The advantage of a needle valve
A needle valve has a relatively small orifice with a long, tapered seat, and a
needle-shaped plunger on the end of a screw, which exactly fits this seat.
As the screw is turned and the plunger retracted, flow between the seat
and the plunger is possible; however, until the plunger is completely retracted,
the fluid flow is significantly impeded. Since it takes many turns of the
fine-threaded screw to retract the plunger, precise regulation of the flow rate
is possible.
The virtue of the needle valve is from the vernier effect
of the ratio between the needle's length and its diameter, or the difference in
diameter between needle and seat. A long travel axially (the control input)
makes for a very small and precise change radially (affecting the resultant
flow). Needle Valves may be used in vacuum systems when a very precise control
of gas flow is required, at low pressure.
