Views: 90

VACUUM TECHNOLOGY

1.a Define  a  vacuum

Vacuum means an enclosure where pressure is lower than the atmospheric pressure.

1. State the pressure ranges of the following vacua

• Low Vacua =760 –25 Torr
• Medium Vacua = 25 – 10^-3Torr
• High  Vacua =10^-3–10^-6 Torr
• Very High Vacua =10 ^–6–10^-9 Torr
• Ultra High Vacua = 10 ^–9Torr And Above

1. (a)State  three uses of vacuum in the laboratory

Vacuums are useful in:

• Aluminizing of mirrors
• Production of  optical films
• Production of  electronic components  e.g. capacitors , resistors  and integrated circuits (ICs)
• Production of thin films
• Mounting of specimens in electron microscopy
• Production of tungsten filaments lamps, discharge lamps e.g. fluorescent tubes etc.
• Manufacture of X-ray tubes, radio receiver valves voltage stabilizes and cathode ray tubes etc.
• In vacuum metallurgy  i.e. production of alloys  and pure metals of varying strength and properties
• Simulation of outer space  and  high altitude environments
• In freeze drying of  food staffs and pharmaceutical products
• In cleaning
• In suctions
• In filtration, distillation , crystallization and evaporation

2. State the meaning of the term ‘Ultimate pressure’ as  used in vacuum pumps

Ultimate pressure is the lowest pressure of a blank-flanged vacuum pump under defined conditions without gas inlet. At ultimate pressure, the usable pumping speed will be zero. It is a theoretical value.

The lowest pressure which can be achieved in a vacuum vessel will be determined by

 

• Pumping speed
• Vapor pressure of lubricants
• Degassing of solved gases in lubricants
• Desorption of gases from internal surfaces of vessel
• Leak tightness of system itself
• Diffusion of gas though vacuum wall or seals
• Compression of vacuum pump system

3. State three safety precautions that must be taken before evacuating desicators and vacuum  filtration apparatus

Desiccators and vacuum filtration apparatus should first be cooled down and slowly have their valves released slowly to reduce the vacuum levels inside the apparatus

4. Define the term
   1. Through-put

Through-put is defined as the quantity of gas at a specified  time  and temperature  passing  through an open  cross- sectional area  of the vacuum system per  unit  area .

1. Pumping speed

Pumping speed is defined as the rate at which a vacuum pump displaces air from a system at a given unit time in the process of creation of  vacuum

5. Describe the operation of a rotor pump

During its operation, air from the vacuum chambers enters the pump through the inlet part and passes into the space created by the eccentric mounting of the rotor in the strator.

6. Two vanes mounted in the rotor sweeps the created space and the trapped air is compressed to a pressure just above the atmosphere. These causes the exhaust flap valves to lift to expel air through the sealing oil –bath to the atmosphere. The process is repeated many times until an ultimate pressure is reached

 

 

7. Describe how the following instruments are used to measure   vacuum
   1. Pirani gauge

It is also one of the most used gauges that measures vacuum within the range of medium to high vacuum. Its action depends upon the variation in thermal conductivity of gas with pressure. A metal filament heated by an electric current  reaches an equilibrium temperature as  heating generated by an electric current  is balanced by the heat loss  due to conduction , convection  and radiation . At higher pressure , the  number of gas molecules  striking the  heated filament  in unit diameter  are large  due to the intermolecular  conduction. While at low pressure, few gas molecules strike the heated filament hence the measurement of the electric  resistance of the filament  provides  a means of determining  the pressure .

Thermocouple gauge

Just like the Pirani gauge, a change in gas pressure causes a change in temperature of a heated filament.

In thermocouple gauge, the temperature is measure by means of a thermocouple fixed to the filament, which is heated by a constant supply of current.

The electromotive force (EMF) generated can be used to operate a micrometer calibrated in pressure units. The operating range of a thermocouple gauge is from   1 torr – 10^-3 torr and the filament can be heated by alternating current

Vacustat

The mercury inside traps and compresses the gas from the system in a closed capillary.  The height which mercury rises indicates the degree of vacuum. Vacustat covers a pressure range from 1 torr – 10^-3 torr.

8. Explain any two types of leaks  in vacuum systems

Leaks can be  classified as real or virtual.

A real or true leak is due to the gas entering  the system through  a hole or any opening  in the system.  

