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WHRB Payback Estimator

Hot gas that would go up the stack, turned into steam, then megawatts or fuel saved, rupees and tonnes of CO₂. A pinch-point heat balance with IAPWS-IF97 steam properties. Change any number and everything updates.

Start from:

Hot gas

Gas flow kg/h

Gas inlet temperature °C

Gas specific heat kJ/kg·K

Lowest stack temperature °C, acid dew point

Casing heat loss %

Steam side

Steam pressure bar(g)

Superheated steam

Steam temperature °C

Feedwater temperature °C

Pinch point °C

Approach temperature °C

Blowdown % of steam

Use of the steam

Condenser pressure bar(a)

Turbine isentropic efficiency %

Generator efficiency %

Auxiliary power % of gross

Value of power ₹/kWh

Grid emission factor tCO₂/MWh

Money

Operating hours per year

Project cost ₹ crore

Operation & maintenance % of cost / year

Life for IRR years

The project cost is a placeholder, not a price. Enter the cost you were quoted.

-Steam, t/h

-Net power, MW

-Saving, ₹ crore / year

-Simple payback, years

-IRR

-CO₂ avoided, t / year

How it works

Pinch. Gas leaving the evaporator can be no colder than saturation + pinch: T_gas,pinch = T_sat + ΔT_pinch. Heat above that point makes the steam: m_s = m_g·c_p·(T_in − T_gas,pinch)·(1 − loss) / [(h_steam − h_eco,out) + b·(h_f − h_eco,out)].
Approach. The economiser leaves the water a little below saturation (T_sat − ΔT_approach) so it does not steam. Its duty sets the stack: T_stack = T_gas,pinch − Q_eco / (m_g·c_p).
Cold end. If that stack would fall below the acid dew point limit, or within 15 °C of the feedwater, the cold end decides instead: steam is cut until the stack sits at the limit, and the real pinch grows.
Steam properties come from IAPWS-IF97 (regions 1, 2 and 4), checked against the official verification values.
Power: isentropic expansion to the condenser, times turbine and generator efficiency, minus auxiliaries. Fuel: steam heat ÷ old boiler efficiency ÷ GCV.
Money: saving − O&M = yearly net; payback = cost ÷ net; IRR assumes the same net every year. CO₂: grid factor (power) or IPCC default fuel factors (fuel).

Not included: part load, gas composition (cp is held constant), fouling, pressure drop, fan power, taxes and financing. Good for a first conversation; a real offer needs a design.