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INDUSTRIAL TRAINING REPORT

AT

Tata Institute of Fundamental Research (TIFR)(From 11-06-2012 to 30-06-2012)

NAME : HAREN B. PARMAR

BRANCH : Mechanical Engineering

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ACKNOWLEGEMENT

It was a great opportunity to have summer training at Cryogenics

Department, Low Temperature Facility, Tata Institute Of Fundamental Research

under the guidance of In-charge Engineer Mr.K.V.Srinivasan.

Cryogenics is a totally new technology(below -150oC) that has its numerous

and mind-boggling applications in field of refrigeration, medical and transport. We

took the training of 20days started on 11/6/2012 to 30/6/2012. In this journey of 20

days we discovered the process of the production of Liguid Nitrogen and Liquid

Helium.In these days treatment given by all the employees and assistants was very

appreciable. They behaved very co-operatively and were always keen to clear our

doubts during training scheduled.

Mr.Arvind, Mr.Jaison, Mr.Vijay, Mr. Sandal always co-operated us

and guided us at every moment. We are sincerely thankful towards all of them of the

institute who supported us our every step.

We are thankful towards our HOD Mrs. Kiran Chaudhary on whom reference we

were in the company. We are also thankful towards the whole other college staff-

members who supported us for the procedure. We are also thankful to the institute

for providing quality canteen and bus facilities. It was indeed a wonderful

experience to all of us.

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INDEX

Sr.no Contents Pageno.

1.About Low Temperature Facility 4

2.About Liquid Nitrogen 4

3.

Principal Of Operation

i. Main Parts of STIRLIN-8 Systemii. Brief Process of Nitrogen

Liquefactioniii. Pipe & Instrument Diagram (P&I

Diagram)

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4.

Design/Function of Various Parts

i. Rotary screw air compressorii. Pressure Swing Adsorption

iii. Cryogenerator

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5. Study Of Rotary Vane Pump 25

6. Conclusion 32

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Low Temperature Facility

Low Temperature Facility of Tata Institute of Fundamental Research, (TIFR)Mumbai meets the requirement of two cryogenic fluids - liquid helium and liquidnitrogen along with other support services to various facilities and laboratories of theinstitute such as, Condensed Matter Physics and Materials Science (DCMP & MS),Nuclear and Atomic Physics (DNAP), Biological Sciences (DBS), Astronomy andAstrophysics (DAA), and Chemical Sciences (DCS).

Low Temperature Facility (LTF) was started in 1961 with liquefaction of helium andthen a liquid nitrogen generator was added in 1968. All the cryogen-demands arebeing met on demand. For enhanced user convenience, 'on-line cryogens' requestsand interfacing has been implemented for liquid helium, nitrogen.

Liquid nitrogen

The category of substances known as liquefied gases includes liquid nitrogen,oxygen, argon, helium and carbon dioxide but these are just a few of the mostcommon ones; there are many others. Liquefied gases are extremely cold liquids, forexample at atmospheric pressures liquid oxygen exists at -183°C and liquid nitrogenat -196°C.

Liquid nitrogen is nitrogen in a liquid state at an extremely low temperature. Liquidnitrogen is a colorless clear liquid with density of 0.807 g/mL at its boiling point anda dielectric constant of 1.4. Liquid nitrogen is often referred to by the abbreviation,LN2 or "LIN" or "LN".

At atmospheric pressure, liquid nitrogen boils at −196 °C (77 K; −321 °F) and is acryogenic fluid which can cause rapid freezing on contact with living tissue, whichmay lead to frostbite. When appropriately insulated from ambient heat, liquidnitrogen can be stored and transported, for example in vacuum flasks. Here, the verylow temperature is held constant at 77 K by slow boiling of the liquid, resulting inthe evolution of nitrogen gas. Depending on the size and design, the holding time ofvacuum flasks ranges from a few hours to a few weeks.

