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Calculate boiler surface blowdown rate to control dissolved solids, prevent scale formation and steam carryover. Includes cycles of concentration formula, TDS limits by operating pressure, and blowdown heat recovery economics.

Impurity Purge Balance

Thermodynamic Evaporation Load

🚰 ENERGY RECOVERY BEST-PRACTICE: If your continuous bottom blowdown purge rate spikes above 5% of physical steam generation, you must mandate the purchase of a high-efficiency 'Flash Tank' or spiral heat exchanger grid. Plumb the brutally hot toxic drain water back through a heat exchanger to violently pre-heat the incoming city feed-water before you finalize dumping it into the cold sewer.

Continuous Purge Flow

526.3 lbs/hr
Required mass blowdown rate.

Mass Blowdown Ratio

5.26 %
Dump vs Vapor output fraction.
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What is Boiler Blowdown: Cycles of Concentration, TDS Limits & Heat Recovery?

Boiler blowdown is the controlled removal of concentrated boiler water to limit the buildup of dissolved and suspended solids. As water evaporates into steam, all non-volatile dissolved minerals (calcium, magnesium, silica, chlorides) remain behind in the boiler drum, concentrating with each evaporation cycle. Left unchecked, excessive Total Dissolved Solids (TDS) causes two serious failure modes: (1) Scale formation — mineral deposits on heat transfer surfaces that reduce efficiency and cause overheating of tube walls, and (2) Carryover — water droplets entrained in steam, carrying dissolved solids into steam distribution systems that foul control valves, strainers, and process equipment. Surface blowdown removes water from the drum waterline (where dissolved solids concentrate) at a controlled rate sufficient to hold TDS below the manufacturer’s maximum for the operating pressure.

Mathematical Foundation

Blowdown Rate (as % of Steam Generation)
Blowdown%=TDSmakeupTDSmax−TDSmakeup×100\text{Blowdown\%} = \frac{\text{TDS}_{\text{makeup}}}{\text{TDS}_{\text{max}} - \text{TDS}_{\text{makeup}}} \times 100
TDSmakeup\text{TDS}_{\text{makeup}}= Total Dissolved Solids of incoming makeup water in ppm (mg/L). Obtain from a water test report. Untreated municipal water typically 150–500 ppm. Softened water retains TDS (only hardness removed). RO-treated water: 10–50 ppm. Well water: highly variable, 50–2,000+ ppm.
TDSmax\text{TDS}_{\text{max}}= Maximum allowable TDS inside the boiler drum in ppm. Set by the boiler manufacturer based on operating pressure. Higher-pressure boilers require lower TDS maxima to prevent steam quality degradation and tube overheating. Typical values: 15 psi (hot water): 3,500 ppm; 150 psi steam: 3,000 ppm; 300 psi: 2,500 ppm; 600 psi: 1,500 ppm; 900 psi: 750 ppm.
Cycles of Concentration (CoC)
CoC=TDSboilerTDSmakeup=TDSmaxTDSmakeup\text{CoC} = \frac{\text{TDS}_{\text{boiler}}}{\text{TDS}_{\text{makeup}}} = \frac{\text{TDS}_{\text{max}}}{\text{TDS}_{\text{makeup}}}
CoC\text{CoC}= Cycles of Concentration: the ratio of boiler water TDS to makeup water TDS at steady state. A CoC of 20 means the boiler water is 20× more concentrated than makeup. CoC = 1 / (Blowdown fraction). If blowdown is 5% (0.05), CoC = 1/0.05 = 20. Higher CoC = less water and energy wasted on blowdown = better efficiency.
Blowdown flow (lbs/hr)\text{Blowdown flow (lbs/hr)}= Physical blowdown flow = Steam generation rate (lbs/hr) × (Blowdown% / 100). Example: 10,000 lbs/hr steam, 5.26% blowdown = 526 lbs/hr drain flow.

