Properties of steam and steam tables

In short

Steam is water vapour used as the working fluid in power plants, process heating and sterilisation, and its properties cannot be found from the ideal-gas equation because water changes phase. Property values (p, T, v, u, h, s) are therefore read from steam tables or the Mollier (h-s) chart, with the dryness fraction x describing how much of a wet mixture is actually vapour. Every Rankine-cycle, boiler, turbine, condenser and heat-exchanger calculation in a university exam, in GATE, and in design work at NTPC, BHEL, L&T, Thermax, Forbes Marshall, ISGEC or any process plant begins with these numbers. It is examined as table-reading problems, dryness-fraction measurement by calorimeter, and property changes across boiling, throttling and expansion.

Explained

Formation of steam at constant pressure

Heat water at a fixed pressure and the temperature rises until it reaches the saturation temperature T_sat for that pressure; after that, temperature stops rising while the water boils.

• Subcooled (compressed) liquid: T < T_sat at that pressure. Heat added is sensible heat.
• Saturated liquid (x = 0): water just at the point of boiling; properties get subscript *f* (h_f, v_f, s_f).
• Wet steam (0 < x < 1): a mixture of saturated liquid and saturated vapour at the same p and T. Temperature alone cannot fix its state.
• Dry saturated steam (x = 1): all liquid just evaporated; properties get subscript *g*. Heat added from x = 0 to x = 1 is the latent heat of vaporisation h_fg (also called enthalpy of evaporation).
• Superheated steam: heated beyond T_sat at the same pressure. Degree of superheat = T_sup − T_sat. Now p and T are independent, so a separate superheat table is needed.
• Saturation pressure and temperature are linked one-to-one: 1 bar ↔ ~99.6 °C, 10 bar ↔ ~179.9 °C, 100 bar ↔ ~311 °C. As pressure rises h_fg falls (≈2257 kJ/kg at 1 bar, ≈1317 kJ/kg at 100 bar) and becomes zero at the critical point.

Key fixed points and the p-v, T-s pictures

• Critical point of water: ~22.064 MPa (220.64 bar), 373.95 °C, v ≈ 0.003106 m³/kg. Above it there is no distinct boiling — liquid and vapour are indistinguishable, so supercritical boilers have no drum.
• Triple point: 0.01 °C and 0.6112 kPa, where ice, water and vapour coexist. Steam-table internal energy and entropy are taken as zero for saturated liquid here (so h_f ≈ 0.0 kJ/kg).
• On a T-s or h-s diagram, the saturated-liquid and saturated-vapour lines meet at the critical point forming a dome; wet states lie inside, superheated states to the right.
• Constant-pressure lines inside the dome are horizontal on T-s (boiling is isothermal) and are straight sloping lines on h-s; outside the dome they curve upward.
• The vapour dome's apex means that near the critical point small property errors matter — one reason supercritical plants use IF-97 software rather than printed tables.

Dryness fraction and mixture properties

• Dryness fraction (quality) x = mass of dry vapour / total mass of mixture. Wetness = 1 − x. Priming/carryover is water droplets carried with steam in a boiler.
• Any specific property of wet steam is the mass-weighted average of its liquid and vapour parts: v = v_f + x·v_fg, h = h_f + x·h_fg, u = u_f + x·u_fg, s = s_f + x·s_fg (where v_fg = v_g − v_f, etc.).
• Because v_f is tiny compared with v_g at low pressure, the approximation v ≈ x·v_g is often used — acceptable below about 10 bar, poor near the critical point.
• Why x matters in practice: wet steam erodes turbine blades; steam quality below about 0.88 at the LP exhaust is avoided by reheating. Wet steam also transfers less heat per kg in a process heater.

Reading and interpolating steam tables

Standard tables used in India (as printed in most textbooks and in IS/BIS-referenced property data) follow the IAPWS-IF-97 formulation, which replaced the older 1967 IFC formulation; small third-decimal differences between textbook editions are normal and not an error. Table structure:

1. Saturated steam by temperature — T, p_sat, v_f, v_g, h_f, h_fg, h_g, s_f, s_g.
2. Saturated steam by pressure — same columns indexed by p (most used in power problems).
3. Superheated steam — for each pressure, v, h, s against temperature.
4. Compressed liquid — rarely needed; for p below ~50 bar, take h ≈ h_f at the same temperature, or h = h_f + v_f(p − p_sat).

