Cooling Tower Blowdown Calculator
Calculate blowdown rate, makeup water requirement, blowdown TDS, and daily water costs at any cycles of concentration. Based on standard cooling tower mass balance principles (Metcalf & Eddy / ASHRAE).
Cooling Tower Parameters
Enter your cooling tower operating parameters to calculate blowdown rate, makeup water requirement, and daily water costs.
Total recirculating water flow
Typical range: 3–6 for industrial towers
Typically 1–2% at standard operating delta-T
From water quality analysis
Cost to treat or dispose blowdown
Municipal / tanker / groundwater cost
How to Use This Calculator
- 1Enter your cooling tower's circulation rate (m³/hr) — this is the total recirculating flow, not the heat load. Refer to your pump nameplate or design data sheet.
- 2Set the cycles of concentration (CoC) — typically 3–6 for industrial towers. If unknown, start with the default of 4 and adjust based on your water treatment programme.
- 3Enter the evaporation rate as a percentage of circulation. For most HVAC and industrial towers operating at a 5–8°C range, this is approximately 1–2%.
- 4Enter your makeup water TDS from a recent water quality report. This determines blowdown TDS and helps assess scaling risk at the chosen CoC.
- 5Fill in water and blowdown treatment costs (₹/m³) to see your daily water expenditure and compare the economics at different CoC values.
Cooling Tower Water Balance: Evaporation, Blowdown & Drift
A cooling tower operates by evaporating a small fraction of the circulating water to reject heat. This evaporation — typically 1–2% of the circulation rate — concentrates dissolved salts in the remaining water. Without blowdown, the concentration of dissolved solids would rise continuously, eventually causing severe scaling, corrosion, and biological fouling.
The mass balance for a cooling tower at steady state has three loss terms: evaporation (E), blowdown (B), and drift (D). Makeup water (M) must replace all three: M = E + B + D. Modern cooling towers with drift eliminators have drift losses as low as 0.001–0.005% of circulation, making evaporation and blowdown the dominant terms.
Evaporation Loss (m³/hr) = Circulation Rate × Evaporation Rate ÷ 100
Blowdown (m³/hr) = Evaporation ÷ (CoC − 1)
Drift Loss (m³/hr) = Circulation Rate × 0.002 ÷ 100
Makeup Water (m³/hr) = Evaporation + Blowdown + Drift
Blowdown TDS (mg/L) = Makeup TDS × CoC
Cycles of Concentration: The Key to Water Conservation
Cycles of concentration (CoC) is the single most important parameter in cooling tower water management. It is defined as the ratio of dissolved solids in the circulating water to dissolved solids in the makeup water — or equivalently, the ratio of makeup volume to blowdown volume at steady state.
Increasing CoC directly reduces makeup water consumption and blowdown volume. A tower operating at CoC 2 discards half its circulating water as blowdown for every unit of evaporation. At CoC 6, blowdown is only one-fifth of evaporation. This translates directly to water savings, reduced chemical consumption per unit of heat rejected, and lower disposal costs.
The maximum achievable CoC is limited by the least-soluble constituent in the makeup water. Calcium carbonate scaling (assessed by the Langelier Saturation Index), silica solubility, and calcium sulphate solubility are the most common limiting factors in Indian industrial water supplies. A competent water treatment programme using scale inhibitors, corrosion inhibitors, and biocides can safely push CoC to 5–7 for many water qualities.
Cooling Tower Blowdown Treatment and Reuse
Cooling tower blowdown is a significant source of high-TDS wastewater in industrial plants. At CoC 4–6, blowdown TDS ranges from 1,200–1,800 mg/L for typical Indian municipal water. Direct discharge is increasingly restricted under CPCB and State PCB regulations, particularly for plants in notified industrial areas or near sensitive water bodies.
Blowdown treatment options depend on volume and TDS. For smaller volumes, conventional softening and filtration may suffice. For high-TDS blowdown, reverse osmosis can recover 70–80% as clean permeate for reuse as makeup water, dramatically improving the economics. Plants targeting zero liquid discharge can integrate cooling tower blowdown into their ZLD system — feeding it to the RO + MEE/MVR chain to recover the water and crystallise the salt cake.
