Twin-screw Refrigeration Compressor Research Articles

Semi-empirical model of the twin-screw refrigeration compressor with capacity control devices

Adopting twin-screw refrigeration compressors with capacity control devices has become the dominant trend to meet varying cooling demands. To analyze the compressor performance with different capacity control methods and further explore the potential, a semi-empirical model with 16 characteristic parameters is proposed. Detailed working processes under the part-load condition (PC) are considered and the corresponding calculations, including pressure loss, power loss, and leakage are innovatively refined. Subsequently, a prototype experiment was carried out for parameters identification and model validation. A good agreement between the experimental and simulated values is attained, with average relative errors of 3.46 % and 1.15 % for shaft power and mass flow rate under the variable frequency condition (VFC) and 3.18 % and 4.63 % under the PC. Thermodynamic simulation results indicate that both shaft power and mass flow rate are linearly correlated with frequency and the efficiencies are nearly stable over the whole interval. However, as the load decreases, the performance decays significantly and the most remarkable change occurs at a position of between 0.9 and 1. The volumetric and adiabatic efficiencies of the VFC are 3.65 and 2.26 times higher than that of the PC when the mass flow rate reaches the lower limit.

Performance evaluation and optimization of the cascade refrigeration system based on the digital twin model

Cascade refrigeration systems (CRSs) have become the priority choice in low-temperature applications, causing severe energy waste due to the lack of practical control strategies. Currently, refrigeration simulations are mainly based on the simplified thermodynamic or Artificial Intelligence (AI) model. However, the former is insufficiently accurate, while the latter is of unclear thermodynamic significance. By connecting the physical and digital space, the digital twin technology provides exciting opportunities for combining thermodynamic and AI methods. In this paper, an integrated digital twin model for the CRS is innovatively proposed based on the modified semi-empirical model of the twin-screw refrigeration compressor. As a major update, the identification and update method of characteristic parameters is developed based on real-time operating data to ensure performance consistency between digital and physical twins. By comparing with the simplified model, results show that the digital twin model can improve prediction accuracy by 7.42%–23.62%. Founded on the proposed model, the thermodynamic performance of the CRS is evaluated. To facilitate on-site applications, the definition of flat intermediate temperature is given, which is fitted to a control correlation with several easily observed variables as the inputs. Taking a food factory as a case study, simulation results reveal that the optimum intermediate temperature should be significantly lower than the original, and the coefficient of performance (COP) has an average improvement potential of 9.1%. By deploying the optimum strategy under the on-site condition, a superior energy-saving rate of up to 13.1% is attained according to test reports.

Performance improvement of twin screw refrigeration compressors for chillers by modifying the suction arrangement

To improve the efficiency of the twin-screw refrigeration compressor at a high speed of about 3000 rpm, the suction arrangement was investigated in this paper. Several simplified CFD models of a twin-screw refrigeration compressor for chillers were established to parametrically analyze the influence of the suction delay, rotor wrap angle, and suction bearing distance. The parametric studies showed that the volumetric efficiency could be improved by 3 % by a suction delay of 18°, and more volumetric efficiency could be improved by shortening the length of it. The optimal delay angle was found to be linear with the rotational speed. Increasing the wrap angle from 300° to 330°, the volumetric efficiency decreased by 2–3 %, and the adiabatic efficiency increased by about 0.2 %. By increasing the distance between the suction bearings and the rotor end face from 5 mm to 80 mm, the isentropic efficiency increases by about 1.4 %. Based on the parametric studies, two modified machines were built and tested by experiments. Experimental results showed that the volumetric efficiency of machines #1 and #2 increased by 1.5 % at 3000 rpm. The isentropic efficiency of machine #2 increased by 0.78 % at 3000 rpm and 1.27 % at 3300 rpm.

