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Surface Finishing Tutorial

#273 Heat-Saving Measures - Steam Properties -1

Category : Environmental conservation
January 6, 2017

(1) Steam properties

[Fig.1] illustrates the process of room temperature water turning into steam. First, prepare water at 15°C in an airtight container. Then, heat the container. The water temperature gradually rises and starts boiling when it reaches at 100°C. If you keep heating it, water evaporates into steam although the temperature stays the same. Once all the water evaporates into steam, the temperature starts rising again. The heat quantity used to be described in units of kilocalories (KCal), but using kilojoules (KJ) is more common nowadays.

[Fig.1] Transition of water by heating

[Fig.2] Three states of water, sensible heat & latent heat

[Fig.3] Saturated water, wet steam, and superheated steam

[Fig.2] shows the relationship between water and heat when this phenomenon occurs. In this phenomenon, heat transforms water into three states: ice (solid), water (liquid), and steam (gas). There are two types of heat: sensible and latent heat. Sensible heat is in a form of heat energy that can be measured by thermometers whereas latent heat cannot be measured by such instruments. In addition, the steam in the right container shown in [Fig.1] can be in various forms as explained in [Fig.3].

Boiling water and steam coexist in a boiler. This steam is called "wet (moist) steam". The latent heat of this steam changes by the quantity of heat energy applied.
In addition, the moisture content of steam is called "wetness". When the wetness becomes 0%, the steam turns into a form called "superheated steam" and the steam temperature increases by heating.

In surface treatment processes, the outlet of a boiler should have superheated steam while the inlet of a heater or a heat exchanger should have dry saturated steam (dryness of steam is 100%). During these processes, the latent heat of steam vaporization travels through the wall of a heater to be transmitted to liquid substances, such as a plating bath.
After losing the latent heat, the steam turns into hot water at 100°C called condensed water. The water used to be lost down the drain is now collected and recycled by most factories to be used as feedwater in boilers. The process is called "drain recovery (sensible heat recovery)".

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