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There are so many parameters that determines steam characteristics. These parameters are structured and readily made available in a table called “Steam Table”.

Before reading a steam table, it is important to know the following terminology and units, which helps inreading and understanding the table better.

1. Steam Pressure- Absolute Pressure and Gauge Pressure

• P_abs,Atmospheric Pressure (Absolute Pressure) –The pressure exerted by atmosphere equals to 1.01325 bar, when water is boiling at 100^oSI unit of pressure is bar (1 bar is 100kPa).
• P_gauge, Gauge Pressure – Gauge Pressure is the pressure above atmospheric pressure. Zero on the dial on the pressure gauge which is equivalent to 1.01325 bar absolute pressure.SI unit of pressure is bar (1 bar is 100kPa).

For practical applications 1.01325 bar absolute is considered as 1 bar absolute, i.e.,

Absolute Pressure =Gauge pressure + 1

2.Heat Content in steam–Sensible Heat, Latent Heat and Superheated Steam

• h_f(Specific enthalpy of saturated water) – Total enthalpy held by every kilogram of liquid water at boiling temperature is called Specific enthalpy of saturated water,h_f. Unit is kCal/kg.
• h_fg (Specific enthalpy of Evaporation) – The extra enthalpy which needs to be added to every kilogram of water which converts liquid water to steam is called Specific enthalpy of evaporation,h_fg.Unit is kCal/kg.
• h_g (Specific enthalpy of steam) – Total Enthalpy in every kilogram of steam is sum of Specific heat of Saturated water and Specific enthalpy of evaporation). Unit is kCal/kg.

h_{g =} h_f + (X xh_fg), where X is the dryness fraction

3.Steam Quality– Dry Steam, Wet Steam, Superheated steam, Dryness Fraction and Specific Volume.

• Dry Steam - Normally steam table shows the data for dry saturated steam, the steam which has been completely evaporated so that it contains no droplets of liquid water.
• Wet Steam - Practically steam carries tiny droplets of water, called Wet steam which cannot be described fully dry saturated, thus uses steam separators and steam traps to keep the steam as dry as possible at user point.
• Superheated Steam – If heat transfer continues after all the water has been evaporated and reaches dry saturation stage, the steam temperature will again rise. This is called Superheated steam, which can reach any temperature above that of saturated steam at corresponding pressure.
• Dryness Fraction (X) –Steam quality is determined through its dryness fraction (X). Water droplets present in the steam carrying space doesn’t contain any specific enthalpy of evaporation. Considering an e.g.

Specific enthalpy at 3 bar gauge pressure and 0.90% dryness fraction means,

h_{g =} h_f + (X xh_fg)
= 144 kCal/kg + (0.90 X 509.7 kCal/kg)
= 602.73 kCal/kg.

• Specific Volume of steam (V_g) – A kilogram mass of water , one litre by volume, if converted all to steam, the result will be one kilogram by mass of steam. But the volume of steam changes with change in pressure. Thus Specific Volume is “The volume of one kilogram of steam at any given pressure”.

Now let us see with an example how to read a steam table considering the parameters above,

E.G.)Find all the parameters of steam at 3 bar gauge pressure, dry saturated ?

Step 1. For conversion to Convert to Absolute pressure, i.e., 3 bar gauge = 3 + 1 = 4 bar absolute

Step 2. Find the parameters corresponding to 4 bar absolute with the steam table,

1. Temperature = 142.9 deg C
2. h_f = 144 kCal/kg
3. h_fg = 509.7 kCal/kg
4. h_g = hf + (X x hfg), where X = dryness fraction = 1 for dry saturated steam

= 144 kCal/kg + (1 x 509.7 kCal/kg) = 653.7 kCal/kg

5. V_g = 0.471 m³/kg