Let's apply the energy balance equation between the upstream and downstream of the throttling valve.
Q + h1= W + h2 + d(KE) + d(PE)
Q = 0
Reason: There is not enough time and surface area available for heat transfer
W = 0
Reason: There is no lifting of any weight (no thermodynamic work)
d(PE) = 0
Reason: There is no potential head difference
d(KE) =~ 0
Reason: there may be an increase in velocity at the outlet compared to the inlet but in many cases the change in kinetic energy is insignificant.
Hence we remain with
Let's apply the energy balance equation between the upstream and downstream of the throttling valve.
Q + h1= W + h2 + d(KE) + d(PE)
Q = 0
Reason: There is not enough time and surface area available for heat transfer
W = 0
Reason: There is no lifting of any weight (no thermodynamic work)
d(PE) = 0
Reason: There is no potential head difference
d(KE) =~ 0
Reason: there may be an increase in velocity at the outlet compared to the inlet but in many cases the change in kinetic energy is insignificant.
Hence we remain with
h1=~h2 (kJ/Kg)