Flow Through an Expansion Valve
Introduction
Refrigerant R717
enters an expansion valve (of cross-sectional area 0.011 m2) at 11 bar, 330 K and 25 m s-1,
and leaves at 2 bar.
This application calculates the temperature and velocity of the refrigerant as it exits the valve.
The First Law of Thermodynamics states
where
is the heat generated by the system
is the rate of change of stored energy within the system
is the rate of work done by the system (except flow work)
and are the mass flowrates into and out of the system
and are the specific enthalpies of the fluid entering and leaving the system
and are the velocities of the fluid entering and leaving the system
and are the elevations of the fluid entering and leaving the system
For steady-state flow through an adiabatic expansion valve and no heat or work effects, the First Law of Thermodynamics reduces to
=
Assuming the input conditions are known, mass conversation implies that and hence
where A is the cross-section area of the valve and is the fluid density at the exit.
The kinetic energy term in the First Law of Thermodynamics is generally small and is normally be ignored - this makes the combined system of equations explicit, and simple to solve. For this analysis, however, the kinetic energy term will remain. The equations are then implicit, and hence require a numerical solution
Parameters
Entrance
Enthalpy at inlet
Density at inlet
Mass flowrate of refrigerant
Exit
Enthalpy and density at outlet
Mass Conservation and First Law of Thermodynamics
Solution of the Equation System
Plot Thermodynamic Process on a PhT Chart