Laws & Principles

  • TDS limits are pressure-dependent (ABMA/ASME BG-112 guidelines): Low-pressure hot water (≤30 psi): 3,500 ppm max. Low-pressure steam (15 psi): 3,000–3,500 ppm. Medium-pressure (150–300 psi): 2,500–3,000 ppm. High-pressure (300–600 psi): 1,500–2,500 ppm. Very high-pressure (600–1,200 psi): 750–1,500 ppm. Supercritical (>3,200 psi): <0.1 ppm (near-pure demineralized water required). These limits prevent steam quality degradation and tube wall overheating from localized scale deposits.
  • Heat recovery mandate at >5% blowdown: If calculated blowdown exceeds 5% of steam generation, the blowdown stream contains significant enthalpy that is economically recoverable. A blowdown heat recovery unit (flash tank + heat exchanger) captures low-pressure flash steam from the blowdown and transfers heat to incoming cold makeup water. At 150 psi, boiler blowdown water contains approximately 330 BTU/lb of sensible heat. A 500 lb/hr blowdown stream = 165,000 BTU/hr wasted heat. A heat recovery system can recapture 50–80% of this, yielding payback periods of 1–3 years for industrial systems.
  • Silica limits are more restrictive than TDS and govern at high pressures: Silica (SiO₂) volatilizes into steam above approximately 400 psi and deposits on turbine blades. ASME guidelines limit dissolved silica to 1–2 ppm in steam above 600 psi, regardless of TDS. Below 100 psi, silica limit is typically 125–150 ppm in boiler water. If raw water silica is high (>20 ppm), blowdown rate must be set by the silica limit first, then TDS checked as a secondary constraint.
  • Intermittent (manual) vs continuous blowdown: Continuous blowdown removes a small, constant flow from the surface and is used for TDS/dissolved solids control. Intermittent (bottom) blowdown is a short, manual purge from the lowest drum point to remove settled suspended solids, sludge, and scale particles. Most boiler systems require both: continuous surface blowdown for TDS control + weekly/monthly bottom blowdown for sludge removal. This calculator addresses surface (continuous) blowdown rate only.

Step-by-Step Example Walkthrough

" A commercial steam boiler generates 10,000 lbs/hr of steam at 150 psi. Untreated city makeup water tests at 150 ppm TDS. Manufacturer maximum drum TDS is 3,000 ppm. "

  1. Blowdown% = TDS_makeup / (TDS_max − TDS_makeup) × 100
  2. = 150 / (3,000 − 150) × 100 = 150 / 2,850 × 100 = 5.26%
  3. Physical blowdown flow = 10,000 × 0.0526 = 526 lbs/hr
  4. Cycles of Concentration = 3,000 / 150 = 20
  5. Blowdown heat content at 150 psi ≈ 330 BTU/lb
  6. Wasted heat = 526 × 330 = 173,580 BTU/hr
Final Result: Blowdown rate = 5.26%, draining 526 lbs/hr. Since blowdown exceeds 5%, install a blowdown heat recovery system. At 173,580 BTU/hr waste heat and a flash tank recovery efficiency of ~60%, approximately 104,000 BTU/hr is recoverable — equivalent to 1.04 MMBtu/hr saved. At natural gas $0.80/therm, this represents ~$8,000+/year in fuel savings.

Quick Answer: How do you calculate boiler blowdown rate?

Blowdown% = TDSmakeup / (TDSmax − TDSmakeup) × 100. Then: Physical blowdown (lbs/hr) = Steam rate (lbs/hr) × (Blowdown% / 100). Example: 10,000 lbs/hr steam, makeup water 150 ppm TDS, drum max 3,000 ppm: Blowdown% = 150 / (3,000 − 150) × 100 = 5.26%. Physical flow = 10,000 × 0.0526 = 526 lbs/hr. Cycles of Concentration = 3,000 / 150 = 20×. Since blowdown >5%, install blowdown heat recovery.

