Capacity Enhancement of Indigenous Expansion Engine Based Helium Liquefier

IOP Conference Series: Materials Science and Engineering

PAPER • OPEN ACCESS Capacity enhancement of indigenous expansion engine based helium liquefier

To cite this article: R S Doohan et al 2017 IOP Conf. Ser.: Mater. Sci. Eng. 171 012011

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International Conference on Recent Trends in Physics 2016 (ICRTP2016) IOP Publishing Journal of Physics: Conference Series 755 (2016) 011001 doi:10.1088/1742-6596/755/1/011001

Capacity enhancement of indigenous expansion engine based helium liquefier

R S Doohan1, P K Kush1, G Maheshwari2 1Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh India. 2Institute of Engineering and Technology, DAVV, Indore, Madhya Pradesh, India. E-mail: [email protected]

Abstract. Development of technology and understanding for large capacity helium refrigeration and liquefaction at helium temperature is indispensable for coming-up projects. A new version of helium liquefier designed and built to provide approximately 35 liters of liquid helium per hour. The refrigeration capacity of this reciprocating type expansion engine machine has been increased from its predecessor version with continuous improvement and deficiency debugging. The helium liquefier has been built using components by local industries including cryogenic Aluminum plate fin heat exchangers. Two compressors with nearly identical capacity have been deployed for the operation of system. Together they consume about 110 kW of electric power. The system employs liquid Nitrogen precooling to enhance liquid Helium yield. This paper describes details of the cryogenic expander design improvements, reconfiguration of heat exchangers, performance simulation and their experimental validation.

1.1.1. Introduction A reciprocating expansion engine based helium liquefier has been developed to support various indigenous development programs [1]. Since then, continuous design improvements in its sub- sections were carried out to enhance its capacity to present 35 liters/hours. Fig. 1 shows the advancement trends in terms of liquid helium yield. Significant improvement in liquid helium yield has resulted from increased capacity of expanders and use of plate fin heat exchangers in its cold box. Present helium liquefier consists of two reciprocating expanders, six cryogenic heat exchangers and a Joule Thomson valve housed in vacuum enclosure called cold box. The system is designed to provide superior performance with high reliability.

Fig. 1. Progress made over the years in term of liquid helium yield enhancement

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2.2.2. Basic system description All refrigerator systems have one element in common that they absorb heat at a low temperature and reject it at a higher temperature. Helium liquefaction system is based on Collins cycle, thermo- dynamically less efficient than other ideal cycles, even than most practical cycle for gas liquefaction [2]. Although modern commercial liquefier plant use expansion turbines, they are well suited for medium and large liquefaction capacities. In comparison with reciprocating expansion engines, expansion turbine normally have small expansion ratio, consequently a small temperature drop. Turbine based system requires high flow rates combined with large heat transfer area for the heat exchangers. Reciprocating expansion engine system additionally get benefited from higher pressure ratios. Most common pressure ratios used for expansion engines liquefaction system are 15 to 20. In our case, reason for high pressure restriction to 15.5 bar absolute, was readily availability of equipments and control valves from commercial sources. Higher pressure than that requires specially ordered equipments and control systems.

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0 3 SECTION II SECTION

2 6 9 1 Cold-Box

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Fig. 2. Simplified schematic of expansion engine based helium liquefier with flow T-s diagram

A simplified flow diagram with important constituent components and two reciprocating expanders is shown in the Fig. 2. Present system has a design flow rate of 31.9 (g/s), at process pressure of 15.5 bar. It is capable of producing 35 liters/hour liquid helium with liquid nitrogen precooling. Approximately 12.76 (g/s) mass flow is diverted through each expander that corresponds to 1.9 kW and 0.6 kW of refrigeration at 54 K and 19.5 K respectively. Nearly 4% of total compressor flow i.e. 1.27 g/s is converted into liquid helium and taken out as product. Refrigeration produced by expanders is theoretically calculated based on enthalpy difference between inlet and exhaust conditions of expansion engines. Enthalpy is calculated from Helium gas properties software HEPAK taking temperature, pressure and mass flow as input [3]. Large losses happen to occur at the exit of expansion engines due to cold end pinch problem of the heat exchangers.The T-s diagram furthermore represents how system cools down the high pressure helium gas from environment temperature to its final liquefaction temperature. This is accomplished by three stages of intermediate refrigeration (two expansions engines & JT) and six heat exchangers in between the expansion stages. System design is performed using fChart Engineering Equation Solver computer simulation program [3] HEPAK® Version 3.40, Horizon Technologies Serial No. 4772, 2013.

