About Seamens Enterprises: Seamens Enterprises is one of Pakistan's earliest adopters of German chemical technology — using imported German raw materials to formulate its own range of organic-based industrial chemical solutions since 1997.
Blowdown rate as a percentage of feed water is calculated as feed water TDS divided by the difference between maximum allowable boiler TDS and feed water TDS, multiplied by 100. Most Pakistani mills run blowdown by rule of thumb rather than this calculation — leaving fuel savings on the table that a five-minute calculation would capture.
How Do I Calculate Boiler Blowdown Rate?
Blowdown rate as a percentage of feed water is calculated as feed water TDS divided by the difference between maximum allowable boiler TDS and feed water TDS, multiplied by 100. A higher feed water TDS relative to the boiler's maximum limit means a higher required blowdown percentage is needed to keep dissolved solids in check.
Formula: Blowdown % = [Feed Water TDS ÷ (Max Boiler TDS − Feed Water TDS)] × 100
Worked Example: Lahore Groundwater Feed
| Step | Value | Note |
|---|---|---|
| 1. Feed water TDS | 1,200 ppm | Mid-range Lahore groundwater |
| 2. Max boiler TDS | 3,000 ppm | Typical manufacturer limit, low-pressure boiler |
| 3. Difference | 3,000 − 1,200 = 1,800 ppm | Max minus feed |
| 4. Blowdown % | 1,200 ÷ 1,800 × 100 = 66.7%... adjusted | See note below |
Note: real-world blowdown percentages are typically 5–10% because feed water is usually pre-treated (softened/RO) to reduce TDS well below raw groundwater levels before entering the boiler — the formula above uses the boiler's actual feed water TDS after any pre-treatment, not raw borewell TDS. This is why confirming your actual feed water TDS after treatment, not assuming raw groundwater figures, is essential before applying the formula.
What TDS Level Requires Blowdown?
The maximum allowable boiler water TDS depends on operating pressure and boiler design, with manufacturers typically specifying a limit that, if exceeded, risks carryover and scale formation. Blowdown should be managed continuously or on a regular schedule to keep boiler water TDS below that specified limit rather than waiting until it's exceeded and correcting reactively.
How Much Fuel Does Excess Blowdown Waste?
Excess blowdown wastes fuel because every litre of blown-down water carries away heat energy that was used to raise it to boiler temperature — energy that then has to be replaced by heating fresh make-up water from a colder starting point. Reducing blowdown to the minimum needed for safe TDS control, rather than an oversized safety margin "just in case," directly reduces this energy loss and lowers furnace oil or gas consumption.
Common inefficiency: Many mills set blowdown valves once and never revisit them even after switching water sources or upgrading treatment — this alone often wastes more fuel than any other single boiler-room inefficiency.
How Often Should Blowdown Be Done?
Blowdown frequency should be set from actual boiler water TDS testing rather than a fixed schedule, since feed water quality, boiler load and treatment programme all affect how fast TDS concentrates. Many Pakistani mills benefit from a combination of continuous low-rate blowdown plus periodic bottom blowdown to remove settled sludge that continuous blowdown alone doesn't clear.
- Test boiler water TDS regularly — daily or per-shift, not just monthly
- Confirm actual feed water TDS post-treatment — not raw borewell figures
- Combine continuous + bottom blowdown — for full sludge and dissolved-solids control
- Recalculate after any water source or treatment change — a one-time setting goes stale fast
Frequently Asked Questions
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