Working out boiler feedwater, blowdown and evaporation
A boiler always takes in more water than it sends out as steam. Steam leaves as pure water while dissolved solids stay behind and concentrate, so part of the boiler water has to be dumped as blowdown to hold the concentration under its limit. Required blowdown is steam rate × feedwater TDS ÷ (allowable boiler TDS − feedwater TDS), and feedwater is simply the steam rate plus that blowdown.
For 2,200 lb/hr of steam with 200 ppm feedwater TDS and a 3,000 ppm boiler limit, blowdown works out to about 157 lb/hr, or 7.1% of the steam rate, so feedwater is roughly 2,357 lb/hr — about 283 gallons an hour. Returning more condensate lowers the feedwater TDS, which shrinks the blowdown, and that is exactly why condensate recovery cuts fuel and water treatment costs at the same time.
Equivalent evaporation puts boilers at different pressures on one scale by converting output to the heat needed to turn 212°F water into 212°F steam, a latent heat of 970.3 BTU/lb. Boiler horsepower follows directly from it, since one boiler horsepower equals 34.5 lb/hr of equivalent evaporation. Allowable TDS varies with boiler type and water treatment program, so confirm the limit with your treatment vendor and have a mechanical engineer or the inspection authority review inspected equipment.
Frequently Asked Questions
Yes. Blowdown water has already been heated to boiler pressure, so dumping it throws that heat away. Condensate recovery and a continuous blowdown heat recovery unit are the usual fixes.
Actual output is what the boiler makes at today's conditions, while equivalent evaporation restates it on the 212°F basis. Cold feedwater or higher steam pressure pushes equivalent evaporation above the actual figure.