Anindya Chakravarty

Validation and performance of the LHC cryogenic system through commissioning of the first sector

The cryogenic system [1] for the Large Hadron Collider accelerator is presently in its final phase of commissioning at nominal operating conditions. The refrigeration capacity for the LHC is produced using eight large cryogenic plants and eight 1.8 K refrigeration units installed on five cryogenic islands. Machine cryogenic equipment is installed in a 26.7-km circumference ring deep underground tunnel and are maintained at their nominal operating conditions via a distribution system consisting of transfer lines, cold interconnection boxes at each cryogenic island and a cryogenic distribution line. The functional analysis of the whole system during all operating conditions was established and validated during the first sector commissioning in order to maximize the system availability. Analysis, operating modes, main failure scenarios, results and performance of the cryogenic system are presented.

In-Field Performance Evaluation of a Large Size Multistream Plate Fin Heat Exchanger Installed in a Helium Liquefier

Abstract A large size multistream plate fin heat exchanger (MSPFHE) is developed for an in-house-developed modified Claude cycle-based helium liquefier. The development of this heat exchanger, including thermal sizing and mechanical design, is described in this article. A pressure drop analysis is also carried out to compute the pressure drops across various components of the MSPFHE. The MSPFHE, after fabrication, pressure and leak testing, is integrated with the rest of the process equipment and piping of the developed helium liquefier cold box. Experiments are conducted to evaluate the performance of the developed MSPFHE under different modes of operations of the helium liquefier. Experimental results are compared with the computed predictions based on the two-dimensional numerical model as well as the commercial software Aspen MUSETM and reported in this article. Good agreement between the computed and measured performances is observed during the field testing of the MSPFHE.

Simulation studies on aerostatic thrust bearings with special reference to cryogenic turboexpanders

The present paper details theoretical studies on aerostatic thrust bearings with special reference to cryogenic turboexpander rotors. A special case of 4.5K, 250W helium refrigerator has also been considered where both the turboexpanders are supported by aerostatic thrust bearings of similar dimensions. The bearings experience continuously changing ambient (discharge) pressure during the start-up and shut-down transients, thereby requiring a proper estimation of bearing characteristics for successful turboexpander operation at all times. Performancecharts, intermsofloadcapacity, stiffnessandbearinggasflowwithrespect to change in clearance have been presented for different bearing gas supply and discharge pressures.

Transient response of a two layer cryogenic compact plate fin heat exchanger

In the present paper, a model for the transient simulation of a 2 layer plate fin heat exchanger having offset strip fins with helium as working fluid is formulated based on unsteady mass and energy conservation equations. The behaviour of the heat exchanger during start up from a given initial condition, cool down and during a step and ramp change in the inlet temperature of one of the fluids is studied with the help of this model. Initially, a model based on average helium properties is developed which is further extended to take into account the temperature dependence of helium properties along with axial heat conduction and variable thermal capacity of the metal separating plate. A computer code has also been developed to this effect.

Development of experimental test facilities for validation of multistream plate finheat exchanger design codes

Compact plate-fin heat exchangers (PFHE) having very high effectiveness (>0.95) are key equipment of modern helium liquefaction/refrigeration systems. Effectiveness (e) of these heat exchangers strongly influences the overall system performance. Apart from basic fluid film resistances, various secondary parameters such as axial heat conduction (AHC) through the heat exchanger metal matrix, parasitic heat in-leak from surroundings, variation in fluid/metal properties and flow mal-distribution etc need to be considered while sizing/rating such high effectiveness PFHE. Need of multiple streams in a single heat exchanger further complicates thermal designs. In-house codes are developed at BARC/IIT Bombay for rating of such high effectiveness PFHEs. For experimental performance evaluation of PFHEs, their characterization at various operating conditions and validation of in-house developed numerical codes, a dedicated closed loop experimental test facility is developed. Development of the dedicated closed loop experimental test facility is presented in this paper along with initial test results.

