Visualization of the solid–liquid equilibria for non-flammable mixed refrigerants

Non-flammable mixed refrigerant (NF-MR) Joule Thomson (J–T) refrigerators have desirable characteristics and wide cooling temperature range compared to those of pure J–T refrigerators. However, the operating challenge due to freezing is a critical issue to realize this type of refrigerator. In this paper, the solid–liquid phase equilibria (i.e. freezing point) of the NF-MR which is composed of Argon (Ar), R14 (CF₄), and R218 (C₃F₈), has been experimentally investigated by a visualized apparatus. The accuracy of the apparatus is experimentally verified with pure refrigerants and selected binary mixed refrigerants. Freezing points of the ternary NF-MRs have been measured with the molar compositions from 0.1 to 0.8 for each component. Each test result is simultaneously acquired by a camcorder for visual inspection and temperature measurement during a warming process. Experimental results reveal that the specific MR, with R14 molar composition higher than 0.4, can achieve remarkably low freezing temperature even below 77 K. These unusual freezing point depression characteristics of the MR can be a useful information for designing a cryogenic MR J–T refrigerator to reach temperatures less than 77 K.     

Performance of an auto refrigerant cascade refrigerator operating in liquid refrigerant supply (LRS) mode with different cascade heat exchangers

Auto refrigerant cascade (ARC) refrigerators operating with zeotropic mixtures provide refrigeration at temperatures less than 173 K (?100 °C) using a single compressor. Different authors have suggested different cascade heat exchangers for ARC refrigerators. There is no study in literature that suggests at what temperature ranges one, two or three cascade heat exchangers are necessary. In this paper the performance of an ARC refrigerator operating in the liquid refrigerant supply mode and operating with optimized hydrocarbon mixtures and different cascade heat exchangers is studied. The optimum number of cascade heat exchangers (stages) to be used for different operating temperatures is suggested.     

Design of high efficiency mixed refrigerant Joule–Thomson refrigerator for cooling HTS cable

The substitution of high temperature superconducting (HTS) cables for existing subterranean electric transmission lines is arising as a solution to continuously increasing electricity demand in urban areas. A cryogenic refrigeration system having the characteristics of high reliability, high efficiency, large cooling capacity, and low capital cost is essential to enable such a substitution. These requirements can be satisfied with a mixed refrigerant Joule–Thomson (MR JT) refrigerator. Unfortunately, usual MR JT refrigerators exhibit good performance at refrigeration temperatures above 80 K. A precooled neon–nitrogen MR JT refrigerator is proposed in this paper that can cool HTS cables at 70 K. The coefficient of performance (COP) of the proposed MR JT refrigerator is predicted to be 0.058 at 70 K (19.2% for exergy efficiency) with the optimized design variables. The COP can be improved further to 0.064 by enhancing the efficiency of the precooling cycle. The maximum achievable COP demonstrates the feasibility of MR JT refrigerator for cooling HTS cable.     

Experimental study of a mixed refrigerant Joule–Thomson cryocooler using a commercial air-conditioning scroll compressor

Mixed refrigerant Joule–Thomson (MR J–T) cryocoolers have been used to create cryogenic temperatures and are simple, efficient, cheap, and durable. However, compressors for MR J–T cryocoolers still require optimization. As the MR J–T cryocooler uses a commercial scroll compressor developed for air-conditioning systems, compressor overheating due to the use of less optimized refrigerants may not be negligible, and could cause compressor malfunction due to burn-out of scroll tip seals. Therefore, in the present study, the authors propose procedures to optimize compressor operation to avoid the overheating issue when the MR J–T cryocooler is used with a commercial oil lubricated scroll compressor, and the present experimental results obtained for a MR J–T cryocooler. A single stage 1.49 kW (2 HP) scroll compressor designed for R22 utilizing a mixture of nitrogen and hydrocarbons was used in the present study. As was expected, compressor overheating and irreversible high temperatures at a compressor discharge port were found at the beginning of compressor operation, which is critical, and hence, the authors used a water injection cooling system for the compressor to alleviate temperature overshooting. In addition, a portion of refrigerant in the high-pressure stream was by-passed into the compressor suction port. This allowed an adequate compression ratio, prevented excessive temperature increases at the compressor discharge, and eventually enabled the MR J–T cryocooler to operate stably at 121 K. The study shows that commercial oil lubricated scroll compressors can be used for MR J–T cryocooling systems if care is exercised to avoid compressor overheating.     