A virtual leak is  caused by  the out gassing (i.e. release of gases and vapors  by the materials used in the construction of the system ) of the inner surface of the system .Virtual leaks  can only be minimized  by proper fabrication and cleaning  techniques  together with proper choice of materials  used.

Both real and virtual leaks produce the same results i.e.   They cause a rise in pressure in the system when it is isolated from the pump or limits the pressure attainable in a continuously pumped system.

9. Discuss any four methods of leak  detection

Leakage detection can be divided into two broad groups

1. High or over-pressure method

This is where the system under test is filled with gas to slightly over atmospheric pressure and the outflow is detected.

Examples of this methods include :

Wet testing

The vessel is immersed in water or liquid of low surface tension such as alcohol, any bubble produced will indicate the presence and origin of a leakage. If the vessel is too large to be immersed, the suspected area is painted with a soap solution or any other search liquid available. The leakage will be indicated by bubbles (The vessel must first be filled with pressure before the search liquid is applied or before emersion)

 

Sealing compound

In this method, suspected soldered joints are covered with plasticine or a Q compound, if the leakage stops then that is exactly where the leakage is. The leakage is then sealed and the vessel pressure is reduced.

The sniffer technique

In this method, gas issuing from the leaks in a pressurized system is sampled by being drawn through a flexible tube into a suitable detector (sniffer) .Within this detectors head, electrons are emitted and these in turn shall ionize any gas molecules present from the system. This can be measured.

Spark coil

A testa coils  is used for locating  leakages in a gas vacuum system .The tip of the coil is passed over the surface  of the vessel and when it comes into  contact  with a leak , a high frequency spark  jumps  from the tip of the coil to the leakage area .

 

1. Low or under -pressure method

In this method, a special device in the system is used to detect the flow of an externally applied gas. The detector is usually a pressure gauge mounted within a continuously pumped system. A leak will be indicated by an apparent change in pressure when a suitable probe gas replaces the leaking air.

10. List any  methods  used in leak hunting in the vacuum system.
    1. Use of high or low pressure detection methods
    2. Use of leak detectors
11. Distinguish between  back migration and  back streaming  as used in vacuum technology.

Back streaming  is a  process whereby  vapor pumps  allow pumping fluids back  into the  vacuum  system  by direct  flight of vapor  molecules   scattered  from hot vapor  jet  or evaporation  at the mouth of the inlet of the vapor pump

• Back migration – this is the transfer of vapor to the higher vacuum side by re-evaporation of molecules which cling to the surfaces within the pump.

 

12. Distinguish between the working mechanisms  of a positive displacement  and moment transfer vacuum pumps

There are different types of vacuum pumps which have basically a common function and that is to remove air, gases and vapour from a confined space and creating a vacuum in that given space. Basically, there are three different types of vacuum pumps, which are:-

13. Positive Displacement Vacuum Pumps

The positive displacement vacuum pumps are used to create low vacuums. This type of vacuum pump, expands a cavity and allows the gases to flow out of the sealed environment or chamber. After that, the cavity is sealed and causes it to exhaust it to the atmosphere.

The principle behind positive displacement vacuum pump is create a vacuum by expanding the volume of a container. For example in a manual water pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure, some fluid from the chamber is pushed into the pump’s small cavity. After that, the pump’s cavity is then sealed from the chamber, opened to the atmosphere and then squeezed back to a minute size. Another example of positive displacement vacuum pumps is like a diaphragm muscle expands the chest cavity, causing the volume of lungs to increase. This expansion results to creating a partial vacuum and reducing the pressure, which is then filled by air pushed in by atmospheric pressure. The examples of positive displacement vacuum pumps are liquid ring vacuum pumps and roots blower which are highly used in various industries to create vacuum in confined space.

14. Momentum Transfer Vacuum Pumps

In the vacuum pump where gas molecules are accelerated from the vacuum side to the exhaust side is known as momentum transfer pump. Based on the laws of fluid dynamics, matter flows differently at different pressures. In the atmospheric pressure and mild volumes, the molecules interact with each other and push on their neighbouring molecules which is known as viscous flow. The molecules interact with the walls of the chamber frequently rather than other molecules when the distance between the molecules increases. This stages is generally called high vacuum which is much more effective than positive displacement. The momentum transfer vacuum pumps are further classified into two types of vacuum pumps – diffusion pump and turbomolecular pump. These both types of pumps blow out gas molecules which are diffused into the pump. The main difference between these two types of pumps is that the diffusion pumps blow out molecules with jets of oil, whereas, turbomolecular pumps use high speed fans. If exhausted directly to atmospheric pressure, these two types of pumps will stall and fail to pump.