2 ICECICMC IOP Publishing IOP Conf. Series: Materials Science and Engineering 171 (2017) 012011 doi:10.1088/1757-899X/171/1/012011

[4] S. A. Klein. (2014) Engineering equation solver Professional V9.697, #3902. . During process simulation, actual available parameters and real gas properties have taken as input. The influence of the process parameters like temperature, pressure ratios, mass flow rate are selected suitably to predict the refrigeration performance of its expanders and whole system. A brief description of important constituent components such as reciprocation expansion engine and heat exchangers is as follows.

2.1. Expansion Engines Expansion engines are the most critical components of the helium liquefaction cycle. Almost entire system refrigeration load is carried by them. Performance of expansion engine depends upon parameters like inlet temperature, pressure ratios, clearance between liner and displacer, re- compression, valves losses and passive heat in-leaks. An ideal expander will perform isentropic expansion followed by exhaust to low pressure stream. The efficiency of the expander will approach to near ideal when exhaust after expansion from the expander is equal to low pressure manifold. Expansion engines are vital components for helium liquefier and its performance is very critical. Therefore, it becomes imperative to improve its design, efficiency and performance. Two reciprocating expansion engines, each having isentropic efficiency more than 81% are used in the system. Typical bore diameter and stroke size for both the expanders liners is 75 mm. The bore size of the cylinder liner is same throughout the length and is made from single stainless steel pipe. Earlier version had first expander liner bore size 75 mm and second expander liner bore size 50 mm with a stroke for both expander as 50 mm. Extensive analysis using finite element methods were performed to guarantee its mechanical performance at its design pressure and temperature. Expander liner was made from solo SS-304 pipe and TIG welded with flange before commencing internal boring and honing operations. Cryogenic expanders were designed using Pro-Engineer 3D modeling software. Assembly related deficiency debugging were rectified in 3D models itself [5]. Contraction between SS-304 liner and G-10 displacer were evaluated using the combination of Pro-Engineer and ANSYS Workbench software [6]. Before expanders integration to the system, extensive test were performed with liquid nitrogen to validate their performance. Fig. 3 shows photo of helium liquefier with cold box and 1000 liters liquid helium Dewar. Expanders are mounted on the top flange of cold box. Expander pistons are connected by connecting rod with flywheel shaft. Work produced by them is used to generate electric power. Using variable load on electric generator speed of expanders is controlled.

3 ICECICMC IOP Publishing IOP Conf. Series: Materials Science and Engineering 171 (2017) 012011 doi:10.1088/1757-899X/171/1/012011

Fig. 3. Actual system photograph with cold box and helium Dewar prominently visible

2.2. Heat exchangers After expansion engines, heat exchangers are next in line as critical components for helium liquefaction system. Helium is cooled in series of counter-flow heat exchanger where first heat exchanger is of aluminium plate fin and rest are of shell-and-tube type. For our earlier version 1.0 all six heat exchanges were shell-and-tube type. Shell-and-tube heat exchangers offer limited heat transfer area and their pressure drop and size increase significantly with higher flow rates. This results in larger passive losses in the system. Helium has a critical point at 5.1953 K, 2.2746 bar absolute. Low critical pressure and lowest liquefaction temperature of helium put strict requirement on pressure drop for helium liquefaction applications. This effect can be mitigated with newer type of plate fin heat exchanger with high compactness factor and low pressure drop, for larger mass flow rates. We could nearly double the compactness factor to 1400 m2/m3 with the help of newly developed plate fin heat exchanger. Fig. 4 shows the side by side view of shell and tube and aluminum plate fin heat exchanger developed for present system.

