Melaaen, E

Owren, G

Wadahl, A

Wagner, U

CERN Document Server

Simulation Program for Cryogenic Plants at CERNE. Melaaen, G. Owren*, A. Wadahl and U. WagnerLHC-Division, CERN, CH-1211 Geneva 23, Switzerland*Department of Refrigeration and Air Conditioning, NTNU, 7034 Trondheim, NoA steady-state simulation program for helium refrigeration plantsbeen developed.  The objective was to have a tool available for anplants in connection with the LEP project, but also for studying sfor future plants in the LHC project.  The program can simulatecomponents up to complete cycles, but it can also be used for closexchangers and expanders in a specified process.  Each component iswriting its energy balances, the equations are put into a unique mSince refrigeration cycles are mainly closed-loop processes, the prcomplete equation system simultaneously.INTRODUCTIONTo reach the necessary beam energy in the Large Electron Positron collidconducting cavities operating at 4.5 K are installed.  Four 12 kW @ 4.5supplied by LINDE and L’AIR LIQUIDE, are installed for the task [1]. ThereCERN used in connection with test facilities.  During his stay at CERN, Gtool (CryoSim) for analyses and studies of existing refrigeration plantsdevelopment of his work.  The tool is now used to simulate existing pevaluation of future cryoplants for the Large Hadron Collider (LHC) projectSCOPE AND CAPABILITIES OF CRYOSIMCryoSim is a robust steady-state simulation program developed for cryogenifor process simulations in general.  Thermophysical properties for heliuavailable package HEPAK [2] are installed.  Data from HEPAK are based on fuSince only helium data are implemented, the program has limitations for anfluids can be installed in the future.CryoSim is especially developed for off-design calculations of existiuseful tool for process calculations and optimization of helium liquefunderstanding and assessing the performance and efficiency for various crprogram can be thermodynamics studies and as a tool during specification ofThe first version of CryoSim had some limitations since the turbine constant and independent of the operation conditions.  In the same way theheat exchangers (UA) did not vary with the mass flow.  This is improved, efficiency and heat-transfer coefficient varying with the flow rate are impbe used for simulating situations with varying cooling power or at off-results.For a given process the flow-scheme has to be described in an input and efficiencies are given here.  In addition inlet and outlet streamsconnections between all components are specified.  The second part of the about all the mass streams in the process.  To specify a stream, flow rateor temperature have to be given.  From the given data in the input filespecified stream represents a boundary or an initial condition.The following components are modelled and included in the program; boexpander, flash drum, heater, heat exchanger, mixer, pipe, splitter, subcomodelling each component is the same, and the structure of the program mcomponent models.  Heat exchangers are modelled in two different ways; omethod (called hx), while the second method is more rigorous (called rhx).the heat exchanger into a specified number of blocks, and each heat-exchangseparately.  With this method it is possible to get out a thermodynamic deheat exchanger (e.g. temperature profile).  Also a control facility is avaused to update boundary conditions until specified set-points (e.g. temperThe program can calculate the flow sheet specified in the input fisimulation or design.  In the simulation mode the process is solved as boundary conditions and specified component parameters.  Design mode uses calculate the heat-transfer coefficient for heat exchangers and isentropic The calculated results are stored in a specific output file.  This filefor mass and energy, exergy analyses, stream data (temperature, enthalpyflow rate and quality), component data (pressure drop, UA, efficiency information about iterations.  Temperature profiles, pinch and NTU for heatTo work with CryoSim it is necessary to have some basic knowledge abouavailable at UNIX ® (e.g. emacs).   For the moment the program is only runnFigure 1 shows an example of one process together with part of its inregarded as the lower end of the cold box for one of the 12 kW plants at Cfile describes the main cluster of components here called EXAMPLE.  The calculation mode is design or simulation.  After this line starts a listcluster EXAMPLE.  