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Case study lnp

  1. 1. Liquid Nitrogen Plant Thermodynamic principle involved in it Submitted to:- Prof R.Jayaganthan (Metallurgy department IITR) By UDIT KUMAR (12118084) SOURABH HIRAU (12118077) SOURABH SINGH (12118078) VAIBHAV KUMAR (12118085) VAMSI PEDAPALLI (12118088)
  2. 2. CONTENTS 1 Introductions 2 Sterling an overview 3 Basic cycle analysis 4 Regenerator analysis 5 Working gas analysis 6 Detail analysis overall 7 Institute Instrumentation centre LN2 8 IIC LN2 Performance and uses analysis 9 References
  3. 3. Introduction Liquid nitrogen is nitrogen in a liquid state at an extremely low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.43.[1] Liquid nitrogen is often referred to by the abbreviation, LN2 or "LIN" or "LN" and has the UN number 1977. At atmospheric pressure, liquid nitrogen boils at −196 °C (77 K; −321 °F) and is a cryogenic fluid that can cause rapid freezing on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks. Liquid nitrogen is a compact and readily transported source of nitrogen gas without pressurization. Further, its ability to maintain temperatures far below the freezing point of water makes it extremely useful in a wide range of applications, primarily as an open-cycle refrigerant, including: Uses  in cryotherapy for removing unsightly or potentially malignant skin lesions such as warts and actinic keratosis  to store cells at low temperature for laboratory work  in cryogenics  in a Cryophorus to demonstrate rapid freezing by evaporation  as a backup nitrogen source in hypoxic air fire prevention systems  as a source of very dry nitrogen gas  for the immersion freezing and transportation of food products  for the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials  to preserve tissue samples from surgical excisions for future studies  as a method of freezing water pipes in order to work on them in situations where a valve is not available to block water flow to the work area - nowadays replaced by electrical heat pumps  in the process of promession, a way to dispose of the dead  for the cryonic preservation in the hope of future reanimation.  to shrink-weld machinery parts together  as a coolant  for CCD cameras in astronomy  for a high-temperature superconductor to a temperature sufficient to achieve superconductivity  for vacuum pump traps and in controlled-evaporation processes in chemistry.  to increase the sensitivity of infrared homing seeker heads of missiles such as the Strela 3  to temporarily shrink mechanical components during machine assembly and allow improved interference fits  for computers and extreme overclocking[4]  for simulation of space background in vacuum chamber during spacecraft thermal testing [5]  in food preparation, such as for making ultra-smooth ice cream.[6] See also molecular gastronomy.  in container inerting and pressurisation by injecting a controlled amount of liquid nitrogen just prior to sealing or capping.[7][8]
  4. 4.  as a cosmetic novelty giving a smoky, bubbling "cauldron effect" to drinks. liquid nitrogen cocktail.  as an energy storage medium.[9][10]  branding cattle.[11] Production Liquid nitrogen is produced commercially from the cryogenic distillation of liquified air. An air compressor is used to compress filtered air to high pressure; the high-pressure gas is cooled back to ambient temperature, and allowed to expand to a low pressure. The expanding air cools greatly (theJoule–Thomson effect), and oxygen, nitrogen, and argon are separated by further stages of expansion and distillation. Liquid nitrogen may be produced for direct sale, or as a byproduct of manufacture of liquid oxygen used for industrial processes such as steelmaking. Liquid-air plants producing on the order of tons per day of product started to be built in the 1930s but became very common after the Second World War; a large modern plant may produce 3000 tons/day of liquid air products. The best way to produce liquid nitrogen plant commercially is sterling cycle because of the following reasons:-  The cycle is regenerative:-The cycle is defined as a closed regenerative cycle with a gaseous working fluid. "Closed cycle" means the working fluid is permanently contained within the thermodynamic system. This also categorizes the engine device as an external heat engine. "Regenerative" refers to the use of an internal heat exchanger called a regenerator which increases the device's thermal efficiency. The cycle is the same as most other heat cycles in that there are four main processes: compression, heat addition, expansion, and heat removal.However, these processes are not discrete, but rather the transitions overlap.  The cycle is efficient:-The Stirling engine is noted for its high efficiency compared to steam engines, quiet operation, and the ease with which it can use almost any heat source.  The cycle is commercially suitable :-This compatibility with alternative and renewable energy sources has become increasingly significant as the price of conventional fuels rises, and also in light of concerns such as peak oil and climate change. This engine is currently exciting interest as the core component of micro combined heat and power (CHP) units, in which it is more efficient and safer than a comparable steam engine. Sterling An Overview The history of the Stirling Cycle The Stirling cycle is a thermodynamic closed cycle invented in 1816 by a Scottish minister Robert Stirling. It was used as an engine and was considered at the time to be capable of replacing the steam engine since boilers used in early steam engines were prone to life-threatening explosions. The counterpart of the hot air motor, the refrigerator, was first recognized in 1832. Both machines experienced high and low points during the nineteenth century. The principle behind the machines was almost condemned to obscurity after the invention of the internal combustion engine (gas-, petrol-, and diesel motors) and compressor refrigerators with external evaporation.
  5. 5. In 1938 the famous Dutch Philips Research Laboratory was looking for a means to power electricity generators for short wave communication systems in remote areas without electricity supply. The practically-forgotten hot air motor attracted attention. In 1946 Philips started optimizing the cooling techniques used in the Stirling cycle. The result was the development of the world conquering cryogenerator, marking the start of significant cryogenic activities at Philips. Though the Stirling hot air motor itself never became a commercial success, the Stirling cryogenerator has been selling by thousands worldwide and has been incorporated in equipment and projects used from Antarctica to the North Pole. Stirling based cryogenerators are used in a wide range of applications, including the production of liquid gases and the cooling of gases, liquids and industrial processes. In the beginning of the 90's the Philips Cryogenic Division became independent and continued its worldwide activities under the name of Stirling Cryogenics BV. Thanks to continuous innovation and considerable investment in R&D, the Stirling cryogenerator is now used in advanced technological machinery for cooling gases and liquids to very low temperatures (i.e. 200K to 20K). The central element in all equipment made by Stirling Cryogenics BV (Stirling) is the Stirling cycle cryogenerator. The cycle is remarkable because it is a closed cycle in which the cryogenerator's working gas never comes into contact with the fluid to be cooled. This two-circuit approach eliminates contamination of the working gas resulting in long continuous operating periods and longevity. The Stirling cryogenerator is extremely environmentally friendly: it does not cause ozone layer depletion in any way, does not contribute to the greenhouse effect and does not emit any harmful or toxic product. It is extremely efficient compared to other cryogenic cooling principles. Stirling is the only company in the world that successfully produces Stirling cycle-based cryogenerators with cooling powers of 1,000-4,000 watt (at 77K) per unit. The Stirling cycle is a thermodynamic closed cycle invented in 1816 by a Scottish minister Robert Stirling. It was used as an engine and was considered at the time to be capable of replacing the steam engine since boilers used in early steam engines were prone to life-threatening explosions. The counterpart of the hot air motor, the refrigerator, was first recognized in 1832. Both machines experienced high and low points during the nineteenth century. The principle behind the machines was almost condemned to obscurity after the invention of the internal combustion The Stirling cycle alternately compresses and expands a fixed quantity of a nearly perfect gas (also known as ideal gas) in a closed cycle. The compression takes place at room temperature to facilitate the discharge of heat caused by compression, whereas the expansion is performed at the required low temperature. For the purpose of explanation, the process may be split up into four distinct phases illustrated in Figure 1 and each indicated by a number.
