Suitable permanent and auxiliary ventilation
Because during the evaporation of the nitrogen within the work section the oxygen in the respiratory air is displaced, the room must be well ventilated. We can differentiate two operational conditions for the ventilation. In order to equilibrate the permanent vaporisation of the liquid nitrogen in the storage containers, the losses in evaporation are mathematically calculated and the permanent ventilation is constructed appropriately. While normal operation the oxygen concentration should not fall below 19 % volume.
If the oxygen concentration falls lower, for example while refrigerating samples or filling dewars, auxiliary ventilation is activated. It is calculated so that the oxygen concentration is returned to the normal value within 5 minutes. If over a longer period the oxygen concentration falls below 18 % volume, a non-defined operational condition emerges. An alarm is sounded and the nitrogen feeder is switched off. But even the oxygen concentration falls at 18 % volume there not yet exists danger for the operating personnel. The error can be rectified and the alarm cancelled.
In order to be able to fill dewars or supply refrigeration devices in the various laboratory areas, transport containers must be refuelled. This is a matter of superisolated pressure containers (1.5 bar) with a content of up to 230 litres. With the refrigeration and filling of these containers, huge quantities of nitrogen gas emerges into the room. The ventilation has to be dimensioned on a large-scale. It is more ingenious to use a special filling station and to guide the expanding nitrogen via a waste gas line into the open air directly.