Continuous Blowdown vs Bottom Blowdown
Blowdown controls dissolved solids in boiler water. Learn the difference between continuous (surface) blowdown and intermittent (bottom) blowdown, and when to use each.
Surface (Continuous) Blowdown
Surface blowdown removes water from just below the surface of the boiler water, where dissolved solids concentrate as steam is produced. By continuously bleeding a small amount of water, TDS (total dissolved solids) is maintained at the target level.
Surface blowdown is typically automated with a conductivity-controlled valve that opens when TDS exceeds the setpoint and closes when it returns to the target. This is the most efficient method of TDS control because it removes the most concentrated water with minimal heat loss.
- Controls: TDS (dissolved solids concentration).
- Frequency: Continuous or intermittent based on TDS measurement.
- Heat recovery: Surface blowdown is hot and can be passed through a heat exchanger to preheat feedwater, recovering 60 to 80% of the heat.
Bottom (Intermittent) Blowdown
Bottom blowdown removes water from the lowest point in the boiler, where sludge and sediment settle. This is done manually by opening a quick-opening valve at the bottom of the boiler for a few seconds, discharging accumulated sludge.
Bottom blowdown does not effectively control TDS because the water at the bottom of the boiler is not representative of the overall dissolved solids concentration. Its purpose is to remove settled solids that surface blowdown cannot reach.
- Controls: Sludge and sediment accumulation.
- Frequency: Typically once per shift or daily, depending on water quality.
- Duration: Short bursts (5 to 10 seconds). Long blowdown wastes energy and can disrupt boiler circulation.
Key Differences
| Feature | Surface Blowdown | Bottom Blowdown |
|---|---|---|
| Location | Near water surface | Bottom of boiler |
| What it removes | Dissolved solids (TDS) | Settled sludge and sediment |
| Control method | Conductivity controller | Manual, scheduled |
| Frequency | Continuous or automatic | Once per shift or daily |
| Volume per event | Small, continuous | Short burst, larger flow |
| Heat recovery | Yes, via heat exchanger | Difficult (intermittent, high flow) |
Best Practice: Use Both
A well-operated boiler uses both methods. Continuous surface blowdown with conductivity control and heat recovery manages TDS efficiently. Regular bottom blowdown removes accumulated sludge that surface blowdown cannot reach.
Bottom blowdown should be performed with a quick-opening valve, opened fully for 5 to 10 seconds, then closed. The rapid flow stirs up and evacuates sludge. Slow or partial opening is ineffective and wastes water and energy.
Never open bottom blowdown on a boiler with low water level
Opening the bottom blowdown valve when water level is low can expose boiler tubes to the furnace heat without water cooling, causing immediate overheating and tube failure. Always verify normal water level before blowing down.
Tip: If you are doing bottom blowdown more than once per shift, your feedwater treatment may be inadequate. Have a water treatment specialist review your chemical dosing and feedwater quality.