Operational analysis and update on modifications to a helium liquefier under development at BARC

An experimental helium refrigerator/liquefier is being designed and developed by Cryo-Technology Division to be installed at Bhabha Atomic Research Centre, Mumbai. During first trial runs, certain limitations of the process were observed and some other fabrication/development issues came to light. After detailed steady state cycle simulation studies, it was decided to undertake some major modifications to achieve the target of about 200W refrigeration capacity at around 4.7K. The most significant modification involved changing the basic thermodynamic cycle from Collins to a modified Claude cycle which is more suited to turboexpanders. To this effect, simulation studies on the new cycle, with two turboexpanders in series interspaced with a multi stream heat exchanger, was taken up and the process computed. The present paper aims to describe in details different studies taken up on the helium refrigerator. It also aims to provide an update on the present status of development of the helium refrigerator/liquefier.

Effects of axial heat conduction, property variation and parasitic heat in-leakon performance of compact plate-fin heat exchangers

Modern helium liquefaction/refrigeration systems employ compact plate-fin heat exchangers having very high effectiveness (>0.95). Performance of such systems is a strong function of effectiveness of heat exchangers used. The calculation of heat exchanger effectiveness in such cases needs considerations of several secondary parameters apart from basic fluid film resistance. In the present paper, the combined effects of secondary parameters like axial heat conduction through heat exchanger matrix and parasitic heat in-leak from the surroundings has been studied numerically. Large temperature changes in cryogenic heat exchangers may result in correspondingly larger changes in fluid properties and metal matrix conductivity, which shall be taken care of during numerical calculations. Numerical model developed in the present work is based on the one given in the literature. Numerical technique to solve the system of equations is implemented in MATLAB®. Real properties of helium at each node are evaluated using HEPAK®, which is linked to the developed code. Using this model, performance of heat exchangers is studied at four different temperature levels of (300-77)K, (77–20)K, (20-8)K and (10-5.1)K. The results highlight the effects of each of the above secondary parameters.

Recent trials with the experimental helium liquefier developed by BARC

An experimental helium liquefier has been designed and fabricated by Cryo-Technology Division and installed at Bhabha Atomic Research Centre, Mumbai. The helium liquefaction process is based on a modified Collins cycle consisting of one pre-cooler turboexpander, a pair of by-pass turboexpanders (warm and cold) and a series of 7 compact brazed plate fin high effectiveness heat exchangers. Liquid nitrogen pre-cooling facility along with another heat exchanger to recover cold of gaseous nitrogenhas also been provided in the system. After the full installation of the process compressor and its integration with the helium liquefier cold box, trial runs were started. A lowest temperature of about 7.8 K was registered in a temperature sensor located downstream of the JT valve.

Design of a triaxial bayonet for connecting a helium liquefier cold box and receiver Dewar vessel

A transferline with a triaxial bayonet for connecting a helium liquefier cold box and Dewar has been conceived. Liquid helium (LHe) settles in the Dewar vessel and the remaining cold gaseous helium (GHe) returns to the cold-box as a part of the liquefier cycle. The two coaxial lines are enclosed in an outermost vacuum line. These three coaxial lines come out of the liquefier through a triaxial bayonet joint which facilitates assembly/disassembly of the system due to the absence of welded joints. The present paper discusses schematic, design, and working of such a triaxial bayonet. It is conceived such that reduction in overall length, ease of fabrication, reduction of thermal stresses due to flexibility of bellows and reduction in fabrication cost are achieved.

Theoretical analysis and experimental validation of double wedge tuner performance at cryogenic temperature

A new type of tuner with double wedge mechanism for tuning of cryo-module cavity has been theoretically analyzed and tested at CDM, BARC. The tuner is a candidate for incorporation in International Linear collider (ILC) operations for tuning and micro-phonics stabilization of RF cavities by changing its length. Mathematical model of tuner performance has been developed for performance estimation at different temperatures. The system has been characterized for various temperatures and its performance has been tested at lower temperatures. In this paper, the performance characteristics of tuner and its dependence on various factors like tuner material, manufacturing and assembly parameters, external environment including external temperature and resistive forces has been discussed. It also describes the low temperature testing of the system.