INTRODUCTION TO
VACUUM
[Type text]
Vacuum resource;
Materials state and
heat input.
Heat energy input.
Uses for Vacuum.
Materials state and heat input
Basic high school science tells us that everything we encounter including our own
bodies is made up from small particles we call atoms. Atoms may be solitary
individuals or, in substances such as air, collect together to form molecules. What
may not be so well understood is the relationship between these particles and the
form or state of the material they make up. The three forms of state that we are most
familiar with are solid, liquid and gas, what makes these forms different is the
bonding between the atoms that form the substance. In a solid material the atoms
are tightly packed and are locked together by the atomic attraction between the
atoms. A liquid on the other hand has atoms which are more widely separated and
so the attraction between each particle is greatly reduced. This allows the atoms to
move in relation to each other due to the looser bonds between the atoms this
imparts the ability to flow to the material. With a gas the bond is so weak that
individual atoms can drift off and become separated from their partners.
Heat energy input.
As we add heat to a material its temperature will increase, this is easily measured
and recorded. What is hidden is the fact that the atoms and molecules within the
material start to vibrate and the violence of the vibration increases as the
temperature rises. At standard temperature/pressure (STP or 1 atmosphere pressure
at 20o Centigrade), one cubic millimetre of air contains approximately 21x1020
molecules moving in random directions and at speeds of around 500 metres per
second. This phenomenon is known as molecular activity. This vibration will oppose
the attraction of the atoms due to the inevitable collisions that occur and so force the
molecules to drift apart, as they move away from each other the forces of attraction
reduce and the state of the material changes. Prior to the change of state the volume
occupied by the substance will increase, this phenomena is known as expansion, or
if the substance is retained within a fixed size container the increase in molecular
activity will cause the pressure exerted by the substance to increase. In liquids or
solids this increase in pressure is difficult to harness or contain. In gases however
the increase in pressure is easily accommodated or utilised.
Atmospheric pressure is generated in this manner with the containment of the gas
achieved by the gravitational attraction of the Earth. Effectively each one of us
supports a column of air equivalent to the surface area of our bodies and the height
of the atmospheric blanket above us. The surface area which supports this column of
air and the weight this column of air generates the pressure which we term
atmospheric pressure.
During the initial stages of generating a vacuum this pressure will force air into the
vacuum pump which can then move the air from the volume or space to be
evacuated and pass it into the general environment so reducing the number of atoms
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in the volume which is being evacuated. Note however that the action of suction only
occurs if there is a connection between a low pressure area and a source of higher
pressure. Molecules of air cannot be removed from a volume unless they enter a
vacuum pump by virtue of their random movements. When removed, the remaining
molecules will spread out to fill the volume but at a lower pressure. Inevitably the
pressure in the volume to be evacuated will fall to such an extent that the molecules
will not enter the pump mechanism. At this point the molecules of air need to be
encouraged to enter the pump mechanism. This can be achieved by employing
electromagnetic means to push the molecules into the pump.
Uses for Vacuum
Vacuum is used for several reasons for example to cause fluid to flow in a pipe, to
avoid some property of air such as friction or oxidation or to improve the
transmission of atomic, molecular or electromagnetic beams in devices such as
thermionic valves, cathode ray tubes, fluorescent tubes and particle accelerators.
In Composites Engineering vacuum is used to De-gas resins particularly very
viscous resins or silicon’s, used to compress or De-bulk fabrics including those pre
impregnated with resin and also vacuum may be used to cause resin to flow into a
laminate.
The use of vacuum in this field enables the production of high quality laminates due
to the force that can be applied to the laminate during the vacuum bagging process.
As I bar (approx. atmospheric pressure) is equivalent to 10332.274528 Kilogrammes
per square metre it can be seen that a Vacuum bag can provide approximately 10
tonnes (one tonne being 1000 kilogramme) per square metre of bag area. The actual
force depends upon the prevailing atmospheric pressure at the time of the process
and the efficiency (approx. 98%) of the Vacuum pump. When used in processes
such as resin infusion there is the added advantage that contamination may be
reduced. This is because at the low pressures used for the process many of the
contaminants which the fabrics may encounter vaporise and are drawn out of the
laminate by the action of the pump.
Jerry Corless
South West Composites Gateway - September 2014
Compositesgateway@cityofbristol.ac.uk
www.swcompositesgateway.co.uk
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