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Gas Purged (Open Loop)
 

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.)

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