  6. 6. Cylinder A is closed by piston B and contains a certain amount of gas. The space inside the cylinder is divided into two sub-spaces, D and E, by a second piston C, called displacer, An annular channel F connects spaces D and E and contains three heat exchangers: regenerator G, cooler H and freezer J.In position I most of the gas is in space D and at room temperature. During phase 1 this gas is compressed by piston B. In phase 2 the gas is displaced by means of the displacer from space D to space E, which is already at a low temperature.During this displacement the gas passes through the heat exchangers. The cooler dissipates the heat caused by compression through cooling water. The regenerator cools the gas almost to the temperature prevailing in space E. In Phase 3 the actual cold production takes place by allowing the precooled gas to expand through movement of the displacer and piston together. Finally, by moving the displacer (phase 4), the gas is returned to space D for a new cycle to begin. While passing the freezer its cold is dissipated to the ambient environment, and in the regenerator it is reheated to nearly room temperature. The initial situation of the cycle has now been restored. It is clear that a large temperature difference will occur between the compression space and the expansion space. The way in which this temperature difference is established and what influence the regenerator has in this is shown in the two graphs on the right.
  7. 7. The working gas in both the compression and expansi- on space is initially at ambient temperature. During the first working cycle the gas is successively cooled by the cooler and by the expansion to temperature T1. When the expanded gas returns to the compression space, a temperature gradient is established in the rege- nerator. This means that, after the second compression stroke, the working gas is slightly pre-cooled in the rege- nerator before it is expanded in the expansion spaces to reach temperature T2. After a number of strokes the temperature gradient in the regenerator reaches an equilibrium, which means that the working gas reaches its lowest temperature, T3 after expansion. It is obvious that the regenerator is the most important component in this cooling process. Basic cycle analysis In this section abrief outline of thebasic principles of the Stirling cycle is presented. The general properties of the cyclewill be derivedwith a discussion of the most important losses. Fundamental Cycle
  8. 8. In the ideal Stirling cycle, the cold isproduced by the reversible expansion of a gas. The gas performs a closed cycle, during which it is alternately compressed at ambient temperature ina compression space and expanded at the desired low temperature' in an expansion space, thereby reciprocatingbetween these spaces through one connecting duct, wherein a regeneratorprovides for theheat exchange between the outgoing and the returninggas flow. Figure 1 shows stages in carrying out the ideal cycle. In this diagram,A is a cylinder, closedby the piston B, and con- taining a nearly perfect gas. A secondpiston, the displacer C, divides the cylinder into two spaces, D at room temperature and E at the low temperature, connectedby the annularpassage F. This passage contains the regenerator G, a porous mass with a high heat capacity: the temperature in thepassage is shown in thegraph. The cycle, consisting of fourphases, runs as follows: I Compression in space Dby thepiston B: the heat of compressionisdischarged through the coolerH. II Transfer of the gasthrough the regenerator to space bymovement of the displacer. The gas is reversibly cooled.down in the regenerator, the heat of the gas being the regenerator mass. III Expansion in the cold space by the combined movement of the piston anal the displacer; the cold produced is discharged through the freezerJ and utilized, IV Return of the gas to space D; thereby the gas is reheated, the heat stored in the regenerator being the gas. With no regenerator present the gas flowing to the expansion space would arrive there at ambient temperature, whereas the returning gas would arrive in the compression space at the low temperature this effect would cause such a tremendous cold loss that the whole process would become futile. In an ideal regenerator, a temperature gradient
  9. 9. is established in the direction of the gas flow. this causes the gas to be c ooled down reversibly, so that it arrive in the expansion space with the temperature prevailing there .Figure2 shows the p-V diagram of this schematic cycle,neglecting the dead space. It consistsof two isotherms and two isochores ;at TC (compressiontemperature) the amount of heat Qc is rejected, atTE (expansiontemperature) the amount of heat QE is absorbed. Actually, the discontinuous movement of the pistons isdifficult to achieve. Inpractice, the pistons are actuated by a crank mechanism and are thus moving harmonically; for the machine to act as a refrigerator, the expansion space E (Figure1)has to lead inphase with respect to the compression spaceD. The harmonic motion and the volume of the heat exchangers (theso- called'deadspace') cause the fourphases of the cycle to merge somewhat, so that they cannotbe distinguished verywell; the gas isnot exclusively compressed in the compression space,but also in the expansion space, and the sameholds for the expansion. It can be shownhowever that the difference in outputbetween the discontinuous and theharmonic process isonly a fewper cent.The fundamental cycle is explained here with the help of the dis- placer machine. The reason is that actual machines are also .of this type. It will be obvious, however, that the cycle may be described mere generally by two synchronously changing volumes interconnected by a cooler,a regenerator, and a freezer, whereby the volume that is leading in phase becomesthe expansion space .Where in the cell is produced. Formulae for the pressure variation, the refrigerating capacity, and the shaft powerfor this general case will be given in the next section Performance of the Ideal (Isothermal) Cycle In the ideal machine, the thermal contact in the heat exchangers is assumed to be perfect, sothat the gas temperature there is equal to the temperature .ofthe walls; the same temperature is supposedto prevail in the adjoining cylinders, which temperatures
  10. 10. thus are constant with time. The regenerator is also assumed to be perfect, so that ne regeneration less .occursand the gas temperature there is alsoconstant with time. The working fluid is suspended to be a nearlyperfect gas; this condition can be met sufficiently by us hydrogenor helium in practical , (1) Pressure variation. For the expansion space V E (temperature (2) and the compression space Vc (.temperature Tc) we write VE =1/2V0(1+cosα) VC=1/2wV0(1+cos(α-ϕ)) Where Vo is the maximum volume .ofthe expansion space, w the ratio of the swept volumes .ofthe compression and the expansion space, and ϕ the phase difference between these spaces; the crank angle α(=0 for VE=Vmax)Changes linearly with time (α = ω t ). volume of the freezer, the regenerator, and the cellar, which form the connecting channel (the dead space), will be indicated by VS with the corresponding temperature TS The variation the pressure p with time (or α) new follows from the condition that the mass .ofthe system as a whole is constant: Where M is the molecularweight of the gas, R is the gas constant, And c is a constant. After introductionofVEandVc fromequation(1) and (reduced to the swept volume of the expansion space V0 andnormalized to the temperature of the compressionspaceTC this reduces to This expression is easily transformed into with the abbreviations’ The constantA may be interpreted as two times the totalchange of volume,while B equals two times themean total volume,both reduced to Vo andnormalized to TC For the pressure p we thus find which may be written more conveniently Expression (2) shows that the pressure variation with time is not purely harmonic. In practice, the deviation of harmonic behaviour is rather small however (the function is symmetrical with respect to (Pmax and Pmin,which points are 1800 apart) as the value of δ
