Steam Vapor Enthalpy

Overview

Engineering reference data for Steam Vapor Enthalpy in thermodynamics.

Key Formulas

First Law

$\Delta U = Q - W$

Energy is conserved — heat added minus work done.

Ideal Gas Law

$PV = nRT$

Relates pressure, volume, and temperature of an ideal gas.

Heat Transfer

$Q = mc\Delta T$

Sensible heat transfer.

Carnot Efficiency

$\eta = 1 - T_C/T_H$

Maximum efficiency between two temperatures.

Variables

Steam Saturation Properties

Property Table Notes

• Suffix f refers to saturated liquid (water at boiling point)
• Suffix g refers to saturated vapor (steam at condensation point)
• v_f changes very little with pressure and is often omitted from tables
• Internal energy (u) can be calculated from the specific enthalpy relation and is often omitted from tables
• At the critical point (221.2 bar, 374.15°C), $v_f = v_g$, $u_f = u_g$, $h_f = h_g$, and $s_f = s_g$ — there is no definite transition from liquid to vapor above this pressure

Phase Change Process

The evaporation process consists of three stages:

1. Preheating — the liquid heats up to the saturation temperature $t_s$
2. Evaporation — the liquid changes state from fluid to gas at constant temperature $t_s$
3. Superheating — the vapor heats above the saturation temperature

Heat transferred during temperature changes is called sensible heat. Heat required for the phase change (evaporation) is called latent heat of evaporation.

Enthalpy of Superheated Steam

$h_s = h_g + c_{ps}(t_s - t_f)$

where $c_{ps}$ is the specific heat of steam at constant pressure. At standard atmospheric pressure, $c_{ps} = 1.860 \text{ kJ/(kg·°C)}$. Note that $c_{ps}$ varies with temperature.

Enthalpy Unit Conversions

Interactive Charts

References

• Original Source