Understanding Enthalpy of Dry Saturated Steam and Pressure

The question asks about the change in enthalpy of dry saturated steam as the pressure increases. Let's first understand what dry saturated steam is and how its properties relate to pressure.

Dry Saturated Steam: This refers to steam that is exactly at its boiling point (saturation temperature) for a given pressure and contains no water droplets. It is in a state of phase equilibrium between liquid and vapor.

Enthalpy of Dry Saturated Steam: The enthalpy of dry saturated steam ($h_g$) is the total energy per unit mass required to raise water from a reference state (usually 0°C or 0.01°C and corresponding pressure) to its saturation temperature and completely vaporize it at that temperature and pressure. It is typically expressed as the sum of the enthalpy of saturated liquid ($h_f$) and the enthalpy of vaporization (latent heat, $h_{fg}$).

Mathematically, this is represented as:

\begin{equation*} h_g = h_f + h_{fg} \end{equation*}

Now, let's consider how $h_f$ and $h_{fg}$ change as pressure increases for saturated conditions:

• As pressure increases along the saturation curve, the saturation temperature also increases. This means more energy is needed to raise the water from the reference temperature to the higher saturation temperature. Therefore, the enthalpy of saturated liquid ($h_f$) increases significantly with increasing pressure.
• The enthalpy of vaporization ($h_{fg}$) is the energy required to change the state from saturated liquid to dry saturated vapor at constant temperature and pressure. As pressure increases, the difference between the properties of saturated liquid and saturated vapor decreases, and the latent heat of vaporization ($h_{fg}$) generally decreases.

So, the enthalpy of dry saturated steam ($h_g$) is the sum of an increasing term ($h_f$) and a decreasing term ($h_{fg}$). However, the increase in $h_f$ is more significant than the decrease in $h_{fg}$ over a wide range of pressures (up to the critical point). Therefore, the net effect is that the enthalpy of dry saturated steam increases as pressure increases.

This relationship can be clearly observed in steam tables, where values for $h_g$ are tabulated against pressure or temperature. Looking at standard steam tables confirms that the enthalpy of dry saturated steam increases as pressure rises, until it reaches a maximum value before the critical point, after which it decreases rapidly to the critical point.

Considering the options provided:

• Option 1: First increases and then decreases - This is true over a very wide range up to and beyond the critical point, but for common engineering pressures, the increase is dominant. The typical context for such questions usually implies pressures below or well below the critical point where the overall trend is increasing.
• Option 2: Remains constant - This is incorrect, as properties change with pressure.
• Option 3: Decreases - This is incorrect over the typical range of pressures below the maximum enthalpy point.
• Option 4: Increases - This is the correct behavior for dry saturated steam enthalpy as pressure increases over the most commonly encountered range, up to the point where the enthalpy peaks before decreasing towards the critical point. Given the general nature of the question, 'Increases' is the expected answer representing the primary trend.

Thus, with an increase in pressure, the enthalpy of dry saturated steam increases.