Boiler Blowdown Frequency: How Often Should You Blow Down?
Blowdown controls TDS, removes sludge, and prevents scale — but too much wastes energy and too little risks carryover. Learn the right blowdown frequency for your boiler and how to optimise it.
Types of Blowdown
- Surface (continuous) blowdown: Removes dissolved solids from the water surface where they concentrate. Controlled automatically by a conductivity sensor that opens a valve when TDS exceeds the setpoint. This is the primary TDS control mechanism.
- Bottom (intermittent) blowdown: Removes settled sludge from the mud drum. Performed manually, typically once per shift. Opens a large valve at the bottom of the boiler for 5–10 seconds to flush accumulated sediment.
Bottom Blowdown Frequency Guidelines
| Boiler Type | Frequency | Duration |
|---|---|---|
| Low pressure (under 4 bar) | Once per shift (8 hrs) | 5–10 seconds per valve |
| Medium pressure (4–15 bar) | Once per shift (8 hrs) | 3–5 seconds per valve |
| High pressure (above 15 bar) | Twice per shift (4 hrs) | 3–5 seconds per valve |
| High suspended solids in feedwater | Every 4 hours | 5–10 seconds |
| Clean feedwater (RO/demin) | Once per day | 3–5 seconds |
These are starting points — adjust based on actual water quality test results.
How to Determine the Right Frequency
The correct blowdown frequency depends on the rate at which dissolved and suspended solids accumulate, which is determined by:
- Feedwater quality: If the feedwater has high TDS or hardness, more blowdown is needed to prevent concentration. Test feedwater regularly.
- Boiler operating pressure: Higher pressure boilers have lower allowable TDS limits, requiring more frequent blowdown. Check the manufacturer's TDS limit for your operating pressure.
- Condensate return rate: High condensate return (80%+) dilutes the feedwater, reducing the blowdown needed. Low return means more makeup water and more blowdown.
- Actual TDS measurement: The ultimate guide. Test boiler water TDS every shift. If it's consistently at the limit, increase blowdown. If well below, reduce it to save energy.
Calculating Blowdown Rate
The required continuous blowdown rate is calculated from the feedwater and boiler water TDS:
Example: Feedwater TDS = 200 ppm, boiler limit = 3,000 ppm:
If the boiler generates 10,000 kg/hr of steam, the blowdown rate is approximately 710 kg/hr. This hot water contains significant energy — consider blowdown heat recovery.
Energy Cost of Excess Blowdown
Blowdown removes hot boiler water at the saturation temperature. Every kilogram of blowdown is a kilogram of water the boiler must reheat from feedwater temperature. At 10 bar(g), blowdown water contains approximately 780 kJ/kg of energy.
- A 10% blowdown rate on a 10,000 kg/hr boiler wastes 1,000 kg/hr of hot water — that's 780,000 kJ/hr of energy
- Over 8,000 hours/year at $12/GJ, that's $75,000/year in wasted fuel
- A blowdown heat recovery system (flash tank + heat exchanger) can recover 50–80% of this energy, with a 1–2 year payback
- Automatic TDS-controlled blowdown reduces blowdown to the minimum required — typically saving 20–40% compared to manual blowdown
Warning
Never reduce blowdown below the level required to control TDS — the result is foaming, carryover, scale formation, and potential boiler damage. Optimise blowdown, don't minimise it blindly.