26-12-2012, 02:24 PM
Pressure-Temperature Relationship of Water & Steam
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Saturated Steam
As indicated by the black line in the above graph, saturated steam occurs at temperatures and pressures
where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization
is equal to the rate of condensation.
Tips
Having said this, it is necessary to be mindful of the following when heating with saturated steam:
Heating efficiency may be diminished if steam other than dry steam is used for process heating.
Contrary to common perception, virtually all of the steam generated from a boiler is not dry saturated
steam, but wet steam, which contains some non-vaporized water molecules.
Radiant heat loss causes some of the steam to condense. The generated wet steam thus becomes
even more wet, and condensate also forms, which must be removed by installing steam traps at
appropriate locations.
Heavy condensate that falls out of the steam flow can be removed through drip leg steam traps.
However, the entrained wet steam will reduce heating efficiency, and should be removed through pointof-
use or distribution separation stations
Steam that incurs pressure losses due to piping friction, etc., may result a corresponding loss in steam
temperature as well
Wet Steam
This is the most common form of steam actually experienced by most plants. When steam is generated using
a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the
distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness. As the water
approaches the saturation state and begins to vaporize, some water, usually in the form of mist or droplets, is
entrained in the rising steam and distributed downstream. This is one of the key reasons why separation is
used to dis-entrain condensate from distributed steam.
Superheated Steam
Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point.
This yields steam that has a higher temperature and lower density than saturated steam at the same
pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not
typically used for heat transfer applications.
Advantages of using superheated steam to drive turbines:
To maintain the dryness of the steam for steam-driven equipment, whose performance is impaired by
the presence of condensate
To improve thermal efficiency and work capability, e.g. to achieve larger changes in specific volume
from the superheated state to lower pressures, even vacuum.
It is advantageous to both supply and discharge the steam while in the superheated state because
condensate will be generated inside steam-driven equipment during normal operation, minimizing the risk of
damage from erosion or carbonic acid corrosion. In addition, as the theoretical thermal efficiency of the
turbine is calculated from the value of the enthalpy at the turbine inlet and outlet, increasing the degree of
superheating as well as the pressure raises the enthalpy at the turbine inlet side, and is thereby effective at
improving thermal efficiency.
Supercritical Water
Supercritical water is water in a state that exceeds its critical point: 22.1MPa, 374 °C (3208 psia, 705°F). At
the critical point, the latent heat of steam is zero, and its specific volume is exactly the same whether
considered liquid or gaseous. In other words, water that is at a higher pressure and temperature than the
critical point is in an indistinguishable state that is neither liquid nor gas.
Supercritical water is used to drive turbines in power plants which demand higher efficiency. Research on
supercritical water is being performed with an emphasis on its use as a fluid that has the properties of both a
liquid and a gas, and in particular on its suitability as a solvent for chemical reactions.