A fluid can be throttled by several means. Examples are: a partly open valve, an orifice or any other sudden
reduction in the cross-section of the flow. The enthalpy remains almost constant and pressure reduces in
this process. Throttling is an irreversible process due to eddying of the fluid.
Consider a perfectly thermally insulated pipe which fluid flows steadily through an orifice. Applying
the first law of thermodynamics to the steady flow system
defined by the control volume between sections 1-1 and 2-2, gives:
dQ/dt+dW/dt=m [h + C/2 + g Z]
dQ / dt = 0 because the system is thermally insulated. dW / dt is also zero. If velocities at sections 1-1
and 2-2 are small or approximately equal and the height difference between these two sections,
Z, is negligible, then we can write:
h + 
/2 + g