Steam Basics - How do you evaluate the condition of steam?

Wet steam has a white color and contains particles of water and the dry saturated steam is blue.

Two methods are adopted to ensure that steam leaving the boiler is dry.  The simplest is the provision of a drier in the steam space.  The steam passage out from the boiler is through  series of baffles so designed that the water particles in the steam shall be deposited and left behind.

The second method is to take the wet steam away from the water to another source of heat.

The extra heat first gives the missing latent heat to any water particles and turns the wet steam into dry saturated steam.  When there is no longer any water present, the steam itself absorbs the extra heat and becomes superheated steam.

Superheated steam at a given pressure can be at any temperature above that of saturated steam.  It expands when heated and contracts when cooled.  Saturated steam, by contrast, condenses when cooled.

Superheating increases the total heat of steam, but not by a very great amount.  For example, the total heat of one pound of steam at 200 PSIG which has been superheated by 100 deg F is 1260 BTU’s.  Although the temperature of the steam has been raised from 387 deg F to 487 def F the total heat has been raised only from 1200 to 1260.

The reason why only 60 BTU’s were added to one pound of steam on heating it 100 deg F at 200 PSIG is because the average specific heat of superheated steam at this pressure and temperature is .6.

The specific heat of any substance is the quantity of heat required to raise the temperature of one pound by 1 deg F.  The specific heat of water is therefore 1.  The specific heat of superheated steam varies according to pressure and temperature.  The higher the pressure, the higher the specific heat; the higher the temperature, the lower the specific heat.

Superheating provides a valuable safeguard against loss of heat content due to wet steam conditions, particularly when the steam is to be used for generating power.

The superheating of steam helps in preventing the formation of condensate in process plants and also increases the power output by delaying condensation during the expansion stage.  In turbines drier steam at the exhaust end will decrease the erosion of blades.