Enthalpy isenthalpic processes

A throttle does not change the enthalpy of the fluid throttling is an isenthalpic process. For a given input state and a specified outlet pressure, one finds the outlet temperature by conducting a one-dimensional search for a temperature at which the enthalpy is equal to the input enthalpy. An enthalpy chart provides a convenient means for the search. Another method of solution is to apply the Joule-Thompson coefficient, defined by... 

One of the classic examples in thermodynamics is the expansion of a fluid across a valve (or a porous plug), which is a constant enthalpy process. The process is shown in Figure 1. The specification of this process is given the temperature and the pressure of the inlet (state 1) and the pressure at the outlet (state 2), calculate the temperature of the outlet stream. Since this is an isenthalpic process, we also know that ... 

Because the enthalpy of the gas does not change, the process is called isenthalpic. What are some consequences of this isenthalpic process ... 

Although the change in enthalpy is zero, the change in temperature is not. What is the change in temperature accompanying the pressure drop for this isenthalpic process That is, what is (dT/dp). We can actually measure this derivative experimentally, using an apparatus like the one in Figure 2.10. 

For an ideal gas, /jlj-j- is exactly zero, because enthalpy depends only on temperature (that is, at constant enthalpy, temperature is also constant). However, for real gases, the Joule-Thomson coefficient is not zero, and the gas will change temperature for the isenthalpic process. Remembering from the cyclic rule of partial derivatives that... 

Thus it happens that in constant pressure processes, the enthalpy change is exactly equal to q, the total heat flow. Or putting it the other way around, q admits a potential H in constant pressure processes. Please note that because H is a state variable, AH is perfectly well-defined between any two equilibrium states. But when the two states are at the same pressure, it becomes equal to the total heat flow during the process from one to the other, and in fact AH is in practice rarely used except in these cases (another kind of use, isenthalpic expansions, is discussed in Chapter 8). 

This implies that this process occurs isenthalpic. The temperature change in this process is expressed by the differential equation (3T/3P)h, which is called the Joule-Thonq>son coefficient (Ijt. If the enthalpy of a gas H is considered to be dependent on T and P then... 

Thus we conclude that in a Joule-Thomson throttling process the enthalpy is conserved. Therefore, the temperature of an ideal gas does not change as the heat capacity Cp and thus the enthalpy H do not depend on pressure. The change of temperature of a real gas during such an isenthalpic expansion is characterized by the Joule-Thomson coefficient... 

Table 2.3 demonstrates the impact of the various thermodynamic paths on a total energy balance for an open system. For the isenthalpic case, AT = 0 for an ideal gas since the enthalpy is a function of temperature only. For the isentropic case, Q = 0 since dS = dQ/T. For the isothermal case, AH = 0 since the enthalpy for an ideal gas is a function of temperature only. For the adiabatic case, AS = 0 for a reversible process only. For both the isentropic and adiabatic cases, the shaft work determined is a maximum for reversible processes. 

Notice that if the choice of piston pressures is such that gas moves from the left to the right, then AVi < 0 and AV2 > 0. Since the process is carried out adiabatically, q = 0, and therefore, AU = w. The enthalpy change for the entire system is zero this is an isenthalpic (constant enthalpy) process. 

We call a process that occurs at constant enthalpy, such as this one, isenthalpic. 

Joule-Thomson expansion results from the unrestrained, free expansion of real gases. Such a process occurs at constant enthalpy and is termed isenthalpic. We can determine the change in temperature that results as the pressure decreases in the isenthalpic throtthng process if we know the Joule-Thomson coefficient, fijr — dT/dP)h- Joule-Thomson expansion is the basis for liquefaction processes, such as those shown in Figure 5.11. 

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