Boiler Blowdown Rate Calculator
Calculate the required blowdown rate to maintain optimal boiler water quality.
How to Calculate Boiler Blowdown Rate
In steam boiler operations, water is evaporated to produce steam. However, the dissolved solids (TDS) present in the feedwater do not evaporate. Instead, they remain in the boiler water, becoming increasingly concentrated over time. If these solids are not managed, they lead to scale formation, corrosion, and "carryover," where water droplets contaminate the steam.
Blowdown is the process of removing a portion of the concentrated boiler water and replacing it with fresh feedwater to maintain the Total Dissolved Solids (TDS) within acceptable limits.
The Blowdown Formula
To determine the correct amount of water to discharge, we use the mass balance formula. The basic calculation for the blowdown rate as a percentage of steam generation is:
Once you have the percentage, you can calculate the mass flow rate of the blowdown water:
Key Variables Explained
- Feedwater TDS: The concentration of dissolved solids in the water entering the boiler (after treatment/deaeration).
- Max Boiler TDS: The maximum allowable concentration inside the boiler drum, usually specified by the boiler manufacturer or water treatment consultant (e.g., 2500–3500 ppm for firetube boilers).
- Steam Generation Rate: The amount of steam the boiler is currently producing (kg/h or lb/h).
Example Calculation
Suppose you have a boiler producing 10,000 kg/h of steam. Your feedwater TDS is 200 ppm, and your maximum allowable boiler TDS is 3,000 ppm.
- Subtract feedwater TDS from max TDS: 3,000 – 200 = 2,800.
- Divide feedwater TDS by that result: 200 / 2,800 = 0.0714.
- Multiply by 100 for percentage: 7.14%.
- Multiply by steam rate for mass: 10,000 × 0.0714 = 714 kg/h.
In this scenario, you must discharge 714 kg of water every hour to keep the boiler clean and efficient.
Why Monitoring Blowdown is Critical
Excessive blowdown results in significant energy loss because you are dumping hot, pressurized water that has already been heated. Conversely, insufficient blowdown leads to scale buildup on heat exchange surfaces. A 1mm layer of scale can reduce boiler efficiency by up to 5%, leading to higher fuel costs and potential tube failure.