Question

Q1. For steady-state flow through a throttling valve with negligible changes in kinetic and potential energies, the isenthalpic variation of temperature with pressure is known as the Joule- Thomson coefficient, μ. a) Write the energy balance and starting from the change for enthalpy (02 in your notes), show that the Joule-Thomson coefficient is given by: b) The throttling process may lead to an increase or decrease in temperature depending on the value of μ, ie, 0 Temperature decreases on throttling u <0 Temperature increases on throttling For a real substance, the Joule-Thomson coefficient for a set enthalpy value changes sign at a temperature called the inversion temperature: μ is positive for temperatures below the inversion temperature and negative at temperatures above. Show that for an ideal gas on the other hand, u 0 at all temperatures.

Answer 1

A Throtling valve is simply a flow restricting device. It can be a thin capillary tube, an adjustable valve or porous plug.

Purpose of this device is to produce a sizable pressure drop in the fluid, which results in an accompanying temperature change.

The pressure drop in the fluid is accompanied by a large drop in Temperature and for this reason these devices are used in refrigeration and air conditioning applications.

Throttling devices are usually small devices and the flow through them may be assumed to be adiabatic and there is also no work exchange involved in throttling devices. There is no change in potential energy and kinetic energy.

The ratio of the change in the temperature of the gas as a result of throttling to the change in the pressure is called Joule - Thomson Coefficient.

Consider isenthalpic curves on a T-P Diagram.

Y axis : TEMPERATURE, X axis : PRESSURE

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Since throttling process causes a decrease in pressure, this process proceed from right to left along a line of constant enthalpy. The inversion curve is the locus of all points of zero slope i.e. μ = 0. The intersection of inversion curve and constant enthalpy line is called inversion point. Joule thomson coefficient is positive to the left of inversion curve and negitive to the right of the inversion curve.

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