Case study lnp

Liquid Nitrogen PlantThermodynamic principle involved in itSubmitted to:-Prof R.Jayaganthan (Metallurgydepartment IITR)By

UDIT KUMAR (12118084)

SOURABH HIRAU (12118077)

SOURABH SINGH (12118078)

VAIBHAV KUMAR (12118085)

VAMSI PEDAPALLI (12118088)

CONTENTS1 Introductions2 Sterling an overview3 Basic cycle analysis4 Regenerator analysis5 Working gas analysis6 Detail analysis overall7 Institute Instrumentation centre LN28 IIC LN2 Performance and uses analysis9 References

IntroductionLiquid nitrogen is nitrogen in a liquid state at an extremely low temperature. It is producedindustrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid withdensity of 0.807 g/mL at its boiling point and a dielectric constant of 1.43.[1] Liquid nitrogen is oftenreferred 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 fluidthat can cause rapid freezing on contact with living tissue. When appropriately insulated fromambient 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 theevolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasksranges 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 itextremely 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 suchas 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 notavailable 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 moleculargastronomy.

in container inerting and pressurisation by injecting a controlled amount of liquid nitrogen justprior to sealing or capping.[7][8]

as a cosmetic novelty giving a smoky, bubbling "cauldron effect" to drinks. liquid nitrogencocktail.

as an energy storage medium.[9][10]

branding cattle.[11]

Production

Liquid nitrogen is produced commercially from the cryogenic distillation of liquified air. An aircompressor is used to compress filtered air to high pressure; the high-pressure gas is cooled backto 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 ofexpansion and distillation. Liquid nitrogen may be produced for direct sale, or as a byproduct ofmanufacture of liquid oxygen used for industrial processes such as steelmaking. Liquid-air plantsproducing on the order of tons per day of product started to be built in the 1930s but became verycommon after the Second World War; a large modern plant may produce 3000 tons/day of liquidair products.

The best way to produce liquid nitrogen plant commercially is sterling cycle because of the followingreasons:-

The cycle is regenerative:-The cycle is defined as a closed regenerative cycle with a gaseousworking fluid. "Closed cycle" means the working fluid is permanently contained within thethermodynamic 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 whichincreases 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 notdiscrete, but rather the transitions overlap. The cycle is efficient:-The Stirling engine is noted for its high efficiency compared to steamengines, 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 energysources has become increasingly significant as the price of conventional fuels rises, and also inlight of concerns such as peak oil and climate change. This engine is currently exciting interestas the core component of micro combined heat and power (CHP) units, in which it is moreefficient and safer than a comparable steam engine.

Sterling An OverviewThe history of the Stirling CycleThe Stirling cycle is a thermodynamic closed cycle invented in 1816 by a Scottish ministerRobert Stirling. It was used as an engine and was considered at the time to be capable of replacingthe steam engine since boilers used in early steam engines were prone to life-threateningexplosions.The counterpart of the hot air motor, the refrigerator, was first recognized in 1832. Bothmachines experienced high and low points during the nineteenth century. The principle behindthe machines was almost condemned to obscurity after the invention of the internal combustionengine (gas-, petrol-, and diesel motors) and compressor refrigerators with external evaporation.

In 1938 the famous Dutch Philips Research Laboratory was looking for a means to powerelectricity generators for short wave communication systems in remote areas without electricitysupply. The practically-forgotten hot air motor attracted attention.In 1946 Philips started optimizing the cooling techniques used in the Stirling cycle. The result wasthe development of the world conquering cryogenerator, marking the start of significantcryogenic activities at Philips. Though the Stirling hot air motor itself never became a commercialsuccess, the Stirling cryogenerator has been selling by thousands worldwide and has beenincorporated 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 ofliquid 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 itsworldwide activities under the name of Stirling Cryogenics BV. Thanks to continuous innovationand considerable investment in R&D, the Stirling cryogenerator is now used in advancedtechnological 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 cyclecryogenerator. The cycle is remarkable because it is a closed cycle in which the cryogenerator'sworking gas never comes into contact with the fluid to be cooled. This two-circuit approacheliminates contamination of the working gas resulting in long continuous operating periods andlongevity.The Stirling cryogenerator is extremely environmentally friendly: it does not cause ozone layerdepletion in any way, does not contribute to the greenhouse effect and does not emit any harmfulor toxic product. It is extremely efficient compared to other cryogenic cooling principles. Stirling isthe only company in the world that successfully produces Stirling cycle-based cryogenerators withcooling 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 ministerRobert Stirling. It was used as an engine and was considered at the time to be capable of replacingthe steam engine since boilers used in early steam engines were prone to life-threateningexplosions.The counterpart of the hot air motor, the refrigerator, was first recognized in 1832. Bothmachines experienced high and low points during the nineteenth century. The principle behindthe machines was almost condemned to obscurity after the invention of the internal combustionThe Stirling cycle alternately compresses and expands a fixed quantity of a nearly perfect gas (alsoknownas ideal gas) in a closed cycle. The compression takes place at room temperature to facilitate thedischarge of heat caused by compression, whereas the expansion is performed at the required lowtemperature. For the purpose of explanation, the process may be split up into four distinct phasesillustrated in Figure 1 and each indicated by a number.