Steps to fix a state:

• Given p and T: compare T with T_sat. T < T_sat → liquid; T = T_sat → wet (need x); T > T_sat → use superheat table.
• Given p and x: use the mixture equations above.
• Given p and h (or s): compute h_f and h_g; if h_f < h < h_g the steam is wet and x = (h − h_f)/h_fg; if h > h_g it is superheated.
• Linear interpolation between tabulated rows: y = y₁ + (y₂ − y₁)(x − x₁)/(x₂ − x₁). Interpolate in the smallest practical interval; properties vary non-linearly, so wide-interval interpolation near saturation introduces error.
• u from h: steam tables often omit u; use u = h − pv with p in kPa and v in m³/kg to get kJ/kg.

Mollier (h-s) chart and when to use it

• The Mollier chart plots enthalpy against entropy with constant-pressure, constant-temperature, constant-dryness and constant-superheat lines. Only the region of practical interest (x above ~0.5 and superheat) is drawn.
• Use: isentropic expansion in a turbine or nozzle is a vertical line down — read h₂ and x₂ directly; throttling is a horizontal line (h constant) to the right.
• Advantage: quick, visual, no interpolation. Limitation: chart-reading accuracy is only about ±1-2%, so numerical work and design use tables or IF-97 software.
• Internal energy and specific volume are not read off the chart; v comes from tables.

Measuring dryness fraction

• Barrel (bucket) calorimeter: condense a known mass of steam in a known mass of water and apply an energy balance. Simple, but heat losses make it inaccurate.
• Separating calorimeter: mechanically separates and weighs the entrained water; gives x from masses alone, but misses fine mist, so it reads high.
• Throttling calorimeter: throttles the steam to about atmospheric pressure, where it becomes superheated at constant enthalpy; measuring p₂ and T₂ fixes h₂ and hence x₁ from h₁ = h₂. Works only if the steam is dry enough (roughly x above 0.95) to become superheated after throttling.
• Combined separating-and-throttling calorimeter: the separator first removes bulk water (giving x₁), then the throttling calorimeter measures the remaining steam (x₂); the true dryness is x = x₁ × x₂. This is the standard method for very wet steam and a favourite exam question.

Properties in use: processes and standards

• Isobaric (boiler): q = h₂ − h₁ (steady flow, no work).
• Isentropic (turbine, nozzle): s₂ = s₁; w = h₁ − h₂. Real machines have isentropic efficiency, so the actual drop is less and the exit is drier.
• Throttling (valve, calorimeter): h₂ = h₁; pressure falls, entropy rises, no work, steam becomes drier or superheated — the basis of the throttling calorimeter.
• Indian practice and codes: boiler and piping work follows the Indian Boiler Regulations, 2026 (in force from October 2026, superseding the 1950 Regulations) under the Boilers Act, 2025 (in force from 1 May 2025, replacing the 1923 Act), administered by state Chief Inspectors of Boilers; BS/IS codes and IS 10496 steam-pipe sizing practice use table-based enthalpy data. Boiler efficiency testing (IS 13979 / BS 845 lines, and the BEE *Energy Manager* material under the Energy Conservation Act, 2001) uses h_steam − h_feedwater straight from steam tables.
• Typical Indian plant conditions: utility subcritical units ~ 150-170 bar, 537/565 °C; supercritical units ~ 247 bar, 565/593 °C; process boilers 10-45 bar saturated.

