Page 27 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

o
T, C

300 4

Superheated
steam
Saturated
2 mixture 3
100

Compressed
liquid
20 1

v, m /kg
3
Figure 2.2.2-2 T-v diagrams for the heating process of water at constant pressure

Imagine that we are to run an experiment. In this experiment, we start with a mass of

water at 1 atm pressure and room temperature. At this temperature and pressure we

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may measure the specific volume (1/ = 1/1000 kg/m ). We plot this state at point 1 on
the diagram.

If we proceed to heat the water, the temperature will rise. In addition, water expands

slightly as it is heated which makes the specific volume increase slightly. We may plot the

locus of such points along the line from State 1 to State 2. We speak of liquid in one of
these conditions as being compressed or sub cooled liquid.

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State 2 is selected to correspond to the boiling point (100 C). We speak of State 2 as
being the saturated liquid state, which means that all of the water is in still liquid form,

but ready to boil. As we continue to heat past the boiling point 2, a fundamental change

occurs in the process. The temperature of the water no longer continues to rise. Instead,

as we continue to add energy, liquid progressively changes to steam phase at a constant
temperature but with an increasing specific volume. In this part of the process, we speak

of the water as being a saturated mixture (liquid + steam). This is also known as the

quality region.

At State 3, all liquid will have been vaporised. This is the saturated steam state.
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