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PRINCIPLE OF OPERATION

The Stirling Liquid Nitrogen plant (StirLIN) is used to produce liquid nitrogen. Thenitrogen is filtered from compressed ambient air (consisting of 78 % Nitrogen).Subsequently, the nitrogen is liquefied and stored. From this description thefunctions of the StirLIN-8 can be deduced:

1. Compressing air

2. Separating the nitrogen from the compressed air.

3. Cooling the nitrogen to produce liquid nitrogen.

4. Storing the liquid nitrogen.

Main parts of StirLIN-8 system

1. An air compressor.

2. A water chiller (optional).

3. An air drier for drying the compressed air.

4. An air vessel, for intermediate storage

5. A Pressure Swing Adsorber (PSA) for separating the Nitrogen from thecompressed air.

6. A mixing vessel for equalizing the N2 gas.

7. A cryogenerator for liquefying the Nitrogen.

8. A storage vessel.

Figure 1. Principle of StirLIN system

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Process:

Figure 2.Nitrogen Liquefaction Process

1. The air compressor

Ambient air enters the compressor (01.00) by means of a suction fan. The air iscompressed up to a pressure of approximately 8 bars. Condensed moisture, oilparticles and dust in the air are removed by an air dryer (23.00) and filter elements(05.00). Because the air feed to the P.S.A. is not constant, an air vessel (36.00) isneeded to provide the P.S.A. with a constant flow of air.

2. The water chiller (optional)

Water, needed for several cooling purposes are cooled down by means of a closedwater circuit. The chiller consists of a water circulation pump and a heat exchanger.A secondary circuit compresses and expands Freon. The phase change betweenFreon gas and liquid Freon provides cold and is fed through the heat exchanger, thuscooling the water. The water is treated with an inhibitor to prevent corrosion andbiological activities.

3. The Pressure Swing Adsorber (PSA)

The ambient air consists of 21 % Oxygen, 78 % Nitrogen and 1 % inert gases (forexample Argon). Producing almost pure Nitrogen is therefore simply a matter offiltering the Oxygen out of the compressed air. This is accomplished by the PressureSwing Adsorber (20.00), which consists of two Oxygen adsorber vessels. These

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vessels are filled with carbon molecular sieves, which have the capacity of bindingOxygen molecules.

After adsorbing a certain amount of Oxygen, the sieves are saturated and must beregenerated. Therefore, the vessels are used alternately. When one vessel isadsorbing Oxygen, the other vessel is being regenerated. Regeneration isaccomplished by de- pressurizing the vessel to the ambient pressure, thus blowing offthe Oxygen. The Nitrogen gas is stored in a mixing vessel (30.00) to equalization ofthe gas.

4. The liquefaction of Nitrogen

In the cryogenerator (25.00 and 26.00), gaseous Nitrogen from the PSA is liquefied.The cold for liquefaction is generated by the Stirling cycle. This is basically acompression and expansion of a perfect gas, mostly helium or hydrogen (the processgas). Compression and expansion take place in two discrete spaces. A water coolerand a regenerator thermally separate the warm compression space and the coldexpansion space. The water cooler removes the heat generated during compression.The regenerator serves as a heat buffer, creating a temperature gradient between thecompression space and the expansion space. The expansion of the process gas coolsthe cryogenerator’s condenser head. The Nitrogen flows along the condenser head,and is thus liquefied. Subsequently, the liquid Nitrogen is stored in a storage tank(40.00). Non condensable gases (Argon) are removed from the condenser head.

Pipe & Instrument Diagram (P&I Diagram)

Pipe and Instrument Diagram shows the process flow of the StirLIN-8.

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Design/Functions of Various Parts:

1. The Screw Compressor

Introduction

Rotary screw air compressors (oil-lubricated) are most commonly used aircompressor for medium to high flow nitrogen applications. Rotary screw aircompressors are positive displacement compressors. The most widely usedcompressor in industry is the single stage oil lubricated rotary screw air compressor.The rotary screw compressor has two rotors encased within a housing, known as anairend. Ambient air enters the airend where the air is compressed by the two rotors.Oil is injected into the airend to create a seal as well as for cooling purposes. The aircompressor used in the process is Screw Compressor BSD 72/11 bar SIGMACONTROL

Figure 3. The Screw Compressor

Item identification

1.Inlet valve 2.Minimum pressure/Checkvalve

3.Drive motor

4.Oil filter 5.Airend 6.Control cabinet

7.Oil separatortank

8.Air filter 9.Oil/air cooler

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Working of Screw Compressor

Ambient air is cleaned as it is drawn in through the filter 8.The air is thencompressed in the airend 5.The airend is driven by an electric motor 3.

