Why entropy remains constant in throttling process in refrigeration?
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Throttling occurs when a fluid flows through a constricted passage, like a porous plug, a partially open valve or an orifice. Since the process occurs within a control volume with adiabatic walls, the heat transfer is zero. The temperature does not decrease in every throttling process.
Let us begin with flow work. Flow work is the work done on the fluid to make it move. Thus Flow work is necessary for maintaining a continuous flow through the control volume. It is given by;
Wf=pvWf=pv …………………………………..(1)
where; WfWfis the flow work per unit mass, p is the pressure and v is the specific volume (All at a particular section.
Now the fluid can flow by imparting kinetic energy to the fluid at constant pressure(eg. a blower) thus there has to be no restriction to the flow path. But if a restriction is created a back pressure will be produced, thus a pressure gradient will be created across the restriction. So pressure at inlet of the restriction will be greater than that of the outlet of the restriction to make the flow possible. So the Flow work at inlet will be greater than that of outlet.
Enthalpy is the thermodynamic potential of the system at a point i.e. it is the sum of internal energy and the product of pressure and volume of the system at that point.
Thus it can be written as;
h=u+pv......................=u+pv......................(2)
If pv increases, with h constant, then u must decrease as a result of the fluid doing work on its surroundings. This produces a decrease in temperature. Conversely, a decrease in pvmeans that work is done on the fluid and the internal energy increases. If the increase in internal energy exceeds the increase in potential energy, there will be an increase in the temperature of the fluid.
Now, Let us see, what happens during a throttling process.
From 1st law;
Q=Δu+WQ=Δu+W
since; Q = 0 and Wf1Wf1 is Work done on the gas as inlet due to the rest of the gas and Wf2Wf2 is Work done by the gas at outlet to the rest of the gas.
(u2−u1)−(Wf1−Wf2)=0(u2−u1)−(Wf1−Wf2)=0
or(u2−u1)+(Wf2−Wf1)=0or(u2−u1)+(Wf2−Wf1)=0
(u2−u1)+(p2v2−p1v1)=0(u2−u1)+(p2v2−p1v1)=0
Therefore;
u2+p2v2=u1+p1v1.............u2+p2v2=u1+p1v1.............(3)
from the definition of enthalpy i.e. eq.(2) and eq(3)
h1=h2h1=h2
thus the enthalpy change during the process is zero i.e. enthalpy remains constant throughout the process.
Suppose an ideal gas is throttled. Since throttling process is isenthalpic, and for an ideal gas enthalpy is a function of temperature only, the temperature of an ideal gas does not change during a throttling process.
For liquids under high pressure pvpv increases as the pressure increases. Due to the fact that the molecules are already forces together so the further increase in pressure does not create considerable change in volume. Thus for liquids at high pressure cooling is always achieved during throttling.
Now comes the Real gas .
The throttling process is also called joule Thomson process. The change in temperature with respect to the pressure at constant enthalpy for fluids is presented in graphs. The plots show that, at a particular enthalpy, as we keep in increasing the constriction (decreasing the output pressure), the temperature keeps on increasing till it reaches the inversion temperature(maximum attainable temperature) and if we further increase the constriction the temperature stops increasing. So cooling is achieved after crossing the inversion temperature. Thus for gasses to produce cooling effect, the initial temperature should be less than inversion temperature. Luckily, almost all gases have maximum inversion temperature less than standard ambient temperature, so cooling can be achieved. Hydrogen and Helium are the exceptions. They need to be pre cooled below their maximum inversion temperatures before throttling.
MARK BRAINLIEST..
Let us begin with flow work. Flow work is the work done on the fluid to make it move. Thus Flow work is necessary for maintaining a continuous flow through the control volume. It is given by;
Wf=pvWf=pv …………………………………..(1)
where; WfWfis the flow work per unit mass, p is the pressure and v is the specific volume (All at a particular section.
Now the fluid can flow by imparting kinetic energy to the fluid at constant pressure(eg. a blower) thus there has to be no restriction to the flow path. But if a restriction is created a back pressure will be produced, thus a pressure gradient will be created across the restriction. So pressure at inlet of the restriction will be greater than that of the outlet of the restriction to make the flow possible. So the Flow work at inlet will be greater than that of outlet.
Enthalpy is the thermodynamic potential of the system at a point i.e. it is the sum of internal energy and the product of pressure and volume of the system at that point.
Thus it can be written as;
h=u+pv......................=u+pv......................(2)
If pv increases, with h constant, then u must decrease as a result of the fluid doing work on its surroundings. This produces a decrease in temperature. Conversely, a decrease in pvmeans that work is done on the fluid and the internal energy increases. If the increase in internal energy exceeds the increase in potential energy, there will be an increase in the temperature of the fluid.
Now, Let us see, what happens during a throttling process.
From 1st law;
Q=Δu+WQ=Δu+W
since; Q = 0 and Wf1Wf1 is Work done on the gas as inlet due to the rest of the gas and Wf2Wf2 is Work done by the gas at outlet to the rest of the gas.
(u2−u1)−(Wf1−Wf2)=0(u2−u1)−(Wf1−Wf2)=0
or(u2−u1)+(Wf2−Wf1)=0or(u2−u1)+(Wf2−Wf1)=0
(u2−u1)+(p2v2−p1v1)=0(u2−u1)+(p2v2−p1v1)=0
Therefore;
u2+p2v2=u1+p1v1.............u2+p2v2=u1+p1v1.............(3)
from the definition of enthalpy i.e. eq.(2) and eq(3)
h1=h2h1=h2
thus the enthalpy change during the process is zero i.e. enthalpy remains constant throughout the process.
Suppose an ideal gas is throttled. Since throttling process is isenthalpic, and for an ideal gas enthalpy is a function of temperature only, the temperature of an ideal gas does not change during a throttling process.
For liquids under high pressure pvpv increases as the pressure increases. Due to the fact that the molecules are already forces together so the further increase in pressure does not create considerable change in volume. Thus for liquids at high pressure cooling is always achieved during throttling.
Now comes the Real gas .
The throttling process is also called joule Thomson process. The change in temperature with respect to the pressure at constant enthalpy for fluids is presented in graphs. The plots show that, at a particular enthalpy, as we keep in increasing the constriction (decreasing the output pressure), the temperature keeps on increasing till it reaches the inversion temperature(maximum attainable temperature) and if we further increase the constriction the temperature stops increasing. So cooling is achieved after crossing the inversion temperature. Thus for gasses to produce cooling effect, the initial temperature should be less than inversion temperature. Luckily, almost all gases have maximum inversion temperature less than standard ambient temperature, so cooling can be achieved. Hydrogen and Helium are the exceptions. They need to be pre cooled below their maximum inversion temperatures before throttling.
MARK BRAINLIEST..
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