Throttling Process
- A significant reduction in pressure can be achieved by introducing a restriction into the flow line. The restriction can be a partially-opened valve, or a porous plug, or a capillary tube.
- Enthalpy (internal energy + flow work) is a constant going through a throttling process: h1=h2.
Example: Saturated liquid refrigerant-134a enters the throttling device at 1 MPa and is exiting at a lower pressure of 0.1 MPa. Determine the temperature drop during this process.
At inlet: from table A-9, T1=Tsat@1MPa =39.39°C, h1=hf@1MPa=105.29 kJ/kg
At outlet: h1=h2, P2=0.1 MPa, T2=-26.43°C
hf=16.29, hfg=215.06, x=(h2-hf)/hfg=0.414 (41.4% of the refrigerant-134a vaporizes during the throttling process). Question: where does the energy come from to vaporize the refrigerant?
DT = -26.43-39.39 = -65.82°C, temperature drops significantly during the throttling process.