15. Entrapment Pumps

Entrapment pumps work using chemical reactions, are known to perform more effectively because they are usually placed inside the space or container to be vacuumed. The molecules in the air create a thin film which is removed by the entrapment pumps as they chemically react to the internal surfaces of the pump. This type of vacuum pump uses cold temperatures to condense gases to a solid or absorbed state. To create ultra high vacuum chambers, the entrapment pumps are used alongwith positive displacement vacuum pumps and momentum transfer vacuum pumps.All the above types of vacuum pumps are further classified into liquid ring vacuum pumps, single cone vacuum pumps, close couple vacuum pumps, two stage vacuum pumps, chemical process pumps and twin lobe roots blower.

 

16. Explain how vacuum  pumps work to create a rough vacuum

A vacuum pump is a device that removes gas molecules or air particles from a sealed volume in order to achieve difference in pressure creating a partial vacuum. Vacuum pumps are designed in a variety of technologies based on the pressure requirements and the application it services. When setting up a vacuum pump system, sizing to the correct parameters is crucial to achieve optimum efficiency.

17. How Does a Vacuum Pump Work?

Vacuum is a space devoid of matter where the gaseous pressure inside this volume is below atmospheric pressure.  A vacuum pump’s main function is to change the pressure in a contained space to create a full or partial vacuum either mechanically or chemically. Pressure will always try to equalize across connected regions as gas molecules flow from high to low to fill the entire area of that volume. Therefore, if a new low-pressure space is introduced, gas will naturally flow from high-pressure area to the new area of low-pressure until they are of equal pressure. Notice this vacuum process is created not by “sucking” gases but pushing molecules. Vacuum pumps essentially move gas molecules from one region to the next to create a vacuum by changing high and low-pressure states.

 

18. Vacuum Pump Basics

As molecules are removed from the vacuum space, it becomes exponentially harder to remove additional ones, thus increasing the vacuum power required. The pressure ranges are placed into several groups:

• Rough/Low Vacuum: 1000 to 1 mbar / 760 to 0.75 Torr
• Fine/ Medium Vacuum: 1 to 10^-3mbar / 0.75 to 7.5^-3 Torr
• High Vacuum: 10^-3 to 10^-7mbar / 7.5^-3 to 7.5^-7 Torr
• Ultra-High Vacuum: 10^-7to 10^-11 mbar / 7.5^-7 to 7.5^-11 Torr
• Extreme High Vacuum: < 10^-11mbar / < 7.5^-11 Torr

Vacuum pumps are classified by the pressure range they can achieve to help distinguish their capabilities. These classifications are:

Primary (Backing) Pumps which handle rough and low vacuum pressure ranges.

Booster Pumps handle low and medium pressure ranges.

Secondary (High Vacuum) Pumps handle high, very high and ultra-high vacuum pressure ranges.

Depending on the pressure requirements and operating application, vacuum pump technologies are considered either wet or dry. Wet pumps use oil or water for lubrication and sealing, while dry pumps have no fluid in the space between the rotating mechanisms or static parts that are used to isolate and compressing gas molecules. Without lubrication, dry pumps have very tight tolerances to operate effectively without wear. Let us look at some of the methods used in a vacuum pump

Capture Pumps

Capture pumps, also referred to as entrapment pumps, have no moving parts and are used for applications that require extremely high vacuum pressures. Without moving parts, capture pumps can create a vacuum environment using two different methods. 

One of the methods used by capture pumps, is by trapping gas molecules through cryogenics to trap gas molecules. Cryopumps use cryogenic technology to freeze or trap the gas to a very cold surface.  By using extremely cold temperatures, they effectively draw molecules inward to create a vacuum.

Sputter Ion Pumps Sputter Ion pumps use highly magnetic fields and ionization of gas molecules to make them electrically conductive as a method of entrapment.  The magnetic field creates a cloud of electropositive ions that are deposited onto a titanium cathode. In this process, the chemically active materials combine with gas molecules to draw them in and create a vacuum.

