It is a Sunday-night shutdown, the checklist asks how much flash steam the blowdown line will throw at the vent, and the answer is sitting in one laminated page of numbers: the steam table. Every serious steam decision — pipe size, trap load, exchanger duty, flash fraction — leans on this table. This lesson teaches you to read it fluently: what each column means, which column answers which question, and the two habits that prevent the classic mistakes.

What the table actually is

The steam table is the saturation curve from What is steam? written out as numbers — one row per pressure, agreed internationally (the modern formulation is called IAPWS-IF97) so that every engineer on earth reads the same values. Habit one, before anything else: the pressure column is absolute. Your gauge reads gauge. Add about 1 to enter the table — the units lesson drills this.

The columns, one by one

ColumnSymbolWhat it tells youYou use it for
Saturation temperatureTsatThe one boiling/condensing temperature at this pressureProcess temperature, trap discharge temperature, superheat checks
Sensible heat (liquid enthalpy)hfHeat held by the water at boiling pointCondensate heat content, flash calculations, feedwater economics
Latent heat (evaporation)hfgHeat to turn boiling water into dry steamHeating duties — this is the payload your process receives
Total heat (steam enthalpy)hghf + hfg — everything invested since 0 °CBoiler duty, fuel calculations
Specific volumevgVolume of 1 kg of dry steam, m³/kgPipe and valve sizing — steam is bought by mass but flows by volume

Units: engineering tables print kJ/kg; Indian plant practice often talks kcal/kg. Divide kJ by 4.187 — or flip the toggle on the widgets below and skip the arithmetic.

habit one: gauge + ~1 the process temperature 1 kg fills 0.42 m³ — sizes the pipe (C2) bar(a) kg/cm²g Tsat °C hf kcal/kg hfg kcal/kg hg kcal/kg vg m³/kg 4.45 3.5 148 148 507 655 0.42 hf 148 — what the condensate keeps when the steam has done its work (lesson A6 claws it back) hfg 507 — the payload: the heat your process actually receives (size heating duties on this) hg 655 — what the boiler paid since 0 °C (boiler duty and fuel arithmetic) 148 + 507 = 655 — hf + hfg = hg: every row adds up
One row, read fluently — the jacketed vessel's row, the same one the worked example below uses. Note that hf ≈ Tsat when you work in kcal/kg: heating a kilogram of water one degree costs about one kilocalorie, so the water's heat roughly counts its own temperature. And the row always adds up: hf + hfg = hg.

One convention to carry everywhere: hf, hfg and hg are counted from water at 0 °C. When a lesson says the practice plant "invests 574 kcal" in a kilogram of steam, that is hg 664 minus the 90 kcal already sitting in its 90 °C feedwater — a different starting line. Keep the two apart and the accounting stays honest: the 148 kcal a kilogram of vessel condensate carries home is 22% of the 655 the steam brought in, not a share of the 574.

One convention to carry everywhere: hf, hfg and hg are counted from water at 0 °C. When a lesson says the practice plant "invests 574 kcal" in a kilogram of steam, that is hg 664 minus the 90 kcal already sitting in its 90 °C feedwater — a different starting line. Keep the two apart and the accounting stays honest: the 148 kcal a kilogram of vessel condensate carries home is 22% of the 655 the steam brought in, not a share of the 574.

One convention to carry everywhere: hf, hfg and hg are counted from water at 0 °C. When a lesson says the practice plant "invests 574 kcal" in a kilogram of steam, that is hg 664 minus the 90 kcal already sitting in its 90 °C feedwater — a different starting line. Keep the two apart and the accounting stays honest: the 148 kcal a kilogram of vessel condensate carries home is 22% of the 655 the steam brought in, not a share of the 574.

Three readings you will make every month

The numbers — three questions, three columns

1. What temperature is my jacketed vessel really at?

Jacket gauge: 3.5 kg/cm²g → 4.45 bar(a) → table: Tsat ≈ 148 °C

2. How much heat does each kilogram give the vessel? Not hg — the process only receives the latent heat. The sensible heat leaves with the condensate (and lesson A6 shows how to claw it back).

hfg at 4.45 bar(a) ≈ 507 kcal/kg. Duty 1,200 kg/h × 507 ≈ 0.61 million kcal/h into the product.

