Blowdown Percentage Calculator

The Blowdown Percentage Calculator calculates the percentage of boiler or cooling tower water purged to control dissolved solids levels.

Blowdown Percentage Calculator Explained

Blowdown is the controlled discharge of water from a system to limit the buildup of dissolved minerals and contaminants. The blowdown percentage is the portion of incoming makeup water discharged as blowdown, expressed as a percent. It connects water, energy, and chemistry by linking flow rates, concentration of salts, and equipment limits.

Boilers and cooling towers concentrate dissolved solids because evaporation or steam production removes water but not salts. As concentration rises, scaling and corrosion risks increase. Blowdown resets the concentration to a safe level by removing a mass of salts, which you replace with fresh makeup water.

Cycles of concentration (C) is the ratio of the concentration in the circulating or boiler water to the concentration in the makeup water. You can measure cycles using a conservative tracer such as conductivity, chloride, or total dissolved solids (TDS). The calculator uses these relationships to compute flow splits and the resulting percentage.

Blowdown Percentage Formulas & Derivations

The calculator applies steady-state mass balance. Evaporation and steam remove water only. Blowdown and drift remove water and dissolved solids. Definitions: M = makeup flow, B = blowdown flow, E = evaporation or steam flow, D = drift losses, c = concentration, and C = cycles of concentration = c_circulating / c_makeup.

• Primary definition: Blowdown percentage of makeup = 100 × B / M.
• Boiler (no drift): Feedwater salt balance gives F = C × B. Therefore blowdown percentage = 100 / C.
• Cooling tower with drift: Mass balance yields B = E / (C − 1) − D and M = E + D + B.
• Alternative expression: At steady state, makeup salt balance M × c_makeup = (B + D) × c_circulating = (B + D) × C × c_makeup.
• Optional reporting: Percent of circulation (if required) = 100 × B / R, where R is recirculating flow.

When drift is negligible, blowdown percentage of makeup is approximately 100 / C for both boilers and cooling towers. If drift is significant, it slightly reduces the percentage because some salt mass leaves via drift. All formulas assume constant density and well-mixed conditions.

How to Use Blowdown Percentage (Step by Step)

Decide whether your system is a boiler or a cooling tower. Identify the known flows and the measured concentrations. Pick a consistent set of units to avoid conversion errors. Then apply the formula that matches your case.

• For boilers, measure or estimate cycles from boiler water and feedwater TDS (C = TDS_boiler / TDS_feedwater).
• For cooling towers, estimate evaporation from heat load or makeup meter and compute cycles from conductivity or chloride ratio.
• If drift is known, include it. If not, use a realistic default (for many towers, 0.001%–0.02% of circulation).
• Compute B using the appropriate formula and then compute the blowdown percentage = 100 × B / M.
• Compare the result with target ranges from your water treatment program.

These steps are quick when your concentration and flow instruments are reliable. Repeat after significant load or chemistry changes. Regular checks help maintain steady chemistry and protect assets.

What You Need to Use the Blowdown Percentage Calculator

Gather a few data points before calculating. You will need one set of consistent units for all flows and concentrations. Use recent lab or instrument readings for accuracy.

• Makeup water concentration of a tracer (for example, TDS, conductivity, or chloride) and circulating/boiler concentration to compute cycles C.
• Evaporation or steam rate (E), estimated from heat load or meters.
• Drift loss (D), if applicable, from manufacturer data or drift eliminator specs.
• Makeup flow (M), if measured; otherwise derive from E, D, and B.
• Optional: Recirculation rate (R) if you want blowdown as a percent of circulation.

Typical cycles range from 3 to 7 for open cooling towers and 5 to 30 for many industrial boilers, depending on chemistry limits. C must be greater than 1. Very high cycles imply low blowdown but can increase scaling risk. Check for leaks or hidden losses if your mass balance does not close.

Using the Blowdown Percentage Calculator: A Walkthrough

Here’s a concise overview before we dive into the key points:

1. Select the application type: boiler or cooling tower.
2. Enter concentrations for makeup and circulating/boiler water to compute cycles of concentration.
3. Enter evaporation or steam rate, and drift if known.
4. Enter makeup flow if measured, or let the calculator solve for it.
5. Choose your preferred units for flow and concentration.
6. Click Calculate to compute B, M (if unknown), and the blowdown percentage.

These points provide quick orientation—use them alongside the full explanations in this page.

Worked Examples

Boiler example: A plant produces 5,000 kg/h of steam. Feedwater TDS is 200 mg/L. Boiler water TDS runs at 3,000 mg/L. Cycles are C = 3,000 / 200 = 15. From the boiler relationship, B% of feedwater = 100 / C = 6.67%. The blowdown flow is B = S / (C − 1) = 5,000 / 14 ≈ 357 kg/h, and feedwater is S + B ≈ 5,357 kg/h. What this means: About 6.7% of feedwater must be discharged to hold TDS at the setpoint.