  11. 11. Seldom exceeds 0.4. This means that the pressure ratio of this type of machine is about 2, a remarkably low value Compared with what is normal in refrigerating apparatus. The presence of the phase angle θ shows that the pressure variation is not in phase with the variation of the expansion or the compression space; it is easily checked that its phase is intermediate between that of these spaces (θ->ϕ for τ->0) It will be found, for this reason, heat is absorbed in the expansion space and liberated in the compression space. For later reference, given here is the expression for the mean pressure pm , deduced by integrating the pressure with respect to the crank angle α (2) Heat absorption in cylinders. The heat absorbed per cycle in the expansion space (QE)and in the ~compression space (Qcanegative quantity) is given by that three exp is used in spaces with gas content, may also 'beusedin the casewhere gas enters and leaves the space can 'beproved by an involved thermodynamic reasoning which isomitted here. It is easily seen that the value of the integrals depends only on the components of p which have the same phase as over and Over; this means that 0< θ <ϕ ifthe machine has to operate as a refrigerator.Evaluation of the integrals leads us ……(4) For practical use, equation (4) may be transformed into qE' the heat absorbed per second (orthe refrigerating capacity) by inserting n,the no of revolutions per minute ,if V0 is expressed in cm3 ,pm in kg cm-2 ,and qe in watts ,it is found that Expression (4a)shows that, besides with Vo and n, the output is proportional toP, and Linearly proportional to the functionB, Which was defined as the mean reducedand normalized volume. The consequences of the dependence on the mean pressure and the temperature ratio (T is also contained in s),Which constitutes one of the features of the process, will be discussed at more length later. According to expression (4a)an increase of the dead space s reduces the output, as couldbe expected. (3) Shaft power and efficiency. :For the work w needed to drive the machine one may write W=-QE-QC ,W=(τ-1)QE=π/a(τ-1)V0pmwsin(ϕ)/B……..(5) Using equation (5) the efficiency of the cycle is the efficiency of the ideal cycle thus equals that of the Carnot cycle this is obvious as the cycle is completely reversible. Although in actual machines the performance is reduced by losses, tc be discussed in the next section; the ideal cycle has to be consideredas the reference process, because most essential facts can be deduced from it.Before closing this section two remarks en the representation of the process will be made. The first remark concerns the representation in a thermodynamic diagram (e.g., p vs V,T Vs S,etc.). These diagrams always relate ,to a fixed quantity .ofthe working fluid,
  12. 12. which has to be in internal equilibrium (equal p and T throughout); this quantity is passed through a number of thermo.... dynamic states and (at least in a closed system) is returned ultimately to its initial state, so that a cycle is described. Looking at’ the Stirling process, it is found that, in so far as cyclic behaviour is concerned, nothing abnormal is at hand.How ever, the system is net homogeneous at all, as different parts of it have a different temperature. AS a consequence different gas particles describe completely different cycles between different temperatures to mention .onlytwo extreme examples, some particles are reciprocating between the compression space and a point in the cooler,while other particles are reciprocating between a point in the freezer and the expansion space. That, because of this situation, normal diagram shave lost their value is shown by the fact that .onewould have to draw an infinite number of diagrams for the diverse particles with different cycles, which obviously leads nowhere. The only diagram which still has some sense is the p-V diagram, as the system is homogeneous inp. But in such a diagram one isnot allowed to draw isotherms or adiabatic, as these lines have lost their meaning. To avoid this difficulty when drawing Figure 2, the dead spacewas assumed to be zero: in that case isothermsmay be drawn,since only then the gas is at thermal equilibrium during the compression and the expansion. When the adiabatic losses are discussed in the next section, this subject will have to be returned to. The second remark concerns the description of the cycle in a schematic form. The transfer of the gas from the compression space to the expansion space and vice versa is performed with constant volume of the gas; this is the simplest representation, as the transfer can beeffected bythe movement of the displacer only. But this way of transfer is only one eg of a multitude of possibilities which exhibit the common property that regeneration is possible. As another example consider transferat constant pressure, which way of trans-for approximates the harmonic cycle much better. The only reason why this manner of transfer is not used to explain the cycle is that it can only be accomplished by simultaneous movement of the piston and displacer, which obviously ismore complicated. This point is stressed because at many places in literature the transferwith constant volume isconsidered to be one of the main characteristics of the Stirling cycle, to contrast itwith the cycle of Claude (withseparate compressor and expander),where the transfer isperformed at constantpressure. As explained, this view does not correctly locate the distinctionbetween the two cycles, the real difference being that in the Stirling cycle the gas is reciprocating through one connecting duct, wherein the heat exchange is brought about by regeneration,whereas in theClaude cycle the connecting circuit consists of a exchanger (withtwo channels). Apart from the restriction contained in the first remark the Stirling cycle thus closely resemblesthe Claude cycle thermodynamically. The Actual Cycle The losses can affect the process in two different ways, viz."by increasing the shaft power and by decreasing the refrigerating capacity; the smaller the ideal values of these quantities, the more pronounced will be the relative effect. Figure 3 illustrates that increase of the shaft power exerts the greatest influence at high refrigerating temperatures, while decrease of the Output createsthe most adverse effects at low temperatures. In this way a temperature range arises, the "optimum working range", wherein the actual efficiency differs least from the ideal one (i.e., where the figure of merit is highest). This range may be shifted both to higher and to lower temperatures by suitable design; moreover, its,limits greatly depend on present and future technological possibilities.
