Steam is water that has been converted to a gas phase through heating. It takes a significant amount of energy, known as the enthalpy of vaporization, to convert water to steam. Steam has many industrial and domestic applications including electricity generation, sterilization, and pasteurization. There are two main systems for producing high pressure steam - steam drums and once through steam generators. Steam drums separate steam and water in a drum, while once through generators convert water directly to steam without a separation drum. Supercritical boilers operate above the critical pressure where water no longer boils, eliminating the need for separation drums.

1. Steam Generation

2. Steam
The vapor into which water is converted, when heated,
forming a white mist of minute water droplets in the
air.
(OR)
Steam is defined as water in gas phase, which is formed
when water boils.

3. Enthalpy of vaporization
It is energy required to turn water into the
gaseous form when it increases in volume by
1700 times at std. temperature and pressure.

4. Application of Steam
• Agriculture purposes
• Domestic
• Electricity generation
• Wood treatment
• Microbial deactivation
• Energy storage
• Sterilization
• Cleaning & Pasteurization

5. Production of High Pressure steam
Two inherently different systems are used to
produce high pressure steam
• Steam drum
• Once Through Steam Generator

6. Steam Drum
• In steam drum, the drum serves as point of
separation of steam from water. Sub-cooled
water enters the tube to which heat is supplied.
As the water flows across the tube, it is heated to
boiling point. Consequently, bubbles are formed
and wet steam is generated. In most boilers, a
steam-water mixture leaves the tube and enters
the steam drum. In steam drum due to difference
in pressure and temperature, vapor gets
separated. The remaining water is then mixed
with the make up water and returned back to the
heated tube and process is repeated.

7. Steam Drum

8. Once Through Steam Generator
Without the use of steam drum, sub cooled water
enters the heated tube and gets converted to
steam along the flow path. The point where
water turns into steam depends on water flow
rate and heat supplied. With close control of both
flow rate of water and heat input value, we can
make sure of all water convert into steam. All
water is evaporated within the tube and only
steam leaves the tube. There is no need of having
a steam drum.

9. Once Through Steam Generator

10. Supercritical Boilers:
At very high pressure, a point is reached where
water no longer exhibits boiling behavior.
Above this pressure (i.e. 221 bar or 3200 psi)
the water temp. keeps on increasing with
added heat. Boiler designed to operate this
critical pressure are referred to as supercritical
boilers. Drums are no longer needed for
supercritical boilers.