Oil-free Linear Compressor Research Articles

Design and component development of a 17 l/d liquefier for helium recondensation

This paper describes the design and component development of a 17 l/d liquefier for helium recondensation. The liquefier uses an oil-free linear motor compressor with suction and discharge pressures at 1 and 22 bar respectively. The liquefier is a hybrid cryocooler consisting of an interchanging component made up of recuperative heat exchangers, an 80 K pulse tube cooler (PTC) and a 25 K two stage PTC. The cooling powers required of the PTC's are 14.2 W at 80 K and 1.6 W at 25 K. The primary cooling mechanism is two stage J-T expansion at 22 bar and 11 bar resulting in liquefaction with a liquid fraction of 0.19. This corresponds to a cooling power of 0.54 W at 4.2 K and a liquefaction rate of 17.8 litres per day. An overview of the design and development of the PTC's, heat exchangers and JT valves is presented.

Comparative study on energy efficiency of low GWP refrigerants in domestic refrigerators with capacity modulation

The ever rapid rise in the number of refrigerators causes a dramatic increase in energy demand and emissions, which represents a small but growing contribution to overall global warming. To mitigate the greenhouse gas emission and energy efficiency, high efficiency linear compressor with capacity modulation and low global warming potential (GWP) refrigerants can be considered for use in refrigerators. This paper presents a comparative study on energy efficiency of R717 (ammonia), R600a (iso-butane), R1234yf and R134a in domestic refrigerators using an oil-free linear compressor. The results show that for given operation conditions, R717 has the highest resonant frequency, power input, cooling capacity while R600a has the lowest. R717 has the potential to be used the proposed linear compressor where there is no contact between the corrosive refrigerant with copper windings. This can significantly improve the energy efficiency ratio (EER) with low cost and minimal environmental impact compared with R134a. R600a requires a larger capacity compressor to achieve the same cooling capacity as R717, R1234yf, and R134a. R1234yf can be considered as an ideal drop-in alternative to R134a for an existing refrigerator because of the similar system performance to R134a.

Experimental study of R1234yf as a drop-in replacement for R134a in an oil-free refrigeration system

R1234yf is a synthetic refrigerant with global warming potential (GWP) of 4 and similar thermodynamic properties to R134a. This paper experimentally compares the performance of R1234yf with R134a in an oil-free vapour compression refrigeration (VCR) system. The oil-free VCR system consisting of oil-free linear compressors, an off-the-shelf condenser and an evaporator with an electric heater avoids the impact of oil lubricant on the heat transfer so that the two refrigerants can be appropriately compared with each other. Experiments for two refrigerants were carried out for compressor strokes of 9–13 mm, operating frequency of 32–38 Hz, pressure ratios of 2–4, and condenser temperatures of 40–50 °C with refrigerant charge of 250 g. The experimental results show that the coefficient of performance (CoP) of R1234yf is 20% lower than R134a with condenser temperature of 40 °C and evaporator temperature of 0 °C. The volumetric efficiency of R1234yf is 5% lower than R134a with condenser temperature of 40 °C and evaporator temperature of −1.5 °C. Results of evaporator pressure drop, superheat, power input, and cooling capacity are also reported.

Comparisons between heat pipe, thermoelectric system, and vapour compression refrigeration system for electronics cooling

Passive systems such as air for electronics cooling have now effectively reached their limits. This paper evaluated three comparable systems for electronics cooling, including heat pipe (HP, passive system), thermoelectric (TE) and vapour compression refrigeration (VCR) systems (active systems). Mathematical model has been built for the heat pipe and the thermoelectric system respectively. Measurements have been conducted to validate the model and to compare the performance among a HP, a single stage TE system and a two-stage TE system, a combination of the HP and the TE system, and a VCR system using an oil-free linear compressor. Close agreements between the modelling and measurements have been achieved in terms of electric power input and cooling capacity at various temperatures. The HP improved the cooling capacity and the coefficient of performance (COP) of the TE system by 53% and 42% respectively at a cold end temperature of 10 °C. Heat pipe is more attractive for cooling large devices at higher temperatures. Two-stage TE system can be used for cooling devices at lower temperatures. VCR system is capable of dissipating much higher heat flux (200 W/cm2) at lower temperature than all other technologies.

