Pinch Valves 
The operation of solenoid valves and
pneumatic valves is distinguished by several attributes:
Response Time
Solenoid valves respond
faster to input commands
than pneumatic valves.
With pneumatic valves,
more time is required for the working fluid
(gas/air) to enter the chamber and build up
pressure than it does for electrons to flow
through the windings of a solenoid coil and
induce a magnetic field.
Secondly, the plunger
of a solenoid valve actually accelerates as
it advances because the magnetic attractive
force acting on it increases exponentially with
plunger travel. As the plunger of a pneumatic
valve travels through its stroke, it is countered
by linearly increasing spring force; consequently,
the plunger decelerates as it advances.
Pinching Forces
Pinching forces of solenoid
valves are restricted by
not only supply voltage,
but also by temperature rise and the ability
to hold-in once actuated. On the other hand,
pneumatic valves are typically capable of producing
stronger pinching forces because the only limitation
on force is supply pressure.
Actuation Noise and
Sound Dampening
Because the plunger of a
pneumatic valve not only moves slower but also
decelerates as it advances, the sound of it
impacting its mechanical limits is less than
for solenoid valves. Acro offers sound dampening options
for noise-sensitive applications using solenoid
valves.
Valve State (Open/Closed)
Pinch valves are further classified as either normally
closed (N/C) or normally open (N/O),
depending on the flow and tubing position when the
valve is in a de-energized state. An alternate way
of making this distinction is that a normally closed
valve fails closed, while a normally open valve
fails open.
Solenoid valves are normally
open (N/O), while pneumatic valves can be normally
open or normally closed.
- Normally closed (N/C):
valve blocks flow and the tubing is pinched
when the valve is in a de-energized state.
- Normally open (N/O):
valve permits flow and
the tubing is not pinched
when the valve is in
a de-energized state.
Heat Generation
Current flows through hundreds
of turns of wire to induce
the necessary magnetic
field when a solenoid valve actuates. Because
the wire has resistance, heat generates as current
flows through it. This resistive heat subsequently
dissipates through conduction to the exterior
surface of the valve and through convection
to the surroundings. When the rates of heat
generation and dissipation equalize, the valve
achieves a steady-state temperature, which must
be less than 190 °F
(88 °C) to maintain
integrity of the coil.
