How to use the waste heat recovery savings calculator
Boiler stacks, ovens, dryers and paint booths commonly exhaust air at 300 to 750°F straight to atmosphere. Routing that stream through an air-to-air or air-to-water exchanger to preheat combustion air, make-up air or boiler feedwater cuts purchased fuel for the same production output. This tool converts exhaust flow, temperature difference and exchanger effectiveness into recovered BTU/hr, then applies heater efficiency and fuel heating value to give annual fuel and dollar savings.
Sensible heat recovery uses the standard air constant: BTU/hr = 1.08 x CFM x effectiveness x temperature difference, where 1.08 comes from 60 min/hr x 0.075 lb/ft3 x 0.24 BTU/lb-°F at standard conditions. Fuel heating values applied are 100,000 BTU per therm of natural gas, 138,500 BTU/gal for No. 2 oil, 91,500 BTU/gal for propane and 3,412 BTU/kWh for electricity. Flue gas density and specific heat differ slightly from standard air, so treat the result as a close engineering estimate.
Detailed design still has to address acid dew point corrosion at low stack temperatures, fouling of heat transfer surfaces, added fan static pressure and the power it costs, condensate handling and physical space for the exchanger. Confirm the numbers with a measured stack analysis and an exchanger manufacturer selection before committing capital.
Frequently asked questions
Sulfur-bearing fuels are usually kept above about 300 to 340°F to stay clear of acid dew point corrosion. Natural gas allows lower stack temperatures but then needs condensate drainage and corrosion-resistant materials.
Yes. This tool counts sensible heat only. A condensing exchanger that recovers water vapor latent heat delivers more than shown, so size that case from manufacturer performance data.