Enthalpy is one of the most useful properties in engineering thermodynamics because it combines internal energy with the flow energy needed to push fluid into or out of a device. Steam power plants, boilers, turbines, heat exchangers, and refrigeration systems all use enthalpy to track energy transfer. Steam tables give engineers measured property values for water and steam at different pressures, temperatures, and phases.
Learning to read them turns a complex phase-change problem into a structured lookup and calculation.
In a boiler, added heat raises water to saturation, vaporizes it, and can then superheat the steam before it enters a turbine. During vaporization, temperature and pressure can remain constant while enthalpy increases greatly because energy is used to separate liquid molecules into vapor. For a saturated mixture, the quality tells what fraction of the mass is vapor, allowing mixture properties to be found by interpolation between saturated liquid and saturated vapor values.
For superheated steam, pressure and temperature together identify the correct table row for properties such as specific volume, internal energy, entropy, and enthalpy.
Key Facts
- Specific enthalpy is h = u + Pv, where h is enthalpy per unit mass, u is internal energy per unit mass, P is pressure, and v is specific volume.
- For steady-flow devices with negligible kinetic and potential energy changes, q - w = h2 - h1.
- Latent heat of vaporization is hfg = hg - hf.
- For a saturated mixture, h = hf + xhfg, where x is quality.
- Quality is x = mvapor / mtotal and applies only in the saturated mixture region.
- Superheated steam properties require two independent properties, usually pressure and temperature, to locate h in the superheated steam table.
Vocabulary
- Enthalpy
- Enthalpy is a thermodynamic property equal to internal energy plus pressure-volume flow energy.
- Steam table
- A steam table is a tabulated set of water and steam properties at specified pressures, temperatures, and phases.
- Saturated liquid
- A saturated liquid is water at the boiling point for a given pressure that is just about to vaporize.
- Saturated vapor
- A saturated vapor is steam at the condensation point for a given pressure that is just about to condense.
- Quality
- Quality is the mass fraction of a saturated mixture that exists as vapor.
Common Mistakes to Avoid
- Using quality for superheated steam is wrong because quality is defined only when saturated liquid and saturated vapor coexist.
- Choosing the wrong steam table is wrong because saturated tables, compressed liquid tables, and superheated tables apply to different phase regions.
- Assuming boiling always occurs at 100 degrees Celsius is wrong because saturation temperature depends strongly on pressure.
- Using hfg instead of the mixture formula is wrong because hfg is only the difference between saturated vapor and saturated liquid enthalpy, not the total mixture enthalpy unless x = 1 with hf accounted for separately.
Practice Questions
- 1 At 200 kPa, saturated water has hf = 504.7 kJ/kg and hfg = 2201.6 kJ/kg. Find the specific enthalpy of a saturated mixture with quality x = 0.75.
- 2 A turbine receives steam at h1 = 3330 kJ/kg and discharges it at h2 = 2550 kJ/kg. If heat transfer and kinetic energy changes are negligible, find the work output per kilogram of steam.
- 3 A student looks up steam at 500 kPa and 250 degrees Celsius. Explain why the superheated steam table is needed instead of using a saturated mixture quality calculation.