3. Why is the steam pipe so much fatter than the water pipe? The vg column. At 4.45 bar(a), vg ≈ 0.42 m³/kg:

1,200 kg/h × 0.42 m³/kg ≈ 500 m³/h of vapour — against 1.2 m³/h when the same mass returns as condensate. Four hundred times the volume: that is the whole pipe-sizing problem.

Reading between the rows

Your pressure will land between two printed rows. Interpolate linearly and move on — the error is far smaller than your pressure gauge's. Need 4.2 bar(a), table lists 4.0 and 4.5? Take 40% of the step. For anything finer, the lookup below does the interpolation for you.

The whole table, for the wall

The working range of most Indian process plants — 0.5 to 25 bar(a) — on one screen. Bookmark this page; the row your boiler lives on is worth memorising.

At site
  • Keep a laminated saturated-steam table in the boiler house and one in maintenance. Phones die; laminate does not.
  • Write every pressure with its suffix — g or a — before you open the table. Most table "errors" are the missing atmosphere.
  • Memorise three anchor rows: 1 bar(a) ↔ 100 °C ↔ 539 kcal/kg latent · 5 bar(a) ↔ 152 °C · 10 bar(a) ↔ 180 °C.
  • Sanity-check any calculation against the trend lines: as pressure rises, Tsat and hf rise, hfg and vg fall. A result that breaks the trend is a wrong lookup.
Pin this
  • The table speaks absolute pressure. Gauge + ~1 = table.
  • hfg is what your process receives; hf is what the condensate keeps; hg is what the boiler paid.
  • vg sizes the pipe — steam is bought by mass but flows by volume.
  • Interpolate between rows without guilt.
Steam stories

The numbers in every steam table trace back to a decade of measurement by the French physicist Henri Victor Regnault in the 1840s — commissioned by a government that ran on steam engines and wanted the fluid understood. His pressure–temperature measurements were so careful that when the international IF97 formulation was computed a century and a half later with electronic instruments, his curve was still recognisably right. You are reading one of the oldest continuously-used datasets in engineering.

FAQ

For a heating duty, do I use hg or hfg?

hfg — the latent heat is the payload your process actually receives. The sensible heat hf leaves with the condensate (and is worth recovering — A6 shows how much it holds), while hg is what the boiler paid to make the steam from cold water. Sizing a duty on hg flatters the steam by a quarter and undersizes your steam demand.

My pressure falls between two table rows. What now?

Interpolate linearly and move on. Need 4.2 bar(a) with rows at 4.0 and 4.5? Take 40% of the step between them. The interpolation error is far smaller than what your pressure gauge contributes, so there is no guilt in the straight line — and the lookup widget on this page does it for you anyway.

Why do two steam tables show different numbers for the same pressure?

Almost always one of two unit traps, not physics — the data behind every modern table is the same IAPWS-IF97 formulation. Either one table prints kJ/kg and the other kcal/kg (divide kJ by 4.187), or you entered one at gauge pressure and the other at absolute. Check both suffixes before doubting the table.

What is the specific-volume column actually for?

Pipe and valve sizing. Steam is bought by mass but flows by volume: 1,200 kg/h at 4.45 bar(a) occupies about 500 m³/h as vapour, yet returns as barely 1.2 m³/h of condensate — four hundred times apart. vg is the column that explains why steam pipes dwarf water pipes, and C2 turns it into line sizes.

Which table rows are worth memorising?

Three anchors carry most shop-floor arithmetic: 1 bar(a) ↔ 100 °C ↔ 539 kcal/kg latent · 5 bar(a) ↔ 152 °C · 10 bar(a) ↔ 180 °C — plus the row your own boiler lives on. With those, you can sanity-check any reading against the trends: pressure up means Tsat and hf up, hfg and vg down.

Check yourself

Five quick questions on this lesson. No marks, no records — the score is for you.