Pre-cooling of helium by means of liquid nitrogen is the oldest and one of the most common process features used in helium liquefiers and refrigerators. Its two principle tasks are to allow or increase the rate of pure liquefaction, and to permit the initial cool-down of large masses to about 80 K. Several arrangements for the pre-cooling process are possible depending on the desired application. Each arrangement has its proper advantages and drawbacks. The aim of this paper is to review the possible process solutions for liquid nitrogen pre-cooling and their particularities

Wagner, U

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsSOLUTIONS FOR LIQUID NITROGEN PRE-COOLINGIN HELIUM REFRIGERATION CYCLESU. WagnerPre-cooling of helium by means of liquid nitrogen is the oldest and one of the most common processfeatures used in helium liquefiers and refrigerators. Its two principle tasks are to allow or increase therate of pure liquefaction, and to permit the initial cool-down of large masses to about 80 K. Severalarrangements for the pre-cooling process are possible depending on the desired application. Eacharrangement has its proper advantages and drawbacks. The aim of this paper is to review the possibleprocess solutions for liquid nitrogen pre-cooling and their particularities.LHC DivisionPresented at the Eighteenth International Cryogenic Engineering Conference (ICEC 18)21-25 February 2000, Bombay Mumbai, IndiaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 387Abstract26 July 2000Solutions for Liquid Nitrogen Pre-cooling in Helium Refrigeration CyclesUdo WagnerLHC Division, CERN, 1211 Geneva 23, SwitzerlandPre-cooling of helium by means of liquid nitrogen is the oldest and one of themost common process features used in helium liqu f ers and refrigerators. Its twoprinciple tasks are to allow or increase the rate of pure liquefaction, and to permitthe initial cool-down of large masses to about 80K. Several arrangements for thepre-cooling process are possible depending on the desired application. Eacharrangement has its proper advantages and drawbacks. The aim of this paper is toreview the possible process solutions for liquid nitrogen pre-cooling and theirparticularities.1 INTRODUCTIONPre-cooling of helium with liquid nitrogen is a very common process feature. Each supplier of heliumrefrigerators has one or several standard solutions for the process arrangement and the nitrogen control. Infact only a limited number of solutions exists of how to integrate the liquid nitrogen pre-cooling into ahelium refrigeration cycle. These different solutions have nevertheless each their proper advantage anddraw back, and not all of them are equally adapted to the different tasks liquid nitrogen pre-cooling has tocover.2 LIQUEFACTION DUTY AND COOL-DOWN DUTYTwo principle tasks for the cooling capacity supplied by liquid nitrogen down to approximately 80 K canbe distinguished, liquefaction duty and cool-down duty, the requirements for which differ in severalpoints.During liquefaction the temperature range in which the cooling must be supplied remains constant,whereas during cool-down heat has to be withdrawn at continuously decreasing temperature. During cool-down the cooling capacity of the nitrogen available above the temperature of the helium returning fromthe load to be cooled is consequently lost.The required capacity for cool-down is usually significantly higher than that necessary forliquefaction, which has to be taken into consideration for the design of the heat exchangers involved.The flow imbalance between warm helium to be cooled and cold helium returning through the heatexchangers is limited for liquefaction, but maximum for cool-down duty. The temperature differencebetween the nitrogen and the helium can therefore become very large for cool-down duty, which againhas to be taken into consideration for the design of the heat exchangers.3 COMPARISON OF POSSIBLE SOLUTIONSThe different solutions for the process arrangement of the liquid nitrogen pre-cooling will be comparedconcerning:q The adaptation to the required function;q The danger of nitrogen freezing in the heat exchanger channels;q The amount of liquid nitrogen necessary for cool-down operation.3.1 Solutions with nitrogen and returning helium in the same heat exchangerFor all solutions where the returning low-pressure helium is passed through the same heat exchanger asthe gaseous nitrogen, the risk of freezing of nitrogen is inherent. These solutions should therefore only beadopted if the probability of excessive amount of cold helium flowing through this heat exchanger isnegligible. Three principle solutions that may be envisaged for the arrangement of heat exchangers,valves and process connections are shown in Figure 1.Figure 1  Solutions for the arrangement of nitrogen pre-cooling with the cold return helium in the same heat exchanger as thegaseous nitrogenThe arrangement given in Figure 1A is most common for liquefiers and is well suitable for continuousliquefaction duty. This arrangement is in general not adapted to cool-down operation, for which twomajor disadvantages can immediately be identified:q Active control of the temperature of the cold helium supplied by the pre-cooler is difficult for the startof the cool-down. A badly adapted control may even destroy the heat exchanger at the warm end;q The main heat exchanger will see high temperature differences around its cold end.The addition of by-pass lines as indicated in the process arrangement in Figure 1B, improves thefunctionality for cool-down operation. This arrangement is fully adapted both to steady state liquefactionload as to continuous cool-down operation. It represents no problem as for the control of the heliumsupply temperature, which is achieved by mixing with warm helium. The temperature difference at thecold end of the main heat exchanger is lower than in the previous example.Finally the by-pass for mixing the warm helium stream in order to achieve smooth temperaturecontrol can be located at a lower temperature level as shown in Figure 2C. It could either be connected tothe intermediate heat exchanger level or just above the liquid nitrogen vaporiser. Energetically this isequivalent whereas the first solution allows for