Cylinder A is closed by piston B and contains a certain amount of gas. The space inside the cylinder isdivided 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 Hand freezer J.In position I most of the gas is in space D and at room temperature. During phase 1 this gasis compressed by piston B. In phase 2 the gas is displaced by means of the displacer from space D tospace E, which is already at a low temperature.During this displacement the gas passes through the heatexchangers. The cooler dissipates the heat caused by compression through cooling water. The regeneratorcools the gas almost to the temperature prevailing in space E. In Phase 3 the actual cold productiontakes place by allowing the precooled gas to expand through movement of the displacer and pistontogether. Finally, by moving the displacer (phase 4), the gas is returned to space D for a new cycle tobegin. While passing the freezer its cold is dissipated to the ambient environment, and in the regeneratorit is reheated to nearly room temperature. The initial situation of the cycle has now been restored. It isclear that a large temperature difference will occur between the compression space and the expansionspace. The way in which this temperature difference is established and what influence the regeneratorhas in this is shown in the two graphs on the right.

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 totemperature T1.When the expanded gas returns to the compression space, a temperature gradient is established inthe rege- nerator. This means that, after the second compression stroke, the working gas is slightlypre-cooled in the rege- nerator before it is expanded in the expansion spaces to reach temperatureT2.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 obviousthat the regenerator is the most important component in this cooling process.Basic cycle analysisIn this section a brief outline of the basic principles of the Stirling cycle is presented. The generalproperties of the cycle will be derived with a discussion of the most important losses.Fundamental Cycle

In the ideal Stirling cycle, the cold isproduced by the reversible expansion of a gas. The gasperforms a closed cycle, during which it is alternately compressed at ambient temperature in acompression space and expanded at the desired low temperature' in an expansion space, therebyreciprocating between these spaces through one connecting duct, wherein a regenerator providesfor the heat exchange between the outgoing and the returning gas flow. Figure 1 shows stages incarrying out the ideal cycle.In this diagram, A is a cylinder, closed by 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 Eat the low temperature, connected by the annular passage F. This passage contains the regeneratorG, a porous mass with a high heat capacity: the temperature in the passage is shown in the graph. Thecycle, consisting of four phases, runs as follows:I Compression in space Dby the piston B: the heat of compression is dischargedthrough the cooler H.

II Transfer of the gasthrough the regenerator to space bymovement of thedisplacer. The gas is reversibly cooled.down in the regenerator, the heatof the gas being stered. in the regenerator mass.III Expansion in the cold space by the combined movement of the piston analthe displacer; the cold produced is discharged through the freezerJ andutilized,IV Return of the gas to space D; thereby the gas is reheated, the heat stored inthe regenerator being restore. to the gas.With no regenerator present the gas flowing to the expansion space would arrive thereat ambient temperature, whereas the returning gas would arrive in the compressionspace at the low temperature this effect would cause such a tremendous cold loss thatthe whole process would become futile. In an ideal regenerator, a temperature gradient

is established in the direction of the gas flow. this causes the gas to be c ooled downreversibly, so that it ar r ive in the expansion space with the temperature prevailing there.Figure2 shows the p-V diagram of this schematic cycle, neglecting the dead space. It consists oftwo isotherms and two isochores ;at TC (compressiontemperature) the amount of heat Qc isrejected, at TE (expansiontemperature) the amount of heat QE is absorbed.

Actually, the discontinuous movement of the pistons is difficult to achieve. In practice, thepistons are actuated by a crank mechanism and are thus moving harmonically; for the machineto act as a refrigerator, the expansion space E (Figure1) has to lead in phase with respect to thecompression space D. The harmonic motion and the volume of the heat exchangers (theso-called'deadspace') cause the four phases of the cycle to merge somewhat, so that they cannot bedistinguished very well; the gas is not exclusively compressed in the compression space, but alsoin the expansion space, and the same holds for the expansion. It can be shownhowever that thedifference in output between the discontinuous and the harmonic process is only a fewpercent.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 volumesinterconnected by a cooler,a regenerator, and a freezer, whereby the volume that isleading in phase becomes the expansion space .Where in the cell is produced. Formulaefor the pressure variation, the refrigerating c a p a c i t y , and the shaft powerfor this generalcase will be given in the next sectionPerformance of the Ideal (Isothermal) CycleIn the ideal machine, the thermal contact in the heat exchangers is assumed to beperfect, sothat the gas temperature there is equal to the temperature .ofthe walls; thesame temperature is supposedto prevail in the adjoining cylinders, which temperatures

thus are constant with time. The regenerator is also assumed to be perfect, so that neregeneration less .occursand the gas temperature there is alsoconstant with time. Theworking fluid is suspended to be a nearlyperfect gas; this condition can be metsufficiently by us hydrogen or helium in practical ,(1) Pressure variation. For the expansion space VE (temperature(2) and the compression space Vc (.temperature Tc) we writeVE =1/2V0(1+cosα)VC=1/2wV0(1+cos(α-ϕ))Where Vo is the maximum volume .ofthe expansion space, w the ratioof the swept volumes .ofthe compression and the expansion space, and ϕ the phasedifference between these spaces; the crank angle α(=0 for VE=Vmax)Changes linearlywith time (α = ω t ). volume of the freezer, the regenerator, and the cellar, whichform the connecting channel (the dead space), will be indicated by VS with thecorresponding temperature TS The variation the pressure p with time (or α) newfollows from the condition that the mass .ofthe system as a whole is constant:Where M is the molecular weight of the gas, R is the gas constant,And c is a constant. After introductionof VE and Vc from equation(1)

