The Joule Thompson Coefficient constant enthalpy process

Equation (4.9) shows that flow of a fluid through a device that exchanges neither heat nor work with the surroundings and involves negligible kinetic energy changes proceeds without change in enthalpy. Valves, porous plugs, and orifices inserted into flow lines are examples of devices through which the flow is isenthalpic. Even though the enthalpies upstream and downstream of such devices are equal, the pressures are not. In fact, the main practical purpose of these devices is to produce an abrupt reduction in pressure, and for refrigerators, of temperature as well. For this reason, they are called throttling devices.

If the fluid is a gas, the change in temperature across the device may be positive, negative, or zero, depending on the equation of state and the upstream temperature. The partial derivative (dT/dp)h representing this process is called the Joule-Thompson coefficient. If the gas is ideal, no change in temperature occurs because the enthalpy is constant.

The relation between the Joule-Thompson coefficient and the equation of state is obtained from Eq (6.22b) by dividing by dp while holding h constant:

This equation is the enthalpy analog of Eq (6.30). It is obtained from the latter by exchanging p for v and h for u.

Example: If N2 at 20oC is reduced in pressure in a throttling device from 10 MPa to 0.1 MPa, what is the temperature change?

The pressure dependence of CP of a nonideal gas is calculated from Eq (6.27). However, this effect is of second order, and it suffices to approximate CP by the ideal gas value: CP = CP0 = 9/2R = 36.4 J/mole-K. Substituting the Van der Waals EOS, Eq (2.5), into Eq (6.31) and using the constants for N2 yields:

The 9.9 MPa pressure drop over the throttling device is accompanied by a 20 OC temperature decrease. Repeating this test at another temperature provides enough data to determine the constants a and b in the Van der Waals EOS.

The pressure dependence of CP of a nonideal gas is calculated from Eq (6.27). However, this effect is of second order, and it suffices to approximate CP by the ideal gas value: CP = CP0 = 9/2R = 36.4 J/mole-K. Substituting the Van der Waals EOS, Eq (2.5), into Eq (6.31) and using the constants for N2 yields:

The 9.9 MPa pressure drop over the throttling device is accompanied by a 20 OC temperature decrease. Repeating this test at another temperature provides enough data to determine the constants a and b in the Van der Waals EOS.

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