Specific Volume

Specific volume vs. Pressure
We can see below, as the steam pressure increases from 1atm to 4 atm, the density of the steam molecules is increasing. As the specific volume is inversely related to the density, the specific volume will decrease with increasing pressure. We can see the reduced volume in the last jar.

This diagram clearly shows that the greatest change in specific volume occurs at lower pressures, whereas at the higher end of the pressure scale there is much less change in specific volume.

The extract from the steam tables below, shows specific volume, and other data related to saturated steam.

At 7 kg/cm2g, the saturation temperature of water is 170°C. More heat energy 'hf' is required to raise its temperature to saturation point at 7 bar g than would be needed if the water were at atmospheric pressure. The table gives a value of 171.96 kcals to raise 1 kg of water from 0°C to its saturation temperature of 170°C.

The heat energy (enthalpy of evaporation 'hfg') needed by the water at 7 bar g to change it into steam is actually less than the heat energy required at atmospheric pressure. This is because the specific enthalpy of evaporation decreases as the steam pressure increases.

However, as the specific volume also decreases with increasing pressure, the amount of heat energy transferred in the same volume actually increases with steam pressure.