Boiler Blowdown Heat Recovery and Efficiency Gain

A boiler concentrates dissolved solids as it makes steam, and blowdown is what holds the concentration at the limit, so the blowdown rate is steam rate / (cycles - 1) -- the same TDS mass balance the blowdown-rate calculator uses, stated in cycles rather than in ppm. A plant running 5 cycles blows down 25% of its steam production; one running 10 blows down 11%. That is why water treatment and blowdown rate are one decision and not two. The heat is the part that goes down the drain. Blowdown leaves at saturation temperature, so 5,000 lb/hr at 150 psig carries about 310 BTU a pound above 60 degF makeup, which is 1.55 MMBTU an hour, and at 80% boiler efficiency, $9 per MMBTU and 8,000 hours that is roughly $140,000 a year of fuel. Recovery has two stages and both are worth taking. Let the blowdown down to a flash tank and about 15% of it flashes to steam that goes straight to the deaerator, displacing live steam; the liquid that remains is still near boiling and goes through a heat exchanger to preheat makeup water. Together they recover most of the stream's energy, with payback measured in months on a plant of any size. But treatment comes first, because it removes the pounds rather than chasing their heat: going from 5 cycles to 10 more than halves the blowdown and is worth tens of thousands a year on its own, before any heat exchanger is bought. The other reason to compute it is that continuous blowdown control is often mis-set -- a plant blowing down on a manual valve rather than on measured conductivity is usually blowing down too much, and this shows what that costs directly. The achievable cycles are set by the boiler water chemistry limits for the pressure and by the makeup water quality, and pushing cycles beyond what the treatment program supports causes scale and carryover, which costs far more than the blowdown saved. Those limits come from the water treatment specialist and the boiler manufacturer, not from an optimization. This does not size the flash tank, the heat exchanger, or the blowdown control valve, and it does not address the separate bottom blowdown that removes sludge and which is not continuous. It does not evaluate discharge temperature limits on blowdown to sewer, which are commonly regulated. Steam boiler operation is a licensed activity in many jurisdictions: the boiler manufacturer, the water treatment program, ASME, and the jurisdiction's boiler inspector govern.

blowdown / steam = feedwater TDS / (boiler TDS - feedwater TDS) = 1 / (cycles - 1); heat in blowdown = blowdown x (saturated liquid enthalpy at boiler pressure - makeup enthalpy); flash fraction = (h_f high - h_f low) / h_fg low; recovered = flash steam x (h_g low - h_makeup) + residual liquid x (h_f low - h_makeup) x exchanger effectiveness.

The continuous surface blowdown mass balance and the flash relation by name -- the same TDS balance the blowdown-rate calculator uses, stated in cycles rather than in ppm, so the two do not disagree about how much blowdown a boiler makes. Saturated water enthalpies are entered from the steam tables. ASME, the boiler manufacturer, the water treatment program, and the jurisdiction's boiler inspector govern.

A mass and energy balance on figures the user enters from the steam tables; no chemistry limit is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: steam_rate_lb_hr, cycles_of_concentration, alt_cycles_of_concentration, blowdown_liquid_enthalpy_btu_lb, flash_liquid_enthalpy_btu_lb, flash_latent_btu_lb, makeup_temp_f, heat_exchanger_effectiveness, boiler_efficiency, fuel_cost_per_mmbtu, hours_per_year, blowdown_lb_hr, blowdown_pct_of_steam, heat_in_blowdown_btuh, flash_steam_lb_hr, recovered_pct, alt_cycles_annual_saving

• Cycles are a chemistry limit not an optimization variablethe water treatment program and the boiler manufacturer
• Surface blowdown only the intermittent bottom blowdown that removes sludge is separateboiler operating practice
• Sewer discharge temperature is regulated and is not evaluated herelocal discharge limits