3. Heat supply using high-pressure steam
Even now, water vapour in the form of high-pressure steam plays an important role in supplying industrial applications with heat. But why is this the case? The mere generation of water vapour is technically very simple using boiler systems. The working medium required, water, is available all over the world. Water vapour has a particularly high energy density because it transitions from gas to liquid form as a result of energy extraction and thus releases its heat of evaporation or enthalpy of evaporation in the process. This process is called condensation.
Heating water from 0°C to 100°C takes just 10% of the energy required to evaporate this water at 100°C. The energy required for evaporation is known as enthalpy of evaporation and becomes usable when steam condenses.
This means that only comparatively small amounts of water vapour need to be generated and transported in order to transport a relatively large amount of energy. There is a very favourable ratio between effort and use. The steam absorbs its energy when heat continues to be added to the water over its boiling point, which depends on pressure, and releases this energy during the condensation process – when the energy extraction reverts the steam back to water in liquid form (condensation).
During the process of condensing 1t of steam, around 700kWh of usable heat is released and approx. 1m³ of condensate fed back into the boiler. By comparison, if the same amount of heat is provided using heating water in a heat exchanger, 31m³ of water would need to be pumped to the heat exchanger and back to the heat generator (assuming the temperature differential is 20K, e.g. flow temperature 90°C, return temperature 70°C). This means that steam is the most suitable medium for delivering heat, especially if a large amount of heat energy is required.
Steam can be provided at higher temperatures than hot water, which is another advantage as some steam consumers need higher temperatures. Added to this is the fact that condensation results in very high heat transfer levels, with relatively small and therefore cost-effective heat transfer surfaces required.