T2 is fixed by intersection of the isobar originating at state 3 and the isentrope starting from state 1. Instead of an efficiency, the quality of the refrigeration operation is described by the coefficient of performance.
The power requirements for a typical household refrigerator with this COP, a cooling capacity QL of 3. The two classes of flow devices differ in two principal ways: fluid velocity changes and reversibility.
In both cases, the open system consists of the section of pipe containing the constriction and imaginary surfaces perpendicular to the flow direction that are sufficiently far upstream and downstream to escape perturbations of the flow by the device. The inlet fluid condition is completely fixed thermodynamic state and velocity and the outlet condition is partially specified.
The First law, and if applicable, the form of the Second law for reversible processes, are used to calculate the remaining downstream conditions. The mass flow rate is constant in time, so the analysis is on the basis of a unit of flowing mass. Example: What is the outlet steam quality in Fig. The assumption that condensation occurs in the outlet stream needs to be verified.
Using the upstream pressure and temperature in the superheated steam table A. This enthalpy is slightly less than the enthalpy of the saturated vapor at the downstream pressure of 1. The quality of the downstream steam is:. Suppose, however, that the downstream pressure is specified as 1. To determine the final state of the gas, steam table A. That flow through an orifice is irreversible seems intuitive because of fluid turbulence or at least laminar friction created by the abrupt reduction in flow cross section.
This qualitative assessment can be verified by comparing the entropies of the gas before and after passage through the orifice. At 8 MPa and oC, steam table A. Because of the smooth shape of the walls, the flow is approximately reversible.
Since the system is also adiabatic, the Second law provides the additional relation:. In order to relate the constant-entropy condition of this particular of open system to the changes in pressure and temperature of the ideal gas, Eq 3. Even though this equation was derived for changes in a closed system, it is applicable to open systems as well.
The reason is that this equation involves only thermodynamic properties, and so depends only on the initial and final states of the fluid, not on the process that caused the change. Another way of viewing isentropic flow is to imagine that the flow consists of small packets of fluid acting as closed systems undergoing reversible adiabatic expansion or compression as they move from the inlet to the outlet.
Once Te is determined from Eq 3. Example: What are the exit temperature and velocity of steam flowing through the nozzle of Fig. Heat Q c is absorbed from a source at low T e. A perfect refrigerator would transfer heat from a colder body to a hotter body without doing any work.
From 2nd law of thermo, this is impossible. Net work taken from the Carnot cycle is the largest possible for a given amount of heat supplied. The cylinder walls and the piston are thermally non—conducting. During the process, the gas absorbs heat Qh from the base of the cylinder and does a work WAB in raising the piston. During the process, the gas expels heat Qc to the reservoir and the work WCD is done on the gas by external agent.
From the 1st law of thermo,. And this of course applies also to heat pumps and refrigerators. If all molecules of a gas in a room move together, this is a very ordered, unlikely state. If the molecules move randomly in all directions, changing speed after collisions, this is a very disordered, likely state. A membrane separating it from a vacuum is suddenly broken and the gas expands irreversibly to Vf.
Since the gas is ideal, U depends only on T.
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