Cooling tower example: A tower evaporates E = 10 m³/h at a steady heat load. Cycles are C = 4 based on chloride ratio. Drift is measured at D = 0.1 m³/h. Compute B = E / (C − 1) − D = 10 / 3 − 0.1 ≈ 3.233 m³/h. Makeup is M = E + D + B ≈ 13.333 m³/h, so blowdown percentage is 100 × B / M ≈ 24.25%. What this means: Roughly one quarter of makeup leaves as blowdown to sustain four cycles.

Assumptions, Caveats & Edge Cases

The formulas assume steady-state operation and a well-mixed system. The tracer used for cycles must be conserved in the liquid phase. Evaporation and steam are treated as pure water loss with no dissolved solids. Drift carries salts, so include it when significant.

• Measurement uncertainty in conductivity or TDS directly affects cycles and the percentage result.
• If C approaches 1, the required blowdown becomes very large, indicating poor water quality or incorrect inputs.
• Hidden leaks or carryover will break the mass balance; reconcile with meter totals before final decisions.
• Rapid load swings create transient imbalances; average values over a stable period for better accuracy.
• Chemical dosing or softening changes effective chemistry limits; update target cycles when treatment changes.

Use these results as part of a broader water treatment strategy. Verify periodically with lab tests and adjust control setpoints to maintain safe concentration limits. Coordination with your treatment vendor is recommended when altering targets.

Units Reference

Correct units keep the mass balance consistent and the percentage meaningful. You can use SI or US customary units as long as you apply them consistently across flows and concentrations. The calculator accepts any consistent set of units.

To use the table, pick one flow unit for all flow values and one concentration unit for both samples. Ratios cancel units automatically. If you switch units mid-calculation, convert first to avoid errors.

Tips If Results Look Off

Unexpected values usually come from cycles, drift, or inconsistent units. Start with a quick mass balance check to see if M ≈ E + D + B. Then verify instrument scaling and sample points.

• Recheck conductivity/TDS meters and ensure samples represent circulating water.
• Confirm that evaporation or steam rate aligns with heat load and ambient conditions.
• Estimate drift realistically; do not treat it as zero if eliminators are damaged.
• Ensure all flows use the same unit and time basis.

If the balance still fails, look for leaks, carryover, or blowdown valve malfunction. Short-term logging with totalizers often reveals the issue.

FAQ about Blowdown Percentage Calculator

What is blowdown percentage?

It is the fraction of makeup water discharged as blowdown, reported as a percent: 100 × B / M. It indicates how much water you must remove to control concentration.

Is blowdown percentage the same for boilers and cooling towers?

The definition is the same, but the operating context differs. For boilers, B% ≈ 100 / C. For towers, B% ≈ 100 / C if drift is negligible, otherwise slightly less.

How do I estimate evaporation if I only know heat load?

Use E = Q / (ρ × h_fg), where Q is heat rejected, ρ is water density, and h_fg is latent heat of vaporization. Keep your units consistent for mass and energy.

How does drift affect blowdown percentage?

Drift carries salts out of the system, so the required blowdown decreases. Use B = E / (C − 1) − D to account for it when computing the percentage.

Glossary for Blowdown Percentage

Blowdown

The controlled discharge of water from a boiler or cooling tower to reduce dissolved solids and maintain target chemistry.

Makeup Water

Fresh water added to replace evaporation, drift, and blowdown losses and restore system volume.

Cycles of Concentration

The ratio of circulating or boiler water concentration to makeup concentration for a conservative tracer such as TDS or chloride.

Drift

Liquid water droplets carried out of a cooling tower by the airflow, removing both water and dissolved solids.

Total Dissolved Solids (TDS)

The mass of dissolved ions and minerals per unit volume of water, often measured as mg/L or estimated via conductivity.

Evaporation or Steam

Phase change that removes water but not dissolved salts. In boilers, steam production is the evaporation term in the balance.

Mass Balance

An accounting of mass in and out of a system to ensure conservation, used to derive blowdown and makeup relationships.

Concentration

The amount of a substance per unit volume, such as mg/L. It determines cycles and guides the blowdown target.

Sources & Further Reading

Here’s a concise overview before we dive into the key points:

• U.S. DOE Energy Tips – Steam: Minimize Boiler Blowdown (PDF)
• Spirax Sarco: Boiler blowdown, water treatment, and controls
• U.S. EPA: Water Efficiency Management Guide – Cooling Towers
• Lenntech: Cooling tower blowdown and cycles of concentration
• Chem-Aqua: Cooling Tower Basics – Cycles of Concentration

These points provide quick orientation—use them alongside the full explanations in this page.