What is the difference between superheated and saturated steam?
Saturated (dry) steam results when water is heated to the boiling point (sensible heating) and then vaporized with additional heat (latent heating). If this steam is then further heated above the saturation point, it becomes superheated steam (sensible heating).
At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 °C (212 °F) at standard pressure. If heated further it becomes superheated steam.
- In the case of wet, or saturated steam (100% dry), it contains 100% of the latent heat available at that pressure. Saturated water, which has no latent heat and therefore 0% dryness, will therefore only contain sensible heat. One may ask if steam dryness can rise above 100%?
- This energy (as heat) is released when the steam condenses to water and the water cools to the temperature of skin. Steam will be substantially more injurious. Steam burns do have to potential to cause more damage than scalding. The reason has to do with the latent heat of vaporization.
- The degree of hotness is defined by its temperature. When the water is boiling its temperature is 100°C(373.15K) under atmospheric pressure. After that the heat provided is latent heat which only changes the phase of the water (to steam). So the temperature of steam formed is also 100°C unless the steam is superheated.
Saturated Steam Temperatures
- 1 pound of steam is equal to 970 BTU (or an easy to remember rule of thumb is 1 pound of steam is equal to 1000 BTU. 1 pound of water is equal to a 16 oz tall boy (by volume)….when you heat that tall boy hot enough to make steam it expands almost 1700 times its original volume.
- pounds per square inch gauge
- For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psia) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential (psid).
Updated: 4th November 2019