Experimental study of one-stage VM cryocooler operating below 8 K

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10 K. This paper describes the experimental development of a VM cryocooler operating below 8 K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er₃Ni to replace the lead sphere and a no-load temperature of 7.8 K was obtained. Then all the lead spheres were replaced by Er_0.6Pr_0.4 material and a no-load temperature of 7.35 K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10 K is about 500 mW with a pressure ratio near 1.6 and a charge pressure of 1.8 MPa. Especially, the magnetic material Er_0.6Pr_0.4 was found to be a potential substitution for the conventional lead.     

Design and optimization of a two-stage 28 K Joule–Thomson microcooler

Micro Joule–Thomson (JT) coolers made from glass wafers have been investigated for many years at the University of Twente. After successful realization of a single-stage JT microcooler with a cooling capacity of about 10 mW at 100 K, a two-stage microcooler is being researched to attain a lower temperature of about 30 K. By maximizing the coefficient of performance (COP) of the two-stage microcooler, nitrogen is selected as the optimum working fluid for the first stage and hydrogen as that for the second stage. A dynamic finite-element model is developed for analyzing the cooler performance and to calculate the smallest cooler geometry. The optimized overall cooler dimensions are 20.4 × 85.8 × 0.72 mm for a net cooling power of 50 mW at 97 K at the first stage and 20 mW at 28 K at the second stage. The cool-down time to 28 K is calculated to be about 1.7 h with mass-flow rates of 14.0 mg/s for nitrogen and 0.94 mg/s for hydrogen at steady state.     

Improving the performance of small Joule–Thomson cryocooler

This paper presents a new cycle for improving the performance of small Joule–Thomson (JT) cryocooler by applying an additional ejector in cycle system. Based on the presented JT cycle with an additional ejector (JTE), the performance of small JT cryocooler operating with pure N₂ and the mixture N₂?CH₄ for cryogenic applications in a temperature range of 80–130 K is investigated by theoretical calculations. It can be found that the refrigeration efficiency of the JTE cryocooler operating with pure N₂ increase by 55.6–25.1%, and the refrigeration capacity increase by a factor of 4.9–1.5 compared to a conventional JT cryocooler. Similar performance improvements are also obtained when the mixture N₂?CH₄ is used as the working gas. The refrigeration efficiency increase by 46.6–17.7% and the refrigeration capacity increase by a factor of 3.8–1.3. It is concluded that the performance of small JT cryocooler can be significantly improved based on the presented JTE cycle.     

Condensation stage of a pulse tube pre-cooled dilution refrigerator

In our article, experiments with a pulse tube (PTR) pre-cooled dilution refrigerator (DR) are presented, where an upgraded ³He condensation stage has been tested. The DR had a ³He flow rate of up to 1.1 mmol/s. The ³He gas entering the refrigerator was first pre-cooled to a temperature of ～50 K at the first stage of the PTR. In the next cooling step, the ³He was run through a recently installed heat exchanger, which was attached to the regenerator of the second stage of the pulse tube cryocooler; at the outlet of this heat exchanger the temperature of the ³He was as low as ～4 K. Due to the non-ideality of the helium gas, the second regenerator of a two stage PTR has excess cooling power which can be made use of without affecting the base temperature of this stage, and it is this effect which was put to work, here. Finally, the ³He was further cooled in a heat exchanger, mounted at the second stage of the PTR, before it entered the dilution unit of the cryostat.The installation of a heat exchanger at the regenerator of the second stage of the PTR is especially important for the construction of DRs with high refrigeration capacities; in addition, it allows for a plain design of the subsequent Joule–Thomson (JT) stage, and herewith facilitates considerably the construction of “dry” DRs. The condensation rate of the ^3,4He mash prior to an experiment was increased. The pressure during condensation could be kept near 1 bar, and thus a compressor was no longer necessary with the modified apparatus.     

Operating modes and cooling capabilities of the 3-stage ADR developed for the Soft-X-ray Spectrometer instrument on Astro-H

A 3-stage adiabatic demagnetization refrigerator (ADR) (Shirron et al., 2012) is used on the Soft X-ray Spectrometer instrument (Mitsuda et al., 2010) on Astro-H (Takahashi et al., 2010) [3] to cool a 6 × 6 array of X-ray microcalorimeters to 50 mK. The ADR is supported by a cryogenic system (Fujimoto et al., 2010) consisting of a superfluid helium tank, a 4.5 K Joule–Thomson (JT) cryocooler, and additional 2-stage Stirling cryocoolers that pre-cool the JT cooler and cool radiation shields within the cryostat. The ADR is configured so that it can use either the liquid helium or the JT cryocooler as its heat sink, giving the instrument an unusual degree of tolerance for component failures or degradation in the cryogenic system. The flight detector assembly, ADR and dewar were integrated into the flight dewar in early 2014, and have since been extensively characterized and calibrated. This paper summarizes the operation and performance of the ADR in all of its operating modes.     

