Steam Basics: Saturated dry vs. unsaturated wet steam
Discover the difference between saturated dry steam and unsaturated wet steam. What are the advantages and where do you use it?
Saturated, unsaturated, dry, wet, flash, supercritical and superheated steam… There is a lot out there which indicates that not all steam is the same. So does the application define which type of steam you need?
The properties depend on volume, pressure, and temperature. This means it's the application that defines which type of steam you need.
Let’s have a look at the different types of steam that exist and their advantages and disadvantages.
Saturated or dry steam
Saturated or dry steam is the type of steam we get if all water molecules remain in the gaseous state. Take a kettle for example that whistles when it’s ready. Steam does not escape freely because the pressure is controlled of its intended use. Sometimes you see mist coming out your kettle: this is dry steam.
Dry steam loses some of its energy when released into the colder atmosphere. It transfers that energy into the ambient air causing it to condense and show as mist. In other words: you produce dry steam by heating water in a closed chamber.
The steam temperature is close to the boiling point at that pressure.
Unsaturated or wet steam
We get unsaturated or wet steam when the steam pulls in tiny droplets of water upon heating. When a steam boiler heats up water, bubbles break through the surface. So, when steam starts to form, it contains liquid. It's this liquid that makes the steam partially wet unless you use a superheater. Even the best steam boilers can release steam containing 3% - 5% wetness.
How pressure and temperature influence water and steam
Generally speaking, steam has a direct relationship with pressure and temperature.
- The higher the pressure in the boiler, the more energy we need to apply to generate steam.
- With increased pressure, steam will be created at higher temperatures. This higher temperature steam contains more energy per kg.
- Superheated steam is an exception to this rule.
This graph shows what the state of water (liquid or gas) will be at a given temperature and pressure. At 1 bar (normal atmospheric pressure) water will freeze at 0°C (32°F) and boil at 100°C (212°F). At 2 bar, your freezing point will be lower and your boiling point higher.
Superheated steam table as guide
Steam tables are essential tools for anyone working with steam. You can compare its importance to a timetable for taking the train, or the map on a GPS when driving somewhere new. We usually use a steam table to determine the temperature at a certain pressure, or the other way around.
What makes this a valuable tool is that it also includes a specific enthalpy and volume. Enthalpy is the quantity of energy contained in 1 kg. You calculate the enthalpy of steam by taking the sum of the enthalpy of the various states (liquid and gas).
To go from water to superheated steam, this steam table shows the relation between pressure and temperature
Steam dryness fraction
As mentioned above, it’s for steam boilers nearly impossible to produce 100% dry steam. We measure the actual level of steam in the dryness fraction.
If steam contains 5% water, it’s said to be 95% dry and have a dryness fraction of 0.95.
The dryness fraction has a direct effect on the total amount of transferable energy. In turn, it affects the heating quality and efficiency.
Remember the difference between latent and sensible heat? 100% dry steam also contains 100% of the latent heat available (at that specific pressure). Saturated water at 0% dryness will only contain sensible heat.
In case you’re not sure on the difference between the two anymore you can read up on it in this article.
Did you know that you can heat steam beyond the boiling point to get superheated steam?
Unlike saturated steam, superheated steam does not have a direct relationship between temperature and pressure. Which means it can exist at a wide range of temperatures. We prefer saturated steam for heating applications and superheated steam for power generation or in turbines.
It seems unlikely, but did you know that steam dryness can be more than 100%? In that case we’re talking about superheated steam. We prefer saturated dry steam for heating applications. While superheated steam is the go-to option for power generation or in turbines.
Saturated (dry) steam makes an excellent heat source for the following reasons:
- Because of rapid and even heating, your product quality and productivity will improve.
- Because pressure controls temperature, we can quickly and precisely achieve a specific temperature.
- Because there is a high heat transfer coefficient, a smaller heat surface is required. This means you can reduce initial equipment outlay.
- Because steam originates from water, it’s safe, clean and low-cost.
Yet, we now know that steam is not 100% dry. Radiant heat loss causes some of the steam to condense making the wet steam even more wet and condensate also forms. If not managed and maintained properly with the right equipment (and accessories), it can have hurtful effects:
- Affect heat transfer efficiency.
- Cause corrosion of piping and critical equipment.
What is the heat transfer coefficient?
We use the heat transfer coefficient to calculate how well heat is transferred. Earlier we wrote: ‘A high heat transfer coefficient requires a small heat surface. Resulting in reduced initial equipment outlay.' Because more water has evaporated into saturated steam, the steam has also absorbed more latent heat. As a result, there is more heat contained in the same mass. And so, it has the ability to do more work.
What’s the difference between saturated (dry) steam and unsaturated (wet) steam?
Dry or saturated steam does not contain any water droplets and you produce it by heating water in a closed chamber. Wet or unsaturated steam does contain water droplets. Saturated (dry) steam is an excellent source for heating. Unsaturated (wet) steam can cause corrosion or decrease heat transfer efficiency if not managed properly.
What is the dryness fraction?
It’s the measure we use to define how dry steam actually is. It has an impact on the heating capabilities of steam.
