A 200 TR cooling tower in an Indian summer evaporates roughly 35 m³/day of water — most of it cooling the chiller condenser, the rest carried by drift and dust loading on the tower fill. Make-up water replaces all losses; blowdown removes concentrated dissolved solids before they scale the chiller condenser tubes. Get the cycles-of-concentration design wrong and you either over-spend on make-up water or under-protect the chiller from scaling.
This guide covers the cycle of concentration calculation, blowdown ratio, drift loss assessment, and the integrated water balance.
The water balance
For a continuously-operating cooling tower:
Make-up = Evaporation + Drift + Blowdown
Where:
- Evaporation removes pure water vapor from the system
- Drift is droplets carried out of the tower by the airstream (water containing dissolved solids)
- Blowdown is intentional removal of concentrated water to maintain solids levels
Evaporation rate
Evaporation removes heat from the circulating water. For typical cooling tower operating at 5 K range (CW supply at 30 °C → 35 °C):
m_evap = Q_total / h_fg = (m_circ × Cp × ΔT) / h_fg
Where h_fg = 2,400 kJ/kg (latent heat of vaporization at 35 °C).
A simpler practical formula:
Evaporation (gpm) = 0.001 × Q (BTU/h)
or
Evaporation (m³/h) = 0.0012 × Q (kW)
For a 1,000 TR cooling tower (3,517 kW heat rejection):
m_evap = 0.0012 × 3,517 = 4.22 m³/h = 101 m³/day
That’s ~1% of circulating flow per hour at typical 6 K range.
Drift loss
Modern drift eliminators reduce drift to:
| Drift eliminator class | Drift loss |
|---|---|
| Standard | 0.05% of circulating flow |
| Premium | 0.005% (10× lower) |
| Code-compliant for environmental | 0.001% |
For a 1,000 TR tower with 425 m³/h circulation (typical) and standard drift eliminator:
m_drift = 425 × 0.0005 = 0.21 m³/h = 5.1 m³/day
Cycles of concentration (COC)
COC measures how concentrated the recirculating water is vs make-up:
COC = TDS_circulating / TDS_make-up
Practical COC values:
| COC | Typical application |
|---|---|
| 2-3 | Aggressive water (high TDS, scale-prone); minimal blowdown management |
| 3-5 | Industrial average; typical Indian projects |
| 5-7 | Premium water treatment (chemical inhibitors); good chiller protection |
| 8+ | Engineered systems with reverse osmosis treatment of make-up |
Higher COC = less blowdown = less make-up. But higher COC also means more concentrated dissolved solids → scaling risk on condenser tubes → chiller efficiency loss.
Blowdown ratio
B = E / (COC - 1) - D
Where E is evaporation, D is drift, and (COC – 1) is the multiplier of make-up beyond evaporation.
For a 1,000 TR system: E = 4.22 m³/h, D = 0.21 m³/h, COC = 4:
B = 4.22 / (4 - 1) - 0.21 = 1.20 m³/h = 28.8 m³/day
Make-up = E + D + B = 4.22 + 0.21 + 1.20 = 5.63 m³/h = 135 m³/day
How COC selection drives make-up cost
For the same 1,000 TR system, comparing COC values:
| COC | Blowdown (m³/h) | Make-up (m³/h) | Annual make-up (m³) |
|---|---|---|---|
| 2 | 4.01 | 8.44 | 73,955 |
| 3 | 1.90 | 6.33 | 55,455 |
| 4 | 1.20 | 5.63 | 49,295 |
| 5 | 0.85 | 5.28 | 46,255 |
| 7 | 0.49 | 4.92 | 43,090 |
| 10 | 0.26 | 4.69 | 41,065 |
Going from COC=2 to COC=5 reduces make-up by 38%. At Indian municipal water tariffs (~₹40-100/m³ for industrial), this is ~₹10-30 lakh/year saving for a 1,000 TR plant.
The trade-off: higher COC requires:
- More aggressive chemical inhibitor (₹2-5 lakh/year for a 1,000 TR plant)
- Better water quality monitoring (TDS, conductivity, hardness logging)
- Possibly better make-up water (RO pre-treatment to lower TDS)
Make-up water sources
Indian projects typically use:
1. Municipal/borewell water — most common; treated and dosed at the cooling tower
2. Treated greywater — for sustainability credit; requires RO + chemistry control
3. Raw water with pretreatment — softeners + filtration + chemistry
4. Reverse osmosis — for high-COC operation (COC 7-10)
Each source requires:
- Make-up flow control valve (modulating, on level switch)
- Conductivity sensor (continuous TDS monitoring)
- Chemical inhibitor injection
- Periodic blowdown automatically based on conductivity
Worked example: 500 TR commercial chiller plant
Office building, 500 TR design at peak. CW circulation 213 m³/h. Site water at 200 mg/L TDS, target COC = 4.
Evaporation: 0.0012 × 1,758 = 2.11 m³/h
Drift: 213 × 0.0005 = 0.11 m³/h
Blowdown: 2.11 / 3 – 0.11 = 0.59 m³/h
Make-up: 2.11 + 0.11 + 0.59 = 2.81 m³/h = 67 m³/day = 24,500 m³/year
At ₹70/m³ municipal water = ₹17 lakh/year on water alone. Add chemicals + treatment ~₹5 lakh = ₹22 lakh/year cooling tower operating cost (water only, excluding fan/pump electricity).
Going to COC=6: make-up drops to 2.45 m³/h = 21,500 m³/year = ₹15 lakh on water + ₹6 lakh chemicals (higher dose) = ₹21 lakh. Marginal saving — sometimes COC optimization beyond 5 isn’t worth the chemical cost.
Five common make-up design mistakes
1. Sizing make-up pipe too small. Make-up flows continuously; size for peak (3× average) plus pressurization losses.
2. No conductivity-controlled blowdown. Manual blowdown either over- or under-shoots; automated control saves 15-25% of blowdown.
3. Chemical inhibitor dosed manually. Dosing pump must be auto-modulated to flow and conductivity, otherwise inhibitor concentration varies by ±30%.
4. Ignoring drift losses. Premium drift eliminators pay back in 2-4 years from reduced make-up alone.
5. Cooling tower sized for design flow at design COC only. Real-world flow varies; check that COC stays in spec across part-load conditions.
Quick checklist
- [ ] Evaporation rate computed from total cooling load
- [ ] Drift loss assumed per drift eliminator class (premium for sustainable design)
- [ ] Target COC selected based on water quality + chemical treatment economics
- [ ] Blowdown rate at design COC
- [ ] Make-up = E + D + B with safety margin
- [ ] Make-up pipe sized for peak demand
- [ ] Auto-blowdown by conductivity controller
- [ ] Chemical inhibitor dosing automated to flow + conductivity
- [ ] Water-quality monitoring schedule
References: CTI ATC-105 Cooling Tower Performance Test Code; ASHRAE Handbook HVAC Sys & Eqp 2024 Ch 39 (Cooling Towers); IS 14661 Indian Code for Cooling Tower Performance Testing; ASHRAE Handbook on Water Treatment.
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