Cooling oil is injected into the airend. It lubricates moving parts and forms a sealbetween the rotors themselves and between them and the airend casing. Thisdirect cooling in the compression chamber ensures a very low airend dischargetemperature.

Cooling oil recovered from the compressed air in the oil separator tank 7 gives upits heat in the oil cooler 9. The oil then flows through the oil filter 4 and back tothe point of injection. Pressure within the machine keeps the oil circulating. Aseparate pump is not necessary. A thermostatic valve maintains optimum coolingoil temperature.

Compressed air, freed of cooling oil in the oil separator tank 7, flows through theminimum pressure/ check valve 2 into the air cooler 9. The minimumpressure/check valve ensures that there is always a minimum internal air pressuresufficient to maintain cooling oil circulation in the machine.

The air cooler brings down the compressed air temperature of only 5 to 10 Kabove ambient. Most of the moisture carried in the air is removed during thiscooling process.

Functions of Parts

Figure 4. Parts in screw compressor

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i. Airend

Figure 5. Screw compressor airend

The screw element is the most important part of any screw-type compressor. It's thatpart of the machine where the actual compression takes place. It is the heart of therotary screw air compressor. Often, the compressor screw element is also called air-end. The rotary screw compressor uses two rotors (helical screws) to compress theair. There's a 'female' rotor and a 'male' rotor. The rotors are of different shape, but fiteach other exactly. When the rotors start turning, air will get sucked in on one sideand gets 'trapped' between the rotors. Since the rotors are continuously turning, theair gets pushed to the other end of the rotors (the 'pressure side') and new fresh airgets sucked in.

ii. Oil injected in compressor

The oil is injected in the compressor element (where the actual compression takesplace), during the compression of the air. This oil is later removed by the oilseparator, so we end up with clean compressed air. Although 99.9% of the oil staysinside the compressor, there is always a little oil that passes through the separatorand leaves the compressor with the compressed air. This is called "oil carryover".

iii. Compressor’s filters elements

In screw air compressor there are oil & air filters. Air filters will protect thecompressor from dust. Oil filters are normally only found on bigger compressors likeoil-injected screw compressors. Lubrication is done by the splashing of thecrankshaft in the oil bath. The oil filter is there to filter out any dirt from the oil. Dirt

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like sand, dust or small pieces of metal will wear-down rotating parts like thecompressor element and the bearings. The air compressor oil filter protects allmoving parts like compressor element, bearings and valves from breaking down.

iv. Compressor coolers

Every compressor has some sort of cooler. A lot of heat is generated. To get rid of allthis heat, air compressor has 1 or more coolers. Small air compressors (thereciprocating piston type) have the simplest form of cooler: fins around thedischarge pipe. Bigger air compressors (of the rotary screw type) have normally 1 oilcooler and 1 after cooler. The oil-cooler will cool the hot oil before it is returned tothe compressor element. The after cooler will cool the oil before it comes out of thecompressor.

2. Compressed Air DryerIntroductionMoisture is one of the major contaminants in compressed air systems. It occursbecause water vapour present in the atmosphere is drawn into the compressor,where its' concentration can rise dramatically as temperature increasesLarge volumes of atmospheric air enter the compressed air system through thecompressor intake. As the air is compressed, its temperature increasessignificantly, causing it to become fully saturated with water vapour. Watervapour retention in air is dependent upon its temperature and pressure; the higherthe temperature, the more water vapour that can be retained; the higher thepressure, the greater the amount of condensed water that will be releasedafter the compression stage, the now saturated air is cooled to a usabletemperature by an aftercooler, causing the retained water vapour to be condensedinto liquid water which is then removed by a condensate drain. The air leaving theaftercooler is now 100% saturated with water vapour.

