Chapter 7: Theory of Operation
100
Control Valve
The control valve is a specially designed solenoid driven valve. The armature is suspended by two
radial springs. This design provides frictionless movement and thus precise control. Mounted at the
front end of the cylindrical armature is the valve plug which incorporates the seal disc of FKM or
NBR or FFKM (ref. to Appendix A, Specifications). By preload force of the two above mentioned
springs the seal disc is pressed against the valve orifice, closing it’s flow channel. Therefore the
valve is closed when not activated. It is a „Normally Closed“ (N.C.) valve.
The inside diameter of the orifice determines the conductance. Each flow controller incorporates a
valve orifice with a conductance in accordance to the full scale range. The valve orifice in standard
units is sized that with a pressure difference between inlet and outlet fitting of typical 0.7 bar to 2.75
bar the specified full scale flow rate will be achieved. (related to air or nitrogen). For more
information refer to Appendix A, Specifications.
For special applications with low pressure conditions, e.g. vaporizer sources, configurations can be
provided. In this case the valve will have an orifice with higher conductance (=larger diameter of
the flow channel). Mass flow controllers for applications where only 200 mbar (or less) are
available have been realized.
At high line pressures in combination with high flow rates it may be necessary to have an orifice
with smaller conductance installed.
The mass flow monitor MF1M has no valve!
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