Lesson E6 chose your heat carrier by fit — temperature reach, control, hygiene, pressure. This lesson prices the running of it, because sooner or later a comparison sheet lands on your desk proving that somebody's favourite carrier is the cheapest. The arithmetic below is the honest version of that sheet: one duty, three carriers, every assumption in the open — including the one assumption that quietly decides the winner almost every time.
Three ways to move the same heat
Take a round duty: 20 lakh kcal/h, 6,000 hours a year. Steam moves it as latent heat — roughly 500 kcal in every kilogram condensed at the user (3.5 kg/cm²g), so the whole duty rides in about 4 t/h of steam from a 10.5 kg/cm²g boiler. A sensible carrier moves only m·cp·ΔT per pass: thermic fluid at cp ≈ 0.55 and a 30 °C loop ΔT carries 16.5 kcal per kilogram-pass, so the same duty needs about 121 t/h in circulation — thirty times the flow. Pressurised hot water (cp 1.0, ΔT ≈ 24 °C) sits between, near 83 t/h.
That single structural fact drives the cost table. Latent transport buys tiny flow at the price of pressure and drainage discipline (the whole of Faculty D). Sensible transport buys low pressure at the price of a large pump running every hour, and — for oil — a fluid that costs real money per kilogram (lesson E3).
The numbers — one duty, four honest columns
Rates used, openly: husk 3,200 kcal/kg GCV at ₹2,500/t · electricity ₹8/kWh · make-up water + treatment ₹50/t · pump-to-wire efficiency 0.6 × 0.8. Steam appears twice — once with all condensate thrown away, once with lesson D6's 85% return and a 90 °C feed tank.
| ₹ lakh / year | Steam — 0% return | Steam — 85% return | Thermic fluid | Hot water (PHW) |
|---|---|---|---|---|
| Fuel | 158.5 | 143.5 | 130.2 | 125.0 |
| Pumping power | 1.1 | 1.1 | 13.2 | 9.1 |
| Medium make-up | 12.0 | 1.8 | 1.4 | ≈ 0 |
| Total | 171.6 | 146.4 | 144.8 | 134.1 |
Why the thermic heater burns more per useful kilocalorie: its flue gas cannot be cooled below metal that must hold 250–300 °C oil, so the stack leaves hotter — a 70–74% band against the steam boiler's 75% (both on GCV, both husk-fired). The oil itself: a ~5,000 kg holdup making up 5–10% a year at a public band of ₹300–500/kg (check current prices) — ₹1–2 lakh, real but small when the fluid is looked after (lesson E5).
The assumption that decides the winner
Look again at the two steam bars. Same boiler, same fuel, same duty — ₹25 lakh a year apart, and the only thing that changed is what happens to the condensate. A comparison that assumes steam's condensate goes to drain has already chosen its winner before the arithmetic starts; it is the oldest trick in the carrier-comparison trade, and it is usually not even malicious — the author simply copied the plant as found.
Priced honestly, the finish is close: steam at 85% return and thermic fluid land within about one percent of each other — well inside the error bars of η, tariffs and pump condition. Hot water is cheapest on this page, but this duty was chosen inside its ~130 °C reach (lesson E1); raise the process temperature and PHW leaves the table, then steam and oil settle it on reach, hygiene and control — which is E6's matrix, not this ledger. Running cost ranks carriers; it rarely vetoes them.
What this page deliberately leaves out: capital, maintenance, IBR compliance effort, operator skill. And its real everyday use is not switching carriers at all — it is pricing what a condensate-recovery project is worth on the steam system you already run (lessons D6 and F1).
- Before believing any carrier sheet, meter the condensate return: make-up water meter ÷ steam meter gives the honest percentage (lessons D8, F8).
- Read real efficiency from the stack, not the brochure — a ₹15,000 flue-gas survey (lesson B4) beats every table, this one included.
- Read the circulation pump's actual kW off the panel; sensible-carrier comparisons stand or fall on it.
- Replace every teaching rate here with today's numbers — fuel, power, water — before showing the sheet to anyone.
- Check the duty's top temperature first (E6): economics only ranks carriers that can actually do the job.
- Latent moves 20 lakh kcal/h in ~4 t/h; a 30 °C-ΔT oil loop needs ~121 t/h — flow, and therefore pumping, is where sensible carriers pay.
- The carrier table is a condensate-return question in disguise: 0% vs 85% return swings steam by ≈ ₹25 lakh/yr on this one duty.
- Thermic fluid's case is 250–300 °C at near-zero pressure (E3), not running cost; PHW's is simplicity inside its ~130 °C reach (E1).
- All three burn the same fuel through similar efficiency — no carrier conjures free heat; they differ in losses, electricity and fluid.
- Teaching rates, stated openly: husk ₹2,500/t · power ₹8/kWh · water ₹50/t — rerun the page with yours.
In one tender season a plant can receive three comparison sheets proving three different carriers cheapest — each internally flawless. The steam vendor's sheet returns every kilogram of condensate; the oil vendor's sends it all to drain; the hot-water vendor's picks a duty two degrees under its ceiling. None of the arithmetic is wrong. The lesson old hands carry: in any carrier comparison, ignore the totals row first and read the assumptions rows — the decision was made there, usually in the line about condensate.
FAQ
Which heat carrier is cheapest to run?
On a low-temperature duty with condensate thrown away: the closed sensible loops. With real condensate recovery the fuel gap nearly closes and the finish is inside the error bars. Above the ~130 °C hot-water ceiling only steam and oil remain, and above ~180–200 °C process temperature the question usually answers itself on reach, not rupees. Treat any table — including this one — as a starting point; meters decide.
Why does steam need thirty times less flow than thermic fluid?
Each kilogram of condensing steam surrenders about 500 kcal of latent heat; each kilogram-pass of 30 °C-ΔT oil carries only cp × ΔT ≈ 16.5 kcal. The ratio is the flow ratio. Steam's price for that compactness is pressure and the whole drainage discipline of Faculty D; oil's price is a large pump that never stops.
Is thermic fluid expensive because the oil costs ₹300–500 a kilogram?
Less than feared: make-up on a healthy system is 5–10% of holdup a year — ₹1–2 lakh class on this duty, smaller than its pumping bill. The oil turns expensive only when overheating degrades it wholesale (lessons E3 and E5). The recurring costs that matter are heater efficiency and electricity.
We already run steam. Should this table make us consider switching?
Rarely — switching carriers is a capital project with its own economics. The table's daily use is different: the gap between the two steam columns prices what a condensate-recovery upgrade is worth on the plant you already have, which is usually the better project (D6, F1).
What does this comparison deliberately leave out?
Capital cost, maintenance, IBR compliance effort and operator skill — this page is running cost only: fuel, pumping power and medium make-up. E6 carries the full decision matrix; use this ledger inside it, not instead of it.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.