Boiler TDS Limits by Operating Pressure (ABMA/ASME Guidelines)

Maximum allowable TDS inside the boiler drum decreases as operating pressure increases. High-pressure boilers produce higher-quality steam and require lower mineral concentrations to prevent carryover and tube overheating from scale deposits. Always confirm manufacturer-specific limits first.

Operating Pressure Max TDS (ppm) Silica Limit (ppm) Typical Application
≤15 psi3,000–3,500125–150Low-pressure heating steam, process steam (small commercial)
15–150 psi2,500–3,00090–125Industrial process steam, commercial heating plants
150–300 psi1,500–2,50040–90Industrial steam, large institutional buildings
300–600 psi750–1,5008–40Power generation, large industrial plants
600–1,200 psi150–7501–8Utility power plants, high-pressure industrial
>1,200 psi<50<1Supercritical utility boilers. Requires demineralized water (DI/RO). Zero suspended solids.
Source: ABMA (American Boiler Manufacturers Association) and ASME BG-112 guidelines. Always use manufacturer’s specified maximum TDS first. At pressures above ~400 psi, silica limits become the binding constraint before TDS limits are reached. Conductivity (μS/cm) is proportional to TDS; approximate conversion: TDS (ppm) ≈ 0.6× conductivity (μS/cm).

Pro Tips & Common Boiler Blowdown Mistakes

Do This

  • ✓Test makeup water TDS regularly and update blowdown settings seasonally — municipal water TDS varies by 30–50% between seasons. Spring snowmelt brings very low-TDS water to municipal systems; summer drought periods concentrate minerals. A blowdown setting correct for 200 ppm summer water may be significantly over-blowing in spring at 100 ppm (wasting energy) or under-blowing if a high-TDS source is switched in. Quarterly water testing and annual blowdown recalculation is good practice. Continuous conductivity monitors with automatic blowdown control valves eliminate this manual adjustment requirement for boilers over 50 HP.
  • ✓Install a blowdown heat recovery system whenever blowdown exceeds 5% — payback is typically 1–3 years. A flash tank on the blowdown line generates low-pressure flash steam (typically 5–15 psi) from the pressure drop of the hot blowdown water. This flash steam can be used for deaeration, domestic hot water preheating, or low-pressure process loads. The remaining hot drain water passes through a heat exchanger to preheat incoming cold makeup water. System efficiency: 60–80% heat recovery of the blowdown enthalpy. For a 150 psi boiler blowing down 500 lbs/hr: ~165,000 BTU/hr waste heat, ~100,000+ BTU/hr recovered. At $8–12/MMBtu natural gas, this saves $7,000–$10,000/year.

Avoid This

  • ✗Don't over-blowdown — excessive blowdown wastes fuel, water, and water treatment chemicals proportionally. A blowdown rate higher than calculated (e.g., 10% when 5% is sufficient) doubles the water makeup requirement, doubles the chemical treatment cost (scale inhibitors, oxygen scavengers), and wastes the chemical oxygen scavenger that was just dosed into the removed water. The energy cost of over-blowdown is direct: you are draining hot pressurized water and replacing it with cold untreated makeup. Every pound of excess blowdown costs approximately 1 lb of boiler makeup + the fuel to reheat it to steam temperature. For a 150 psi boiler, this means approximately 1,200 BTU/lb of unnecessarily wasted enthalpy per pound of excess blowdown.
  • ✗Don't use softened water TDS directly in this formula without understanding what softening does and doesn't remove. Water softening (ion exchange) replaces calcium and magnesium ions with sodium ions — it does NOT reduce total TDS. A 400 ppm hard water becomes 400 ppm soft water; the TDS is unchanged, but the scale-forming minerals are replaced with sodium salts. The blowdown formula uses TDS and the TDS/conductivity-based limit, so the calculation still applies. However, silica is NOT removed by softening. If raw water silica is >5 ppm, a softened-water boiler at elevated pressure may hit the silica limit before the TDS limit. Reverse osmosis removes both TDS and silica and allows dramatically higher cycles of concentration.