Study on non-uniform internal pressure distribution of twin-screw refrigeration compressor

To achieve higher efficiency of twin-screw compressors for refrigeration, the non-uniform internal pressure distribution in the rotor chamber and the flat section in the p-θ diagram were investigated in this paper. A three-dimensional CFD model of a twin-screw refrigeration compressor for chillers was established to analyze the generating mechanism of the flat section in the p-θ diagram. The p-θ diagrams of the compressor were compared under different rotational speeds and working fluids. Analysis results showed that the flat section in the p-θ diagram is caused by the periodic pressure wave propagation and reflection along the working chamber, and the pressure along the chamber is non-uniform when the sound speed is relatively slow compared with the chamber length in one compression cycle. A dimensionless ratio of chamber length and pressure wave travel distance is proposed to estimate the pressure distribution in the working chamber, based on which the flat section is prone to appear in large-scale compressors with low sound speed refrigerants like R134a. Some improvement directions were put forward to avoid efficiency losses, i.e., appropriately delaying the suction end angle and modifying the discharge port to match the pressure distribution.

Study on suction pressure loss near suction end and stagnant pressure rise in p-θ diagram of Twin-screw refrigeration compressor for chiller

• A twin-screw refrigeration compressor is tested under different operating conditions. • Two phenomena of efficiency loss are analyzed and causes are founded. • A three-dimensional CFD model is established to further analyze these phenomena. • Some optimization to avoid these efficiency losses is proved to be effective. To improve the efficiency of the twin-screw refrigeration compressor, two pressure characteristics in the p-θ diagram, such as suction pressure loss near the suction end (SESPL) and stagnant pressure rise in the compression process (CPSPR), were investigated in this paper. A three-dimensional CFD model of a twin-screw refrigeration compressor for chillers was established to analyze the generating mechanism of these phenomena. The p-θ diagram and performance of the compressor were tested and compared under variable working conditions, to verify the simulation results. Analysis results showed that the SESPL is caused by the small flow area between the working volume and suction chamber, and the CPSPR is mainly caused by inflow and outflow in the closed cavity of the vapor injection port. On the basis, some improvement directions were put forward to avoid these efficiency losses, i.e. appropriately delaying the suction end angle and blocking the vapor injection port. Experimental results indicated that the volumetric efficiency and total efficiency both increased by about 2% under different condensing temperatures after delaying the suction end angle of the axial suction port by 18° and blocking the vapor injection port of the compressor.

Experimental investigation on twin screw refrigeration compressor with different capacity control methods

Slide valve and variable-speed control are commonly used in twin screw refrigeration compressors independently to regulate the refrigerating capacity of the compressor. In this paper, the pressure fluctuation characteristics, noise characteristics, and thermodynamic performance of a twin screw refrigeration compressor with slide valve and variable-speed control at the same volume flow rate are tested and compared. The relationship among pressure fluctuation, noise characteristics, and thermodynamic performance is investigated, when the evaporating temperature and the condensing temperature are set to 6 °C and 36 °C, respectively. Results show that the compressor with variable-speed control has better comprehensive characteristics than that with slide valve. The noise of the compressor with variable-speed control is lower than that of the compressor with the slide valve and the reduction of noise reaches 10.52 dB(A) at the volume flow rate of 381.07 m3•h−1. The thermodynamic performance under part-load conditions of the compressor with variable-speed control is better than that of the compressor with slide valve. The improvement of COP and total efficiency reaches 33.57% and 38.16% at the volume flow rate of 381.07 m3•h−1.

Minimum clearance technology to improve performance of twin-screw refrigeration compressors by spraying coating on rotors

During the operating of twin-screw refrigeration compressors, the leakage has a significant influence on the performance of compressors. In this paper, a novel minimum clearance technology that applies the abradable coating to the rotor surface is proposed to lessen the leakage. In order to adapt to the application of the abradable coating, the deformation of rotors during operation is analyzed and a suitable clearance is designed based on the fluid-solid coupling analysis. Then, a suitable coating pre-treated by sandblasting is obtained according to the results of friction coefficient and abradability test. Finally, a prototype is tested and the total efficiency, volumetric efficiency, refrigerating capacity, and COP of the prototype are compared before and after spraying the coating. Test results show that the volumetric efficiency and refrigerating capacity increase significantly after spraying the abradable coating, and the input power decreases slightly. As a result, the COP increases by 6.71% and 7.89% at two nominal operating conditions.

