-CCM+. The pressure distributions were calculated and graphically depicted. Results also showed that the volumetric and adiabatic efficiencies of the screw compressor measured by the experiments were 78% and 71%, respectively.

Numerical investigation on mass and heat transfer in an ammonia oil-free twin-screw compressor with liquid injection

Ammonia is one of the most frequently employed chemical materials, while oil-free screw compressors have recently been recognised as a quite suitable compression tool for an ammonia refinement system. In this paper, a heterogeneous model with mass and heat transfer was developed to investigate the characteristics of an ammonia screw compressor. The mass diffusion equation was solved based on the proper calculation of the physical properties of ammonia water mixtures. Then the model was validated by experimental data using a two-stage compression ammonia refining system. With validated simulation results, the impact of liquid injection on the compressor power consumption and temperature glide was then predicted and analysed. It was predicted that the isentropic efficiencies of the two compressors were 75% and 64% respectively. The simulation results revealed that the mean droplet diameter of the injected liquid should be less than 100 μm in order to guarantee sufficient cooling effect and improve the compressor performance. Finally, an optimization method for the location of injection nozzle and the amount of injected liquid was proposed. A second liquid injection nozzle is recommended to be added and located after the start point of compression process where the temperature glide is maximum. And the optimal flow rate of the new liquid injection nozzle is about 70% of the liquid amount injected to the suction port.

Analysis of performance of two-stage screw compressor under various operating conditions

The screw compressor has a promising application in the process industry due to its appropriate pressure and flow rate. In this paper, a mathematical model of a two-stage oil-free screw compressor is developed to simulate the working process under various operating conditions. The results show that the interstage pressure of the two-stage oil-free screw compressor is built automatically depending on the equal mass flow rate of each stage. It increases with the suction pressure, discharge pressure, and clearance, but decreases with rotating speed. Although the power consumption of the 2nd stage decreases with the suction pressure, the power consumption of the whole compressor always increases with the suction pressure, discharge pressure, rotating speed, and clearance. The volumetric efficiency increases with the suction pressure and rotating speed, but decreases with the discharge pressure and clearance for each stage. The indicated efficiency increases with the rotating speed but decreases with the clearance. A maximum indicated efficiency is found. The volumetric and indicated efficiency of the 2nd stage is more sensitive to the operating condition than that of the 1st stage.

Economics of water injected air screw compressor systems

There is a growing need for compressed air free of entrained oil to be used in industry. In many cases it can be supplied by oil flooded screw compressors with multi stage filtration systems, or by oil free screw compressors. However, if water injected screw compressors can be made to operate reliably, they could be more efficient and therefore cheaper to operate. Unfortunately, to date, such machines have proved to be insufficiently reliable and not cost effective. This paper describes an investigation carried out to determine the current limitations of water injected screw compressor systems and how these could be overcome in the 15-315 kW power range and delivery pressures of 6-10 bar. Modern rotor profiles and approach to sealing and cooling allow reasonably inexpensive air end design. The prototype of the water injected screw compressor air system was built and tested for performance and reliability. The water injected compressor system was compared with the oil injected and oil free compressor systems of the equivalent size including the economic analysis based on the lifecycle costs. Based on the obtained results, it was concluded that water injected screw compressor systems could be designed to deliver clean air free of oil contamination with a better user value proposition than the oil injected or oil free screw compressor systems over the considered range of operations.

The Benefits of Installing Remote IO to a Two Stage Oil Free Screw Compressor Package in an Upstream Oil and Gas Application

A two stage oil free screw compressor package was recently delivered to a major blue chip oil and gas giant for use on one of their oil platforms located in the North Atlantic. During initial design it became clear that the amount of instrumentation required on the package would be substantial. As a result of this an elegant solution was required that would move non-critical inputs and outs (IO) from the control panel to the package itself.This paper first describes the problems caused by the substantial amount of instrumentation required on the package. This description includes how the instrumentation is classed as critical and non-critical and what signals must still be sent over a hard wire and why. As the final location of the package is in an extremely hazardous environment, a highly corrosive marine environment, and the compressor itself is compressing highly combustible hydrocarbon gas, the protection methods employed by the instruments to prevent explosion is then discussed. The same protection methods must also be employed by the control systems IO that have been moved from the safe area to the hazardous area and this is all detailed within the paper.The multiple communications networks that allow the remote IO system to send its values to the package control system and then onto the client's distributed control system are also discussed and how redundancy is built into this network. The paper concludes with how the changes made to this package have resulted in cost savings to the order of tens of thousands of Dollars.

The Design, Selection, and Application of Oil-Free Screw Compressors for Fuel Gas Service

Fuel gas compressors installed in cogeneration systems must be highly reliable and efficient machines. The screw compressor can usually be designed to meet most of the gas flow rates and pressure conditions generally required for such installations. To an ever-increasing degree, alternative sources are being found for the fuel gas supply, such as coke-oven gas, blast-furnace gas, flare gas, landfill gas, and synthesis gas from coal gasification or from pyrolysis. A feature of the oil-free screw compressor when such gases are being considered is the isolation of the gas compression space from the bearing and gear lubrication system by using positive shaft seals. This ensures that the process gas cannot be contaminated by the lubricating oil, and that there is no risk of loss of lubricant viscosity by gas solution in the oil. This feature enables the compressed gas to contain relatively high levels of particulate contamination without danger of “sludge” formation, and also permits the injection of water or liquid solvents into the compression space, to reduce the temperature rise due to the heat of compression, or to “wash” any particulate matter through the compressor.

Analysis of Internal Clearance of Oil-Free Screw Compressor and Its Application to Performance Simulation.

Clearance between male and female rotors and that between the rotors and casing of an oil-free screw compressor in operation are calculated. Surface displacement vectors of both rotors due to, for example, thermal deformation, profile errors, and deviation of the center distance, are converted to such that the clearance between the rotors changes assuming that the rotor surface is approximated by a plane in the vicinity of the calculating point and that the normal line of the plane does not change direction before or after the displacements. The calculation is applied to a computer simulation for predicting air compressor performance. Change in volumetric efficiency related to suction air temperature was calculated and the results agreed well with experimental ones.

Paper 19: The Oil-Free Screw Compressor

The initial development of oil-free screw compressors took place during the 1940s and 1950s. The research work carried out demonstrated that the compressor had a number of highly desirable features, many of which were not obtainable from existing compressor types. In addition, it was determined that the operating range of pressure and volume for which the compressor proved most suitable was effectively between the ranges of reciprocating and aerodynamic compressors. This paper discusses the principles used in designing a range of high-performance, oil-free screw compressors incorporating the knowledge learnt from earlier development work. The effect of operating conditions on performance is described and the limiting parameters identified. Certain effects of multi-staging are described, and a short description given of developments in screw compressors other than those that are oil free.