All components with their characterization are written inThe first line for a component starts with an identification of the comporhx) and an identification name.  The information describing a component The example shows the structure of one heat exchanger and one expander minput file specifies information of each stream in the process.  Every liboundary or initial condition for one stream.PROGRAM STRUCTURECryoSim is based on an object-oriented technique and written in the progragives the program large flexibility.  The equation system for each componecovering the equations for the whole plant.  Since refrigeration cycles mthe program solves the total number of equations as one large system (siGauss-Jordan elimination method.  The results is a robust program.  The coenergy balances; therefore, the enthalpy for each stream is solved.  Theupdated separately by following the streams from boundary conditions and fin the flow-scheme.  For stream splitters and flash drums either split faspecified in the input file.  Similarly must the pressure drop inside eacha component be specified in the input file.  For a heat exchanger an opcalculating the pressure drop dependent on the flow rate.str1str2str3str4str5str6str9str7str8H1T1V1M1F1HE1Qwb1compound EXAMPLEmode simulationrhx HE1  feeds mass str1 mass str5  prods mass str2 mass str6  param 1 10 1 -1  pspec 1 0.1 0.05  design 0 38000.0  design 1 38000.0endexpander T1  feeds mass str2  prods mass str3 work wb1  pspec 1 4.5  param 1 0.7end...endendmass   str1   tp    7.17    6.36   524.0mass   str2   tp    4.72    6.36   524.0..mass   str8   hp  11.12   1.20   524.0mass   str9   tp    7.10    1.20   524.0endFigure 1 Shows the principle for an input filedescribing a given process [3]        The equations based on the energy balances are put into the matrixthe Gauss-Jordan method normally requires constant coefficients, butthermodynamic process has non-constant coefficients.  An iteration procedusolving the system.  For each iteration the coefficients are kept constancoefficients are updated between each iteration.  When the solution vectoriterations, the solution is reached.        Figure 2 shows the program structure.  Each box represents one ddirectory are compiled in a library.  All libraries are compiled togetherresult is the executable file sim.  Directory CryoSim is organized into threpresents the mathematical library with equation solvers, while cryodata afor thermophysical properties.  Basic contains the main structure of thorganization and storing of data.  Two important structures defined in streams and components.  The two subdirectory steady and dynsim contain and a transient simulation program.  dynsim is under preparation and, thpaper.  In the directory steady steady-state models of the components arprogram compiled to the executable file sim.  To run the program, sim must is meant to be a tool for graphical presentation of calculated results asmain CryoSim directorycryodatacryodatamat library basic plothepakdynsim steadyexecutableprogram simFigure 2 Showsthe directorstructure of tprogram andlinks to otheapplications [      CryoSim partly testeout at CERNby comparingsimulatedresults andmeasured datafrom runningplants.  These studies show good validity for the program, but a closer stin the near future.  Especially important is it to test the heat-exchangedesign and off-design situation.  A thorough description of the program Cthe validity of the models are given in [3-5].CONCLUSIONCryoSim is a robust calculation program for helium processes and cryoplasimulation and for limited design cases.  The program is useful during anevaluation of new processes.  Since the total equation system is solved adjusted to solve  closed-loop processes as refrigeration cycles. The objprogram large flexibility and possibilities for further development.  Crpreliminary results show good validity [3-5].REFERENCES1 Güsewell, D. et al., Cryogenics for the LEP200 Superconducting CParticle Accelerator Conference (PAC93), Washington D.C., 19932 Cryodata inc., User’s Guide to HEPAK, Version 3.21, Colorado, Januar3 Høye, G.K., A Simulation Program for Helium Cryoplants, Thesis, NorwTechnology, Trondheim, 19944 Wadahl, A., CryoPlant Process Simulation Program, Thesis, NorwegTechnology, Trondheim, 19955 Melaaen, E., User’s Guide to CryoSim and System Documentation fopreparation,  CERN, Geneva, 1996

Simulation program for cryogenic plants at CERN

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