  13. 13. The increase in shaft power is mainly due to three causes, namely, the mechanical loss of the drive, the flow loss (that is, the power needed to force the gas through the narrow connecting circuit) and the adiabatic loss. The first two losses need no further comment, but the adiabatic loss will be discussed at greater length. In the ideal isothermal process it was assumed that the temperature in the cylinders is constant with time. This means that thethermal contact between the gas and the wall in the cylinder spaces is assumed to be so perfect that these walls can be employed as well to establish the contact between the gas and the surroundings, that is, to serve as cooler and freezer. In this case, separate heat exchangers are of no use and must be omitted; this ideal machine thus consists of the two cylinders and the regenerator and is therefore referred to as the "three-element machine”. Actually, the separate heat exchangers are introduced because the thermal contact between the gas and the cylinder walls is always so poor that insufficient heat exchange with the surroundings can be established through these walls; for this case the name "five-element machine" isused. As a consequence, the temperature of thegas in the cylinders changes nearly adiabatically with time. This adiabatic behaviour has only a minor influence on the efficiency (it would be too involved to give the full explanation here; it is based on the fact that these processes occur in both cylinders with the same phase, so that the temperature ratio is independent of time) but in this case the heat (or the cold) must be transported from the cylinders to the heat exchangers. This transport can only be effected by the reciprocating gas which performs it by assuming different temperatures when flowing in opposite directions, with the resultthat the mean temperature in each cylinder deviates from that in the adjoining heat exchanger. Figure 4 shows schematically the temperature distribution in the machine. It will be observed that the mean temperature in the expansion cylinder is lower than that of the freezer and that the mean temperature in the compression cylinder is higher than that of the cooler. This means that the ratio of the cylinder temperatures is higher than that of the temperatures of the
  14. 14. heat exchangers, which causes an increase in shaft power. Strictly speaking the expression "adiabatic loss" is therefore misleading and should be replaced by IL transport loss". It is adhered to though ,because ultimately the adiabatic behaviour is the fundamental cause that has necessitated the introduction of separate heat exchangers. Because the value of the temperature ratio T is larger in the adiabatic than in the isothermal case, one would expect also a decrease in refrigerating capacity; this influence is very small, however, as the larger value of T is nearly compensated by a decrease of the quantity S, the mean relative volume of the working circuit, which quantity has another form in the adiabatic case.In the above discussion the use of the expressions "adiabatic compression" or “expansion” has been intentionally avoided. The reason is that the gas is compressed and expanded everywhere in the working space its behaviour during these processes, however, depends largely on the condition of heat transfer prevailing in the various sections of the volume. While in the cylinders the gas temperatures change nearly adiabatically, this is not the case in the connecting circuit in the regenerator, for example, the heat transfer is so high, that the behaviour is practically isothermal. Thus, while the actual Stirling process is certainly not isothermal, it would be incorrect to call it adiabatic. This situation thus furnishes still another example for the previously made remark that thermo- dynamic diagrams are of little value for this process.The decrease of the x- refrigerating capacity is mainly due to two effects,the flow loss and the insulation loss. Again no comment is made on the flow loss. Also the insulation (and conduction) loss proper needs no discussion. There exists another loss, however, that acts as an insulation loss as well which has the utmost importance for the quality of the process. This loss, caused by the non-ideal behaviour of the r e generator, will now be discussed.It is obvious that in the Stirling process ideal regeneration is possible in principle as the same amount of gas passes the regenerator in both directions with the same temperature difference, so that the amount of heat rejected and absorbed by the gas for both directions of flow is the same,since the specific heat is independent of pressure (which is practically the case for nearly perfect gases).The following argument shows the extreme importance of a Small departure from idealistic nature of the regenerator, caused by non-ideal heat transfer. In the regenerator a quantity of heat Qr must be absorbed and rejected in each cycle. Owing to imperfections, this amount is reduced to eta(r)*Qr where eta(r) is efficiency of regenerator.This means only part of heat is available to regenerator. The rest that is (1-eta(r))*Qr carried along with the gas through the regenerator , so that gas arrives too hot in the expansion space. This remainder deltaQr constitutes the regenerator loss .As this loss must be made up from the cold produce Qe,it must be compared with this quantity. calculation show that
  15. 15. The constant Cr depends mainly on compression ratio it’s value mainly is 10 eg we take Tc=300K and Te=75K and regeneration loss 1% then δQr/Qe=30%Thus each per cent of regeneration loss involves .a loss of 30 % in refrigerating power;below 30K this figure even increases to greater than 90 per cent, meaning that at such temperatures the entire cold production is consumed by the regenerator. It is thus no exaggeration to call the regenerator the heart of the machine. The regenerator used in’ actual gas refrigerating machines consists of a mass of fine metal wire, forming a light felt-like substance. With this type of material, efficiencies of 99 per cent and higher can be obtained; the thermal conductivity of the material is very low.This section will be ended with a short discussion on the problem of how to minimize the total sum of the losses; this problem is very involved indeed, so that only a very broad outline can be given. As an example, the regenerator will be considered. The losses of the regenerator make themselves felt in three different ways, viz., the regeneration loss (reducing the cooling power), the flow loss, and the dead space (this is not a real loss as the shaft power is also decreased; it exerts its influence through the other losses, as these increase relatively). The 'regeneration loss can be reduced by making a longer regenerator: this, however, increases the flowloss and the dead space. One may also give the regenerator alarger cross-section; this reduces the flow loss and somewhat the regeneration loss, but increases the dead space. Thus it is possible to find optimum dimensions for the regenerator. The same hold for the freezer and the cooler, where the regeneration loss is replacedby the loss due to insufficient heat transfer. Moreover, the losses are governed by the choice of the values of and phai. Finally, the outside of the cooler and the freezer must be made optimal. Thus a very large number of design parameters are to be fixed such that the total result gives an optimum condition; it will be obvious that a lot of experience is needed to find the right solution quickly.This is compensated by the fact that once a system of calculation is worked out, it holds for any size of machine. Regenerator Analysis Condition of perfect regeneration For a Stirling refrigeration cycle with perfect regeneration, the heat Q 1 transferred to the regenerator from the working substance in a constant generalized co-ordinate cooling process must be equal to the heat Q2 transferred back to the working substance from the regenerator in the constant generalized co-ordinate heating process. When QI is not equal to Q2' the Stirling refrigeration cycle cannot achieve perfect regeneration. Because QI and Q2 are dependent on the heat capacity of the working substance, a Stirling refrigeration cycle can achieve perfect regeneration only if the heat capacity of the working substance satisfies certain conditions. In order to determine these conditions, we first give the fundamental equation of thermodynamics for a working substance dU = TdS + Ydy ….(1)
