Design and Development of LN 2 subcooler for

Design and Development of LN 2 sub-cooler for TIFR, Mumbai Mr. Aniket Bhimarao Pachimbare Second year M-Tech Thermal Engg. , Department of Mechanical Engg. , V. J. T. I. Mumbai, Maharashtra Dr. M. V. Tendolkar Department of Mechanical Engg. , V. J. T. I. Mumbai, Maharashtra K. V. Srinivasan Low Temp facility, T. I. F. R. Mumbai, Maharashtra Abstract My present study deals with thermal designing of a suitable helical coil & bath type Liquid Nitrogen Sub-cooler. The operating parameters of this sub-cooler will handle liquid nitrogen at 2 bar pressure with the quality factor of about 0. 25 is sub-cooled by using liquid nitrogen bath at 1 bar pressure& 77. 34 K temperature. Helically coiled heat exchanger is used in order to obtain a large heat transfer area per unit volume and to enhance the heat transfer coefficient on the inside surface. . Heat load, coil length & pressure drop calculations are done to estimate theoretical values using heat & mass transfer as well as fluid mechanics principles and numerical correlations. This study also deals with the computational fluid dynamics (CFD) simulation of helical coiled tubular heat exchanger used for liquid Nitrogen flowing in turbulent condition at 2 bar pressure Introduction Low temperature facility of Tata Institute of fundamental Research Mumbai provides liquid helium & nitrogen along with the various support services to various facilities & laboratories within the institute. Liquid nitrogen is produced in STIRLIN-8 plant at rate of 400 lit/hr at 4 bar pressure. That Liquid nitrogen is flowing through a 310 meters long vacuum jacketed and super-insulated pipeline to the Pelletron LINAC Facility. While passing through pipe, properties of liquid nitrogen changes due to various frictional losses, structural losses and heat in leaks. In order to meet the required properties at receiving end, a suitable helical coil & bath type Liquid Nitrogen Sub-cooler is required design. Cryostat of sub-cooler is equipped with all necessary safety system such as inner vessel safety valve, rupture disc combination pressure gauge, liquid nitrogen level gauge. Ø Helical coil heat exchangers have a more complex flow pattern due to the geometrical configuration of helical coils, which also impart additional centrifugal force on the inner coil flow and increasing the pressure drop on the shell side Ø For flow through helical coil another dimensionless number is introduced called as Dean number (De) subcooler CFD analysis of single phase liquid nitrogen flow through helical coil where d= tube diameter D= coil/curvature diameter Ø Kalb & Seader developed concise correlation of numerically calculated fully developed Nusselt number for Prandtl number range of 0. 7 to 5 Where h= het transfer coefficient k=Thermal conductivity Ø Finally total coil length is calculated for effective heat transfer, based on heat transfer coefficient. Conclusions Pressure Drop Calculation On the basis of analysis, solution algorithm and results discussed, the following conclusions have been drawn. 1. Estimated coil turns are sufficient to desire temperature drop or to subcool the single phase liquid nitrogen. ØSeparation model for calculation of pressure drop has been found to be most suitable for liquid nitrogen hence using same following calculation were made. 2. Dividing the total coil length into parts, are favorable to enhance heat transfer also leads to reduce in pressure drop. Problem Statement Source Results ØThe parameters used in correlating adiabatic two phase frictional drop according to Lockhart Martinelli separation model as follows Future Work state point Pressure(bar) Quality Temp (K) Enthalpy (KJ/kg) 1 2 3 4 5 2 2 1 0 0. 25 0 1 94 83. 6 78 77. 34 64. 26 -61. 415 -120. 53 Design Procedure Heat Transfer Rate Calculation Ø Fluid is present at inlet in 2 phase , so we require to absorb latent heat as well as sensible heat for sub-cooling. Q= m*(h 2 –h 3) Where Ø CFD analysis of 2 phase liquid nitrogen flow through helical coil Ø Installing Diffuser at inlet of sub-cooler, to improve LN 2 quality Model of LN 2 Sub-cooler References: 1. Barron, R. F. , Cryogenic systems, Oxford University Press(1985) 2. Charles Kalb & J. D. Seader , fully developed viscous-flow heat transfer in curved circular tubes with uniform wall temperature, Al. Ch. E Journal, Vol. 20, March 1974. 3. J. K. Partridge, J. W. Tuttle, mathematica model and experimental result for cryogenics densification and sub-cooling using a submerged cooling source, cryogenics 52 Elsevier(2012) PP 262 -267. 4. Andy Kuwazaki & Todd Leicht (Fermi-Lab), Liquid N 2 Subcooler Coil Sizing for D-Zero Upgrade, Oct 3, 1995’ 5. NIST Website - www. nist. gov m= mass flow rate (kg/sec) h= Enthalpy (KJ/kg) Heat Transfer Coefficient Calculation Ø In an effort to provide the same amount of heat transfer as a straight tube heat exchanger in a smaller space, engineers replace the straight inner pipe with a helical coil. Ø This allows for more heat transfer surface area in a smaller length shell, but increases the pressure drop across the heat exchanger.