and(reduced to the swept volume of the expansion space V0 andnormalized to the temperature of thecompression space TC this reduces toThis expression is easily transformed intowith the abbreviations’The constantA may be interpreted as two times the total change of volume, while B equals two timesthe mean total volume,both reduced to Vo and normalized to TC For the pressure p we thus find

which may be written more convenientlyExpression (2) shows that the pressure variation with time is not purely harmonic. Inpractice, the deviation of harmonic behaviour is rather small however (the function issymmetrical with respect to (Pmax and Pmin,which points are 1800 apart) as the value of δ

Seldom exceeds 0.4. This means that the pressure ratio of this type of machineis about 2, a remarkably low value Compared with what is normal in refrigeratingapparatus. The presence of the phase angle θ shows that the pressure variation is notin phase with the variation of the expansion or the compression space; it is easilychecked 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 inthe compression space. For later reference, given here is the expression for the meanpressure 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)andin the ~compression space (Qca negative quantity) is given bythat three exp is used in spaces with gas content, may also 'beused in the case where gas enters andleaves the space can 'beproved by an involved thermodynamic reasoning which is omitted here. It iseasily seen that the value of the integrals depends only on the components of p which have the samephase as over and Over; this means that 0 < θ <ϕ if the machine has to operate as arefrigerator.Evaluation of the integrals leads us

……(4) For practical use, equation (4) may betransformed into qE' the heat absorbed per second (orthe refrigerating capacity) by inserting n,the noof 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 to P, and Linearlyproportional to the function B, Which was defined as the mean reduced and normalized volume. Theconsequences of the dependence on the mean pressure and the temperature ratio (T is also containedin 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 could be expected.(3) Shaft power and efficiency. :For the work w needed to drivethe machine one may writeW=-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 thatof the Carnot cycle this is obvious as the cycle is completely reversible. Although in actualmachines the performance is reduced by losses, tc be discussed in the next section; the ideal cyclehas to be consideredas the reference process, because most essential facts can be deduced fromit.Before closing this section two remarks en the representation of the process will bemade. The first remark concerns the representation in a thermodynamic diagram (e.g., pvs V,T Vs S,etc.). These diagrams always relate ,to a fixed quantity .ofthe working fluid,

which has to be in internal equilibrium (equal p and T throughout); this quantity is passedthrough a number of thermo.... dynamic states and (at least in a closed system) is returnedultimately 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 athand.How ever, the s y s t e m is net homogeneous at all, as different parts of it have adifferenttemperature. AS a consequence different gas particles describe completely different cyclesbetween different temperatures to mention .onlytwo extreme examples, some particles arereciprocating between the compression space and a point in the cooler,while other particlesare reciprocating between a point in the freezer and the expansion space. That, because ofthis situation, normal diagram shave lost their value is shown by the fact that .onewould haveto draw an infinite number of diagrams for the diverse particles with different cycles, whichobviously leads nowhere. The only diagram which still has some sense is the p-V diagram, as thesystem is homogeneous in p. But in such a diagram one isnot allowed to draw isotherms oradiabatic, as these lines have lost their meaning. To avoid this difficulty when drawing Figure 2, thedead space was assumed to be zero: in that case isotherms may be drawn, since only then the gas is atthermal equilibrium during the compression and the expansion. When the adiabatic losses arediscussed in the next section, this subject will have to be returned to. The second remark concernsthe description of the cycle in a schematic form. The transfer of the gas from the compressionspace to the expansion space and vice versa is performed with constant volume of the gas; this is thesimplest representation, as the transfer can beeffected bythe movement of the displacer only. Butthis way of transfer is only one eg of a multitude of possibilities which exhibit the common propertythat regeneration is possible. As another example consider transfer at constant pressure, which wayof trans-for approximates the harmonic cycle much better. The only reason why this manner oftransfer is not used to explain the cycle is that it can only be accomplished by simultaneousmovement of the piston and displacer, which obviously is more complicated. This point is stressedbecause at many places in literature the transfer with constant volume isconsidered to be one of themain characteristics of the Stirling cycle, to contrast itwith the cycle of Claude (withseparatecompressor and expander),where the transfer isperformed at constant pressure. As explained, thisview does not correctly locate the distinction between the two cycles, the real difference being thatin the Stirling cycle the gas is reciprocating through one connecting duct, wherein the heat exchangeis brought about by regeneration, whereas in the Claude cycle the connecting circuit consists of acounter-flow he.at exchanger (withtwo channels). Apart from the restriction contained in the firstremark the Stirling cycle thus closely resembles the Claude cycle thermodynamically.