Transfer Pumps Transfer pumps can operate using two types of methods; Kinetic energy or Positive Displacement. Unlike like Capture Pumps, Transfer pumps are pushing the gas molecules out of the space through the system. What they have in common is they all use a method of mechanically pushing gas and air through the system at different system intervals. Its common that multiple transfer pumps are used together in parallel to provide higher vacuum and flow rate. It is also common to utilize multiple transfer pumps in a system to allow for redundancy in the event of a pump failure.

Kinetic Pumps Kinetic Pumps use the principle of momentum through impellers (blades) or introducing vapor to push gas towards the outlet.

Turbomolecular Pump All Kinetic pumps are secondary pumps as they are used for high pressure applications. One dry method is the Turbomolecular pump, which use high speed rotating blades inside the chamber that propel the gas molecules. Transferring momentum from the rotating blades to the gas molecules increasing their rate of moving towards the outlet. These pumps provide low pressures and have low transfer rates.

Vapor Diffusion Vapor Diffusion Pump use high velocity heated oil steam that uses kinetic energy to drag gas molecules from the inlet to the outlet. There are no moving parts and there is a reduced pressure at the inlet

Positive Displacement Pumps The basic principle of a Positive Displacement pump is by expanding the original volume into the chamber they move small, isolated volumes of gas at different stages, compressing to a smaller volume and at a higher pressure expelled to the outside. These pumps operate at lower pressure ranges and are categorized under primary or booster pumps and incorporate wet or dry technologies. Here are the various types of positive displacement primary vacuum pumps:

Oil Sealed Rotary Vane Pump Oil Sealed Rotary Vane pumps compress gases with an eccentrically mounted rotor that turns a set of vanes. Due to centrifugal force, these vanes slide out and form chambers between themselves and the housing. The pumped medium is trapped inside these chambers. During further rotation, their volume is constantly reduced. Thereby, the pumped medium is compressed and transported to the outlet. Rotary vane vacuum pumps are available in single- and two-stage versions.

Liquid Ring Pump Liquid ring pumps have an off-center impeller with vanes bent towards rotation that form a moving cylindrical ring of liquid around the casing from centrifugal acceleration. The vanes create crescent shaped spaces of different sizes as they rotate and are sealed by the liquid ring. Near the suction or inlet, the volume becomes larger causing the pressure in the each one to drop and draw in gas. As it rotates, the volumes between each vane decreases because of the eccentrical positioned impeller and liquid ring formation. This compresses the gas as it discharges, creating a continuous flow.

Diaphragm Pump Diaphragm pumps are dry method positive displacement vacuum pumps. A diaphragm sits on a rod connected via crankshaft that moves the diaphragm vertically as it rotates. When the diaphragm is in the low position, volume in the chamber increases, lowering the pressure and pulling air molecules in. As the diaphragm moves up, the volume is decreased, and gas molecules are compressed while flowing to the outlet. Both the inlet and outlet valves are spring loaded to react to the pressure changes.

Roots Style Pumps Root pumps push gas in one direction through two lobes that mesh without touching as the counter  rotate. This counter rotating creates maximum flow rate as the volume increases at the inlet at the simultaneously decreasing at the outlet compressing the pressure. These pumps are designed for applications where removal of large volumes of gas is required

19. Highlight three advantages and disadvantages of diffusion pumps

 Main advantages of a diffusion pump is that it operates using silicone-based diffusion pump oil , and has not a single moving part. With no moving parts, and only the chamber of boiling oil that creates the vacuum, diffusion pumps are easy to maintain and the risk of breaking will be on a minimum. Another major advantage is the fact that diffusion pump oils are very accessible and affordable.

One major disadvantage of diffusion pumps is the tendency to backstream oil into the vacuum chamber. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits.

20. State four advantages of using Mcleod  gauge to measure  vacuum pressure

• McLeod gauge is an inexpensive standard that measures vacuum pressure without any electronics or sophisticated equipment.
• It is used for calibrating other low pressure measuring gauges.
• It is not influenced by gas composition.
• The readings obtained from McLeod gauge do not require any correction.

21. Draw a labeled block diagram of a diffusion pump vacuum system.

 

22. State three parts of Mcleod gauge

 

23. Differentiate between valve and flange

The flange hand is connected by direct welding, etc. Compared with the faucet with threaded connection, this connection method has great resistance and high safety.

The valve is the control part of the flow of liquid, gas, etc., and has the functions of cutting off, regulating, and diverting. The flange is a fixed connecting part with a hole in the middle of the flange and a disk around which there are many connecting bolt holes.