Analysis of oil-free linear compressor operated at high pressure ratios for household refrigeration

Compared with conventional reciprocating compressor for vapour compression refrigeration (VCR) system, linear compressor offers higher energy efficiency and oil-free operation, which allows the use of mini/micro-channel heat exchangers. However, there are key challenges when oil-free linear compressors are used for household refrigeration with typical high pressure ratios (above 10), such as high clearance loss, high piston offset, and very non-linear gas spring. Previous papers by the author have demonstrated the feasibility of oil-free linear compressor for electronics cooling at lower pressure ratios (below 3.5). This paper presented comprehensive analysis of these issues as a key step towards developing oil-free linear compressor for household refrigeration. The model of non-linear gas spring at high pressure ratios is validated by measurements of a previous prototype linear compressor with minimum flow. Piston offset can be effectively controlled by solenoid valve at 1 Hz. Gas leakage increases by a factor of 2.5 if the piston is fully eccentric in the cylinder. The gas leakage loss can be 27% of power input for pressure ratio of 13.6 using R600a.

A compact refrigeration system based on multijet sprays for electronics thermal management

The present study integrates a spray impingement heat sink and a compact vapor compression cooling system driven by a R-134a small-scale oil-free linear compressor. The cooling strategy is based on the impact of multiple cylindrical jets at a single point to create a low-pressure spray that impinges on a heated surface. An experimental performance evaluation is presented focusing on the influence of the applied thermal load, orifice-to-surface distance (jet chamber height), number of colliding jets and hot reservoir temperature. For a hot reservoir temperature of 25 °C, the heat sink configuration with three colliding jets and a jet chamber height of 23.8 mm sustained imposed thermal loads as high as 225 W (heat flux of 354.7 kW/m2), maintaining the impingement surface temperature below 40 °C with a refrigerant mass flow rate measured at approximately 5 kg/h. The resulting average heat transfer coefficients ranged from 7.5 to 18.1 kW/(m2 K), and calculated vapor mass qualities as high as 0.93 have been achieved at the outlet of the cooling unit. High-speed video sequences showing the interaction between the impinging spray and the heated surface were collected to help illustrate the phase change heat transfer mechanisms and the influence of the independent variables.

Modeling and measurement of a moving coil oil-free linear compressor performance for refrigeration application using R134a

A prototype of the compressor with moving coil type linear motor has been developed. The developed compressor is integrated with test loop using R134a refrigerant to confirm its performance. The simulation results are validated with the experimental data. The experimental results from the test loop with strokes of 10, 11 and 12 mm for three different pressure ratios of 4, 7, 10 are presented and discussed in this paper. The COP of the system calculated is 1.4 from the test results (for 54 °C condenser temperature and −20 °C evaporator temperature) with the stroke of 10 mm, pressure ratio of 10 and cooling capacity of 134 W. The maximum COP of 2.13 is achieved (for 54 °C condenser temperature and 2 °C evaporator temperature) with the stroke of 12 mm, pressure ratio of 4 and cooling capacity of 325 W. Normal refrigerator compressors utilize lubricant oil but as the refrigerant is changed the oil suitable for particular refrigerant also needs to be changed. The novel linear compressor tested does not utilize lubricant oil. The linear compressor has only one friction point i.e. between the piston and cylinder. The linear compressor utilizes Rulon (low coefficient of friction material) as a special material coated on the piston surface in contact with the cylinder. The oil-free operation of the compressor helps in adapting the refrigerator to different refrigerants without having to consider a change of lubricating oil. Refrigeration system performance with both linear compressor and the conventional reciprocating compressor is measured and compared. System COP with the linear compressor is 18.6% more than with the commercially available reciprocating compressor. The absence of connecting rod and crank mechanism accompanied with oil-free operation due to reduced friction enhance the performance of the linear compressor.

