Gas purged (also known as open loop) glove boxes are primarily
specified where the quality of contained gas can be relaxed, and
where the longer term cost of gas consumption are less of a problem
than the initial capital outlay of a gas purification system.
The supply gas is constantly passed through the glove box (at a
rate of flow to create and maintain the conditions required) and
then lost to atmosphere. Gas consumption can be high and measured
in bottles per day, and the contained glove box quality may not
reach higher specification where this is a requirement.
Generally three points of consideration need to be decided, the
required and attainable oxygen and/or moisture levels, the introduction
or production of any by-products in the contained process and their
suitability for direct disposal to ambient atmosphere, and the required
or possible integrity of the containment.
The last and first points are interrelated in that the quality
of purge gas will, in conjunction with the integrity of the glove
box, determine the overall performance of contained atmosphere quality.
The integrity of the glove box is effectively its molecular leak
rate, and this can vary considerably in differing materials of construction,
design and quality of build. Generally the quality of bottled gas
or bulk storage gas is the same, but some piped systems cause the
quality to deteriorate between source and glove box.
It is therefore impossible to provide any general guide on the
expected performance of a gas purge glove box, however our gas purge
glove boxes which are designed to a "high integrity" specification
have been tested for consistency of performance of contained atmosphere
quality using bottled nitrogen gas. After initial gas purge down
the gas flow can be adjusted between 3 and 10 litres per minute
to provide a non-working environment of between 10 and 20 ppm oxygen
and < 30* ppmv moisture.
Initial purge down duration from air to 30 ppm oxygen is variable
to the size of glove box containment and gas flow but other considerations
may influence the time. A general guide is for the containment to
be gas purged three x the volume, so the time is a function of the
glove box volume and gas purge flow rate. Normal gas purge flow
rates are 30 to 50 litres per minute but can be significantly increased
with design input and the provision of facilities and equipment
to cater for the increased gas flow.
The frequency of initial gas purge depends on the type of entry
port specified, with a single door requiring this operation on each
door opening, and a double door only on the initial set up, and
any loss of integrity thereafter. Thus the selection of entry or
transfer port is an important decision at the point of purchase.
Most purchasers specify a double door load-lock (antechamber),
which being of substantially less volume than the glove box is gas
purged in a shorter time and with less consumption of gas. Other
purchasers will specify a single door only due to either the low
utilisation of the glove box or frequency of entry, and others may
consider the large size of transfer item does not warrant the cost
of a large antechamber and the resulting time and gas volume needed.
(*Moisture readings taken after the glove box has had time, to
flush out surface and absorbed moisture from the materials of construction.)