As the compressed air moves downstream to storage vessels and through piping,its temperature falls and concentrated vapour will sublimate as droplets of liquidwater. If not removed, this will cause corrosion of the distribution system,

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blocked or frozen valves and machinery, as well as providing an ideal breedingground for micro-organisms and bacteria.To eliminate these moisture problems, all viable water vapour must be removedby adsorption dryers, before it can enter the compressed air system.

Working of Dryer

PNEUDRI comprises of high tensile extruded aluminiumcolumns each containing twin chambers filled with desiccantmaterial which dries the compressed air as it passes through.

One chamber is operational (drying), whilst the oppositechamber is regenerating by Pressure Swing Adsorption (PSA).

A small volume of the dried compressed air is used toregenerate the saturated desiccant bed by expanding air fromline pressure to atmospheric pressure, removing the watervapour adsorbed by the desiccant material, and thereforeregenerating the dryer. Heat regenerative models have electricheaters built into the desiccant beds to further reduce purge airconsumption and increase operating efficiency.

3. Pressure Swing Adsorption

Introduction

The technology of air-to-nitrogen production with the use of adsorption processesin nitrogen generators is well studied and widely applied at industrial facilities forthe recovery of high-purity nitrogen.

The operating principle of a nitrogen generator utilizing the adsorption technology isbased on the dependence of the adsorption rates featured by various gas mixturecomponents upon pressure and temperature factors. Among nitrogen adsorptionplants of various types, pressure swing adsorption (PSA) plants have found thebroadest application world-wide.

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The system's design is based on the regulation of gas adsorption and adsorbentregeneration by means of changing pressures in two adsorber–adsorbent-containingvessels. This process requires constant temperature, close to ambient. With thisprocess, nitrogen is produced by the plant at the above-atmospheric pressure, whilethe adsorbent regeneration is accomplished at below-atmospheric pressure.

Figure 6. Process of nitrogen generation

The swing adsorption process in each of the two adsorbers consists of two stagesrunning for a few minutes. At the adsorption stage Oxygen, H2O and CO2 moleculesdiffuse into the pore structure of the adsorbent whilst the nitrogen molecules areallowed to travel through the adsorber–adsorbent-containing vessel. At theregeneration stage the adsorbed components are released from the adsorbent ventedinto the atmosphere. The process is then multiply repeated.

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Working of PSA

Figure 7. Working of PSA

PSA nitrogen generators operate on the Pressure Swing Adsorption (PSA)principle to produce a continuous stream of nitrogen gas from compressed air.Pairs of extruded aluminum columns are filled with carbon molecular sieve(CMS).

Pretreated compressed air enters the bottom of the on-line column and followsup through the CMS. Oxygen and other trace gasses are preferentially adsorbedby the CMS, allowing nitrogen to pass through. After a pre-set time, the on-linecolumn automatically switches to regenerative mode, venting contaminantsfrom the CMS.

Carbon molecular sieve differs from ordinary activated carbons in that it has amuch narrower range of pore openings. This allows small molecules such asoxygen to penetrate the pores and be separated from nitrogen molecules whichare too large to enter the CMS. The larger molecules of nitrogen by-pass theCMS and emerge as the product gas.

During adsorption in one tower the second tower is totally regenerated just bydepressurization to ambient pressure. The Oxygen enriched off gas with 30-35vol.-% Oxygen content is vented to the outside atmosphere. After about oneminute adsorption in one adsorption tower the process controller is switchingover to the second tower and the first one is regenerated. This cycle operates ona continuous basis, ensuring a constant stream of nitrogen gas, 24/7 if required.

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Figure 8. PSA block diagram

4.Cryogenerator

Introduction

The 4-cylinder cryogenerator is used for the cooling of a fluid or the condensation ofa gas at a system pressure of 20 bar maximum and a minimum temperature down to60K. The cryogenerator operates according to the Stirling Cycle. It generates atemperature of 60K in the condenser head. The generated heat is removed by meansof cooling water. The working pressure in the cryogenic system may be up to 20 bar.Fluid to be cooled has to be pumped through the condenser. The process gas to becondensed is, depending on the liquefaction rate, sucked in automatically.The threemain flows are shown in Figure 9. Flow diagram.