  16. 16. where U and S are, respectively, the internal energy and entropy of the system, and Y is the corresponding generalized force of the generalized co-ordinate y. For a gas system, y = V and Y = - P, where V and Pare, respectively, the volume and pressure of the system. For a ferromagnetic system (the change in volume in the ferro-magnetic system is ignored),y=M and Y=(H+ƛM),where µ0 is the permeability of vaccum,ƛ is the molecular field constant, H is the magnetic field intensity and M is the magnetization intensity of the ferromagnetic substance. Then QI and Q2 may be expressed as ..(2) and …..(3) where Cy represents the heat capacity at constant volume Cv for a gas system or the heat capacity at isomagnetization CM for a ferromagnetic system it is seen from Equations (2) and (3) that when Cy is only a function of temperature T but does not depend only (v olume or magnetization), Q1 is equal to Q2" Hence ………(4) may be taken as a general criterion. When this holds true, the Stirling refrigeration cycle possesses the condition of perfect regeneration. Using thermodynamic relations, Equation (4) may be expressed in another useful form …………(5) Using the equation of state of the working substance, one can determine directly whether Equation (5) is true. When Equation (5) or Equation (4) is not true, the Stirling refrigeration cycle cannot, in general, possess the condition of perfect regeneration. Regenerative characteristics of gas stirling refrigeration cycle For an ideal Vander wall gas we have ……(11) For a gas which is described by the Redlich-Kwong, Beattie-Bridgeman, Benedict-Webb-Rubin, Dieterici, Berthelot or Martin-Hou equation of state" we can prove that ……………..(12) It can be determined from Equation (5) that a Stirling refrigeration cycle using an ideal or van der Waals gas as regeneration while a Stirling refrigeration cycle using a the working substance possesses the condition of perfect regenration cycle using a gas which is described by the Redlich Kwong Beattie-Bridgeman, Benedict-Webb-Rubin, Dieterici, Berthelot or Martin-Hou equation of state as the working substance does not,in general, possess the condition of perfect regenration Effect of non-perfect regeneration on performance of gas Stirling refrigeration cycle For a Stirling refrigeration cycle with perfect regeneration, the coefficient of performance is equal to that of a Carnot refrigeration cycle for the same temperature range. For a Stirling refrigeration cycle with non-perfect regeneration, the coefficient of performance is always smaller than that of a Carnot refrigeration cycle for the same temperature range and is heavily dependent on the specific properties of the working substance in the cycle.for illustration the gas Stirling refrigeration cycle as an example and assume the working substance in the cycle is a gas described by a strict equation of state": ………..(13) where R is the universal gas constant, a and b are two positive parameter and n is a real parameter. The generality and significance of Equation (13) lie in thefact that the values of a, b and n may be obtained
  17. 17. for different gases 9. For example, for three low temperature gases (He, H 2 and Ne), the values of a, b and n are listed in Table 1. In particular, when n = 1/2, Equation (13) becomes the Redlich-Kwong equation of state, which has a wide range of application in low temperature technology. From Equation (13), we obtain the entropy of the gas ………….(14) And the heat capacity at constant volume …………….(15) where both So(T) and C~T) are a function of temperature only. However, the term on the right-hand side of Equation (15) is dependent on the volume, so Cv is dependent on volume V as well as temperature T. This shows that a Stirling refrigeration cycle using a gas described by Equation (13) as the working substance does not, in general, possess the condition of perfect regeneration.Using Equations (2), (3) and (15), we obtain ……(16) and ……(17) where V1 and Vl are the volumes of the working sub- stance in the constant volume cooling and heating pro- cesses, respectively. On the other hand, during isothermal compression, the volume of the working substance changes from Vl to V1and heat QH is released to the high temperature heat reservoir at temperature TH• During isothermal expansion, the volume of the working substance changes from V1 to Vl and heat QL is absorbed from the low tempera- ture heat reservoir at temperature TL. Then, using Equation (14), we find ……(18)and From eqn 16 and 17 we obtain ……..(19)
  18. 18. From Equation (20), one may come to the conclusion that when n # 0, -1, and a # 0, AQ ~ 0, so that the Stifling refrigeration cycle does not possess the condition of perfect regeneration For a gas Stirling refrigeration cycle operating only between two heat reservoirs at temperatures Tn and TL,the temperature of the working substance in the two constant volume processes is always lower than that of the high temperature heat reservoir and higher than that of the low temperature heat reservoir. When the heat transferred into the regenerator is larger than that transferred out of the regenerator, the redundant heat in the regenerator can only be released to the low temperature heat reservoir in a controlled manner, such that the refrigeration heat is reduced from QL to Q~. If not, the temperature of the regenerator would be changed and the regenerator would not operate normally. Similarly, when the heat transferred into the regenerator is smaller than that transferred out of the regenerator, the heat deficit in the regenerator can only be supplied from the high temperature heat reservoir in a controlled manner, while QL remains unchanged. Thus, we find that the level of refrigeration per cycle is given by Qr = …….(21) Using eqn 16 and 19 we obtain work input per cycle ………(22) Therefore coefficient of performance of sterling refrigeration cycle is given Equation (23) shows that when n ~ 0, - 1, and a ~ 0, the coefficient of performance e of the Stirling refrigeration cycle is always smaller than that of the Carnot refrigeration
  19. 19. cycle for the same temperature range because the Stirling refrigeration cycle does not possess the condition of perfect regenration from eqn(23a) we can determine that when ……..(24) the coefficient of performance of a Stirling refrigeration cycle operating between two heat reservoirs is equal to zero. It is obvious that when a gas with n > 0 or n < - 1 is used as the working substance, the Stirling refrigeration cycle can only operate under the circumstances where …………(25) That is to say that for such a Stirling refrigerator, the temperature spans of the two heat reservoirs between which the Stirling refrigerator operates are restricted.It is seen from Table 1 that for a Stirling refrigeration cycle using H e or Ne as the working substance, the level of refrigeration and the coefficient of performance should be given by Equations (21a) and (23a), respectively; while for a Stirling refrigeration cycle using He as the working substance, the level of refrigeration and the coefficient of performance should be given by Equations (21b) and (23b), respectively. When the working substance in a Stirling refrigerator is a gas described by the Redlich-Kwong equation of state the level of refrigeration …..(26) And the coefficient of performance is of the cycle may be directly deduced from Equations(21a) and (23a), respectively. For such a Stirling refrigerator ,the temperature spans of the two heat reservoirs should be restricted by the equation
  20. 20. Which is deduced directly from eqn 25 Conclusions The above results show clearly that the performance of a Stirling refrigeration cycle is heavily dependent on the specific properties of the working substance in the cycle. Thus we cannot ignore the properties of the working substance and simply assume that a Stirling refrigeration cycle will achieve theoretically perfect regeneration through the use of a reversible regenerator. In general, Stirling refrigeration cycles may be divided into two categories according to their regenerative characteristics. The first category is Stirling refrigeration cycles with the condition of perfect regeneration, and the second is those with the condition of non-perfect regeneration. By using the equation of state of the working substance and Equation (5), one can, in general, easily distinguish whether a Stirling refrigeration cycle possesses the condition of perfect regeneration. For a Stirling refrigeration cycle possessing the condition of perfect regeneration, the performance has been described in several textbooks. However, for a Stirling refrigeration cycle with the condition of non-perfect regeneration, some other characteristics must be taken into account: such as the effect of non- perfect regeneration on the coefficient of performance of the cycle, the restriction of the temperature spans of the two heat reservoirs between which the Stirling refrigerator operates, and so on, as revealed here, there are factors on which it depends but not discussed here
  21. 21. Working gas analysis Performance characteristics Cryocooler which schematic diagram is shown on right side developed by Zimmerman which work at room temperature and just above the critical temperature of helium. Calculations are taken from his original work .Measured performance characteristic are given in Table 1. The lowest temperature at which we measured the cooling capacity was 16 K where it was 1 mW K-1 Further optimization of the cycle allowed us to achieve operation at9 K, but unfortunately the metal film resistor cracked due to thermal stress, and we were not able to repeat the cooling capacity measurement .At lower temperatures we expect that the thermal resistance between the copper tip and the helium gas limit the cooling capacity. This could be improved by providing a larger contact area. The heat leak to the tip was estimated from the warmup rate when the pressure was held constant and the displacer raised, assuming that the thermal mass of the tip was that of 8 grams of copper knowing the thermal conductivity of plastic (o.5mW cm-1 k-1 at 12K) we can also estimate the thermal gradient at the tip to be 3.3 Kcm-1 This is in close agreement with the mean temperature gradient between top and bottom of 4.8 K cm- 1 Similarly, the regeneration loss was estimated from the increase in the warmup rate due to pressure cycling alone, and the shuttle heat loss was estimated from the increase in warmup rate due to displacer cycling at constant pressure. These losses can only be estimated since they are not completely decoupled under the quoted measuring conditions. As can be seen from Table 1 the total heat load determined from these measurements amounts to 2.1 mW at 12 K.
