Lesson 3 - Steam Theory SAQ

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a)    Describe what the following terms mean in relation to steam:

(i)             Saturated liquid

(ii)          Saturated vapour

(iii)         Superheated vapour

b)    Rank the terms below in order of smallest to largest specific enthalpy:

(i)            Saturated liquid

(ii)          Saturated vapour

(iii)         Superheated vapour

c) What type of steam is used in a brewery? Explain why.

Response:

a)

(i) Water can exist as solid ice, liquid water, vapour (steam), or as a mixture. As water is heated, it gains sensible energy which increases its temperature. Water will keep accepting more heat and not change phase until it reaches the boiling point (at a specified pressure). Just before the first drop of water becomes steam, it is known as a saturated liquid.

(ii)  As more heat is added to a saturated liquid, some of the water begins to vaporise into steam. As more heat is added, more liquid evaporates, until all the liquid water has become steam. When the last drop of water has become steam, it is classed as a saturated vapour.

(iii) If more heat is added to saturated vapour (steam), it becomes a superheated vapour.

b) The more heat that the water is holding, the higher the specific enthalpy. With this in mind, we can determine that a saturated liquid holds the smallest amount of enthalpy, while a superheated vapour holds the largest amount of enthalpy.

c) Ideally, due to its large enthalpy, superheated steam should be used in a brewery. However, the heat transfer via a stable vapour (superheated steam) is not very good. So, for heat transfer applications, dry saturated steam is the optimal steam to use in a brewery. However, because dry saturated steam is difficult to produce with an everyday boiler, and virtually impossible to maintain through distribution piping, the steam used in a brewery is in fact likely to be wet saturated steam.

So in summary, wet saturated steam is used in the brewery.