B. Types, Categories and Techniques

Read text B. Pay attention to the words in bold type.

Answer the questions to text A.

Decide whether the following statements are true or false according to text A.

1.Otto von Guericke was the first to demonstrate the properties of vacuum in a practical way.

2.The scientist used eight dogs to separate the hemispheres.

3.The vacuum pump was originally invented to help teachers to study the properties of gases.

4.The history of progress in the study of vacuum is the history of vacuum pump design.

5.Before 1900, vacuum pumps were not produced.

6.In the 20^th century the rotary oil pump, the molecular pumps were developed.

7.The pumps developed in the 20^th century made a high vacuum possible.

8.With modern techniques all but one of every million molecules can be removed.

9.Efficient pumps can reduce the pressureof the gas to between 100 and 0.1 pascal.

10.The air of our atmosphere exerts low pressure to enter an enclosed space.

11.A pump isatoy.

12.Perhapsthe best-known pump is the oil diffusion pump.

1.What is vacuum pump?

2.Did a German or British scientist demonstrate the properties of vacuum in a practical way?

3. What does stand for the “Magdeburg Experiment”?

4. What was the advantage of the first vacuum pumps?

5.Did scientists pay attention to the design of pumps?

6. How are the pumps developed in the 20^th century called?

7. What made high vacuum possible?

8. Does a perfect vacuum exist?

9. Can the human heart be compared with a pump?

10.What does a vacuum pump do?

11.How many force pumps can you name?

12.Is the vacuum pump a necessity in the 21^st century?

Pumps can be broadly categorized into three techniques:

Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.

Momentum transfer pumps, also called molecular pumps, use high-speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber.

Entrapment pumps capture gases in a solid or absorbed state. This includes cryopumps, getters, and ion pumps.

Positive displacement pumps are the most effective for low vacuums, and their high back-stream flows through mechanical seals generally limit their usefulness in high vacuums. Momentum transfer pumps in series with positive displacement pumps are the most common configuration used to achieve high vacuums, but they stall at low vacuums. Entrapment pumps can be added to reach ultrahigh vacuums, but they have a maximum operational time since they do not exhaust materials. They periodically saturate and require regeneration, which usually means bringing the system back up to higher pressures and temperatures. The available operational time is usually unacceptably short in low and high vacuums, thus limiting their use to ultrahigh vacuums. Pumps also differ in details like manufacturing tolerances, sealing material, pressure, flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to liquids and vibration.

In a momentum transfer pump, gas molecules are accelerated from the vacuum side to the exhaust side. Momentum transfer pumping is only possible below pressures of about 1 kPa. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than positive displacement pumping. This regime is generally called high vacuum.

Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds. They do this at the expense of the seal between the vacuum and their exhaust. The two main types of molecular pumps are the diffusion pump and the turbo-molecular pump. Turbo-molecular pumping systems are the most reliable and cost-effective means producing high vacuum. Besides, they are portable. Both types of pumps blow out gas molecules that diffuse into the pump. Diffusion pumps blow out molecules with jets of oil, while turbo-molecular pumps use high-speed fans. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump.

Дата добавления: 2014-12-26