Throttling Process

Thermodynamics THERMODYNAMIC SYSTEMS AND PROCESSES original conditions if the process was reversed. For example, an automobile engine does not give back the fuel it took to drive up a hill as it coasts back down the hill. There are many factors that make a process irreversible. Four of the most common causes of irreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finite temperature difference, and mixing of two different substances. These factors are present in real, irreversible processes and prevent these processes from being reversible. Adiabatic Process An adiabatic process is one in which there is no heat transfer into or out of the system. The system can be considered to be perfectly insulated. Isentropic Process An isentropic process is one in which the entropy of the fluid remains constant. This will be true if the process the system goes through is reversible and adiabatic. An isentropic process can also be called a constant entropy process. Polytropic Process When a gas undergoes a reversible process in which there is heat transfer, the process frequently takes place in such a manner that a plot of the Log P (pressure) vs. Log V (volume) is a straight line. Or stated in equation form PVⁿ = a constant. This type of process is called a polytropic process. An example of a polytropic process is the expansion of the combustion gasses in the cylinder of a water-cooled reciprocating engine. Throttling Process A throttling process is defined as a process in which there is no change in enthalpy from state one to state two, h₁ = h₂; no work is done, W = 0; and the process is adiabatic, Q = 0. To better understand the theory of the ideal throttling process let’s compare what we can observe with the above theoretical assumptions. An example of a throttling process is an ideal gas flowing through a valve in midposition. From experience we can observe that: P_in > P_out, vel_in < vel_out (where P = pressure and vel = velocity). These observations confirm the theory that h_in = h_out. Remember h = u + Pv (v = specific volume), so if pressure decreases then specific volume must increase if enthalpy is to remain constant (assuming u is constant). Because mass flow is constant, the change in specific volume is observed as an increase in gas velocity, and this is verified by our observations. Rev. 0 Page 29 HT-01