CFD simulation for the internal pressure characteristics of an oil-injected twin-screw refrigeration compressor

3• A 3D model of an oil-injected twin-screw compressor was simulated with CFX. 3• The CFD model accounted well for the pressure in both time and frequency domains. 3• The highest pressure pulsation acted on the inner surface of the rotor housing. 3• The fluid hammer effect was found at the oil injection outlets. The internal pressure characteristics of an oil-injected twin-screw refrigeration compressor was studied using a full 3D CFX model, and the hexahedral moveable meshes of the twin-screw domain were generated by the professional mesh tool SCORG. The gas pressure throughout the whole working process was investigated to disclose its distribution feature. The working pressure at different locations, such as the inner surface of the screw rotor housing, the oil injection paths, the male screw groove, the internal suction and discharge flowing paths, were all captured and analyzed. The simulated pressure inside the screw working chamber agreed well with the experimental results in both time and frequency domains. The pressure pulsations in the compression section acting on the inner surface of the screw rotor housing presented higher levels than the data in the other domains. The highest pulsation level occurred on the location where could record the whole discharge process of the internal ports. Under the high oil flowrate conditions, due to the different influences of the fluid hammer effect, the pressure pulsation increment of the axial oil injection outlet was much larger than that of the radial oil injection outlet. The orifice on the oil path could help reduce the influence of the fluid hammer. Finally, several suggestions were given based on the simulation findings.

Study on the noise reduction methods for a semi-hermetic variable frequency twin-screw refrigeration compressor

Abstract The semi-hermetic twin-screw refrigeration compressors have been widely applied in middle size refrigeration and heat pump systems. The operation noise of the twin-screw refrigeration compressor is becoming a crucial limitation for their applications. Two noise reduction methods for a semi-hermetic variable frequency twin-screw refrigeration compressor were developed. One is the end face attenuation passage and the other is the discharge pipe damping. The former reduces the gas pulsation amplitude in the discharge chamber which is the main noise source. The discharge pipe damping is comprised of several Helmholtz resonators and is installed in the discharge pipe within the compressor. Methods for the flow field and sound field simulation of the compressor were further proposed to verify the noise reduction effect of each method. Moreover, an experimental study was carried out to evaluate the general effect when the two method were applied in a real compressor. Both simulation results and experimental results demonstrated that both noise reduction methods had good effect. According to the experimental results, the actual operation noise level was no more than 85.5 dBA and the mean noise reduction value was in the range of 5.0 dBA–10.0 dBA when the rotational speed increased from 3000 rpm to 5100 rpm.

Noise control of a twin-screw refrigeration compressor

• The noise characteristics of compressor were measured experimentally. • Make full use of structure and put forward optimization measures. • Each optimization scheme is verified by experiments. • Compare the noise value before and after optimization. Twin-screw compressors are widely used in refrigeration systems, but noise is a serious problem during operation. Gas pulsation is the main noise excitation source in twin-screw compressors, and the noise has significant fundamental harmonic characteristics. In this study, the noise characteristics of a screw compressor were experimentally investigated, which provides insight into the noise control technology of the compressor. The analysis incorporated the noise control theory and the structural characteristics of the experimental compressor. The noise reduction schemes including half-wavelength tube, Helmholtz resonator, and multi-cavity series muffler were proposed, and the noise attenuation performances of these methods were studied experimentally. The results showed that application of these noise reduction strategies significantly reduced the noise level by 10.7 dBA, 10.9 dBA, 7.2 dBA, and 10.2 dBA at the male rotor side, the female rotor side, the suction side, and the exhaust side, respectively. Comparative test results indicated that the use of the exhaust muffler and the attenuation scheme on the exhaust bearing block was most effective and it reduced the compressor noise level by 8.17 dBA. These results provided useful guidance for designing a quiet compressor.

