# Thermodynamic energy analysis of nozzle vs. throttling valve/capillary

by mohnishp
Tags: capillary, isenthalpic, nozzle, thermodynamics, throttling
 P: 1 Hello experts! This is my very first post on PF. I have read the rules, and I am hoping that I post this in the right section. Also, I did some research on existing threads, and found quite a few interesting discussions. However, none of them specifically addressed this difference. If I have missed something, I apologize in advance. My question is: How to differentiate between a nozzle(non-isenthalpic) vs. a throttle (isenthalpic)? I am taking an engineering thermodynamics course, and trying to decipher the logic behind this differentiation. Of course, I am looking for something other than "check the wording of the problem" :P The energy exchanged per unit mass with a flowing fluid stream through a steady-state device: e = Δu + ΔPv + Δpe + Δke where, e = q - w → net effect of heating & work extracted from the fluid flow Δu → internal energy ΔPv → flow energy (a concept still a bit cloudy in my mind) Δpe → (gravitational) potential energy Δke → kinetic energy here, the Δ terms refer to changes between the inlet and outlet of the device. My textbook shows two examples, one with steam flowing through a nozzle, and the other with refrigerant flowing through a capillary tube. The nozzle example uses h2 = h1 - Δke, where h = u + Pv is enthalpy, and the velocities at inlet and outlet are given in order to compute Δke. The capillary example uses h2 ≈ h1 and the inlet and outlet pressures are given, allowing us to complete the required analysis. The text says, for throttling processes "even though the exit velocity is often considerably higher than the inlet velocity, in many cases, the increase in kinetic energy is insignificant (Δke ≈ 0)". This makes me feel uncomfortable, since above point implies that there is some sort of a difference between squeezing the end of a garden hose, and putting a valve at the end of a garden hose, both of which can create a sharp spray. Please help me resolve this, so that I can look at any such device and know whether or not to employ an isenthalpic analysis. Thank you! Mohnish
PF Patron
HW Helper