The Actual Cycle

The losses can affect the process in two different ways, viz."by increasing the shaft powerand 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 shaftpower exerts the greatest influence at high refrigerating temperatures, while decrease of theOutput createsthe most adverse effects at low temperatures. In this way a temperature rangearises, the "optimum working range", wherein the actual efficiency differs least from theideal one (i.e., where the figure of merit is highest). This range may be shifted both to higherand to lower temperatures by suitable design; moreover, its,limits greatly depend on presentand future technological possibilities.

The increase in shaft power is mainly due to three causes, namely, the mechanical loss ofthe drive, the flow loss (that is, the power needed to force the gas through the narrowconnecting 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 processit was assumed that the temperature in the cylinders is constant with time. This meansthat thethermal contact between the gas and the wall in the cylinder spaces is assumedto be so perfect that these walls can be employed as well to establish the contact betweenthe gas and the surroundings, that is, to serve as cooler and freezer. In this case, separateheat exchangers are of no use and must be omitted; this ideal machine thus consists ofthe two cylinders and the regenerator and is therefore referred to as the "three-elementmachine”. Actually, the separate heat exchangers are introduced because the thermalcontact between the gas and the cylinder walls is always so poor that insufficient heatexchange with the surroundings can be established through these walls; for this case thename "five-element machine" isused. As a consequence, the temperature of thegas in thecylinders changes nearly adiabatically with time. This adiabatic behaviour has only aminor influence on the efficiency (it would be too involved to give the full explanationhere; it is based on the fact that these processes occur in both cylinders with the samephase, so that the temperature ratio is independent of time) but in this case the heat (orthe cold) must be transported from the cylinders to the heat exchangers. This transportcan only be effected by the reciprocating gas which performs it by assuming differenttemperatures when flowing in opposite directions, with the resultthat the meantemperature in each cylinder deviates from that in the adjoining heat exchanger. Figure4 shows schematically the temperature distribution in the machine. It will be observedthat the mean temperature in the expansion cylinder is lower than that of the freezer andthat 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 thetemperatures of the

heat exchangers, which causes an increase in shaft power. Strictly speaking theexpression "adiabatic loss" is therefore misleading and should be replaced by IL transportloss". It is adhered to though ,because ultimately the adiabatic behaviour is thefundamental 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 theisothermal case, one would expect also a decrease in refrigerating capacity; this influenceis very small, however, as the larger value of T is nearly compensated by a decrease of thequantity S, the mean relative volume of the working circuit, which quantity has anotherform in the adiabatic case.In the above discussion the use of the expressions "adiabaticcompression" or “expansion” has been intentionally avoided. The reason is that the gas iscompressed and expanded everywhere in the working space its behaviour during theseprocesses, however, depends largely on the condition of heat transfer prevailing in thevarious sections of the volume. While in the cylinders the gas temperatures change nearlyadiabatically, 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 theactual 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 thatthermo- 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) lossproper needs no discussion. There exists another loss, however, that acts as an insulationloss 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 obviousthat in the Stirling process ideal regeneration is possible in principle as the same amountof gas passes the regenerator in both directions with the same temperature difference, sothat the amount of heat rejected and absorbed by the gas for both directions of flow is thesame,since the specific heat is independent of pressure (which is practically the case fornearly perfect gases) .The following argument shows the extreme importance of a Smalldeparture from idealistic nature of the regenerator, caused by non-ideal heat transfer. In theregenerator a quantity of heat Qr must be absorbed and rejected in each cycle. Owing toimperfections, this amount is reduced to eta(r)*Qr where eta(r) is efficiency of regenerator.Thismeans only part of heat is available to regenerator. The rest that is (1-eta(r))*Qr carried along withthe gas through the regenerator , so that gas arrives too hot in the expansion space. This remainderdeltaQr constitutes the regenerator loss .As this loss must be made up from the cold produce Qe,itmust be compared with this quantity. calculation show that

The constant Cr depends mainly on compression ratio it’s value mainly is 10 eg we take Tc=300Kand Te=75K and regeneration loss 1% then δQr/Qe=30%Thus each per cent of regenerationloss involves .a loss of 30 % in refrigerating power;below 30K this figure even increasesto greater than 90 per cent, meaning that at such temperatures the entire coldproduction is consumed by the regenerator. It is thus no exaggeration to call theregenerator the heart of the machine. The regenerator used in’ actual gas refrigeratingmachines consists of a mass of fine metal wire, forming a light felt-like substance. Withthis type of material, e f f i c i en cie s of 99 per cent and higher can be obtained; thethermal conductivity of the material is very low.This section will be ended with a shortdiscussion on the problem of how to minimize the total sum of the losses; this problemis 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 feltin three different ways, viz., the regeneration loss (reducing the cooling power), theflow loss, and the dead space (this is not a real loss as the shaft power is alsodecreased; 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 alargercross-section; this reduces the flow loss and somewhat the regeneration loss, butincreases the dead space. Thus it is possible to find optimum dimensions for theregenerator. The same hold for the freezer and the cooler, where the regeneration lossis replacedby the loss due to insufficient heat transfer. Moreover, the losses are governedby the choice of the values of and phai. Finally, the outside of the cooler and thefreezer must be made optimal. Thus a very large number of design parameters are to befixed such that the total result gives an optimum condition; it will be obvious that a lotof experience is needed to find the right solution quickly.This is compensated by thefact that once a system of calculation is worked out, it holds for any size of machine.Regenerator Analysis