Heat transfer simulation in cavity of twin screw compressor under coupling of clearance leakage-heat by utilizing fuzzy beamlet finite element model

In order to optimize structure of twin screw compressor, and improve the working efficiency of twin screw refrigeration compressor, the heat transfer rules of cavity in twin screw refrigeration should be obtained correctly. Because the coupling of clearance leakage flow and heat transfer has a significant effect on heat transfer of cavity medium of twin screw refrigeration compressor, the heat transfer model of cavity medium under clearance leakage-heat coupling is constructed. To promote the computation accuracy and efficiency, the fuzzy beamlet finite element model is constructed by considering fuzzy affecting parameters and taking beamlet transform as interpolating function of element. The heat transfer laws of medium in cavity are obtained based on the theoretical an experimental analysis. Based on the simulation results, the computation error of fuzzy beamlet finite element method has least computation error and diversity than the other two methods for comparison, and the running time of fuzzy beamlet finite element method is least. Heat transfer analysis of medium in cavity under three conditions is carried out, and results show that temperature of cavity medium of twin screw refrigeration compressor is highest when the clearance leakage-heat coupling is considered and agrees better with reality; therefore, the proposed method has higher computing precision and efficiency than selected numerical methods according to computing results and running time. The impact of different clearances on heat transfer of medium in cavity is analyzed based on fuzzy beamlet finite element method, results show that the temperature of cavity medium increases accordingly as tooth top clearance, clearance between teeth, suction end clearance and exhaust end clearance increase, and the suction end clearance and exhaust end clearance have less effect on temperature of cavity medium.

Design and Experimental Research of Noise Reduction Method for Semi-hermetic Variable Frequency Twin-screw Refrigeration Compressor

Design and Experimental Research of Noise Reduction Method for Semi-hermetic Variable Frequency Twin-screw Refrigeration Compressor

Experimental study of lubricating oil impact on pressure pulsation for twin-screw refrigeration compressor

An experimental investigation on pressure pulsations of twin-screw refrigeration compressor was conducted. The pressure pulsation energy at different radial locations on the screw housing, inside the discharge chamber, and the energy transmitting to downstream components during the working process were all tested. Various oil parameters have been investigated to disclose its influence on the pressure pulsation energy of twin-screw refrigeration compressors. With the total oil flow rate increasing, pressure pulsation was found attenuating above the 1st order of pumping frequency, while the amplitude reduction was found depended on the oil injection locations and oil distributions obviously. The final compression pressure inside the screw lobe cavity before the internal discharge port opening was found to be much lower than the expected value based on the built-in radial Vi, and the pressure continued increasing until the axial port opening around 5.45°. Several suggestions about design for lower pressure pulsations were recommended based on test findings.

Characteristicsx and suppression of NVH in twin screw refrigeration compressors

Characteristics of NVH (Noise, vibration and harshness) in twin screw refrigeration compressors, mainly caused by mechanical vibration and turbulent gas flow, are discussed in detail. According to the different generation mechanism, suppression methods of mechanical NVH and hydrodynamic NVH are proposed, respectively. Mechanical NVH is related to rotor profile, bearings, lubricating conditions and maching precision, while hydrodynamic NVH is inevitable and severe for all positive displacement compressors due to gas compression and turbulent flow. Concluded from three cases of NVH suppression for semi-hermetic twin screw refrigeration compressors, half-wave method based on acoustic interference theory, pulsation damper evolving from Helmholtz resonator and broadband perforated tube silencer are effective paths to alleviate hydrodynamic NVH in twin screw refrigeration compressors without bringing in extra energy losses.

CFD Simulation and Experimental Study of Working Process of Screw Refrigeration Compressor with R134a

Twin-screw refrigeration compressors have been widely used in many industry applications due to their unique advantages. The performance of twin-screw refrigeration compressors is generally predicted by one-dimensional numerical simulation or empirical methods; however, the above methods cannot obtain the distribution of the fluid pressure field and temperature field inside the compressor. In this paper, a three-dimensional model was established based on the experimental twin-screw refrigeration compressor. The internal flow field of the twin-screw compressor was simulated by computational fluid dynamics (CFD) software using structured dynamic grid technology. The flow and thermodynamic characteristics of the fluid inside the compressor were analyzed. The distribution of the internal pressure field, temperature field, and velocity field in the compressor were obtained. Comparing the P-θ indicator diagram and the performance parameters of the compressor with the experimental results, it was found that the results of the three-dimensional numerical simulation were consistent with the experimental data. The maximum error was up to 2.578% on the adiabatic efficiency at the partial load working condition. The accuracy of the 3D numerical simulation of the screw compressors was validated and a new method for predicting the performance of twin-screw refrigeration compressors was presented that will be helpful in their design.

