How Do Vacuum Pumps Work?
To create a high vacuum in a system it is necessary to
move all of the molecules of gas out of the system. The molecules
will move only if there is a pressure difference between the two
regions of the space (see Figure 4). The low pressure region is the
space with the smaller number of molecules, while the high pressure
region is the space with the larger number of molecules.
Any device which can induce a pressure difference between
the two regions in the space is called a pump. The pump which
creates the vacuum in the certain system is called a vacuum
pump.
Exercise: you need only a cup of beverage and
a straw. Try to suck a drink by the straw and feel how do the mouth
muscles move. When one sucks on a straw the mouth muscles create
the region of low pressure, while atmospheric pressure on the
surface of your beverage pushes it up the straw. Pumps operate on
the same principles.
Figure 4: Vacuum pump operation.
There are two categories of vacuum
pumps: transfer pumps and trapping
pumps.
The transfer pumps are also
called kinetic pumps since they impart the momentum
to the gas which is being pushed in a such a way that the gas is
transferred continuously from the inlet of the pump to the outlet.
This is usually done by mechanical moving parts of the pump, as
shown in Figure 5. The moving (usually rotating) part of the pump
accelerates the molecules of the gas and makes the region of lower
pressure. Therefore, the molecules from the tank will start moving
towards the region of lower static pressure, with the procedure
continuously repeating until all (or most) of the molecules are
taken from the container where we would like to have a vacuum. When
we get the wanted level of vacuum we isolate the tank by a high
vacuum valve. This valve stops any exchange of gas between the
container and the pump.
Figure 5: Operation of the transfer
pump.
The trapping or capture pumps
are usually located in the container being evacuated. The trapping
pumps remove gas molecules by sorption or condensation on its
internal surfaces (see Figure 6). If the gas molecules chemically
react with the internal material of the pump, the new material
created by the reaction of gas and inner material molecules will be
deposited as a thin film. This is called sorption of the gas
molecules. Furthermore, if the gas molecules come in touch with the
refrigerated surfaces of the pump, gas will be condensed and
removed as a liquid.
Figure 6: Operation of the trapping vacuum
pump.
The low vacuum levels can be measured also with the
simplest manometer; i.e.; U - tube (see again Figure 2). In this
case, instead of having a container with pressurized gas, we will
have a vacuum container. Since the atmospheric pressure is higher,
the air molecules on the open end of U - tube will push the fluid
towards vacuum container. Again, the difference in heights of fluid
between the two sides of U - tube will determine how much the
pressure in the container is less than the atmospheric
pressure.
In general, the special devices which measure the vacuum
levels are called vacuum
gauges.
The simplest mechanical gauge you can imagine as a chamber
divided by a membrane. One side of the chamber is connected to the
vacuum container and the other to the known pressure tank (it can
be simply open, therefore it will be under the atmoshperic
pressure). The membrane is attached to the pointer mechanism (see
Figure 7). Depending on the membrane deformation, the pointer will
show the vacuum level. A gauge like this is considered to be a
coarse or rough guage.