Condition of perfect regenerationFor a Stirling refrigeration cycle with perfect regeneration, the heat Q 1 transferred to the regeneratorfrom the working substance in a constant generalized co-ordinate cooling process must be equal to theheat Q2 transferred back to the working substance from the regenerator in the constant generalizedco-ordinate heating process. When QI is not equal to Q2' the Stirling refrigeration cycle cannotachieve perfect regeneration. Because QI and Q2 are dependent on the heat capacity of theworking substance, a Stirling refrigeration cycle can achieve perfect regeneration only if the heatcapacity of the working substance satisfies certain conditions. In order to determine these conditions,we first give the fundamental equation of thermodynamics for a working substancedU = TdS + Ydy ….(1)

where U and S are, respectively, the internal energy and entropy of the system, and Y is thecorresponding 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 thepermeability of vaccum,ƛ is the molecular field constant, H is the magnetic field intensity and M is themagnetization 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 heatcapacity 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 thecondition of perfect regeneration. Using thermodynamic relations, Equation (4) may be expressed inanother 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 vander Waals gas as regeneration while a Stirling refrigeration cycle using a the working substancepossesses the condition of perfect regenration cycle using a gas which is described by the Redlich KwongBeattie-Bridgeman, Benedict-Webb-Rubin, Dieterici, Berthelot or Martin-Hou equation of stateas the working substance does not,in general, possess the condition of perfect regenration

Effect of non-perfect regeneration on performance of gas Stirlingrefrigeration cycle

For a Stirling refrigeration cycle with perfect regeneration, the coefficient of performance isequal to that of a Carnot refrigeration cycle for the same temperature range. For a Stirlingrefrigeration cycle with non-perfect regeneration, the coefficient of performance is alwayssmaller than that of a Carnot refrigeration cycle for the same temperature range and is heavilydependent on the specific properties of the working substance in the cycle.for illustration the gasStirling refrigeration cycle as an example and assume the working substance in the cycleis 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

for different gases 9. For example, for three low temperature gases (He, H 2 and Ne), the values of a, b and nare listed in Table 1. In particular, when n = 1/2, Equation (13) becomes the Redlich-Kwong equation ofstate, which has a wide range of application in low temperature technology. From Equation (13), we obtainthe 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 theright-hand side of Equation (15) is dependent on the volume, so Cv is dependent on volume Vas well as temperature T. This shows that a Stirling refrigeration cycle using a gas describedby Equation (13) as the working substance does not, in general, possess the conditionof 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 volumecooling 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 thehigh temperature heat reservoir at temperature TH• During isothermal expansion, the volume ofthe working substance changes from V1 to Vl and heat QL is absorbed from thelow tempera- ture heat reservoir at temperature TL. Then, using Equation (14), we find

……(18)and

From eqn 16 and 17 we obtain……..(19)

From Equation (20), one may come to the conclusion that when n # 0, -1, and a # 0, AQ ~ 0, so that theStifling refrigeration cycle does not possess the condition of perfect regenerationFor a gas Stirling refrigeration cycle operating only between two heat reservoirs at temperatures Tn andTL,the temperature of the working substance in the two constant volume processes is always lower thanthat of the high temperature heat reservoir and higher than that of the low temperature heat reservoir. Whenthe heat transferred into the regenerator is larger than that transferred out of the regenerator, the redundantheat in theregenerator can only be released to the low temperature heat reservoir in a controlled manner, such that therefrigeration heat is reduced from QL to Q~. If not, the temperature of the regenerator would be changedand the regenerator would not operate normally. Similarly, when the heat transferred into the regenerator issmaller than that transferred out of the regenerator, the heat deficit in the regenerator can only be suppliedfrom the high temperature heat reservoir in a controlled manner, while QL remains unchanged. Thus, wefind 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 sterl ing refrigeration cycle is given

Equation (23) shows that when n ~ 0, - 1, and a ~ 0, the coefficient of performance e ofthe Stirl ing refrigeration cycle is always smaller than that of the Carnot refrigeration

cycle for the same temperature range because the Stirling refrigeration cycle does notpossess 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 twoheat reservoirs is equal to zero. It is obvious that when a gas with n > 0 or n < - 1 is usedas the working substance, the Stirling refrigeration cycle can only operate under thecircumstances where

…………(25)

That is to say that for such a Stirl ing refrigerator, the temperature spans of the two heatreservoirs between which the St irl ing refr igerator operates are rest ricted. It is seen fromTable 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 theworking substance, the level of refrigerat ion and the coefficient of performance should begiven by Equations (21b) and (23b), respectively. When the working substance in aStirling refrigerator is a gas described by the Redlich -Kwong equation of state the levelof refrigeration

… . . (26)

And the coefficient of performance is

of the cycle may be directly deduced from Equations(21a) and (23a), respectively. Forsuch a Stirling refrigerator ,the temperature spans of the two heat reservoirs should berestricted by the equat ion