Performance Assessment of Single and Multiple Jet Impingement Configurations in a Refrigeration-Based Compact Heat Sink for Electronics Cooling

The performance of a novel impinging two-phase jet heat sink operating with single and multiple jets is presented and the influence of the following parameters is quantified: (i) thermal load applied on the heat sink and (ii) geometrical arrangement of the orifices (jets). The heat sink is part of a vapor compression cooling system equipped with an R-134a small-scale oil-free linear motor compressor. The evaporator and the expansion device are integrated into a single cooling unit. The expansion device can be a single orifice or an array of orifices responsible for the generation of two-phase jet(s) impinging on a surface where a concentrated heat load is applied. The analysis is based on the thermodynamic performance and steady-state heat transfer parameters associated with the impinging jet(s) for single and multiple orifice tests. The two-phase jet heat sink was capable of dissipating cooling loads of up to 160 W and 200 W from a 6.36 cm2 surface for single and multiple orifice configurations, respectively. For these cases, the temperature of the impingement surface was kept below 40 °C and the average heat transfer coefficient reached values between 14,000 and 16,000 W/(m2 K).

Predicting the temperature distribution and suction gas superheating of an oil-free linear compressor

It is well established that the volumetric and isentropic efficiencies of reciprocating compressors used for household refrigeration are significantly reduced by suction gas superheating. Moreover, excessive levels of temperature may affect the reliability of some components, such as the electrical motor. This paper reports a modeling approach to predict the temperature distribution in the gas and solid components of an oil-free linear compressor. A simulation model based on the finite volume method was used to solve the heat conduction in the solid components and gas flow inside the compressor. On the other hand, the compression cycle in the cylinder was solved with a transient lumped formulation, but in a coupled manner with the remainder of the solution domain. The prediction of the suction gas superheating obtained with the model was in good agreement with the measurements, despite discrepancies being observed in some solid components and in the gas path along the discharge system. The model does not require any experimental calibration and hence is suitable to analyze different compressor designs. To illustrate this, the model was applied to predict the temperature distribution with respect to two design parameters.

The effect of clearance control on the performance of an oil-free linear refrigeration compressor and a comparison between using a bleed flow and a DC current bias

A moving magnet type oil-free linear compressor has been designed for applications such as electronics cooling. A control system using a solenoid operated valve was developed for DC offset and axial clearance control. Operation of the linear compressor with a fixed clearance of 0.8 mm was compared with a zero DC offset operation. The fixed clearance operation requires a higher power input.The mean piston position can be controlled by using a DC bias on the drive voltage or by using a bleed flow from the compressor body to the suction side of the compressor. Using a DC bias on the drive voltage induces a higher power loss than using a bleed flow when there is a small radial clearance between the piston and cylinder. This provides evidence that the piston control in a refrigeration system with capacity control should be for a zero DC offset using a bleed flow.

Thermal modelling and analysis of an oil-free linear compressor

Gas superheating in the suction system may significantly reduce the volumetric and isentropic efficiencies of small reciprocating compressors adopted for household refrigeration. This paper reports a thermal modelling approach developed to predict superheating in an oil- free linear compressor. A simulation code based on the finite volume method was adopted to solve heat conduction in the solid components and gas flow inside the compressor shell. In order to reduce the computational cost, the compression cycle inside the cylinder was modelled with a transient lumped formulation, but in a coupled manner with the remainder of the solution domain. Comparisons between numerical and experimental results of temperature showed discrepancies in some solid components and in the gas path along the discharge system. However, the model was able to predict suction gas superheating in good agreement with measurements. A sensitivity analysis of the temperature distribution with respect to two design parameters was also carried out. The model is particularly useful for compressor design since no experimental calibration is required.

