What is Dry Steam?
No steam boiler or steam generator produces 100% dry steam (dry steam). This is only possible in theory.
Thermodynamically speaking, when a steam boiler heats water, not all of the H2O molecules can turn into the steam phase due to the efficiency losses of the components of the boiler. At this time, the bubbles emerging from the water surface attract small water droplets along with the steam and drag them into the system. The presence of water droplets in the steam leaving the steam boiler in this way causes the steam to be partially wetted, thus producing wet steam.
Steam Dryness Rate
The steam dryness ratio is used to measure the amount of water in the steam. If the steam contains 10% water by mass, it is said to be 90% dry or has a dryness ratio of 0.90.
Steam dryness is important because it has a direct impact on the total amount of transferable energy contained in the steam (usually only latent heat), which in turn affects heating efficiency and quality.
For example, saturated steam (100% dry) contains 100% of the latent heat available at that pressure. Saturated water, which has no latent heat and is therefore 0% dry, will contain only sensible heat.
Importance of Steam Dryness Rate
When it comes to the efficiency figure of a steam boiler, all efficiency losses will affect the dryness rate of the final product, the steam.
How much of 100 units of water entering the boiler evaporates indicates both the boiler efficiency and the steam dryness.
Due to the low heat capacity, more steam needs to be transported to the plant in mass in order to carry the heat energy required by the plant.
In other words, the higher the efficiency of a boiler, the higher the dryness rate of the steam. The reverse is also true.
If the manufacturer gives the boiler efficiency as 96%, we should understand from this statement that almost dry steam is produced. However, steam boiler and especially steam generator manufacturers claim that they produce dry steam by separating the entrained water after placing 1 or even 2 separators at the outlets of their machines.
Effect of Steam Dryness Rate on Fuel Consumption
Example:
What is the amount of heat that can be transported by 1,000 kg/h steam at 6 bar pressure and 10% (x=0.90) dryness?
x = 0.90 x 2.066 kJ/kg = 1.859.4 kJ/kg
1,859.4 x 1,000 = 1,859,400 kJ/ton
However, the amount of heat energy that will be carried by the steam produced in a steam generator/boiler producing steam at 96% dryness;
2,066 Kj/kg x 0.96 x 1,000 = 1,983,360 kJ/ton
will be.
When we divide the difference between the two by the fuel lower calorific value (34,530 kJ/Nm³ for natural gas)
Q=(1.983.360 – 1.859.400) / 34.530
Q=3.59 Nm³/h will mean fuel savings.
Monthly for 24 hours, 25 days; (3.6 x 24 x 25 = 2.154 Nm³/month x 12.47 TL/Nm³)
A monthly saving of 26,860 TL will be achieved. (8,950 Euro/year)
