"How much steam does it need?" is the first question of every sizing job — the line, the valve, the trap, even the boiler all inherit their size from the answer. The physics is one line of arithmetic. The craft is knowing which load you are calculating, because every heated process has at least two: the warm-up load and the running load, and they can differ by a factor of five.
The one-line method
Heat something and the duty is mass × specific heat × temperature rise: Q = m · cp · ΔT, in kcal when m is kg and ΔT is °C. Steam supplies that duty by condensing, delivering its latent heat hfg per kilogram (lesson A6). So:
steam (kg/h) = Q per hour ÷ hfg at the steam pressure
That is the entire method. Steam demand is condensing rate — the kilograms per hour that turn back into water inside the equipment. Everything downstream (trap sizing, condensate lines) is built on this same number.
Worked: the practice plant's jacketed vessel
A 4,000-litre batch of water-like product (cp = 1) must go from 30 °C to 90 °C in 45 minutes. Steam in the jacket: 3.5 kg/cm²g, hfg = 507 kcal/kg.
Product: 4,000 × 1 × 60 = 240,000 kcal, in 0.75 h → 320,000 kcal/h.
The vessel itself: 2,500 kg of steel (cp ≈ 0.115) warming ~90 °C alongside
the batch ≈ 26,000 kcal → 35,000 kcal/h. Cold metal is a real customer — never skip it.
Losses: jacket surface to the room, take 10% of the product duty →
32,000 kcal/h.
Total warm-up duty ≈ 387,000 kcal/h → 387,000 ÷ 507 ≈ 760 kg/h of steam.
And once the batch reaches 90 °C? Holding it there needs only the losses — perhaps 60–80 kg/h. The vessel that demanded 760 kg/h for forty-five minutes idles at a tenth of it for the rest of the shift. That is the shape in the figure below, and it is the shape of almost every batch process in India.
Worked: the dryer, a running load
Continuous processes skip the drama: one steady load. The practice plant's dryer heats 65,000 kg/h of air (cp ≈ 0.24) from 30 °C to 140 °C in its steam radiator bank at 7 kg/cm²g (hfg ≈ 489):
Q = 65,000 × 0.24 × 110 ≈ 1,716,000 kcal/h → 1,716,000 ÷ 489 ≈ 3,500 kg/h of steam — the figure on the dryer's nameplate, and the load its PRS, control valve and trap set were sized around.
Why the datasheet says more
The vessel we just calculated at 760 kg/h carries 1,200 kg/h on its datasheet. That is not error — it is margin with reasons: a winter cold start from 15 °C, a fouled jacket a year after cleaning, the ambition to cut batch time next season, and the control valve's need to modulate rather than run saturated wide-open. Sizing culture: calculate honestly, then state the margin and its reasons — a line sized on bare arithmetic has no future, and one sized on fear wastes capital and control quality alike. What you must not do is let the margins compound silently down the chain: valve margin on top of load margin on top of boiler margin ends in oversized everything (lesson B3 showed where that road goes).
Run your own numbers — batch or continuous:
- The truth-teller for an existing process: collect and weigh the condensate for a timed run. Kilograms condensed per hour is the steam demand, no theory required.
- Calculating a batch duty? Add the vessel metal and the losses — the product alone understates warm-up by 15–25% on typical jacketed vessels.
- Ask every equipment supplier for the duty in kcal/h and their assumed steam pressure. A "500 kg/h" heater means nothing without the pressure behind it.
- Peak vs running matters most for the trap: it must pass the warm-up flood at low start-up pressure differential, not just the running dribble (lesson D5).
- Steam demand = duty ÷ hfg at the working pressure. Duty = m·cp·ΔT per hour, plus metal, plus losses.
- Every batch process has two loads: warm-up (design case) and running (control case). They differ by 3–10×.
- Steam demand is condensing rate — the same kg/h your traps and condensate lines must handle.
- Margins need reasons, stated once — not compounded silently at every link in the chain.
James Watt sized the world's first steam loads with a horse. To sell engines against animal power he measured a brewery horse turning a mill and settled on 33,000 foot-pounds per minute — one horsepower — so a customer could ask "how many horses does my mill need?" and buy that engine. The unit was marketing arithmetic in 1783; it survives on every motor nameplate in your plant. Steam sizing has always been about answering the customer's question in the customer's units.