Figure 9. Flow diagram of cryogenerator

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Construction

The cryogenerator performs the cooling by compression and expansion of a gas(Helium or Hydrogen) in a closed cycle. This cycle is called the Stirling Cycle.

The cryogenerator is driven by an electric motor, coupled via a flexiblecoupling and a flywheel. On starting the cryogenerator, the starting valveremains open to lower the compression in the working space and thus allow theelectric motor to change from Y to Delta before it comes under full load.

When the lubricating oil reaches its operating pressure, the starting valvecloses. The compression in the working space increases and normal runningcommences.

The cooling cycle

The cryogenerator is filled with Helium gas or Hydrogen gas under highpressure, which effects the cooling process in a closed cycle. This cycle iscalled the Stirling cycle. In the internal process, it is required to use a gas,which does not liquefy at the operating temperature of the cryogenerator. Forthis reason either helium or hydrogen can be used as the cooling medium.

The cryogenerator performs the cooling by compression and expansion of theworking. The internal process is displayed in Figure 3 Stirling cycle.

Figure 10. Stirling cycle

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Step 1

With the displacer (2) in its highest position, the rotation of the crankshaft moves thepiston (1) upward to compress the refrigeration gas (He) in the compression space(7). The pressure will rise from P1 to P2.

Step 2

As the crankshaft continues its rotation, the piston (1) remains almost stationary andthe displacer (2) moves downward. The refrigerant gas is forced through the cooler(4), where the gas is cooled to the temperature of the cooling water. The gas isfurther cooled as it passes through the regenerator (5), which has a low temperaturefrom the previous cycle, before entering the expansion space (8).The pressure dropfrom P2 to P3.

Step 3

The rotating crankshaft moves both the displacer (2) and the piston (1) downwardsand the gas expands in expansion space (8). As the gas expands, its temperaturedrops and the cold is transferred to the cool surface (6). Here the cold is transferredto the heat exchanger in the condenser head. The pressure drop from P3 to P4.

Step 4

The displacer (2) now returns to its highest point and the piston (1) remains in itsbottom position. The cold gas is forced back to the compression space (7), and isbrought back to the temperature of the cooling water by passing through theregenerator (5) and the cooler (4). The cycle, now completed, is then repeated. Thepressure rise P4 to P1.

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Figure 11. PV-diagram Figure 12. Actual PV diagram

So far, the working principle has been presumed to be an intermittent movement. Inpractice, the movement of the piston and the displacer is a continuous harmonicmotion.

The driving mechanism

The crankshaft drives the piston (1) via a twin-shanked connecting rod(11).The displacer (2) is operated by a second connecting rod (10) and adisplacer rod (3), passing through the centre of the piston (1).

The cryogenerator is completely sealed-off from the ambient atmosphere, withthe emerging crankshaft being encircled by a gas and oil-tight seal. Thecryogenerator is filled with a refrigerant such as helium or hydrogen to thefilling pressure of either 22 (50Hz) or 18 bar (60Hz).

Once in operation the maximum working pressure will rise to 30 (50Hz) or 25(60Hz) bar. Lubrication is by pressurised oil, derived from acrankshaft-driven,spur-gear oil pump.

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Figure 13. The drivingmechanism

Item identification

1.Piston 4.Displacerrings

7. Scraperring

13. Bearinghousing

2.Displacer 5.Guidebussing

8. Crosshead

3.Displacerrod

6.Pistonrings

9. Crosshead pen

10.Connectingrod

11. Twinshankedconnectionrod

12. Crankshaftbearings

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The Gas System

Figure 14. Gas system shows a schematic image of the cryogenerator.

The cryogenerator is a completely sealed machine, which is filled with Heliumor Hydrogen gas. The Helium is called: working gas. The liquefactiontemperature of Helium is 4 to 5K. This means that the helium will never liquefywhen using Nitrogen gas as product.

The cooling cycle, the compressed helium gas flows through the cooler and theregenerator towards the expansion space. Because of the expansion, the heliumbecomes colder. The energy needed for this expansion is extracted from theprocess gas.

This gas flows, completely independent of the helium gas, through the heatexchanger. Because of the repeating cycle the temperature of the process gasbecomes lower.

