A throttling
process is defined
as a process in which there is no change in enthalpy from
state one to state
two, h1 =
h2;
no work is done, W = 0; and the process is adiabatic, Q =
0. To better understand
the theory of the ideal throttling process let’s compare
what we can observe with the above
theoretical assumptions. An
example of a throttling process is an ideal gas flowing
through a valve in mid position.
From experience
we can observe that: Pin > Pout,
velin < velout (where
P = pressure and vel = velocity). These
observations confirm the theory that hin = hout.
Remember h = u + Pv (v = specific volume),
so if pressure decreases then specific volume must
increase if enthalpy is to remain constant
(assuming u is constant). Because mass flow is constant,
the change in specific volume is
observed as an increase in gas velocity, and this is
verified by our observations.
The theory also states W = 0. Our
observations again confirm this to be true as clearly no "work" has been
done by the throttling process. Finally, the theory states
that an ideal throttling process
is adiabatic. This cannot clearly be proven by observation
since a "real" throttling process
is not ideal and will have some heat transfer.
