Boiler blowdown.
Recover the heat.

Steady continuous surface blowdown only; dry steam and returned process condensate contain negligible dissolved solids. Treatment-chemical solids, carryover, sludge and intermittent bottom blowdown are excluded. B = S(1-r)Cm/(Cb-Cm); M = S(1-r)+B. B/M = Cm/Cb. Percent makeup is not percent feedwater.

Boiler TDS is a user-approved operating limit, not automatically selected. Historical ABMA values are cited as reported by the National Board (1999); consult current manufacturer guidance and water chemistry, including silica, alkalinity and hardness. The default is an example, not an operating recommendation.

Pressures are gauge; atmospheric pressure is fixed at 1.01325 bar. Saturation properties use a 0.25°C numerical table generated from CoolProp IF97. Liquid enthalpy is approximated by saturated-liquid enthalpy at the same temperature, neglecting the small compressed-liquid pressure correction. Dissolved-solids effects on properties are neglected.

Flash fraction = (hf,boiler - hf,flash)/hfg,flash, assuming adiabatic throttling and equilibrium separation. All generated vapor counts toward flash-vessel flow, including any unused vapor. Diameter shown is only a vapor-disengagement screen at 3 m/s, not a vessel selection or pressure-vessel design.

The residual liquid stays at flash-vessel pressure through the liquid-liquid exchanger; pressure reduction occurs downstream to avoid secondary flashing in the exchanger. The makeup stream first passes through this exchanger, then an indirect flash-steam condenser. Flash condensate is NOT returned to the boiler in this model.

One shell pass and two tube passes: duty follows selected drain temperature; secant heat capacities preserve the enthalpy balance. Area = NTU*Cmin/U using the shell-and-tube effectiveness relation. U is a user-entered fouled value. Final area, tube velocity, fouling, corrosion allowance, pressure drop, traps and materials require vendor design. The flash condenser area is not included in the liquid-liquid HX area.

Only captured flash latent heat is credited, capped by remaining makeup heating demand. Flash-condensate sensible heat, unused vapor and residual liquid heat remain in drain/unused losses. Waste heat is referenced to incoming makeup enthalpy, so the three diagram branches sum to 100%. No condensate water-reuse savings are claimed.

Continuous coincident makeup and blowdown flow is assumed. Annual useful heat replaces fuel at the entered boiler efficiency; efficiency and fuel-price heating-value bases must match. Net savings subtract auxiliary electricity and annual maintenance. Installed cost includes the controller, flash vessel, condenser, exchanger and installation.

The 10-year chart shows cumulative undiscounted cash and discounted cash (NPV), beginning with installed cost at year zero. Fuel price alone escalates; operating hours and other annual costs stay fixed. Simple payback uses year-one net savings. Taxes, financing, incentives, replacements and automatic-controller water/chemical savings are excluded.

This is a preliminary thermal/economic sizing estimate, not an ABMA certification or a stamped mechanical design. Verify actual load coincidence, water chemistry, controls, drain conditions and pressure-vessel requirements before equipment selection.

References: National Board: historical ABMA water limits · DOE Steam Tip Sheet 9 · Spirax Sarco heat recovery guidance · IAPWS-IF97 properties · F-Chart effectiveness–NTU model.