Side-stream treatment — where a fraction of the circulating water is continuously softened or filtered and returned to the tower — can also enable higher CoC operation without increasing blowdown volume, by reducing the concentration of scale-forming ions.
Water Conservation in Indian Industrial Cooling Systems
India's industrial sector accounts for a substantial share of freshwater withdrawals, with cooling systems being among the largest industrial water users. Power plants, refineries, steel mills, fertiliser plants, and large HVAC systems all rely on cooling towers, and makeup water demand can represent 60–80% of a plant's total water consumption.
The Bureau of Energy Efficiency (BEE) and the Central Pollution Control Board (CPCB) both identify cooling tower optimisation as a priority water conservation measure. Increasing CoC from 3 to 5 in a 500 m³/hr cooling system can save over 1,000 m³/day of freshwater — equivalent to the daily water needs of approximately 5,000 people. For water-stressed states like Gujarat, Maharashtra, Rajasthan, and Tamil Nadu, such savings are increasingly a regulatory expectation and a board-level ESG priority.
Spans Envirotech works with industries across India to optimise cooling tower water management — from water quality audits and CoC maximisation studies to blowdown treatment and ZLD integration. Our engineering team can assess your specific water quality, recommend the maximum safe CoC, and design a treatment system to achieve it.
Frequently Asked Questions
What is cycles of concentration (CoC) in a cooling tower?
Cycles of concentration (CoC) is the ratio of dissolved solids in the circulating water to the dissolved solids in the makeup water. A CoC of 4 means circulating water TDS is 4× the makeup TDS. Higher CoC reduces blowdown and makeup water but increases scaling and corrosion risk.
How do I calculate cooling tower blowdown rate?
Blowdown rate (B) = Evaporation loss (E) ÷ (CoC − 1). For example, with 10 m³/hr evaporation and CoC 4: B = 10 ÷ 3 = 3.33 m³/hr. Total makeup = Evaporation + Blowdown + Drift.
What is the ideal CoC for a cooling tower?
The ideal CoC for most industrial cooling towers is 4–6. Below CoC 3, excessive blowdown wastes water and chemicals. Above CoC 6, scaling, corrosion, and biological fouling risk rises sharply. The limiting CoC depends on your makeup water quality — primarily calcium carbonate, sulphate, and silica content.
How much water can I save by increasing CoC?
Increasing CoC from 2 to 4 reduces makeup water by approximately 33%. From CoC 3 to 6 saves around 20% more. Savings diminish at higher CoC — the incremental benefit above CoC 6 is small but the chemistry risk increases. Use this calculator to compare savings at your specific CoC values.
What causes scaling and fouling in cooling towers?
Scaling is caused by supersaturation of calcium carbonate, calcium sulphate, and silica as water concentrates through evaporation. Biological fouling — including Legionella risk — arises from warm, nutrient-rich circulating water. Risk increases with higher CoC, alkaline pH, and high temperatures. A balanced treatment programme of scale inhibitors, corrosion inhibitors, and biocides is essential.
What TDS limit should cooling tower blowdown not exceed?
Circulating water TDS is generally kept below 2,000–3,500 mg/L for most industrial towers. Specific limits apply: hardness below 700 mg/L as CaCO₃, silica below 150 mg/L, chlorides below 750 mg/L. Your water treatment provider should define limits using the Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI) for your water quality.
Can cooling tower blowdown be treated and reused?
Yes. Cooling tower blowdown can be treated using softening, filtration, and RO to recover 70–80% as clean permeate. The RO reject can be sent to a ZLD system for further concentration. Side-stream softening of circulating water can also enable higher CoC operation without increasing blowdown TDS.
Related Tools
RO Recovery Calculator
Size the RO system for cooling tower blowdown treatment and water recovery.
ZLD Water Recovery Calculator
Calculate ZLD system water recovery and estimate blowdown reuse potential.
Chemical Dosing Calculator
Calculate scale inhibitor, biocide, and corrosion inhibitor dosing for your cooling tower.
Optimise Your Cooling Tower Water Management
Spans Envirotech provides cooling tower water audits, CoC optimisation studies, blowdown treatment design, and ZLD integration for industrial plants across India. Talk to our engineers about reducing your cooling water footprint.
Or call us: +91-98100 00233