This continues until there is a balance between the temperature of the processgas and the energy needed for the cycle. During operation some helium gas

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leaks through the piston rings into the crankcase. In the crankcase most of thecycle time the pressure is lower than the pressure in the working space.

To prevent a pressure equalisation, the gas leaking towards the crankcase has tobe returned to the working space. In the crankcase a mixture is formed betweenleaked Helium gas and the lubricating oil.

This mixture ends up in the oil-gas separator where both substances areseparated. The oil flows back to the crankcase, while the helium gas flows tothe gas filter. This filter takes out the last bit of oil and collects the waste oil.

In the buffer space the now clean helium gas can pass the suppletion valve. Thepressure in the buffer space is equal to the maximum pressure in the crankcase,which is 21 bar (50Hz). The suppletion valve opens when the minimumpressure in the working space is lower than the (constant) pressure in thecrankcase. At that time helium gas flows back into the working space.

The Oil System

Figure 15. The oil system

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An oil pump (1), driven by the crankshaft, pumps lubricating oil from thebottom of the crankcase to an oil-filter (3). Here dust-particles and otherimpurities are removed. After the filter the oil is going through the Michellbearing (4).

This special bearing is capable of coping with high lateral forces that occur inthe crankshaft. Some oil lubricates the crankshaft bearings (5) and the rest ofthe oil enters the oil cooler (2).

The low temperature of the water cools down the oil. Via small canals (6)inside the crankshaft bearings oil is pushed up (7) trough the crankshaft canalsto the gudgeon pins. These pins are lifted every stroke, enabling the oil tolubricate them. Partly also an oilflow enters the shaftseal chamber via a“Cimring”(8). This ring acts like a one-way valve. Inside this chamber the oillubricates the shaftseal and its components (9). When the pressure in thechamber is high enough, the oil flows back, via the oil-pressure regulator (10),to the crankcase.

The Cooling Water System

Either a closed water system or external equipment supplies cooling water. Aclosed water system consists out of a chiller unit, piping and valves.The maincontrol box controls the chiller, which automatically controls the watertemperature. The quality of the water has to be checked when new water is putinto the system. Inhibitors have to be added once, to ensure the quality.

The flow has to be sufficient to ensure maximum process operation. Beforeentering the cryogenerator, the water is filtered to prevent any dirt going intothe coolers. After running through the two coolers, a flow switch measures thewater flow. The cryogenerator cannot run without water. Therefore the waterflow 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 themain control box.

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The Condenser

The condenser for the SPC-04 is specially developed to obtain a natural flow bygravity. The outer jacket is cast iron which is suitable for a vacuum space to preventlosses.

Figure 16. The condenser

The re-condenser forms the upper part of the cryogenerator assembly and is securedto the cylinder of the cryogenerator by threaded studs and nuts. Flexible VJ-lines areused to make a connection between the cryostat and the re-liquefier. The re-condenser comprises an outer cast iron shell that houses the condenser headassembly, pipes, and a pressure safety valve .

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Study Rotary Vane Vacuum Pump

Introduction

Mechanical vacuum pumps work by the process of positive gasdisplacement, that is, during operation the pump periodically createsincreasing and decreasing volumes to remove gases from the system,and exhaust them to the atmosphere. In most designs a motor drivenrotor spins inside a cylindrical stator of larger diameter.

Rotary Vane Mechanical Vacuum Pumps

Rotary vane pumps typically have an electric motor driven rotor(either belt or directly driven) which has one to three sliding vanesthat maintain close contact with the inner wall of the cylindricalstator. The vanes are metal in oil sealed pumps, and carbon in drypumps.

Centripetal force acts upon the vanes in the spinning rotor so as toforce them against the inner sealing surface of the stator. In somemechanical pumps springs are used to augment this action. Rotaryvane pumps may be of the single or double stage design. Single stagepumps are simpler, having only one rotor and stator, and are lessexpensive.

In a two stage design, the exhaust port of the first stage is connectedto the inlet port of the second stage which exhausts to atmosphericpressure. Two stage pumps may attain a base pressure of one to twomillitorr, but are more expensive than single stage pumps.

