You need to prepare to perform a First Law analysis on a device. Upon inspection, you see that the device is small, uninsulated, has one inlet and one outlet, and has no obvious work inputs or outputs. The operator tells you that steam enters through the inlet and liquid water exits through the outlet. The operator says that he turns it on at the beginning of the day, and turns it off at the end, and never has to pay any attention to its operation. Beginning with the general equation for the First Law of Thermodynamics, develop a simplified expression that should be suitable for use with this device.
What will be an ideal response?
From the information given and observations, one should assume
(1) There may be heat transfer (no insulation).
(2) While the device is small, the entering steam condenses so the heat transfer is probably not negligible.
(3) The device has a single inlet and a single outlet
(4) Ignore potential work interactions
(5) As there is a large change in enthalpy between vapors and liquids (for the most part), any change in kinetic energy is likely small in comparison to the changes in enthalpy, and so the change in kinetic energy can be ignored.
(6) As the device is small, changes in potential energy can also be ignored.
(7) As the operator sees no need to interfere with operation, and as it runs all day long, one can treat it as steady-state; steady-flow.
Therefore, starting with the general first law on a rate basis:
dE/dt = 0
W? = ?ke = ?pe = 0
m? 1 = m? 2 = m? , taking
Inlet = State 1; Outlet = State 2.
These cause the first law to reduce to
???? ? = ????? (???????? ? ????????)
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