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy_0.9Gd_0.1Ni₂ enclosed in a thin walled stainless steel container (22.2 mm in O.D., 0.3 mm in thickness and 40.0 mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0 T to 3.0 T (0–130 A) with an average ramp rate of 0.24 T s⁻¹. The cooling capacities of the ADR system at 20 K which is the normal boiling point (NBP) of hydrogen, are 11.1 J cycle⁻¹, 6.3 J cycle⁻¹ and 1.9 J cycle⁻¹ when the temperature spans are 1 K, 2 K and 3 K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20 K region.     

Pressure drop correlation for oil–refrigerant R134a mixture flashing flow in a small diameter tube

This paper presents an experimental investigation of the ester oil ISO VG10/refrigerant R134a mixture flashing flow in a 6.0 m long, 3.22 mm ID tube, which is one of the primary steps towards the construction of a methodology for the study of the lubrication and gas leakage in refrigeration compressors. The phase change starts with solubility reduction of the refrigerant in the oil as the pressure decreases due to the friction forces. In this flashing flow the foam pattern is observed at the end of the tube as vapor quality reaches high values, and this is a particular phenomenon of this kind of mixture flow. In order to study this pressure drop, an experimental apparatus was designed to allow the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns. Pressure and temperature distribution along the flow were measured for saturation pressure ranging from 450 to 650 kPa, mass flux ranging from about 2000 to 3000 kg/(m²s), temperatures around 303 K, and inlet refrigerant concentration varying between 0.2 and 0.4 kg ref/kg mixt. An available correlation proposed to predict the frictional pressure drop for a mixture composed by the mineral oil SUNISO 1GS and refrigerant R12 flowing in small diameter tubes yielded large deviations in predicting the ester oil and refrigerant R134a mixture flow. A new correlation has been proposed that fitted the experimental data with rms deviations of 24%.     

Characterization of a thermoelectric/Joule–Thomson hybrid microcooler

Micromachined Joule–Thomson (JT) coolers are attractive for cooling small electronic devices. However, microcoolers operated with pure gases, such as nitrogen gas require high pressures of about 9 MPa to achieve reasonable cooling powers. Such high pressures severely add complexity to the development of compressors. To overcome this disadvantage, we combined a JT microcooler with a thermoelectric (TE) pre-cooler to deliver an equivalent cooling power with a lower pressure or, alternatively, a higher cooling power when operating with the same pressure. This hybrid microcooler was operated with nitrogen gas as the working fluid at a low pressure of 0.6 MPa. The cooling power of the microcooler at 101 K operating with a fixed high pressure of 8.8 MPa increased from 21 to 60 mW when the precooling temperature was reduced by the thermoelectric cooler from 295 to 250 K. These tests were simulated using a dynamic numerical model and the accuracy of the model was verified through the comparison between experimental and simulation results. Based on the model, we found the high pressure of the microcooler can be reduced from 8.8 to 5.5 MPa by lowering the precooling temperature from 295 to 250 K. Moreover, the effect of TE cooler position on the performance of the hybrid microcooler was evaluated through simulation analysis.     

Evaluating the effect of the space surrounding the condenser of a household refrigerator

The paper presents an analytical and computational modeling of the effect of the space surrounding the condenser of a household refrigerator on the rejected heat. The driving force for rejecting the heat carried by the refrigerant from the interior of a refrigerator is the temperature difference between the condenser outer surface and surrounding air. The variation of this difference, because of having an insufficient space, increasing the room air temperature, or blocking this space, is of interest to quantify its effectThe results showed that having an enough surrounding space width (s > 200 mm) leads to a decrease in the temperature of the air flowing vertically around the condenser coil. Accordingly, this would significantly increase the amount of heat rejected. Moreover, blocking this space retards the buoyant flow up the condenser surface, and hence increases the air temperature around the condenser. This would also decrease the heat rejected from the condenser. Predicted temperature contours are displayed to visualize the air plumes' variation surrounding the condenser in all cases.     

Optimization of the working fluid for a sorption-based Joule–Thomson cooler

Sorption-based Joule–Thomson coolers operate vibration-free, have a potentially long life time, and cause no electromagnetic interference. Therefore, they are appealing to a wide variety of applications, such as cooling of low-noise amplifiers, superconducting electronics, and optical detectors. The required cooling temperature depends on the device to be cooled and extends into the cryogenic range well below 80 K. This paper presents a generalized methodology for optimization in a sorption-based JT cooler. The analysis is based on the inherent properties of the fluids and the adsorbent. By using this method, the working fluid of a JT cooler driven by a single-stage sorption compressor is optimized for two ranges of cold-tip operating temperatures: 65–160 K and 16–38 K. The optimization method is also extended to two-stage compression and specifically nitrogen and carbon monoxide are considered.     

