Boiler Blowdown Calculator
Calculate boiler blowdown rate from steam flow and TDS levels, and estimate blowdown flow, feedwater requirements, water losses, and energy losses.
Calculation Results
What Is Boiler Blowdown?
Boiler blowdown is the controlled removal of boiler water to limit the concentration of dissolved and suspended solids. As feedwater is heated and water evaporates into steam, many nonvolatile impurities remain in the boiler water and become more concentrated. If those concentrations rise too far, operating problems can include deposits, foaming, carryover, sludge accumulation, and poorer steam quality.
Blowdown removes a portion of the concentrated boiler water so it can be replaced by lower-solids feedwater. The correct rate is a balance: insufficient blowdown can allow solids to build up, while excessive blowdown wastes hot treated water, treatment chemicals, and energy. [Source: U.S. DOE]
Why Is Boiler Blowdown Necessary?
Blowdown is one part of boiler-water management. It is used to control total dissolved solids and to remove suspended solids or sludge that would otherwise remain in the boiler. Surface or continuous blowdown is commonly associated with dissolved-solids control, while bottom or mud blowdown is used to remove material that settles in lower regions.
- Total dissolved solids
- Suspended solids
- Sludge
- Foaming and carryover risk
- Deposits associated with poor water control
Blowdown alone does not solve every corrosion or chemistry problem. Treatment chemistry, deaeration, condensate return, testing, and manufacturer limits remain important.
How Boiler Blowdown Is Calculated
A useful preliminary method is a steady mass balance. The feedwater entering the boiler is split into steam and blowdown:
If the dissolved solids are assumed to leave primarily with blowdown, the solids balance is:
Combining the relationships gives:
Here B is blowdown flow, S is steam flow, F is feedwater flow, Fw is feedwater TDS and Bw is boiler-water TDS. The boiler-water TDS must be greater than the feedwater TDS for this calculation.
How TDS Affects Blowdown Rate
Higher feedwater TDS means more dissolved solids enter the boiler for a given feedwater flow, so more blowdown may be required to hold boiler-water TDS at the selected limit. Conversely, a higher allowable boiler-water TDS permits greater concentration and can reduce the calculated blowdown requirement.
The allowable TDS cannot be selected from the equation alone. It must reflect boiler type, pressure, load, steam-purity requirements, water-treatment program and manufacturer guidance. Spirax Sarco notes broad ranges for some conventional shell boilers but explicitly describes such figures as guidance and recommends consulting the boilermaker for specific limits. [Source: Spirax Sarco]
Blowdown as a Percentage
Blowdown percentage can be stated on different bases. A percentage of total feedwater is not the same number as a percentage of steam generation. If B is blowdown and F is total feedwater, the feedwater-basis percentage is B/F × 100. If the reference is steam flow S, the percentage is B/S × 100.
This calculator keeps those values separate. In the “Calculate from Blowdown %” mode, the entered percentage is specifically defined as a percentage of total boiler feedwater, so the flow relationship is B = S × b / (1 − b).
Continuous vs Intermittent Blowdown
Continuous or surface blowdown removes a controlled stream of boiler water and is commonly used to manage dissolved-solids concentration. Intermittent or bottom blowdown is typically performed periodically to remove sludge and suspended solids that settle in the boiler. The two functions are related to water-quality control but are not identical.
DOE guidance describes surface blowdown as often continuous and bottom blowdown as a manual or periodic procedure for settled solids. The actual arrangement and frequency depend on the boiler and operating procedures. [Source: U.S. DOE]
How Much Does Boiler Blowdown Cost?
Blowdown can create costs through water loss, water-treatment chemical loss, sewer or disposal charges, and the thermal energy carried out with hot boiler water. Those costs depend on the local water and fuel prices, blowdown temperature, flow, treatment system and disposal arrangement, so a fixed dollar value is not appropriate for every site.
The calculator therefore estimates physical water and energy quantities rather than claiming a universal monetary saving. DOE notes that excessive blowdown wastes energy, water and chemicals, which is why chemistry-based control can have both operating and efficiency benefits.
Boiler Blowdown Heat Recovery
Hot blowdown water contains thermal energy that may be recoverable. Common approaches include a blowdown heat exchanger, a flash tank, and preheating boiler makeup water. The economic case depends on the continuous blowdown rate, temperature, pressure, hours of operation, heat sink, and recovery-system effectiveness.
DOE guidance identifies continuous-blowdown systems as candidates for heat recovery and includes examples at several boiler pressures. Any example threshold should be treated as screening guidance, not a universal design standard. [Source: U.S. DOE]
How to Reduce Boiler Blowdown
- Improve feedwater treatment where appropriate.
- Monitor boiler-water TDS or conductivity consistently.
- Use automatic surface-blowdown control when justified.
- Improve condensate return to reduce makeup-water solids and demand.
- Verify boiler-water chemistry against the treatment program.
- Recover blowdown heat where technically and economically practical.
DOE notes that automatic blowdown control can regulate discharge in response to dissolved-solids concentration and reduce water, chemical and energy losses compared with poorly matched fixed/manual control. [Source: U.S. DOE]
Limitations of This Calculator
This tool is intended for preliminary engineering estimates. Actual boiler blowdown control may require boiler-manufacturer requirements, the water-treatment program, conductivity measurements, operating pressure, steam-purity requirements, boiler type, feedwater chemistry, condensate return, and applicable engineering procedures.
The energy model uses a constant liquid-water specific heat and temperature difference. It does not calculate saturation enthalpy, flashing to a lower pressure, flash-steam fraction, heat-exchanger pinch points, or detailed fuel economics.