  22. 22. Theoretical analysis The Stirling cycle, with an isothermal decompression, should result in a cooling capacity of 16 mW at the tip, due to the displaced gas." Due to the poor regeneration in this device, the decompression may have been closer to adiabatic, which would give a cooling capacity of only 6mw Shuttle heat loss, the net heat carried down by the motion of the displacer, due to its varying internal energy, was only 0.2 mW as calculated .Regeneration loss, the net heat flow down with the movement of the helium gas, was the largest loss. Assuming stationary concentric cones with a separation s, and a uniform vertical temperature gradient dT/dz, we calculate the regeneration heat flow through a given cross-section where the mean cone diameter is D. Let the mass of helium below this point be M(t) = MI + Mo (sin(wt)+ 1)/2 where w is 2(pie) times the cycling frequency. Since the thermal skin depth of the helium gas is larger than the half-width of the annular gap, the rate of heat transfer to the plastic Op(t) is in phase with the loss of heat by the helium (1) where CpHe is the specific heat of helium gas. From this heat transfer rate we may compute the temperature profile in the helium gas, and also the surface temperatures of the plastic cones. (2) where Pp, K p and Cp are the density, thermal conductivity and specific heat of the plastic respectively. The heat capacity of the plastic PpCp is 35 mJ K-1 cm^3 at 12 K. We assume that the helium in the annular gap is in laminar flow with a parabolic velocity profile, the temperature gap in between the surface of the gap and a point a distance y into the helium is given by (3) Where K He is the mean thermal conductivity of the helium gas. By multiplying (2) and (3) by the specific heat of helium and the helium velocity and integrating over the gap and over one cycle, we find that the regenerative heat leak is comprised of two terms: (4) (5) The regeneration leak due to the thermal impedance of the plastic Qrp was 11 mW at 12 K, and the leak due to the impedance of the helium QrHe was 3 mW. The sum of these two leaks is of the same order as the theoretical cooling capacity. However the measured regeneration loss was smaller, possibly due to poor thermal contact between the thermometer and the helium gas. Regeneration
  23. 23. efficiency is better higher up on the cone since the thermal conductivity and heat capacity of the plastic increase rapidly with temperature Conclusions In conclusion, the operation of a closed-cycle, single conical stage, split Stirling cryocooler has been demonstrated to temperatures as low as 9 K. The theoretical cooing capacity at the cold tip is 6 mW assuming an adiabatic decompression of the displaced helium. A larger thermal contact area between the helium and the load would allow some reasonable fraction of that cool ing capacity to be utilized. Regeneration losses, at low temperatures, limit the ultimate performance of this prototype, emphasizing the importance of minimizing dead volume at the cold end, and having a uniform rate of gas flow. The use of different materials at the low temperature end, such as a laminated stack of metal foils and plastic sheets, would be desirable for better regenration Detail analysis overall Symbols which are going to be used
  24. 24. Assumed conditions of operation 1. The volume variations in the compression and expansion process are sinusoidal. The three expansion spaces are all in phase so that all achieve their maxima and minima simultaneously The stroke of the piston is the same for all three, although the swept volumes are different because of different cylinder diameters. Volume variations in the expansion spaces lead those in the compression space by crank angle α 2. The clearance volumes with the pistons at the end of the stroke (crank at top dead centre) are negligibly small. 3. The temperatures of the working fluid in the compression and expansion spaces and the associated cooler and freezer heat exchangers remain constant at the defined temperatures of the cycle, Tc for the compression space and TEl TEZ and TE3 for the three expansion spaces of the cryocooler. The processes of compression and expansion are therefore isothermal, necessitating either infinite rates of heat transfer or very slow speed operation. 4. The regenerative processes are perfect .The regenerators have a large heat capacity compared with that of the working fluid per pass so that the local temperatures of the matrix remain unaltered. The surface area and heat transfer coefficient are large enough to change the temperature of the working fluid passing through to the terminal values, which at each end of the regenerator are equal to th temperature limits of the cycle Or the temperature of the intermediate expansion spaces. Heat losses due to longitudinal and transverse heat conduction are zero. 5. There is a linear variation in the matrix temperature in the axial direction and no variation in matrix temperature in the radial direction. 6. The working fluid is a perfect gas and the characteristic gas equation PV= MRT applies throughout. 7. The mass of the working fluid remains constant i.e. there is no leakage. 8. Aerodynamic friction and velocity head effects are negligible so that the instantaneous pressure is the same throughout the system. 9. Rotational speed of the engine is constant. 10. Steady state conditions for the overall operation of the engine are established so that volumes and pressures are subject to cyclic variations only. Analysis Let the instantaneous volumes of expansion spaces be represented as: And the instantaneous value of compression space as The volumes of the dead spaces, being the constant volumes of the working space not included in the volumes of the expansion or compression spaces (including the void volumes of regenerators, the freezers and cooler heat exchangers and all the associated ducts and ports), may be represented as: The instantaneous mass of the working fluid in the expansion space is:
  25. 25. The instantaneous mass of the working fluid in the compression space is: The instantaneous mass of the working fluid in the dead spaces: Since total mass of working fluid remains constant: If the instantaneous pressure is the same throughout the system (equal to P say) and if Te1 Te2 Te3 and T; are constant at TEl ,TE2, TE3 and Tc then, substituting for the volumes, eliminating R and rearranging gives If the temperature variations in the dead spaces are assumed to be linear in the axial direction then the mean temperatures are:
  26. 26. and
  27. 27. The mean cylinder pressure …(18) Heat transferred and work done Since the processes of expansion and compression take place isothermally the heat transferred is equal to the work done P. Therefore: then
  28. 28. Thus the heat transferred in the expansion space is of opposite sign to the heat transferred in the compression space and is numerically different by the temperature ratio Tc/TE = τ. By analogy the work done in the two spaces has the same relation, Pc=τPE and so the net power is: …...(28) Multiple expansion spaces VEl,VE2, VE3 would all require a related component of compression work, so that: ……(28) …(30) It is customary to express the ratio heat lifted/work done as the coefficient of performance for a refrigerating machine but this does not appear to be appropriate for a multiple expansion Stirling refrigerator where the refrigeration effect is available at several different temperatures, TE1, TE2,TE3 etc. Mass distribution in the machine From the characteristics gas eqn M=PV/RT where we obtain expression of mass distribution in the expansion ,compression and dead space as follows. Expansion space
  29. 29. Compression space Dead space