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

Working gas analysisPerformance characteristics

Cryocooler which schematic diagram is shown onright side developed by Zimmerman which work atroom temperature and just above the criticaltemperature of helium. Calculations are taken fromhis original work .Measured performancecharacteristic are given in Table 1. The lowesttemperature at which we measured the coolingcapacity was 16 K where it was 1 mW K-1 Furtheroptimization of the cycle allowed us to achieveoperation at9 K, but unfortunately the metal filmresistor cracked due to thermal stress, and wewere not able to repeat the cooling capacitymeasurement .At lower temperatures we expect thatthe thermal resistance between the copper tip andthe helium gas limit the cooling capacity. Thiscould be improved by providing a larger contactarea. The heat leak to the tip was estimated fromthe warmup rate when the pressure was heldconstant and the displacer raised, assuming thatthe thermal mass of the tip was that of 8 grams ofcopper knowing the thermal conductivity of plastic(o.5mW cm-1k-1at 12K) we can also estimate thethermal gradient at the tip to be 3.3 Kcm-1 Thisis in close agreement with the mean temperaturegradient between top and bottom of 4.8 K cm-

1Similarly, the regeneration loss was estimatedfrom the increase in the warmup rate due topressure cycling alone, and the shuttle heat losswas estimated from the increase in warmup ratedue to displacer cycling at constant pressure.These losses can only be estimated since they arenot completely decoupled under the quotedmeasuring conditions. As can be seen from Table1 the total heat load determined from thesemeasurements amounts to 2.1 mW at 12 K.

Theoretical analysis

The Stirling cycle, with an isothermal decompression, should result in a cooling capacity of 16mW at the tip, due to the displaced gas." Due to the poor regeneration in this device, thedecompression may have been closer to adiabatic, which would give a cooling capacity of only6mw Shuttle heat loss, the net heat carried down by the motion of the displacer, due to itsvarying internal energy, was only 0.2 mW as calculated .Regeneration loss, the net heat flowdown with the movement of the helium gas, was the largest loss. Assuming stationaryconcentric cones with a separation s, and a uniform vertical temperature gradient dT/dz, wecalculate the regeneration heat flow through a given cross-section where the mean conediameter is D. Let the mass of helium below this point be M(t) = MI + Mo (sin(wt)+ 1)/2where w is 2(pie) times the cycling frequency. Since the thermal skin depth of the helium gasis larger than the half-width of the annular gap, the rate of heat transfer to the plastic Op(t) isin 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 thetemperature 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 plasticrespectively. The heat capacity of the plastic PpCp is 35 mJ K-1 cm^3 at 12 K. We assume that thehelium in the annular gap is in laminar flow with a parabolic velocity profile, the temperature gapin 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) bythe specific heat of helium and the helium velocity and integrating over the gap and over onecycle, 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 theleak due to the impedance of the helium QrHe was 3 mW. The sum of these two leaks is of the sameorder 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

efficiency is better higher up on the cone since the thermal conductivity and heat capacity of theplastic increase rapidly with temperature

Conclusions

In conclusion, the operation of a closed-cycle, single conical stage, split Stirling cryocooler has beendemonstrated to temperatures as low as 9 K. The theoretical cooing capacity at the cold tip is 6 mWassuming an adiabatic decompression of the displaced helium. A larger thermal contact areabetween the helium and the load would allow some reasonable fraction of that cool ing capacity tobe utilized. Regeneration losses, at low temperatures, limit the ultimate performance of thisprototype, emphasizing the importance of minimizing dead volume at the cold end, and having auniform rate of gas flow. The use of different materials at the low temperature end, such as alaminated stack of metal foils and plastic sheets, would be desirable for better regenration

Detail analysis overall

Symbols which are going to be used

Assumed conditions of operation

1. The volume variations in the compression and expansion process are sinusoidal. The threeexpansion spaces are all in phase so that all achieve their maxima and minima simultaneously Thestroke of the piston is the same for all three, although the swept volumes are differentbecause of different cylinder diameters. Volume variations in the expansion spaces lead thosein the compression space by crank angle α2. 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 theassociated cooler and freezer heat exchangers remain constant at the defined temperatures of thecycle, Tc for the compression space and TEl TEZ and TE3 for the three expansion spaces of thecryocooler. The processes of compression and expansion are therefore isothermal, necessitatingeither infinite rates of heat transfer or very slow speed operation.4. The regenerative processes are perfect .The regenerators have a large heat capacity comparedwith that of the working fluid per pass so that the local temperatures of the matrix remainunaltered. The surface area and heat transfer coefficient are large enough to change thetemperature of the working fluid passing through to the terminal values, which at each end ofthe regenerator are equal to th temperature limits of the cycle Or the temperature of theintermediate expansion spaces. Heat losses due to longitudinal and transverse heat conduction arezero.5. There is a linear variation in the matrix temperature in the axial direction and novariation in matrix temperature in the radial direction.6. The working fluid is a perfect gas and the characteristic gas equation PV= MRTapplies 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 instantaneouspressure 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 thatvolumes and pressures are subject to cyclic variations only.