Key points

• Saturation temperature - the boiling temperature fixed by pressure; p and T are not independent inside the dome.
• Sensible heat h_f - heat to bring water from 0 °C to saturation at that pressure.
• Latent heat h_fg - heat to evaporate 1 kg at constant p and T; falls with pressure, zero at the critical point.
• h_g = h_f + h_fg - enthalpy of dry saturated steam.
• Dryness fraction x - mass of vapour per unit mass of mixture; x = 0 saturated liquid, x = 1 dry saturated.
• Mixture rule - any property = f-value + x × (fg difference); apply to v, u, h and s alike.
• Superheated steam - needs both p and T; degree of superheat = T − T_sat.
• Critical point - 220.6 bar, 373.95 °C; above it, no phase change and no latent heat.
• Triple point - 0.01 °C, 0.6112 kPa; datum where u_f = s_f = 0 and h_f ≈ 0.
• Enthalpy of superheated steam ≈ h_g + c_ps(T_sup − T_sat), with c_ps ≈ 2.0-2.3 kJ/kg·K.
• Throttling - constant enthalpy; raises dryness, never produces work.
• Isentropic expansion - constant entropy; work = h₁ − h₂; wetness increases down the turbine.
• Mollier chart - h-s plot for quick turbine and nozzle work; tables for accurate numbers.
• Separating + throttling calorimeter - true x = x₁ × x₂.
• u = h − pv - internal energy when the table omits it.
• Specific volume rises hugely as pressure falls (0.1944 m³/kg at 10 bar vs 14.67 m³/kg at 0.1 bar) - why LP turbine blades and condensers are large.
• IAPWS-IF-97 - the current international property formulation behind modern tables and software.
• Indian Boiler Regulations, 2026 (successor to the 1950 Regulations) under the Boilers Act, 2025 (successor to the 1923 Act), govern steam-plant practice in India.

Formulas and examples

Reference values used below (standard tables, IF-97):

Example 1 - properties of wet steam. Steam at 10 bar, x = 0.85.

• h = 762.8 + 0.85 × 2015.3 = 762.8 + 1713.0 = 2475.8 kJ/kg
• v = v_f + x(v_g − v_f) with v_f = 0.001127: v = 0.001127 + 0.85 × 0.19327 = 0.1654 m³/kg
• s = 2.138 + 0.85 × (6.586 − 2.138) = 2.138 + 3.781 = 5.919 kJ/kg·K
• u = h − pv = 2475.8 − 1000 × 0.1654 = 2310.4 kJ/kg

Example 2 - heat to raise steam. Feedwater at 40 °C (h ≈ 167.5 kJ/kg) is converted to dry saturated steam at 10 bar, 2 t/h.

• Heat per kg = 2778.1 − 167.5 = 2610.6 kJ/kg.
• Mass flow = 2000/3600 = 0.5556 kg/s → heat load = 0.5556 × 2610.6 = 1450.3 kW.
• With boiler efficiency 85%, fuel heat input = 1450.3/0.85 = 1706.3 kW.

Example 3 - isentropic expansion in a turbine. Dry saturated steam at 40 bar expands isentropically to 0.1 bar.

• s₁ = s_g = 6.070 kJ/kg·K = s₂ = 0.649 + x₂(8.149 − 0.649)
• x₂ = (6.070 − 0.649)/7.500 = 5.421/7.500 = 0.723 (very wet — real plants superheat and reheat)
• h₂ = 191.8 + 0.723 × 2392.8 = 191.8 + 1730.0 = 1921.8 kJ/kg
• Ideal work = 2801.4 − 1921.8 = 879.6 kJ/kg; at 85% isentropic efficiency, actual work = 747.7 kJ/kg.

Example 4 - throttling calorimeter. Steam at 10 bar is throttled to 1 bar and 120 °C. Superheated steam at 1 bar, 120 °C has h ≈ 2716.6 kJ/kg.

• h₁ = h₂ = 2716.6 → x₁ = (2716.6 − 762.8)/2015.3 = 1953.8/2015.3 = 0.9695
• Wetness = 3.05%. (The 20.4 °C of superheat after throttling confirms the instrument is valid.)

Example 5 - combined separating and throttling calorimeter. A separator drains 0.45 kg of water while 9.55 kg of steam passes on; the throttling section then gives x₂ = 0.97.

• x₁ = 9.55/(9.55 + 0.45) = 0.955; true dryness x = 0.955 × 0.97 = 0.926, i.e. 7.4% wetness.