4 ICECICMC IOP Publishing IOP Conf. Series: Materials Science and Engineering 171 (2017) 012011 doi:10.1088/1757-899X/171/1/012011

Fig. 4. Different type of heat exchanger developed for helium liquefier

3.3.3. Analysis and performance prediction Based on the experience and using some critical measurements plus varying the efficiency ranges, one can reasonably predict the system refrigeration performance at different temperature level. Predicted mass flow and refrigeration produced by the expanders agrees with the simulation results within ± 10%. It was observed that predicted and observed performances of expanders have good agreement for the selected cycle pressure ratios. Overall system performance degrades, when subjected to off- design operating conditions. A simulation program was developed to predict the refrigeration performance of system at higher temperature. The expander operating characteristics are taken according to the inlet and outlet conditions. Helium gas properties are taken from Hepak [3]. Fig. 5 represents the calculated output of expected refrigeration performance at higher temperature [7]. Present system is designated as version 2.0 in the graph. For version 1.0 and 2.0 simulated refrigeration performance is validated with experimental results at temperature 4.7 K taking effectiveness of process as unity. Whereas version 1.0 and 3.0 represent the earlier and future version respectively. More than 700 W of refrigeration can be produced with the present system at 20 K temperature level. Although system can be tuned to operate as liquefier or refrigerator for different applications. The optimum pressure ratio for these operations will be different and are beyond the scope of present paper. Understanding of available refrigeration at higher temperature will be useful for integral purifier design, to be incorporated in the next version. Present system has been operated several times without major breakdown and performance degradation. Numerous temperature sensor have been mounted on different locations for performance prediction and further improvement in system.

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Fig. 5. Expected refrigeration from the system at different temperature level

4.4.4. RESULTS AND DISCUSSION Compressor isothermal compression work requirement increases proportionally with the cycle pressure ratio. For maximum total system efficiency, it is essential to match the cold box pressure ratio to the compressor system. Compressor sole job is to supply high pressure pure helium to cold box that lead to the increase in its exergy (availability). Further improvement in system performance is possible by looking at the share of exergy utilization by the cold box different components. Exergy utilization study is performing for different components while performing their intended operations. This study could be used to identify relative importance of individual component with respect to overall performance. Slight performance improvements could results in substantial saving in the power consumption. Fig. 6 shows the relative share of exergy utilization by different cold box components. Replacing JT with relatively high efficiency wet expansion engine could increase liquid yield. Miscellaneous losses occurs mainly in interconnecting piping, valves, helium transfer line, mounting arrangement of cold box components, storage Dewar, adsorber and improper matching of heat exchangers temperature schemes, etc. However due care shall be taking during system fabrication and assembly, as it directly effect these losses. Miscellaneous or unaccountable losses are still high in the system and ways can be devised to efficiently mitigate them.

Fig. 6. Relative share of exergy utilization by cold box different component

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5.5.5. CONCLUSION High liquefaction rate were achieved as a results of meticulous design improvement of expanders and heat exchangers. Large size compressor system with oil removal system was put into operation and successfully commissioned. Experimental results from the system yielded liquid helium average production at a rate of 35 liters/hour. System functional analyses were performed. Developed model predicts system performance with good accuracy in normal operating range. With the help of this developed models, influence of major input parameters on the system efficiency can be studied. From above analysis it may be concluded that replacing JT with more efficient wet expander and mitigating miscellaneous loses to certain extend could increase system yield significantly.

REFERENCES [1] P. K. Kush, et al., "India’s first indigenously developed helium liquefier," in Indian Particle Accelerator Conference, New Delhi, 2011. [2] U. Wagner, "Refrigeration," in CERN Accelerator School, superconductivity and cryogenics for accelerators and detectors, , Erice, Italy, 2002. [3] HEPAK® Version 3.40, Horizon Technologies Serial No. 4772, 2013. [4] S. A. Klein. (2014) Engineering equation solver Professional V9.697, #3902. [5] Pro/ENGINEER® Foundation Advantage, 2007, ID 19344719, Waltham, (MA): Parametric Technology Corporation. [6] ANSYS® Inc., revision 12.0.1, 2009. [Online]. www.ansys.com [7] P. Lebrun, L. Tavian, G. Vandoni and U. Wagner, "Cryogenics for particle accelerators and detectors," No. CERN-LHC-2002-011, Geneva, Switzerland, 2002.