In steam-using industries, two commonly referred to types of steam are dry steam (also called
Saturated steam) and wet steam.
Dry steam applies to steam when all its water molecules remain in the gaseous state. It's a transparent gas.
Wet steam applies to steam when a portion of its water molecules have given up their energy (latent heat) and condense to form tiny water droplets. Take the example of kettle boiling water. Water is first heated using an element. As water absorbs more and more heat from the element, its molecules become more agitated and it starts to boil. Once enough energy is absorbed, part of the water vaporizes, which can represent an increase as much as 1600X in molecular volume.
In a boiler, energy from the fuel is transferred to liquid water in order to create steam. At first, cold water gets warmer and receives energy in the form of “sensible heat”, right until the boiling point.
If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state. However, not all steam is the same. The properties of steam vary greatly depending on the pressure and temperature to which it is subject. Steam is then called saturated steam.
Advantages of saturated steam
- Improved product quality and productivity
- Temperature can be quickly and precisely established
- Smaller required heat transfer surface area, enabling reduced initial equipment outlay
- Safe, clean, and low-cost
When all the water is vaporized, any subsequent addition of heat raises the steam’s temperature. Steam heated beyond the saturated steam level is called superheated steam.
The best steam for industrial process heating is the dry saturated steam. Wet steam reduces total heat in the steam. Also water forms a wet film on heat transfer and overloads traps and condensate equipment. Super heated steam is not desirable for process heating because it gives up heat at a rate slower than the condensation heat transfer of saturated steam.
It must be remembered that a boiler without a super heater cannot deliver perfectly dry saturated steam. At best, it can deliver only 95% dry steam. The dryness fraction of steam depends on various factors, such as the level of water to be a part of the steam. Indeed, even as simple a thing as improper boiler water treatment can become a cause for wet steam.
As steam flows through the pipelines, it undergoes progressive condensation due to the loss of heat to the colder surroundings; the extent of the condensation depends on the effectiveness of the lagging. For example, with poor lagging, the steam can become excessively wet.
Since dry saturated steam is required for process equipment, due attention must be paid to the boiler operation and lagging of the pipelines.
In a turbine, steam must be dry. Water droplets are not allowed, because they could break the vanes. At the high rotation speed of the vanes in a turbine, the centrifugal force applied on small drops of water can become very important, unbalance the vanes and cause them to break. That is the first reason why superheated steam is better in a turbine.
Advantages of superheated steam to drive turbines:
To improve thermal efficiency and work capability.
To maintain the dryness of the steam for steam-driven equipment.
Water when it is heated after boiling and steam is formed still contains water vapour in steam. Since it is not completely dry, some amount of water is present, it is represented by a dryness fraction between zero to one, such as 0.89 dry or 0.79 dry etc., The total heat of such a steam will be less than the steam which is superheated, which does not contain water vapour at all and condensation does not take place easily. Hence superheated steam is preferred for steam engines.
Wet steam can reduce plant productivity and product quality, and can cause damage to most items of plant and equipment. Whilst careful drainage and trapping can remove most of the water, it will not deal with the water droplets suspended in the steam. To remove these suspended water droplets, separators are installed in steam pipelines.
The steam produced in a boiler designed to generate saturated steam is inherently wet. Although the dryness fraction will vary according to the type of boiler, most shell type steam boilers will produce steam with a dryness fraction of between 95 and 98%. The water content of the steam produced by the boiler is further increased if priming and carryover occur.