Where it is used

• Thermal power plants: NTPC, Tata Power, Adani Power — heat rate, turbine heat balance and condenser vacuum calculations all read enthalpies from tables; BHEL and L&T design boilers and turbines on them.
• Process industry: sugar, paper, textile, distillery, fertiliser and refinery plants size steam headers, PRVs and heat exchangers using h_fg at header pressure; Thermax, Forbes Marshall and ISGEC supply the boilers and steam traps.
• Energy audits: BEE-certified Energy Auditors compute boiler efficiency by the direct method (steam enthalpy rise ÷ fuel energy) and value steam loss in rupees per tonne.
• Food, pharma and hospitals: autoclave and CIP sterilisation specify dry saturated steam quality; wet or superheated steam fails sterilisation validation.
• Statutory inspection: boiler attendants and Boiler Inspectors under the Indian Boiler Regulations work with saturation pressure-temperature pairs for hydraulic tests and safety-valve settings.
• Job roles: graduate engineer trainee (thermal), performance engineer, boiler commissioning engineer, HVAC and utility engineer, energy auditor.

Interview questions

• Q: Why can't the ideal-gas equation be used for steam? A: Steam near saturation is close to condensation, so intermolecular forces and phase change make pv = mRT badly wrong; property tables or IF-97 correlations are used instead.
• Q: What is dryness fraction and why does it matter in a turbine? A: It is the mass fraction of vapour in wet steam; low dryness at the LP stages causes blade erosion and efficiency loss, which is why reheat is used.
• Q: How many properties fix the state of wet steam, and of superheated steam? A: Wet steam needs pressure (or temperature) plus dryness fraction; superheated steam needs pressure and temperature, which are independent there.
• Q: What happens to latent heat as pressure rises? A: It decreases steadily and becomes zero at the critical point, 220.6 bar and 373.95 °C.
• Q: Why does throttling make wet steam drier? A: Enthalpy stays constant while pressure falls, and h_f falls faster than the enthalpy available, so more of the mixture must be vapour; it is an irreversible process with rising entropy.
• Q: Why can a throttling calorimeter not measure very wet steam? A: Its reading needs the steam to be superheated after throttling; below about x = 0.95 the exit is still wet, so a separating stage is added first.
• Q: What is the significance of the triple point in steam tables? A: It is the datum — internal energy and entropy of saturated liquid are set to zero at 0.01 °C — so tabulated values are relative, not absolute.
• Q: Difference between superheated steam and supercritical steam? A: Superheated steam is above T_sat at a pressure below critical, so it was formed by boiling; supercritical fluid is above 220.6 bar, where no boiling or distinct phases occur.
• Q: Why is saturated steam preferred for process heating but superheated for turbines? A: Saturated steam gives a high film coefficient and isothermal condensation heat transfer; superheat avoids wetness in a turbine and adds usable enthalpy drop.
• Q: How do you find internal energy if the table gives only h? A: u = h − pv, with p in kPa and v in m³/kg, giving kJ/kg.
• Q: What does a vertical line on the Mollier chart represent? A: Isentropic expansion or compression — the basis of turbine and nozzle ideal-work calculations.
• Q: At 1 bar, how much more heat does evaporating water need than heating it from 0 °C? A: About 2258 kJ/kg of latent heat against roughly 418 kJ/kg of sensible heat — over five times as much, which is why condensing steam is such an effective heating medium.

Common mistakes

• Treating temperature and pressure as independent inside the vapour dome, or looking up a wet state in the superheat table.
• Using h_g when the steam is wet, which overstates enthalpy by (1 − x)·h_fg.
• Forgetting v_f in v = v_f + x·v_fg at high pressure, where it is no longer negligible.
• Mixing units: p in bar with v in m³/kg in u = h − pv (must convert to kPa), or kJ/h with kW.
• Interpolating over a wide pressure range, or interpolating across the saturation line.
• Reporting the separating-calorimeter value alone as the dryness fraction instead of x₁ × x₂.
• Assuming superheated steam is always better — in a heat exchanger its low heat-transfer coefficient means a larger surface area.

Related topics

Power cycles: Rankine, Otto, Diesel and Brayton · Boilers and steam power plants · Steam turbines and nozzles · Thermodynamics: laws and properties · Second law, entropy and availability · Heat transfer and heat exchangers