Every time you boil a kettle of water, you create steam.
However, rather than being a byproduct of boiling water, steam is a powerful, versatile and efficient source of heat energy. That is why steam is widely used across industries.
In this article, we will explore where steam is used, how to control the energy within steam and why steam is so efficient for transferring heat into industrial processes.
Which industries use steam for thermal energy?
Many industries rely on steam for heat or thermal energy. Companies and organisations use thermal energy to provide high temperatures for critical processes.
In the food and beverage industry, steam is used at various stages during manufacturing, including pasteurisation, cooking, cleaning, and drying. For example, the heat from steam cooks raw baked beans within their tins.
While in the healthcare sector, hospitals use high-pressure steam to sterilise surgical equipment within an autoclave. The steam kills microorganisms and spores, ensuring that medical instruments are safe to use without the need for chemicals.
These industries, and many others, use steam because of its unique properties as an efficient thermal energy source.
Where is steam produced and used?
Steam is produced in a boiler house (A) and distributed through pipework to the processes needing thermal energy. Each process uses a heat exchanger (B) to transfer some of the thermal energy within steam into that process. Steam releases this energy by condensing against a colder surface and forms a liquid condensate; the liquid returns to the boiler house, where it feeds the boiler and produces further steam. The whole process is known as the steam-condensate loop.
How much thermal energy is within steam?
You can calculate the amount of thermal energy within steam using steam tables1. These show the relationship between pressure, temperature, volume and above all, how much energy steam contains. With this knowledge, you can adjust the energy within steam for different processes.
Steam tables
At atmospheric pressure, 0 bar g, water boils at 100℃ (as in a kettle). The first row in the steam tables shows that 419kJ/kg of energy, known as the enthalpy of water, is needed to bring water to its boiling point.
To produce steam though, you need to add a further 2,257kJ/kg of energy, which is called the enthalpy of evaporation or the useful energy. While the total amount of energy within the steam is now 2,676kJ/kg, only the useful energy will transfer into the process when the steam condenses in a heat exchanger.
The steam tables assume you are producing dry saturated steam. However, it is also possible to produce wet steam and superheated steam, which are shown in the steam phase diagram.
Steam phase diagram
Point A shows the equivalent information from the first line of the steam tables, where at 0 bar g it takes 419kJ/kg of enthalpy (heat energy) for water to reach the boiling point of 100℃ and start evaporating.
The line between point A and point B shows the journey of the boiling water as you add a further 2,257kJ/kg of heat energy. At B, the water has completely evaporated into dry saturated steam. The red line on the diagram is the dry saturated steam line. Steam produced between A and B is wet steam because it still contains moisture. Steam to the right of the dry saturated steam line is superheated steam.
Which type of steam is the most efficient for heat transfer?
Dry saturated steam
Dry saturated steam is the ideal type of steam for heat transfer as it is the most efficient for the process and creates fewer engineering challenges. The steam is dry because it contains no moisture and is saturated because it is full of energy. The steam cannot contain any more energy at that temperature and pressure.
Wet steam
Wet steam is less efficient than dry saturated steam because it contains less energy. With wet steam, the process will receive less energy and a smaller mass of steam, which will likely extend the process time, miss the required target temperature or lead to product spoilage.
Superheated steam
Superheated steam2 has a higher temperature than saturated steam at the same pressure. While some steam plants produce superheated steam for power generation, superheated steam is not recommended for heat transfer because it has a lower heat transfer rate. Superheated steam also requires a larger heat transfer area than dry saturated steam.
Why is steam more efficient than low-temperature hot water (LTHW) for heat transfer?
Dry saturated steam is more efficient for heat transfer3 than alternatives like low-temperature hot water (LTHW) because of its unique properties.
Pressure controls the thermal energy within steam
Changing pressure allows you to manipulate steam to meet the needs of the process, including the amount of thermal energy it contains.
When you heat water under pressure, it has a higher boiling point, so more thermal energy is needed. From the steam tables, at 5 bar g, water boils at 159℃ and needs 671kJ/kg of energy to reach the boiling point. However, turning that boiling water into steam takes less thermal energy than at lower pressures. At 5 bar g, you need 2,086kJ/kg of energy, compared to 2,257kJ/kg at 0 bar g.
Knowing the impact of pressure, you can produce steam under high pressure in the boiler house and then distribute it to the process. When the steam reaches the heat exchanger, lowering the pressure will increase the amount of useful energy transferred into the process.
Steam has a higher heat content than LTHW
Steam is produced at much higher temperatures and pressures than LTHW, so it has a significantly higher heat content and can transfer more heat energy into the process. In other words, less steam is required than LTHW to produce the same heating effect.
For example, compare a steam-to-water heat exchanger with a water-to-water exchanger designed to transfer 11℃ into the process. With a kilogram of steam, there is 50 times more useful heat energy than in a kilogram of LTHW. That means you need 50 times less steam for the same heating effect, because water has a specific heat capacity of 4.19kJ/kg °C while the useful heat energy within steam is 2,200kJ/kg.
Steam transfers heat three times faster than LTHW
Steam transfers heat energy faster than LTHW because it gives up its heat energy by condensing. The heat transfer coefficient, which measures the speed of heat transfer, is approximately three times greater with a steam-to-water heat exchanger than with a water-to-water heat exchanger.
As steam provides a faster heat transfer rate, a smaller surface area is also required within the heat exchanger to transfer the same amount of heat energy. As a result, a steam-to-water heat exchanger needs a smaller footprint than an equivalent water-to-water heat exchanger.
Steam transfers heat uniformly across a surface area
Steam is a gas, so it spreads out to fill a space. Within a steam-to-water heat exchanger, there is an even spread of heat across the heat transfer surface area compared to a water-to-water heat exchanger .
Using steam for heat transfer minimises cold spots within the heat exchanger and provides more consistent heat transfer.
Steam: An efficient choice for heat transfer
Steam is an attractive thermal energy source for many industries because of its efficiency. Changing pressure allows you to adjust the useful energy within steam. Steam's heat content and transfer speed are significantly greater than LTHW. In addition, steam needs smaller heat transfer surface areas than other energy sources.
Learn more about steam
1 What is steam? - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/what-is-steam
2 Superheated steam - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/superheated-steam
3 Heat transfer - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/heat-transfer