Research on Characteristics of Twin-screw Refrigeration Compressor

Research on Characteristics of Twin-screw Refrigeration Compressor

Investigation of characteristics of discharge pressure pulsation in a twin-screw refrigeration compressor

In this paper, characteristics of discharge pressure pulsation in a twin-screw refrigeration compressor are investigated. A thermodynamic model is developed and validated using data from a comprehensive experimental study. This validated model is then applied to investigate effects of key parameters including condensing temperature, compressor rotational speed, super-feed pressure, part-load operation, and design parameters on the discharge pressure pulsation. The results showed that the discharge pressure pulsation was mainly due to periodic variations of mass and energy flow from the working volumes to the discharge chamber. As the condensing temperature increased or decreased from the design condition, the compressor was in either over- or undercompression leading to an increase in the amplitude of the pressure pulsation. The gas super-feed pressure could increase the pressure pulsation at condensing temperatures below the design value and reduce the pressure pulsation at condensing temperatures above the design value. The analysis also demonstrated that the pressure pulsation was lower at part-load conditions. However, the compressor rotational speed increased both the amplitude and frequency of the pressure pulsation. Theoretical analysis of design parameters indicated that a large discharge volume with a high number of lobes could lower the pressure pulsation. These analyses provide useful information for the compressor design and optimization.

Research on axis orbit of the journal bearing lubricated with oil and refrigerant mixtures in a twin-screw refrigeration compressor

In twin-screw refrigeration compressor for high pressure applications, journal bearing has been a preferred option to bear larger and larger radial force. Hence, the stability of rotor-journal bearing system is worth investigating to ensure the reliability of a twin-screw refrigeration compressor, which can be reflected on axis orbit. This paper presents a homogeneous two-phase flow model to calculate the axis orbit of journal bearing used in a twin-screw compressor, considering gas-induced force, unbalanced force and gravitational force. In order to compute gas-induced force acting on bearings by FEM, the p-V indicator diagram is measured experimentally. In addition, axial orbit under different evaporating temperature and different built-in volume ratio was investigated experimentally. Meanwhile it verifies that the proposed two-phase flow model has a good agreement in terms of orbit shape with the experimental result. Moreover, the effects of refrigerant mass fraction at supply groove and structural parameters are analyzed theoretically.

Performance investigation of a pressure pulsation dampener applied in the discharge chamber of a twin screw refrigeration compressor

Intermittent gas flow generates pressure oscillations in the twin-screw refrigeration compressor that cause serious problems such as structural vibration and noise. In order to reduce the amplitude of this pressure pulsation, a pressure pulsation dampener (PPD) applied in the discharge chamber of a twin screw refrigeration compressor was proposed based on the theory of Helmholtz resonator. A mathematical model was developed to design an optimal PPD by incorporating the R134-oil mixture sound speed model and the pressure pulsation simulation model. A comprehensive experimental study was then performed to validate the model and evaluate the effect of key parameters such as oil flow rate and cavity volume on attenuation performance of the PPD. Vibrational characteristics of the compressor equipped with and without the PPD were also measured and compared. Under the design frequency of 250 Hz, the vibrational acceleration of compressor under-chassis reduced by 36.2% to 41.1% when the compressor was fitted with the proposed PPD.

Experimental study of the axial force on the rotors in a twin-screw refrigeration compressor

The high operating pressure of a twin-screw compressor employed in high temperature heat pump or NH3/CO2 cascade refrigeration systems often leads to extremely large axial force on the compressor rotors. Accurate calculation of this axial force is very important in selection of bearing and design of balancing pistons. In this paper, an experimental study of the axial force on the twin-screw compressor rotors was conducted. The variations of the axial force under different operating conditions were investigated. Furthermore, the axial force was also calculated by employing the mean pressure model and sector pressure model, and was compared with that of the experimental results. It was concluded that the sector pressure model is more accurate in predicting the axial force on the rotors in comparison with that of the mean pressure model. However, the mean pressure model was more accurate under operating conditions with large built-in volume ratio. Moreover, the axial force on the rotor end face was found to have larger influence than that on the rotor helical surface.