  30. 30. Institute Instrumentation centre Liquid Nitrogen plant [IIT Roorkee] 1. TECHNICAL SPECIFICATIONS 1.1 Cryogenerator         Operating Temperature Production capacity Rotation Direction Piston Stroke Displacer stroke Cylinder bore Cooling down period Weight : 77K – 200K : 1000W at 77K, 2500W at 200K : Clockwise : 52 mm : 30 mm : 80 mm :Approx. 4 – 5 minutes : 500 kg. for cryogenerator assembly including  motor and condenser head Noise level at one level : 68dB(A)  Cooling water Consumption : 1.7 – 3.5 bar at inlet 1.2 Electric Motor  Rated Power :11kW at working pressure of 30bar  Rated speed :1460 per min   Specification class Max. cut in frequency :IP55 :10 times / hour 1.3 Electrical Connection  Mains Voltage :220V/380V/415V/480V 3phase ±5%, N and E  Frequency :50Hz/60Hz ±2%  Max. Mains Fusing :3 × 80A/35A/35A  Power supply Cable :5 × 16mm² ; 5×6mm²; 5×6mm²; 5×6 mm²  Max. Rated Current IN : 39A/22.5A/20.5A/17.7A 1.4 System Connections  Gas inlet connection : Ø 25 mm  Liquid outlet connection : Ø 25 mm  Cooling water inlet connection : Hose pillar 0.5”  Cooling water outlet connection : Hose pillar 0.5 “ 1.5 Setting Values of Safety provisions  Lubricating oil pressure switch : 4 bar  Relieve valve working gas : 22 bar  Pressure switch working gas : 32 bar  Cooling water flow switch : 0.6 m³/h
  31. 31.  Cylinder temperature switch : 62 °C   Relieve Valve condenser head Thermal Overload : 0.2 bar : 220V/25A;280V/14.5;415V/13A;480V/11.5A 1.6 Installation Requirements  Maximum height above sea level : 1000 m  Maximum ambient temperature : 45 °C  Minimum ambient temperature : 0 °C   Maximum Relative humidity Minimum Relative humidity : 95% non-condensing : 20% 1.7 Recommended Lubricating Oil  Lubricating oil : Shell T32 Turbo Oil  Oil Capacity : 1.1 litres 1.8 Working Gas  Refrigerant : Helium or hydrogen gas with a minimum purity of 99.99%.  Filling Pressure : 22 bar at a mains frequency of 50 Hz : 18 bar at a mains frequency of 60Hz 1.9 Dimensions  L × W × H : 980 × 750 × 1065 mm 2. PRINCIPLE OF OPERATIONS 2.1 Introduction The 1-cylinder cyrogenerator is used for the cooling of a fluid or a condensation of a gas at a system pressure of 20bar max. and min. temperature down to 60K.If the working condition for the installation are different from standard condition, the cold capacity of the installation will be different as well from the given capacity. Correction factors to be used to calculate the output under the working conditions.The cyrogenerator operates according to the Stirling cycle. It generates temperature of 60K in the condenser head. The generated heat is removed by cooling water.The working pressure in the cryogenic system may be up to 20bar. Fluid to be cooled has to be pumped through the condenser. The process has to be condensed is, depending on the liquefaction rate, sucked in automatically.If applicable, the non- condensable gases from the process gas have to exhausted from the condenser. The three main flows are shown in figure 1 Flow diagram :
  32. 32. 2.2 Construction The cyrogenerator performs the cooling by compression and expansion of a gas (Helium or Hydrogen) in a closed cycle. This cycle is called Stirling Cycle. The cyrogenerator is driven by an electric motor coupled via a flexible coupling and a flywheel. On starting the cyrogenerator the starting valve remains open to lower the compression in the working space and thus allow the electric motor to change from Y to Delta before it comes under full load. When the lubricating oil reaches its operating pressure, the starting valve closes. The compression in the working space increases and normal running commences.
  33. 33. 1. Piston 8. Cross head 2. Displacer 9. Cross head pen 3. Displacer rod 10. Connecting rod 4. 5. Displacer rings connection rod Guide bussing 11. Twin shanked 12. Crank shaft bearings 6. Piston rings 13. Bearing housing 7. Scraper ring Installation identification list 1. Protection bellow 13. Pressure block 2. Water lines 14. Oil cooler 3. Buffer vessel 15. Crankcase 4. Cooling water out 16. Oil drain plug 5. Electric motor 17. Lift point 6. Protection cover 18. Flexible coupling 7. Coupling process gas in 19. Lift point 8. Condenser head 20. Cooling water temperature 9. Coupling process gas out 21. Cooling water in 10. Water intercooler 22. Mounting skid 11. Cylinder housing 23. Cushy foot 12. Oil fill nut 2.3 The Driving Mechanism The crankshaft drives the piston (1) via a twin-shanked connecting rod (10) and displacer rod (3), passing through the centre of piston (1). The cyrogenerator is completely sealed off from the ambient atmosphere, with the emerging crankshaft being encircled by a gas and oil-tight seal. The cyrogenerator is filled with a refrigerant such as helium or hydrogen to the filling pressure of either 22 (50Hz) or 18 bar (60Hz). Once in operation the maximum working pressure will rise to 30 (50Hz) to 25 (60Hz) bar. Lubrication is by pressurised oil, derived from a crankshaft-driven, super-gear oil pump. Item identification 2.4 The Gas System
  34. 34. Figure shows a systematic image of cyrogenerator. The cyrogenerator is a completely sealed machine, which is filled with Helium or Hydrogen gas. The Helium is called Working Gas. The liquefaction temperature of Helium 4-5 K. this means that helium will never liquefy when using Nitrogen gas as product.The pressure inside cyrogenerator varies between 18 to 43 bar.The working space consists of the compression space and the expansion space. Connecting to the working space are cooler and the regenerator. The cooling cycle, compressed Helium gas flows through the cooler and the regenerator towards the expansion space. Because of the expansion, the helium becomes cooler. The energy needed for this expansion is extracted from the process gas. This gas flows, completely independent of the helium gas, through the heat exchanger.Because of the repeating cycle the temperature of the process gas becomes lower. This continues until there is a balance between the temperature of the process gas and the energy needed for the cycle.During operation some Helium gas leaks through the piston rings into the crankcase. In the crankcase most of the cycle time the pressure is lower than the pressure in the working space. To prevent pressure equalization, gas leaking towards the crankcase has to be returned to the working space.In the crankcase a mixture is formed between leaked Helium gas and the lubricating oil. This mixture ends up in the oil-gas separator where both substances are separated. The oil flows back to the crankcase, while the Helium gas flows to the gas filter. This filter takes out the last bit of oil and collects the waste oil. The pressure in the buffer space is equal to the maximum pressure in the crankcase, which is 21 bar (50Hz) or 18 bar (60Hz).The suppletion valve opens when the minimum pressure in the working space is lower than the (constant) pressure in the crankcase. At that time Helium gas flows back into the working space. 2.5 The Oil System