slightly lower temperature differences at the cold end ofthe main heat exchanger.3.2 Solutions with nitrogen and returning helium in separate heat exchangersIn order to avoid with certainty the risk of freezing of nitrogen in the heat exchanger channels, theexchanger including the nitrogen channels has to be separated from that with channels for heliumreturning from the cold end of the installation. Two principle solutions that may be envisaged for thearrangement of heat exchangers, valves and process connections are shown in Figure 2.Compared to the solutions with common exchangers for all streams, these arrangements allowselecting high efficient aluminium plate fin exchangers for the helium-to-helium part, in which thetemperature differences will always be small. For the nitrogen exchangers a technology that is probablyless efficient but tolerant to high temperature difference can be selected. The maximum temperaturedifference in the nitrogen exchangers will on the other hand always be larger as for the solutionsintegrating all channels.Figure 2  Solutions for the arrangement of nitrogen pre-cooling with the cold return helium in a separate heat exchanger as thegaseous nitrogenThe arrangement given in Figure 2A is perfectly adapted to pure liquefaction duty. For cool-down duty onthe contrary it would require a continuously increasing helium mass flow through the nitrogen heatexchanger and consequently the cooling capacity from the returning helium could not be recovered. Thisarrangement is thus badly adapted to cool-down duty.The addition of a by-pass line as shown in Figure 2B allows for cool-down operation. Thisarrangement is fully adapted both to steady state liquefaction load and to continuous cool-down operation.It represents no problem as for the control of the helium supply temperature, which is achieved by mixingof helium. The temperature difference at all headers of the helium-to-helium heat exchanger is small asthis exchanger is always balanced.A particularity of this arrangement, if used for cool-down operation, is that the nitrogen leaving thecold box will continuously decrease in temperature down to the temperature of saturated vapour. Thismust be considered when designing the nitrogen exhaust piping.3.3 Thermal performanceThe thermal performance for the different solutions is only analysed for cool down operation, as it istrivial for liquefaction duty. The arrangement in Figure 2A is th refore not considered. For thecalculations, the following simplifications are used:q Ideal heat exchange, i.e. down to zero temperature difference is assumed in order to neglect theefficiency effect of different heat exchanger solutions;q The heat capacity of helium is considered constant;q The nitrogen is assumed to evaporate at 80 K with a heat of evaporation of 200J/g and with gaseousnitrogen having a constant heat capacity from vapour to ambient of 1.05 J/g.It is evident that these simplifications, especially of the properties of state, lead to errors in theresults. These errors are nevertheless sufficiently small to get qualitatively significant data. All data areplotted over the temperature of the cold returning helium and have as parameter the temperaturedifference between cold supply and return helium of 50 K, 75 K and 100 K. During cool-down, thistemperature difference can of course only be maintained until the supply helium reaches the minimumtemperature obtainable with the liquid-nitrogen pre-cooling.The minimum amount of nitrogen necessary expressed in percent of the helium flow for cool-down,is shown in Figure 3. This performance can be realised with the arrangements as shown inFigures 1C and 2B. The nitrogen consumption for the arrangement given in Figure 1B is higher comparedto these solutions until the supply temperature of the helium is at its minimum value. This results from themixing with warm gas instead of gas with constantly decreasing temperature. The relative mass flow isshown in Figure 4.With decreasing helium return temperature a part of the nitrogen cooling capacity is lost. This lossresults in decreasing temperatures at the warm end of the heat exchanger arrangement. For the solutionwith separated heat exchangers in Figure 2B this loss is expressed in a decreasing temperature of thegaseous nitrogen only. For the solutions with combined flow in the heat exchangers, this loss results in adecreasing temperature of the gaseous nitrogen and the returning helium. Assuming the same temperaturefor both streams, one can plot the outlet temperature at the warm end as function of the temperature forthe returning helium, again with the temperature difference of the helium at the cold end as parameter.Figure 3  Minimum values for the relative nitrogenconsumption versus cold helium return temperatureFigure 4  Relative nitrogen consumption versus cold heliumreturn temperature as for solution of Figure 1BFigure 5  Warm end helium return temperature versus coldhelium return temperature as for the solution for Figure 1CFigure 6  Warm end helium return temperature versus coldhelium return temperature as for the solution for Figure 1BFigure 5 shows the helium return temperature at the warm end as it results for the arrangement inFigure 1C and Figure 6 the one for the arrangement in Figure 1B. It is evident that the higher losses forthe solution of Figure 1B that are already expressed in the nitrogen consumption result as well in lowerreturn temperatures.4 SUMMARYFive principle solutions with their main characteristics for the process arrangement of the pre-coolingwith liquid nitrogen have been presented. Pure liquefaction duty is of little issue and can be realised in asimple and straightforward way. For cool-down duty dedicated by-pass lines are required and the heatexchangers have to be designed for high temperature difference.In order to decide on a specific solution to employ, one should also take into consideration theprobability of excess flow of cold helium, the frequency of cool-down operation, the required temperaturedifference between supply and return helium, and the overall size of the refrigerator and the heatexchangers.

Solutions for Liquid Nitrogen Pre-Cooling in Helium Refrigeration Cycles

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