AnalysisLet 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 includedin 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:

The instantaneous mass of the working fluid in thecompression space is:

The instantaneous mass of the working fluid in the deadspaces:

Since total mass of working fluid remains constant:

If the instantaneous pressure is the same throughout thesystem (equal to P say) and if Te1 Te2 Te3 and T; are constantat 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 themean temperatures are:

and

The mean cylinder pressure

…(18)

Heat transferred and work done Since the processes of expansion and compression take place isothermally theheat transferred is equal to the work done P. Therefore:

then

Thus the heat transferred in the expansion space is of opposite sign to the heat transferred inthe 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 componentof compression work, so that:

……(28) …(30)

It is customary to express the ratio heat lifted/work done as the coefficient of performance fora refrigerating machine but this does not appear to be appropriate for a multiple expansionStirling refrigerator where the refrigeration effect is available at several differenttemperatures, TE1, TE2,TE3 etc.

Mass distribution in the machine

From the characteristics gas eqn M=PV/RT where we obtain expressionof mass distribution in the expansion ,compression and dead space as follows. Expansion space

Compression space

Dead space

Institute Instrumentation centre Liquid Nitrogen plant[IIT Roorkee]1. TECHNICAL SPECIFICATIONS

1.1 Cryogenerator

Operating TemperatureProduction capacityRotation DirectionPiston StrokeDisplacer strokeCylinder boreCooling down periodWeight

: 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 headNoise 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 classMax. 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

Cylinder temperature switch : 62 °C

Relieve Valve condenser headThermal 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 humidityMinimum 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 minimumpurity 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 systempressure of 20bar max. and min. temperature down to 60K.If the working condition for the installation aredifferent from standard condition, the cold capacity of the installation will be different as well from thegiven capacity. Correction factors to be used to calculate the output under the working conditions.Thecyrogenerator operates according to the Stirling cycle. It generates temperature of 60K in the condenserhead. The generated heat is removed by cooling water.The working pressure in the cryogenic system maybe up to 20bar. Fluid to be cooled has to be pumped through the condenser. The process has to becondensed 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 :

2.2 Construction

The cyrogenerator performs the cooling by compression and expansion of a gas (Helium or Hydrogen) in aclosed cycle. This cycle is called Stirling Cycle. The cyrogenerator is driven by an electric motor coupled viaa flexible coupling and a flywheel. On starting the cyrogenerator the starting valve remains open to lowerthe compression in the working space and thus allow the electric motor to change from Y to Delta before itcomes 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.

1. Piston 8. Cross head2. Displacer 9. Cross head pen3. Displacer rod 10. Connecting rod4.

5.

Displacer ringsconnection rodGuide bussing

11. Twin shanked

12. Crank shaft bearings6. Piston rings 13. Bearing housing7. Scraper ring

Installation identification list1. Protection bellow 13. Pressure block2. Water lines 14. Oil cooler3. Buffer vessel 15. Crankcase4. Cooling water out 16. Oil drain plug5. Electric motor 17. Lift point6. Protection cover 18. Flexible coupling7. Coupling process gas in 19. Lift point8. Condenser head 20. Cooling water temperature9. Coupling process gas out 21. Cooling water in10. Water intercooler 22. Mounting skid11. Cylinder housing 23. Cushy foot12. 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 ambientatmosphere, with the emerging crankshaft being encircled by a gasand oil-tight seal. The cyrogenerator is filled with a refrigerant such ashelium or hydrogen to the filling pressure of either 22 (50Hz) or 18bar (60Hz). Once in operation the maximum working pressure will riseto 30 (50Hz) to 25 (60Hz) bar. Lubrication is by pressurised oil, derivedfrom a crankshaft-driven, super-gear oil pump.Item identification

2.4 The Gas System

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 liquefactiontemperature of Helium 4-5 K. this means that helium will never liquefy when using Nitrogen gas asproduct.The pressure inside cyrogenerator varies between 18 to 43 bar.The working space consists of thecompression space and the expansion space. Connecting to the working space are cooler and theregenerator. The cooling cycle, compressed Helium gas flows through the cooler and the regeneratortowards the expansion space. Because of the expansion, the helium becomes cooler. The energy neededfor this expansion is extracted from the process gas. This gas flows, completely independent of the heliumgas, through the heat exchanger.Because of the repeating cycle the temperature of the process gasbecomes lower. This continues until there is a balance between the temperature of the process gas andthe energy needed for the cycle.During operation some Helium gas leaks through the piston rings into thecrankcase. In the crankcase most of the cycle time the pressure is lower than the pressure in the workingspace. To prevent pressure equalization, gas leaking towards the crankcase has to be returned to theworking 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 tothe crankcase, while the Helium gas flows to the gas filter. This filter takes out the last bit of oil and collectsthe waste oil. The pressure in the buffer space is equal to the maximum pressure in the crankcase, which is21 bar (50Hz) or 18 bar (60Hz).The suppletion valve opens when the minimum pressure in the workingspace is lower than the (constant) pressure in the crankcase. At that time Helium gas flows back into theworking space.2.5 The Oil System

