Steam is the gas formed when water passes from the liquid to the gaseous state. At the molecular level, this is when H2O molecules manage to break free from the bonds (i.e. hydrogen bonds) keeping them together.
How steam works?
In liquid water, H2O molecules are constantly being joined together and separated. As the water molecules are heated, however, the bonds connecting the molecules start breaking more rapidly than they can form. Eventually, when enough heat is supplied, some molecules will break free. These 'free' molecules form the transparent gas we know as steam, or more specifically dry steam.
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.
Types of steam:
1. Saturated Steam (Dry):
Saturated steam, or dry steam, is the most common type for industrial use. This type of steam has excellent heat transfer capabilities, making it a great solution for most process heating projects.
In addition to its heat transfer coefficient, saturated steam is quick to create and offers a great degree of control. It supplies even heating and temperature is accurately managed by controlling pressure.
This type of steam is created by boiling water with a boiler heater. The heat is increased until it vaporizes the steam. This method removes the moisture from the vapor, hence why it is also known as dry steam. The process is simple and cost-effective, further contributing to saturated steam’s widespread use.
2. 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.
3. 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.
4. Culinary Steam
As the name implies, culinary steam is common in the food and beverage industry. Because it creates products for human consumption, this type of steam is carefully regulated. It needs to be free of contaminants and corrosion. The heater uses stainless steel materials, and the steam must pass through a filtration system. As well, they typically use corrosion inhibitors.
Although the FDA regulates the quality of culinary steam, it is still separate from pure and clean steam. As well, culinary steam generation is less expensive than pure steam.
5. Filtered Steam
As with culinary steam, filtered steam must pass through a filtration system. This includes a filter and strainer that removes condensation as well as all sorts of contaminants and impurities that can enter the system.
This type of steam is primarily used for sterilization. The strictness of regulations and the items to be sterilized determine whether filtered, clean, or pure steam is required.
6. Pure Steam
High-quality electric feedwater and boiler heaters are used to generate cleaner steam. Pure steam has the highest purity grade for steam and is used for the most sensitive and sterile conditions. For instance, all injectables require pure steam for sterilization in the pharmaceutical industry.
Pure steam generation requires careful material selection. The feedwater must be double osmosed, and the boiler should be stainless steel to prevent impurities. Pure steam is only used where necessary because of its higher production cost.
7. Clean Steam
Clean steam is similar to pure steam but requires a slightly lower purity grade. Feedwater can use demineralized or osmosed water, although a stainless steel boiler is still required.
Cheaper than pure steam, it is an economical solution where a lesser degree of purity is required. It is still used for sterilization in pharmaceuticals and biotech — but only for non-injectables and processes that don’t require strict endotoxin regulation.
8. Utility Steam
Utility steam comes from a conventional boiler. It’s cheaper to generate and accounts for most of the saturated steam we use. Also known as plant steam, it can be used to create other types of steam. For instance, filtered steam often starts as utility steam that is then run through a filter and strainer.
Different types of water are used to generate plant steam depending on its end-use. Still, utility steam is not used in pharmaceutical or medical settings.
9. Flash Steam
Flash steam is utility steam produced through a specific method. It uses high pressure and high temperatures to rapidly generate/discharge steam. Hence the name flash steam. Although its creation method is different, its use and quality are the same as utility steam.