Containers
for many chemicals and nuclear materials
have to be exceptionally clean and dry to prevent
unwanted reactions. Cleaning processes often leave
traces of water or solvent that are difficult to remove,
particularly from crevices and construction joints.
Traditional drying methods involve time consuming
and costly oven drying at atmospheric pressure,
which can also damage or distort the material being
dried.
Rather than heating the whole vessel to boil off
liquids, vacuum drying lowers the pressure inside the
vessel so that liquids boil at room temperature, and
are pumped away. Some heat is required to counteract
the cooling effect of vaporisation and prevent
liquids freezing on the vessel surface. However this is
small and, depending on the vessels thermal mass
and the rate of vaporisation, no external heat source
may be needed.
In the past vacuum pumps have been a source of
contamination themselves, requiring maintenance
intensive traps to prevent hydrocarbons
backstreaming into the vessel being dried. The traps
also quickly became saturated with water or solvent
vapours, reducing pumping efficiency. Current totally
oil free pump designs eliminate the need for traps and
reduce both complexity and drying times.
The system shown was built to meet the specific
requirements of a customer. Tubes 4m long x 11mm
diameter, with a welded plug at one end have to be
dried to <10 ppm moisture.
In this case the tubes could contain up to 5ccs of
demineralised water each, and have to be dried
within a 10 minute process window at room temperature,
without the application of heat.
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The system uses an oil-free scroll pump coupled to the tube through
a manifold with an external o-ring seal. Optimum pumping speed was set
to control the rate of vaporisation and achieve the shortest drying time,
without causing the demineralised water to freeze.
A
capacitance manometer is fitted to the manifold as close as possible
to the tube connection point, indicating when the pressure inside
the tube
falls below the vapour pressure of water, and is “dry”. To
ensure the moisture level is below 10 ppm a separate UHP sample line and
moisture monitor is also fitted. Purging of the moisture sensor and venting
of the dry tube is carried out via a built in gas purifier, which ensures
the gas has <1 ppb moisture content.
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