Throttling Process

Whenever a fluid flows through a constricted path like partially opened valves, there is decrease in pressure and the process is called throttling process.

To explain the phenomenon of throttling, let us consider the process of throttling by partially opened valves and fluid is flowing in a pipe.

Fig. Pipe with partially opened valve

Let the pipe is completely insulated so that no heat transfer to the surrounding or to the system take place. Due to the fall in pressure at the constricted passes, the temperature of the fluid should also decrease and the fluid should come out with high velocity. But as high fictional resistance between the fluid and the constricted passes is present therefore there is no considerable change in velocity. The pressure energy of gas is converted into heat which is used to warm the gas to its initial temperature.

As this is open device so we can use steady flow energy equation.

2 2

(½)V₁+gZ₁+h₁+ Q₁ = (½)V₂+gZ₂+h₂+ W₂ -----(1)

As the pipe is insulated

∴ Q₁ = 0

As there is no shaft

∴ W₁ = 0

As the pipe is horizontal

∴ Z₁ = Z₂

As in throttling process increase in velocity is very insignificant

∴ V₁ ≃ V₂

Inserting all these values in eq. (1)

∴ h₁ = h₂

It means in Throttling Process

dH = 0

∴ enthalpy is constant in throttling process.

Points to remember

1. In both the free expansion and throttling process, enthalpy remains constant but the basic difference is that during free expansion process the gases leave the system with high velocity but during throttling process the velocity at exit is approximately equal to the velocity at inlet.

2. As enthalpy is constant therefore temperature will also remains the same.

3. As the temperature is same therefore internal energy will also remain the same.