Engineering References
2026 Update. This page uses established engineering relationships and currently accessible boiler/steam references. There is no single “2026 boiler blowdown percentage” that applies to every boiler.
Explains why blowdown controls dissolved and suspended solids and why excessive blowdown wastes water, energy and treatment chemicals.
DOE Steam Tip Sheet #9Provides screening guidance and examples for blowdown heat recovery. Example data are not universal design values.
DOE Steam Tip Sheet #10Describes conductivity-related automatic control and how improved control can reduce unnecessary water, chemical and energy losses.
DOE Steam Tip Sheet #23Explains TDS measurement, broad boiler-water TDS guidance, and the engineering approach for determining blowdown from steam generation and TDS.
Controlling TDS in the Boiler WaterSupplementary calculation reference only; not used as the sole authority for boiler-water limits.
Boiler BlowdownFrequently Asked Questions
1. What is a normal boiler blowdown rate?
There is no single blowdown rate that applies to every boiler. The appropriate rate depends on feedwater quality, the allowable boiler-water TDS, boiler type, operating pressure, steam-purity needs, condensate return and the water-treatment program. Published typical ranges can be useful as context, but the operating target should be established from actual chemistry and boiler or water-treatment requirements.
2. How do I calculate boiler blowdown rate?
For a TDS mass-balance estimate, the calculator uses steam flow together with feedwater TDS and the selected boiler-water TDS limit. With S as steam flow, Fw as feedwater TDS and Bw as boiler-water TDS, blowdown flow is B = S × Fw / (Bw − Fw). The calculation assumes the dissolved-solids balance is represented by those concentrations.
3. What TDS should boiler water have?
The allowable boiler-water TDS is not a universal number. It varies with boiler design, pressure, steam loading, steam-purity requirements, alkalinity, treatment chemistry and manufacturer guidance. Spirax Sarco publishes broad guidance for conventional shell boilers, but also states that the boilermaker should be consulted for specific recommendations. Use the value established for the actual boiler and water-treatment program.
4. Is boiler blowdown percentage based on steam flow or feedwater flow?
Both conventions are encountered, and they give different percentages. This calculator labels them separately. In the percentage-input mode, the entered blowdown percentage is explicitly treated as a percentage of total boiler feedwater. Results also show blowdown as a percentage of steam generation so the two bases can be compared without mixing them.
5. What happens if boiler blowdown is too high?
Excessive blowdown sends more hot treated water to disposal than necessary. That can increase makeup-water demand, treatment-chemical use, sewer or disposal volume, and thermal energy loss. The correct response is not simply to minimize blowdown as far as possible; the rate still has to maintain the required boiler-water chemistry and steam quality.
6. What happens if boiler blowdown is too low?
Insufficient blowdown can allow dissolved and suspended solids to concentrate beyond acceptable limits. Depending on boiler conditions, this can contribute to foaming, carryover, deposits, sludge accumulation or water-quality problems. Blowdown settings should therefore be tied to verified boiler-water chemistry, operating conditions and the treatment program rather than selected solely to reduce water loss.
7. How does conductivity relate to boiler blowdown?
Electrical conductivity is commonly used as an indirect indicator of dissolved-solids concentration. Automatic surface-blowdown systems can use a conductivity measurement and controller set point to regulate the amount of boiler water discharged. Conductivity-to-TDS relationships depend on water chemistry and temperature, so controller calibration and the treatment supplier’s procedures remain important.
8. What is the difference between continuous and bottom blowdown?
Continuous or surface blowdown removes a controlled stream of boiler water and is commonly associated with dissolved-solids control near the water surface. Bottom or mud blowdown is typically intermittent and is used to remove sludge and suspended solids that settle in lower parts of the boiler. Their purposes overlap only partially, so one should not automatically be treated as a substitute for the other.
9. Can boiler blowdown heat be recovered?
Yes. Hot blowdown contains useful thermal energy. Depending on pressure, temperature, flow and plant arrangement, heat can be recovered with a blowdown heat exchanger, a flash tank, or by preheating makeup water. The simplified estimate on this page does not model flash-steam production, so detailed recovery design should use steam-table enthalpy and actual downstream pressure conditions.
10. Does boiler pressure affect blowdown?
Pressure can affect allowable boiler-water chemistry, saturation temperature, flash-steam behavior and the energy carried by blowdown. The TDS mass-balance equation itself uses steam flow and solids concentrations, but pressure can still influence the correct TDS target and the detailed heat-recovery calculation. Follow the boiler manufacturer and water-treatment guidance for the actual operating pressure.
11. Can this calculator be used for industrial steam boilers?
It is intended for preliminary industrial steam-boiler estimates where steam flow, TDS values or a defined blowdown percentage are known. It can help quantify flow, water loss and a simplified sensible-heat loss. Final blowdown control should still be based on boiler chemistry, treatment procedures, manufacturer limits, instrumentation and the actual operating system.
12. Can I use this calculator for a power plant boiler?
The mass-balance relationship can be useful for conceptual checks, but power-plant boiler chemistry and blowdown systems can require much more detailed treatment, purity and cycle-chemistry analysis. Do not use a simple TDS target or this preliminary energy model as a substitute for plant chemistry procedures, OEM limits, heat-and-mass balances, flash calculations or applicable engineering standards.
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Engineering Disclaimer
This calculator provides preliminary engineering estimates and does not determine the correct boiler-water chemistry or operating blowdown setting for a specific boiler. Verify boiler manufacturer requirements, water-treatment procedures, actual conductivity/TDS measurements, operating conditions, applicable codes, and heat-recovery design before procurement, installation, control changes, or operation.