BeginnerTopic 20 of 105Thermodynamics & Fluids
Throttling
Pressure drop at constant enthalpy; basis of refrigeration expansion valves.
In depth
A valve or porous plug drops pressure with no work or significant heat, so h₁=h₂. Temperature may fall (Joule-Thomson effect) for real gases; ideal gas stays constant. Used in refrigeration expansion valves and gas liquefaction. Entropy rises (irreversible).
Key points
- Isenthalpic
- Joule-Thomson coefficient
- Inversion temperature
ExampleRefrigerant expands through a throttle to the evaporator pressure.
What is Throttling?
Pressure drop at constant enthalpy; basis of refrigeration expansion valves.
Why is it important to understand throttling?
Throttling lowers pressure at constant enthalpy, which is the step that cools fluid in a refrigerator or a Joule-Thomson cooler.
Why does throttling not change enthalpy?
The process happens without work or heat transfer and with negligible change in kinetic and potential energy, so enthalpy is constant.
Why does throttling cool some gases but not others?
It depends on the Joule-Thomson coefficient, which is positive for many gases at normal conditions and negative for others, such as hydrogen at room temperature.
How is Throttling used in practice?
Typical applications include cooling refrigerant in the expansion valve of a fridge, liquefying gas in a Linde cycle and reducing steam pressure in a pressure reducing valve.
What should I learn before Throttling?
Study "Steady-flow devices" first (topic 19 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Throttling?
Isenthalpic. Joule-Thomson coefficient. Inversion temperature.
Can you give an example of Throttling?
Refrigerant expands through a throttle to the evaporator pressure.
What comes after Throttling?
The next topic is "Second law of thermodynamics", which builds directly on this one.
How difficult is Throttling?
It is classed as Beginner in Thermodynamics & Fluids (topic 20 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.