  35. 35. Oil pump (1) driven by the crankshaft, pumps lubricating oil from the bottom of the crankcase to the oil cooler (2). The low temperature of the water cools down the oil. After the cooler the oil is going through an oil filter (3). Her dust particles and other impurities are removed. After filtering the oil is pumped through the Mitchell bearing (4). This special bearing is capable of coping with high lateral forces that occur in the crankshaft. Some oil lubricates the crankshaft bearings (5) and the rest of the oil enters the crankshaft. Via small canals (6) inside the crankshaft oil is pushed up (7) to the gudgeon pin. This pin is lifted every stroke, enabling the every stroke, enabling the oil to lubricate it.Another oil-flow enters the shaft seal chamber via a “Cimring”. This ring acts like a one-way valve. Inside this chamber the oil lubricates the shaft seal and its components (9). When the pressure in the chamber is high enough, the oil flows back, via the oil-pressure regulator (10), to the crankcase. 2.6 The Cooling Water System Either a closed water system or external equipment supplies cooling water. When cooling water is used from an external installation, the water should be analysed by a consultancy office that will advise which measures are required (filter, inhibitor). This is very important as deposits in the pipes and the cooler will seriously impair the efficiency of the installation.A closed water system consists out of a chiller unit, piping and valves. The main control box controls the chiller, which automatically controls the water temperature. The quality of the water has to be checked when new water is put into the system. Inhibitors have to added once, to ensure quality.A flow meter (FI) checks the amount of flow needed for cooling down the compressed air.The flow has to be sufficient to ensure maximum process operation.Before entering the cyrogenerator, the water is filtered (FLT) to prevent any dirt going into the coolers. After running the two coolers, a flow switch (FC) measures the water flow. The cyrogenerator cannot run without water. Therefore the water flow switch is connected to the main control box. If the water flow is too low, the machine will stop immediately. An error message will be displayed on the main control box. The pressure drop over the cyrogenerator , Cooling water pressure drop over cyrogenerator. 2.7 The Condenser The condenser described in this chapter can be used for condensing a gas at a system pressure up to 20bar and a temperature down to 60K. Also liquefaction of Nitrogen gas is possible.The cooling of liquids can also be carried out, but it is advised that specialist advice should be obtained before operating any such system. 2.7.1 Construction
  36. 36. The re-condenser forms the upper part of the cyrogenerator assembly and is secured to the cylinder of the cyrogenerator by ten threaded studs and nuts. Flexible couplings may be used to connect the inlet outlet connections to the customer equipment. The re-condenser comprises an outlet steel shell of soldered and brazed construction that houses the condenser head assembly, a pipe, through which the non-condensable gases are drawn off, and a pressure safety valve. The space between the outer shell and the condenser head is filled with a granular insulating material called “Perlite”. A fill cap is fitted to enable topping up the insulation material. 2.7.2 Operation Process gas enters the inlet at the top of the condenser head. Inside the head the copper heat exchanger is cooled down by Stirling cycle. The gas streams through the exchanger and the temperature decreases, resulting in liquefaction of gas. The low liquefied gas flows out of the head towards the storage vessel. Because of vibrations a flexible tube connects the condenser head and vessel. Non- condensable gases i.e. gases which liquefies at temperatures lower than achieved by the cyrogenerator (Ar, Ne, etc.), are sucked off through a line in the top of the hand and are blown off. 2.7.3 Safety Device The safety valve operates in case the pressure within the insulation space exceeds 0.2 bar and prevents damage to the outlet jacket of the condenser. 2.8 Survey of Safety Devices Figure shows the safety devices contained on the installation. Basically all the safeties are “normally open” types. In case of any problems with the power supply to the sensors or the main control box, the installation will automatically shut down. Attention: The cyrogenerator can restart automatically after a shut down due to a power failure or water flow failure.
  37. 37. NR Name safety device Type Working Principle 1 Oil pressure safety switch Electrical The oil pump pressurises the oil. When the pressure exceeds 2 bar the switch is activated. An “Okay signal” is given to the PLC in the main control box. The safety is factory set and sealed. 2 Stationary gas pressure safety Mechanical If the pressure in the buffer vessel exceeds 22 bar, the safety opens to release the pressure. The safety is factory set and sealed. 3 Working gas pressure safety switch Electrical A micro switch is activated if the working gas pressure exceeds the maximum allowed pressure. A signal is given to the PLC in the main control box. The installation is stopped and the startingvalve opens to release the pressure. The safety is factory set and sealed. 4 Water flow switch Electrical If the water flow is sufficient, the flow switch gives a “Ok signal” to the main control box. If the water flow drops below minimum set point, the main control box will switch off installation. 5 Temperature sensor Electrical The temperature sensor gives a signal to the PLC when the working space exceeds a temperature of 60° C. The PLC will switch off the system to prevent damages. The safety is factory set and sealed. 6 Pressure switch non- condensable gasses Electrical This switch gives an “Okay signal” to the main control box when the process gas pressure exceeds the system pressure minus 0.5 bar. If the pressure drops below this pressure, the main control box will switch off the installation. The safety is factory set and sealed. 7 Safety valve insulation space Mechanical If the insulation pressure exceeds 0.2 bar, this valve blows off, preventing damage to the condenser head interior. 8 Thermal motor overload switch Electrical This automatic fuse switches when the electric motor consumes too much power. The main control box will switch off the installation. The safety is factory set and sealed. IIC LN2 performance and uses  It produces 200L of liquid nitrogen (as per record) per day.  Working hour is 16 to 18 h per day.  Purity as reported(no equipment to measure) 98% to 99%. Uses  Chief users are Biotechnology department, physics department and metallurgy department
  38. 38. References 1. Philips cryogenic equipment hydrocarbon processing (manual). 2. Product sheet StirLIN Compact – DL. 3. Cryogenics Volume 19 issue 10 1979 [doi 10.1016_0011-2275(79)90033-x] G. Walker -- Generalized ideal reference cycle for regenerative refrigeration- part 1 isothermal systems 4. Cryogenics Volume 24 issue 7 1984 [doi 10.1016_0011-2275(84)90088-2] G. Walker -- Generalized ideal reference cycle for regenerative refrigeration- Part 2. Adiabatic systems. 5. Cryogenics Volume 28 issue 1 1988 [doi 10.1016_0011-2275(88)90227-5] P.R. Tailor_ K.G. Narayankhedkar -- Thermodynamic analysis of the Stirling cycle 6. Cryogenics Volume 31 issue 12 1991 [doi 10.1016_0011-2275(91)90126-h] M.D. Atrey_ S.L. Bapat_ K.G. Narayankhedkar -- Theoretical analysis and performance investigation of Stirling cycle regenerators 7. Stirling cycle - Wikipedia, the free encyclopaedia. 8. Kohler, J. W. L., Joakers, C. O, Fundamentals of a gas refrigerating machine Philips Tech Rev (1954) 16 69-78. 9. Kolfler, J. W. L, JQakera, C. O. Construction of a gas refrigerating machine Philips Tech Rev (1954)16 105-i 15 10. Sdmfidt, G. Theorie der Gesehlossenen Calorisehen Masehine van Laubroy and Sehwartzkopff in Berlin Z Ver Oster lng (1861) 79 11. Walker, G. Cryocoolers Vols i and 2, Plenum Publishing Corp., New York (1983) Zimmerman, J. E., Raddmagh, R. Operation of a SQUID in a very low power cryocooler Application of Closed Cycle Cryocoolers tO Superconducting Devices NBS S~al Publication 5-8, US Government Printing Office, Washington. DC (!978) 13 . Zimmezman, J.E., Radebaugh, IL Applications of closed cycle cryocoolers to small superconducting devices, Proc Conf Boulder (1977), National Bureau of Standards Special Pub 508 (1978) 59 14. Zimmerman, J.E.,SuUivan, D.B.,Cryogenics, 19 (1979) 170 15. MeCarty, R.D., Thermophysical properties of He-A, NBS Technical Note 631 (1972) 16. Radebaugh, IL, Zirnrnerman, J.E., ibid 1, 67 17 . Jakob, M. Heat transfer, Wiley New York 1 (1949) 18. Institute Instrumentation centre of IIT Roorkee LN2 manual and records.