The benefits of linear compressors
Among the positive displacement compressors, linear compressors are the focus of special attention for small-scale refrigeration applications. Instead of being driven by a rotary motor through crankshaft, the piston of a linear compressor is driven directly by the linear motor. Beyond their compactness, linear compressors offer advantages such as lower friction losses, possible oil-free operation and easy refrigerating capacity modulation. This translates into higher energy efficiency forrefrigeration systems using this type of compressors.
Linear compressors are the subject of intense research and development work, as evidenced by three articles published in the January 2020 issue of the International Journal of Refrigeration.(1) (2) (3)
- In a first article 1, X. Zhang et al. stress that improving the dynamic performance of linear compressors has been a major theme of scientific research and industrial development for several decades. This article presents a comprehensive and generalized model that is able to predict both transient and steady-state behaviours of the piston movement, internal pressure and temperatures as well as the overall performance.
The model allowed the researchers to demonstrate that the maximum mass flow rate and highest motor efficiency occur when the excitation frequency is at the resonance frequency of system and to evaluate the penalties associated with operation away from resonance. A preliminary validation was conducted utilizing experimental results from a commercial linear compressor for domestic refrigerators.
- In another article 2, H. Zou et al. point out that linear compressor is a promising alternative for commercial refrigerator using R290 (propane). They have modelled the design of a linear compressor and then have analysed by using this model the performance of a linear compressor using R290 with different piston diameters in order to get the optimal structural configuration for its application in commercial refrigerators.
By considering the leakage from compression chamber to the suction chamber and by taking as an example a linear compressor with 12 cm3 swept volume, the optimal piston diameter for a commercial refrigerator using R290 was found to range from 30 mm to 32 mm.
- In a third article 3, H. Jiang et al. underline that linear compressors are attractive for domestic refrigeration. Minimizing the clearance volume of a linear compressor is a valuable approach to increase the volumetric efficiency thus cooling capacity. This is particularly beneficial for refrigerants that require high displacement due to resulting high friction. However, operation of linear motor with offset may deteriorate the motor efficiency due to earlier saturation.
Therefore the authors have experimentally compared the refrigeration performance of a linear compressor with R1234yf as a refrigerant for various clearance volumes using an offset of 0 mm. In particular they found that when clearance was reduced from 0.8 mm to 0.6 mm, an average 4% increase of volumetric efficiency was achieved. They also found that the cooling capacity for a pressure ratio of 2.5 and a stroke of 13 mm increased by 12% when the clearance decreased from 1.07 mm to 0.4 mm.
The authors conclude that minimizing the clearance is significant for part load operation and improves the overall COP particularly for high pressure ratios. They recommend this approach for linear compressors used in fridges and freezers.
The cited IJR articles can be downloaded free of charge by IIR members on ScienceDirect after activating their account (https://sciencedirect.com/science/activate/iif) or in Fridoc using the links provided below.
1 Xinye Zhang et al, Theoretical analysis of dynamic characteristics in linear compressors
2 Huiming Zou et al, Performance analysis of linear compressor using R290 for commercial refrigerator
3 Hanying Jiang et al, Performance of a linear refrigeration compressor with small clearance volume