Engineering Thermodynamics: Boilers
Thread Content
The steam used in industry is usually generated in open-system equipment with continuous inflow and outflow of the working fluid, assuming that the resistance to flow is ignored. Then the pressure of the working fluid can remain constant; this is a constant-pressure process, such as the generation of steam in boilers. I would like to ask why it is said that the pressure of the working fluid remains constant The working fluid is the boiler feed water; does that mean the feed water pressure remains constant?
In other words, if the resistance during flow is 0 (and the hydrostatic head resulting from height differences throughout the boiler should also be ignored), then the steam pressure generated is equal to the boiler feedwater pressure； The processes of heat absorption and vaporization of boiler feedwater, as well as superheating, are all pressure-controlled processes.
Sigh~ I just don’t understand why it is said that water vapor is generated by maintaining a constant pressure in the boiler The pressure of the water supply isn’t constant either
The boiler feedwater pressure and the boiler outlet vapor pressure can be considered equal.
You’ve confused the two ‘constant pressure’ concepts; they are not the same thing. Water vapor is generated at a constant pressure in the boiler, meaning that the pressure remains unchanged as water turns into steam inside the boiler. The enthalpy equation is H=U+pV; when water is heated and vaporizes, its enthalpy increases, which is reflected in an increase in internal energy and an expansion of volume. It is a constant-pressure process because, if flow resistance is ignored, the water supply pipes, the boiler, and the equipment that use gas form a connected volume; ignoring height differences, the pressure is the same everywhere within them. Another issue is the constant pressure of the water supply. When discussing the first issue, it is assumed that the feed water pressure is a constant value.
The boiler steam pressure can be considered constant once equilibrium is reached. The feed water pressure is the pressure of the pump, which is higher than the steam pressure.