Please tell about Vaccum pump basic calculation?

Question

I am doing project please help if u know.

Answer 1

A vacuum pump is a pump that removes gas molecules from a sealed volume in order to leave behind a partial vacuum.

Pumps can be broadly categorized into three techniques:[1]

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.
Performance measures
Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in litres per second, cubic feet per minute, or cubic metre per hour. Because of compression, the volume flow rate at the outlet will always be much lower than at the inlet. Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping speed can be simultaneously different for each of the gases being pumped, and the average pumping speed will vary depending on the chemical composition of the gases remaining in the chamber.
Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured in units such as torr-litres. At a constant temperature, throughput is proportional to the number of molecules being pumped per unit time, and therefore to the mass flow rate of the pump. (Think PV=nRT) When discussing a leak, backstreaming or outgassing, throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput.
Positive displacement and momentum transfer pumps have a constant volume flow rate, (pumping speed,) but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant, the throughput and mass flow rate drop exponentially. Meanwhile, the leakage, evaporation, sublimation and backstreaming rates produce a constant throughput into the system. When the pump's mass flow drops to the same level as the mass flows into the chamber, the system asymptotically approaches a constant pressure called the base pressure.

Evaporation and sublimation into a vacuum is called outgassing, and the most common source is water absorbed by materials in the chamber. If the dominant mass flow into the vacuum system is chamber leakage or outgassing of materials under vacuum, then the vacuum can be improved simply by installing bigger pumps with a higher volume flow rate. However, there is a point where backstream leakage through the pump and outgassing of the pump oils become the dominant mass flows into the chamber. In this situation, the vacuum will approach the pump's ultimate pressure - the best vacuum that this type of pump can achieve under ideal conditions. Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed, but they will not reduce the base pressure below ultimate. Better pumping technologies must be used to go beyond this barrier.

FOR MORE PL. VISIT:
http://en.wikipedia.org/wiki/Vacuum_pump
http://www.dream-models.com/eco/vacuumpump.html

Answer 2

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Answer 3

Try here; http://en.wikipedia.org/wiki/Vacuum_pump

Answer 4

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