Steam Production and Utilization

Understanding Steam

The diagram below helps to explain the various principles involved in the thermodynamics of steam. It shows the relationship between temperature and enthalpy (energy or heat content) of water as it passes through its phase change.

Graph showing the relationship between temperature and enthalpy (energy or heat content) of water as it passes through its phase change

The reference point for enthalpy of water and steam is 0oC, at which point an enthalpy value of 0 kJ/kg is given to it (but of course water at 0o has a lot of energy in it, which is given up as it freezes - it's not until 0K, absolute zero, when it truly has no enthalpy!). As we increase the temperature of water, its enthalpy increases by 4.18 kJ/kg oC until we hit its boiling point (which is a function of its pressure - the boiling point of water is 100oC ONLY at 1 atm. pressure). At this point, a large input of enthalpy causes no temperature change but a phase change, latent heat is added and steam is produced. Once all the water has vaporized, the temperature again increases with the addition of heat (sensible heat of the vapour).

Steam Production and Distribution

 Steam is produced in large tube and chest heat exchangers, called water tube boilers if the water is in the tubes, surrounded by the flame, or fire tube boilers if the opposite is true. The pressure inside a boiler is usually high, 300-800 kPa. The steam temperature is a function of this pressure. The steam, usually saturated or of very high quality, is then distributed to the heat exchanger where it is to be used, and it provides heat by condensing back to water (called condensate) and giving up its latent heat. The temperature desired at the heat exchanger can be adjusted by a pressure reducing valve, which lowers the pressure to that corresponding to the desired temperature. After the steam condenses in the heat exchanger, it passes through a steam trap (which only allows water to pass through and hence holds the steam in the heat exchanger) and then the condensate (hot water) is returned to the boiler so it can be reused. The following image is a schematic of a steam production and distribution cycle.

schematic of a steam production and distribution cycle