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The temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment. This procedure is called a Throttling process. This temperature can be described by the “Joule–Thomson effect” or “Joule–Kelvin effect” or “Kelvin–Joule effect” or “Joule–Thomson expansion”.In the Joule experiment, the gas expands in a vacuum and the temperature drop of the system is zero, if the gas were ideal.
In this process here 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. Let’s take an example of a throttling process is an ideal gas flowing through a valve in mid position.
We can observe that: Pin > Pout, velin < velout (where P = pressure and vel = velocity). These observations confirm the theory that hin = hout. 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.
The theory also states W = 0. Our observations again confirm this to be true as clearly no "work" has been done by the throttling process. Finally, the theory states that an ideal throttling process is adiabatic. This cannot clearly be proven by observation since a "real" throttling process is not ideal and will have some heat transfer.
In this process, steam becomes drier and nearly saturated steam becomes, superheated.
As a gas expands, the average distance between molecules grows. Because of intermolecular attractive forces , expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process and no heat is transferred, the total energy of the gas remains the same because of the conservation of energy. The increase in potential energy thus implies a decrease in kinetic energy and therefore in temperature.
A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature). Below the Joule–Thomson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature. Above the inversion temperature, gas molecules move faster and so collide more often, and the latter effect (reduced collisions causing a decrease in the average potential energy) dominates: Joule–Thomson expansion causes a temperature increase.
SEPARATING CALORIMETER: -
It consists of two concentric chambers, the inner chamber and the outer chamber, which communicates with each other through an opening at the top. As the steam discharges through the metal basket, which has a large number of holes, the water particles due to their heavier momentum get separated from the steam and collect in the chamber. The comparatively dry steam in the inner chamber moves up and then down aging through the annular space between the two chambers and enters the Throttling Calorimeter.
It is a vessel used initially to separate some of the moisture from the steam, to ensure superheat conditions after throttling. The steam is made to change direction suddenly; the moisture droplets, being heavier than the vapor, drop out of suspension and are collected at the bottom of the vessel.
It consists a narrow throat (Orifice). Pressure and temperature are measured by pressure gauge and thermometer. The steam after throttling process passes through the heat exchanger and condensate is collected. Steam Generator is also provided to supply the saturated steam (Max) at 2kg/cm2 pressure. There is no need of boiler.
It is a vessel with a needle valve fitted on the inlet side. The steam is throttled through the needle valve and exhausted to the condenser.
The quality of wet steam is usually defined by its dryness fraction. When the dryness fraction, pressure and temperature of the steam are known, then the state of wet steam is fully defined. In a steam plant it is at times necessary to know the state of the steam. For wet steam, this entails finding the dryness fraction. When the steam is very wet, we make use of a separating calorimeter.
Separating calorimeter does not give an accurate result and the throttling calorimeter fails if the steam is not superheated after throttling. A combination of separating and throttling calorimeter is therefore found most suitable for accurate measurement of dryness of steam