Frequently Asked Questions

What is the difference between surface blowdown and bottom blowdown?

Surface (continuous) blowdown removes water from the drum at the waterline, where dissolved solids concentrate as steam evaporates. It runs continuously at a calibrated rate to hold TDS below the maximum limit. This is what this calculator determines. Bottom (intermittent) blowdown is a manual purge from the lowest point of the boiler drum or mud drum. It removes settled suspended solids, sludge, and scale particles that accumulate at the bottom from water treatment chemistry precipitation and particulate carryover. Bottom blowdown is typically performed daily to weekly, for short durations (30–60 seconds), and is intentionally aggressive (full-open valve, rapid flushing). It does not serve the TDS control function of surface blowdown. Most properly maintained boiler systems require both: surface blowdown for TDS management + regular bottom blowdown for sludge and sediment removal. This calculator addresses surface blowdown only.

What happens if TDS exceeds the maximum limit?

Exceeding the maximum TDS limit causes two primary failure modes: (1) Scale formation: Calcium carbonate, calcium sulfate, and silica minerals precipitate out of solution onto heat transfer surfaces (boiler tubes and drum walls). Scale is an extreme thermal insulator: even 1/16” of calcium carbonate scale reduces heat transfer efficiency by 10–15%. Continued operation with heavy scale causes localized tube overheating, thermal fatigue, tube blistering, and ultimately tube failure (blowout). A tube failure at boiler pressure is an explosive event. (2) Steam carryover/priming: High-TDS water has elevated surface tension and foaming tendency. When steam rises rapidly through high-TDS boiler water, it entrains water droplets in the steam stream. This wet steam delivers dissolved solids into the steam distribution system, fouling control valves, traps, heat exchangers, and process equipment. In steam turbines, even small amounts of entrained moisture cause turbine blade erosion and potential catastrophic blade failure at speed.

How does water softening vs reverse osmosis affect blowdown rate?

Softening (ion exchange): Removes hardness (calcium, magnesium) but NOT total dissolved solids or silica. TDS is unchanged. Blowdown rate calculation remains the same. Benefit: eliminates hard-scale deposits from CaCO₃ and CaSO₄ (the most common scale types). Does not allow higher CoC unless TDS limit allows it. Reverse Osmosis (RO): Removes 95–99% of TDS, silica, and virtually everything dissolved. RO water at 10–20 ppm TDS dramatically increases allowable CoC: at 3,000 ppm max TDS and 15 ppm makeup, CoC = 200×, meaning blowdown = 0.5% instead of 5.26% with untreated water. This reduces makeup water consumption and chemical treatment cost by >80%. RO is economically justified for boilers above ~200 HP where water treatment chemical costs are significant. Downside: RO water is very aggressive (corrosive to carbon steel at low pH — deaeration and pH adjustment required). Demineralized water also risks caustic embrittlement in older boiler designs.

How do I set and verify the continuous blowdown valve position?

Setting blowdown: Calculate the required flow (lbs/hr), then convert to gallons/hour (divide by 8.33 for room temp approximation; actual density at boiler temperature varies). Set the continuous blowdown valve to produce this flow rate, measured at the blowdown outlet (timed bucket test or inline flow meter). Verification: Allow the boiler to reach steady state (typically 4–8 hours of normal operation). Then draw a drum water sample using a proper cooled sample cooler (never sample hot; use ASME-recommended valve sampling assembly). Test TDS or conductivity. The measured drum TDS should be at or near: TDS_makeup × CoC = TDS_makeup × (TDS_max / TDS_makeup). If drum TDS is significantly below TDS_max, you are over-blowing (open valve slightly). If near or above TDS_max, increase blowdown rate immediately. Automatic conductivity controllers with motorized blowdown valves maintain ±2% of target TDS continuously.

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