Ballast nitrogen flow at vacuum drying

Adiabatic compression on pressure side of vacuum pump can heat explosive mixture to ignition temperature. Therefore it is important to assure inert conditions also on pressure side of the vacuum pump if vacuum pump is not equipped with flame arresters. Inert conditions can be lost even with minimal leakage of vacuum system if minimum flow of ballast nitrogen is not assured during drying.

The abscise shows absolute pressure in the drying chamber while ordinate shows minimum nitrogen norm flow in the units of vacuum pump capacity. The unit for nitrogen flow is calculated to norm conditions (room temperature and atmospheric pressure) while vacuum pump capacity is not (manufacturers usually quote vacuum pump capacity in l/s at given vacuum pressure, not in norm l/s). For example consider drying at temperature 60°C and absolute pressure 100 mbar and let solvent have LOC of 8% (at 20°C). Also let the vacuum pump capacity at 100 mbar be 100 l/min. In this case necessary nitrogen flow through drying chamber would be 9 norm liters per minute. Minor jump or graph discontunuity (in given example at approximately 150 mbar) forms because at pumping from very low pressure the mixture on the pressure side of vaccum pump heats so much that at this temperature LOC drops below 5%. Below this value of LOC the document CEN/TR 15281 requires increased safety margin between LOC and working oxygen concentration.

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