Comparison between a crank-drive reciprocating compressor and a novel oil-free linear compressor

Reciprocating compressors, driven by induction motors through a crank mechanism, have been commercially used over many years for refrigeration. An oil-free linear compressor driven by a moving magnet motor was designed, for a refrigeration system with a compact heat exchanger. Measurements using nitrogen are reported here to compare the motor performance and overall efficiencies of the two types of compressor with comparable design parameters. The experimental results show that the moving magnet linear motor has a much higher motor efficiency than the conventional induction motor, particularly at low power inputs. However, with a much smaller clearance volume (approaching zero), the crank-drive compressor demonstrates a higher volumetric efficiency based on the swept volume, that is approximately 20% higher than the linear compressor when operated at its maximum stroke (13 mm). It is anticipated that with a revised design, the overall performance of the linear compressor could be enhanced further.

Clearance seal compressors with linear motor drives. Part 2: Experimental evaluation of an oil-free compressor

Oil-free compressors are required for various applications, including cryocoolers and refrigeration systems that use compact evaporators. A novel low-cost moving magnet oil-free linear compressor is described here, and follows a companion paper on the linear motor system. The experimental apparatus to evaluate the thermodynamic performance of the linear compressor is comparatively straightforward, but the data analysis is involved, since key measurements have to be corrected for the phase response of the transducers. Harmonic fitting, using fast Fourier transform with zero-padding and minimisation algorithms, have been used for the voltage, current and displacement signals. This enables corrections to be made for the phase lag in the voltage, current and displacement measurements. The experiments using nitrogen, with a constant pressure ratio, show that the linear motor system under resonance achieves a high overall efficiency. For the design point, the motor efficiency is 74%, while for a pressure ratio of 3.0, the average overall adiabatic efficiency is 54% and the isothermal efficiency is 46%, all of which are within a reasonable range for a small compressor.

Mini Vapor Cycle System for high density electronic cooling applications

This paper reports the experimental analysis of a mini Vapor Cycle System (VCS) for electronic thermal management in avionic applications. The water cooled miniature scale refrigeration system uses R134a as working fluid and implements a new concept oil-free linear compressor prototype. In the range of operating test conditions investigated the pressure ratio varies from 1.54 to 3.75, the cooling capacity of the system varied from 37 W to 374 W, while the coefficient of performance (COP) ranged between 1.04 and 5.80. Particular attention was dedicated to the cold plate design, which had to meet the requirements established by aeronautical standards. The cold plate was equipped with 15 thermocouples in order to monitor the wall temperature distribution. The suitability and feasibility of the proposed cold plate for electronic thermal management in an aeronautical environment is critically discussed, based on the experimental results.

Investigation on High-Power Stirling-Type Pulse Tube Coolers for Cooling HTS Motors

High-power Stirling-type pulse tube coolers (HSPTCs) driven by oil-free linear compressors are promising candidates for cooling HTS devices for the advantages of simple configuration, low noise and stable operation. Three U-shaped two-stage pulse tube coolers driven by a 10 kW linear compressor were built and studied. The effects of flow inhomogeneity on the temperature profiles along the regenerator and on the cooling capacity were investigated experimentally. By optimizing the impedance matching and operating conditions, the first cooler can reach a base temperature of 44.2 K; the second cooler reaches a base temperature of 27.1 K and provides a cooling power of 50 W at 45.6 K with 4.8 kW of power; the last cold head reaches a base temperature of 13.27 K and provides a cooling power of 18.3 W at 30 K. The present results form a foundation for building a HSPTC system capable of providing a cooling power of 80 W at 30 K.

Advances in High Power Stirling-Type Pulse Tube Cooler

High power Stirling-type pulse tube coolers (HSPTCs) with oil-free linear compressors are promising solutions for cooling superconducting magnets. The cooling capacity of HSPTC ranges from 50 W to a few kilowatts below 100 K. At present the lowest refrigeration temperature of HSPTC has reached below 30 K. After a brief introduction of Stirling-type PTC, this paper reviews the newest developments in HSPTC. At the same time, the potential of HSPTC to be applied in cooling superconducting magnets is analysed. Possible future advances and prospective research emphases are also presented.