Optimal refrigerant charge and energy efficiency of an oil-free refrigeration system using R134a

This article presents a design of a high thrust density moving magnet linear actuator for linear refrigerator compressor. The shape of permanent magnet (PM) shows great effect on cost, fabrication and magnetic flux density generation in the actuator. Axially magnetised disk shaped PMs are low cost and convenient for fabrication. Moreover, axially magnetised PMs generate high flux density in a specific direction compared to radial magnets. This paper investigates the design of axially magnetised moving magnet linear oscillating actuator (LOA) which operates on single phase alternating supply. Additionally, an alternative path is proposed for the return of magnetic flux from mover to the stator. Using finite element method (FEM) tools, all the parameters of LOA are optimised. Moreover, with the help of a specially designed experimental setup, FEM simulation results are validated. This actuator demonstrates great advancement in thrust force density, motor constant and output power compared to the size of proposed LOA.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-epa.2019.0789

Design of a high thrust density moving magnet linear actuator with magnetic flux bridge

Without crank mechanism, linear compressors can provide significant advantages such as selection of refrigerants, miniaturization, high efficiency, and non-start/stop. Free motion of piston means that displacement can be modulated according to heat load and friction loss of refrigerants. R1234yf is a synthetic refrigerant with global warming potential (GWP) of only 4 which is considered as an ideal replacement for R134a for automobile air conditioner and domestic refrigerator. In this paper, both modelling and measurements were conducted to study the thermodynamic performance of linear compressor using R1234yf in terms of P–V loop, polytropic index, motor efficiency, piston offset, mass flow rate, cooling capacity and coefficient of performance (CoP). The results from the simplified model agree well with measurements. For a pressure ratio of 2.5, a stroke of 12 mm with a condenser temperature of 40 °C, the linear compressor using R1234yf can achieve a motor efficiency of 86%, a cooling capacity of 120 W and a CoP of 3.0. The thermodynamic efficiency of R1234yf is 20–50% lower than R134a. The simplified model is capable of evaluating linear compressor performance with refrigerants.

Thermodynamic analysis of linear compressor using R1234yf

The piston of linear compressors is subjected to low side loads and metal contact can be avoided by feeding high-pressure refrigerant gas from the discharge system into the piston-cylinder clearance. This paper reports a thorough study on the inefficiencies associated with leakage and viscous friction brought about by the transient compressible fluid flow in the piston-cylinder clearance of an oil-free linear compressor. The main objective is to understand the effect of design parameters, such as compressor speed, piston oscillation amplitude, pressure ratio and radial clearance, on the volumetric and isentropic efficiencies. The compressor efficiency was found to be mainly affected by the clearance and pressure ratio. A reduction of 2.5% in the volumetric efficiency and 4.2% in the isentropic efficiency, for instance, were predicted when the radial clearance was varied from 2.5 to 10.0 µm.

Scrutinizing the sources of inefficiencies in the piston-cylinder clearance of an oil-free linear compressor.

This paper develops a Failure Mode, Effects and Criticality Analysis (FMECA) for a heating, ventilation and air conditioning (HVAC) system in railway. HVAC is a safety critical system which must ensure emergency ventilation in case of fire and in case of loss of primary ventilation functions. A study of the HVAC’s critical areas is mandatory to optimize its reliability and availability and consequently to guarantee a low operation and maintenance cost. The first part of the paper describes the FMECA which is performed and reported to highlight the main criticalities of the HVAC system under analysis. Secondly, the paper deals with the problem of the evaluation of a threshold risk value, which can distinguish negligible and critical failure modes. Literature barely considers the problem of an objective risk threshold estimation. Therefore, a new analytical method based on finite difference is introduced to find a univocal risk threshold value. The method is then tested on two Risk Priority Number datasets related to the same HVAC. The threshold obtained in both cases is a good tradeoff between the risk mitigation and the cost investment for the corrective actions required to mitigate the risk level. Finally, the threshold obtained with the proposed method is compared with the methods available in literature. The comparison shows that the proposed finite difference method is a well-structured technique, with a low computational cost. Furthermore, the proposed approach provides results in line with the literature, but it completely deletes the problem of subjectivity.

/pdf/fmeca-assessment-for-railway-safety-critical-systems-2qqj3d9bba.pdf

FMECA Assessment for Railway Safety-Critical Systems Investigating a New Risk Threshold Method

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.

Pravin Khedkar

Papers

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

Failure modes effects and criticality analysis of the linear compressor

Modeling and simulation performance of reed valve in linear compressor