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Figure 17. Rotary Vane Pump

Material specification

Externals (head, casing) - Cast iron, ductile iron, steel, andstainless steel.

Vane, Pushrods - Carbon graphite.

End Plates - Carbon graphite.

Shaft Seal - Component mechanical seals, industry-standardcartridge mechanical seals, and * magnetically-driven pumps.

Oil used – Alcatel 120.

Operation cycle of single stage rotary vane pump

In the rotary vane pump there are a hollow cylindrical stator with inletand exhaust valves, a rotor mounted eccentrically inside the stator for

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pumping, two vanes sliding in the rotor, forced against the stator bycentrifugal force and springs

Inlet

As the vane passes in front of the inlet orifice, an increasing space isformed into which the gas from the chamber to be evacuated expands.When the second vane passes, the space is closed.

Transfer

The gas trapped in the space between the two vanes is transferred to theexhaust orifice as the rotor rotates.

Compression

The space communicates with the exhaust, which is fitted with a valve:the gas is compressed until the safety valve is opened.

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Exhaust

The gas is expelled into the oil casing when the pressure is sufficient toopen the valve.

Application

Single stage rotary vane pumps are the best choice for continuouspressures above 1.0 Torr (1.3 mbar), as well as applications where largeamounts of condensable gases are present.

Two stage rotary vane pump

To improve the backing pressure and flowrate at low pressure, twostages are connected in series. The second is similar to the first bothstructurally and operationally. The gases pulled in by the first (lowpressure) stage are transferred to the second (high pressure) stage anddischarged through the high pressure (HP) valve.

Application

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Two stage rotary vane pumps are the best choice for applicationrequiring an ultimate vacuum as low as 103Torr (1.33 x 103mbar).

Function of oil in rotary vane pump

Oil has several important functions in the pump: It lubricates mechanicalcomponents (bearings, seals, rotor, vanes, etc.). It makes moving partsrelatively tight by limiting internal leakage. It carries away the heatproduced by the compressed gases. Not all oils produce the sameultimate pressure in a given pump. Ultimate pressure depends on thesaturated vapour pressure of the oil, its viscosity and its ability todissolve gases. Good pumping conditions are related to the type of oilused. The choice depends on: - Expected pump performance, chemicalaggression and corrosion of pumped gases, accessories used, desiredmaintenance intervals and total operating cost.

Function of gas ballast

When condensable vapours are being pumped, gas is compressedbeyond its saturated vapour pressure in the "compression" phase.

Vapors can condense (liquefy) in the compression cycle and mix withthe pump oil to contaminate the oil and at worst, form an emulsion. Inmany industrial applications the vapor is water vapor that condenses towater, but it can be other liquids too. This mixture of oil and liquidcontaminant circulates through the pump and causes:

• Reduced vacuum performance

• Loss of lubrication and sealing

• Possible Pump corrosion

• Possible Pump seizure

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The gas ballast can be used to inject a certain quantity of air (neutral ordry gas) into the last stage of the pump during the "compression" phaseso that the partial pressure of the pumped gas is less than its saturatedvapour pressure at the temperature of the pump. Condensation istherefore impossible if this limit is not reached. The maximumadmissible vapour pressure is obtained at pump inlet for this value.

At the end of "compression", the pressure in the exhaust chamber isgreater than atmospheric pressure. An anti-suckback device (valve +spring) prevents the gases and oil from being drawn back into the inlet.

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The saturated vapour pressure of a body is higher when the system is hotthan when it is cold; therefore, the pump must reach operatingtemperature before pumping condensable vapours.

Photos of disassembling & assembling of Alcatel rotary vane pump

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CONCLUSION:

Thus, we have successfully studied the production of liquid nitrogen inlow temperature facility of TIFR & also studied the Alcatel Rotary VaneVacuum Pump. The process of nitrogen liquefaction of is closelymonitored by a number of sensors and also the plant personnel. Theworking of the various valves is completely controlled by aprogrammable logic circuit (PLC) which ensures the elimination ofhuman errors and boosting the efficiency. We got a whole newexperience of how things work in a company or an institute. It wasindeed a true learning experience.