Development of 1K-class Joule–Thomson cryocooler for next-generation astronomical mission

This paper reports on the development of a 1K-class Joule–Thomson (JT) cryocooler in Japan for application to upcoming next-generation astronomy missions. In this development, engineering models (EMs) were designed and manufactured for verification tests. The survival of the models in the mechanical and thermal vacuum environment tests of the JT compressors was proven to be possible with stable compression performance. In addition, the electromagnetic noise and disturbance force associated with the JT compressors were evaluated. Gas analysis showed that the estimated total amount of CO₂ gas contaminant was less than the getter capacity for the required lifetime. A nominal cooling power of 10 mW at 1.7 K was verified using the EM test units.     

Cryogenic low noise 2.2–3 GHz amplifier

A simple design of one stage, low power cryogenic amplifier at 2.2–3 GHz range is presented. The design was numerically simulated by freely available microwave library SuperMix. The amplifier constructed according to the numerically optimized design was measured in cryogen-free refrigerator. The measured noise temperature T_N ≈ 6 K and gain G ≈ 15.5 dB are in good agreement with numerical simulations.     

Analysis and optimisation of a mixed fluid cascade (MFC) process

A mixed fluid cascade (MFC) process that comprises three refrigeration cycles has great capacity for large-scale LNG production, which consumes a great amount of energy. Therefore, any performance enhancement of the liquefaction process will significantly reduce the energy consumption. The MFC process is simulated and analysed by use of proprietary software, Aspen HYSYS. The effect of feed gas pressure, LNG storage pressure, water-cooler outlet temperature, different pre-cooling regimes, liquefaction, and sub-cooling refrigerant composition on MFC performance are investigated and presented. The characteristics of its excellent numerical calculation ability and the user-friendly interface of MATLAB™ and powerful thermo-physical property package of Aspen HYSYS are combined. A genetic algorithm is then invoked to optimise the MFC process globally. After optimisation, the unit power consumption can be reduced to 4.655 kW h/kmol, or 4.366 kW h/kmol on condition that the compressor adiabatic efficiency is 80%, or 85%, respectively. Additionally, to improve the process further, with regards its thermodynamic efficiency, configuration optimisation is conducted for the MFC process and several configurations are established. By analysing heat transfer and thermodynamic performances, the configuration entailing a pre-cooling cycle with three pressure levels, liquefaction, and a sub-cooling cycle with one pressure level is identified as the most efficient and thus optimal: its unit power consumption is 4.205 kW h/kmol. Additionally, the mechanism responsible for the weak performance of the suggested liquefaction cycle configuration lies in the unbalanced distribution of cold energy in the liquefaction temperature range.     

Low cryogen inventory,forced flow Ne cooling system with room temperature compression stage and heat recuperation

We present design and commissioning results of a forced flow cooling system utilizing neon at 30 K. The cryogen is pumped through the system by a room-temperature compression stage. To decouple the cold zone from the compression stage a recuperating counterflow tube-in-tube heat exchanger is used. Commissioning demonstrated successful condensation of neon and transfer of up to 30 W cooling power to the load at 30 K using only 30 g of the cryogen circulating in the system at pressures below 170 kPa.     

Development of mechanical cryocoolers for the cooling system of the Soft X-ray Spectrometer onboard Astro-H

Astro-H is the Japanese X-ray astronomy satellite planned for launch in 2014. The Soft X-ray Spectrometer (SXS) onboard Astro-H, is a high energy resolution spectrometer utilizing an X-ray micro-calorimeter array, which is operated at 50 mK by the ADR with the 30-L superfluid liquid helium (LHe). The mechanical cryocoolers, 4 K-class Joule Thomson (JT) cooler and 20 K-class double-staged Stirling (2ST) cooler are key components to achieve a LHe lifetime for over 3 years in orbit (5 years as a goal). Based on the existing cryocoolers onboard Akari (2006) and JEM/SMILES (2009), modifications for higher cooling power and reliability had been investigated. In the present development phase, the Engineering Models (EMs) of these upgraded cryocoolers are fabricated to carry out verification tests for cooling performance, mechanical performance and lifetime. Nominal cooling power of 200 mW at 20 K for the 2ST cooler and 40 mW at 4.5 K for the JT cooler were demonstrated with temperature and power margin. Mechanical performance test for the 2ST cooler units proves tolerability for pyro shock and vibration environment of the Astro-H criteria. Continuous running of the 4 K-class JT cooler combined with the 2ST precooler for lifetime test has achieved over 5000 h without any degradation of cooling performance.