Oil pump (1) driven by the crankshaft, pumps lubricating oil from the bottom of the crankcase to the oilcooler (2). The low temperature of the water cools down the oil. After the cooler the oil is going through anoil filter (3). Her dust particles and other impurities are removed. After filtering the oil is pumped throughthe Mitchell bearing (4). This special bearing is capable of coping with high lateral forces that occur in thecrankshaft. Some oil lubricates the crankshaft bearings (5) and the rest of the oil enters the crankshaft. Viasmall 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 viaa “Cimring”. This ring acts like a one-way valve. Inside this chamber the oil lubricates the shaft seal and itscomponents (9). When the pressure in the chamber is high enough, the oil flows back, via the oil-pressureregulator (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 usedfrom an external installation, the water should be analysed by a consultancy office that will advise whichmeasures are required (filter, inhibitor). This is very important as deposits in the pipes and the cooler willseriously impair the efficiency of the installation.A closed water system consists out of a chiller unit, pipingand 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 toadded once, to ensure quality.A flow meter (FI) checks the amount of flow needed for cooling down thecompressed air.The flow has to be sufficient to ensure maximum process operation.Before entering thecyrogenerator, the water is filtered (FLT) to prevent any dirt going into the coolers. After running the twocoolers, 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, themachine will stop immediately. An error message will be displayed on the main control box. The pressuredrop over the cyrogenerator , Cooling water pressure drop over cyrogenerator.

2.7 The CondenserThe condenser described in this chapter can be used for condensing a gas at a system pressure up to 20barand a temperature down to 60K. Also liquefaction of Nitrogen gas is possible.The cooling of liquids can alsobe carried out, but it is advised that specialist advice should be obtained before operating any such system.

2.7.1 Construction

The re-condenser forms the upper part of the cyrogenerator assembly and is secured to the cylinder of thecyrogenerator by ten threaded studs and nuts. Flexible couplings may be used to connect the inlet outletconnections to the customer equipment.The re-condenser comprises an outlet steel shell of soldered and brazed construction that houses thecondenser head assembly, a pipe, through which the non-condensable gases are drawn off, and a pressuresafety valve. The space between the outer shell and the condenser head is filled with a granular insulatingmaterial called “Perlite”. A fill cap is fitted to enable topping up the insulation material.2.7.2 OperationProcess gas enters the inlet at the top of the condenser head. Inside the head the copper heat exchanger iscooled 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 thecyrogenerator (Ar, Ne, etc.), are sucked off through a line in the top of the hand and are blown off.2.7.3 Safety DeviceThe safety valve operates in case the pressure within the insulation space exceeds 0.2 bar and preventsdamage to the outlet jacket of the condenser.

2.8 Survey of Safety DevicesFigure 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, theinstallation will automatically shut down.

Attention: The cyrogenerator can restart automatically after a shut down due to a power failure or waterflow failure.

NR Name safety device Type Working Principle1 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 maincontrol box. The safety is factory set and sealed.

2 Stationary gas pressuresafety

Mechanical If the pressure in the buffer vessel exceeds 22bar, the safety opens to release the pressure.The safety is factory set and sealed.

3 Working gas pressuresafety switch

Electrical A micro switch is activated if the working gaspressure exceeds the maximum allowedpressure. A signal is given to the PLC in the maincontrol box. The installation is stopped and thestartingvalve opens to release the pressure. Thesafety is factory set and sealed.

4 Water flow switch Electrical If the water flow is sufficient, the flowswitch gives a “Ok signal” to the maincontrol box. If the water flow drops belowminimum set point, the main control boxwill switch off installation.

5 Temperature sensor Electrical The temperature sensor gives a signal to thePLC when the working space exceeds atemperature of 60° C. The PLC will switch offthe system to prevent damages. The safetyis factory set and sealed.

6 Pressure switch non-condensable gasses

Electrical This switch gives an “Okay signal” to the maincontrol box when the process gas pressureexceeds the system pressure minus 0.5 bar. Ifthe pressure drops below this pressure, themain control box will switch off the installation.The safety is factory set and sealed.

7 Safety valve insulationspace

Mechanical If the insulation pressure exceeds 0.2 bar,this valve blows off, preventing damage tothe condenser head interior.

8 Thermal motor overloadswitch

Electrical This automatic fuse switches when the electricmotor consumes too much power. The maincontrol box will switch off the installation. Thesafety 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

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 systems4. 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 cycle6. 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 cycleregenerators

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 1510. Sdmfidt, G. Theorie der Gesehlossenen Calorisehen Masehine van

Laubroy and Sehwartzkopff in Berlin Z Ver Oster lng (1861) 7911. 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 Applicationof Closed Cycle Cryocoolers tO Superconducting Devices NBS S~al Publication 5-8, US GovernmentPrinting Office, Washington. DC (!978)

13 . Zimmezman, J.E., Radebaugh, IL Applications of closedcycle cryocoolers to small superconducting devices, ProcConf Boulder (1977), National Bureau of Standards SpecialPub 508 (1978) 59

14. Zimmerman, J.E.,SuUivan, D.B.,Cryogenics, 19 (1979) 17015. MeCarty, R.D., Thermophysical properties of He-A, NBS

Technical Note 631 (1972)16. Radebaugh, IL, Zirnrnerman, J.E., ibid 1, 6717 . Jakob, M. Heat transfer, Wiley New York 1 (1949)18. Institute Instrumentation centre of IIT Roorkee LN2 manual and records.