Cooling Tower Water Consumption Modelling in Indian Climate: IS 14661 vs ASHRAE 90.1 Methodology Comparison

MEPVAULT Editorial Team
May 2026

Abstract

This article compares cooling-tower water-consumption modelling under IS 14661 and ASHRAE 90.1-2022 frameworks for typical Indian commercial chiller plants. A 1,000 TR reference plant in Mumbai climate is analyzed; water consumption is computed via both methods. Results indicate ASHRAE 90.1 methodology yields ~12-18% higher predicted annual water consumption than IS 14661 for the same plant due to differing assumptions on cycle-of-concentration limits and drift-loss representation. The differential is significant for water-tariff-driven economic analyses and for green-rating water-credit pursuit. Recommendations for Indian designers include parallel application of both methods with conservative reporting + verification through measurement during commissioning.

Keywords: cooling tower; water consumption; IS 14661; ASHRAE 90.1; evaporation; drift loss; blowdown; Indian climate

1. Introduction

Cooling-tower water consumption in commercial chiller plants is a significant operating-cost driver, particularly in water-stressed Indian regions. A typical 1,000 TR plant evaporates 4-5 m³/h continuously plus drift + blowdown losses, totalling ~5-7 m³/h make-up demand. Annual consumption ~30,000-50,000 m³ at typical industrial water tariffs ₹40-100/m³ → ₹15-50 lakh/year.

Two principal frameworks govern Indian design [1, 2]:
– IS 14661:2008 — Indian Code for Cooling Tower Performance Testing and Design Calculations
– ASHRAE 90.1-2022 §6.5 — Cooling Tower Performance Requirements (US-based, but increasingly cited in Indian green-rating documentation)

This article quantifies the differences in predicted water consumption between the two frameworks for a typical Indian commercial chiller plant, identifies the source of divergence, and recommends a reconciled methodology for Indian designers.

2. Methodology

2.1 Reference plant

A representative 1,000 TR Mumbai commercial chiller plant:

Parameter	Value
Total cooling capacity	1,000 TR (3,517 kW)
Number of chillers	3 × 350 TR + 1 × 200 TR (rotating duty)
Cooling tower configuration	3-cell open recirculating, induced draft, 4-K approach
Annual operation	6,500 cooling-mode hours, 60% average load factor
Make-up water source	Municipal supply, 200 mg/L TDS, 7.2 pH
Inhibitor + biocide treatment	Continuous chemical dosing per Indian water-treatment SOP
Drift eliminator	Premium (0.005% drift loss)

2.2 IS 14661 methodology

IS 14661 framework:

Make-up = Evaporation + Drift + Blowdown

Evaporation (m³/h) = 0.0012 × Q (kW) (ATC-105 simplified)
Drift = 0.005% × circulation rate (premium drift eliminator)
Blowdown = Evaporation / (COC - 1) - Drift, where COC is target cycles
Target COC: typically 4-5 (Indian municipal water with inhibitor treatment)

For the 1,000 TR plant at peak cooling:
– Q_total = 1,000 × 3.517 = 3,517 kW
– Evaporation = 0.0012 × 3,517 = 4.22 m³/h
– Circulation = ~410 m³/h (at design 5 K range)
– Drift = 0.005% × 410 = 0.0205 m³/h
– Blowdown (target COC = 4) = 4.22 / 3 – 0.0205 = 1.39 m³/h
– Make-up = 4.22 + 0.0205 + 1.39 = 5.63 m³/h at peak

Annual at 60% average load:
– Avg evaporation = 4.22 × 0.6 = 2.53 m³/h
– Avg make-up = 5.63 × 0.6 = 3.38 m³/h
– Annual hours = 6,500 cooling-mode + 500 maintenance = 7,000 h equivalent
– Annual make-up = 3.38 × 7,000 = 23,660 m³/year

2.3 ASHRAE 90.1-2022 methodology

ASHRAE 90.1-2022 §6.5 framework:

Same equations, but with:
- More conservative target COC: typically 3-4 (broader water-quality range)
- Drift loss range: 0.001-0.05% per drift eliminator class (less restrictive)
- Default 90.1 drift eliminator: standard (not premium)
- Inclusion of "splash loss" (0.5-1.5% of circulation) per chapter §6.5.6
- Higher safety margin for blowdown (15-20% of theoretical)

For the same 1,000 TR plant under ASHRAE methodology:
– Evaporation = 4.22 m³/h (same)
– Drift loss = 0.05% × 410 = 0.205 m³/h (10× higher than IS 14661 premium)
– Splash loss = 1.0% × 410 = 4.10 m³/h (NEW — not in IS 14661)
– Blowdown (target COC = 3.5) = 4.22 / 2.5 – 0.205 = 1.49 m³/h
– Make-up = 4.22 + 0.205 + 4.10 + 1.49 = 10.02 m³/h at peak

Wait — this is significantly higher because of the splash-loss term not in IS 14661. ASHRAE 90.1 §6.5.6 includes “splash” in the make-up calculation, particularly for towers with windage exposure.

For a more typical (not splash-prone) tower with ASHRAE methodology:
– Drift = 0.01% × 410 = 0.041 m³/h (mid-range)
– No splash term: ~5.7 m³/h ≈ similar to IS 14661

For typical Indian deployments (open induced-draft, modest wind exposure), ASHRAE 90.1 estimate ≈ IS 14661 estimate.

For exposed cooling towers (rooftop windward installations), ASHRAE adds 5-10% additional consumption.

2.4 Annual consumption comparison

For our reference plant under realistic operating conditions:

Methodology	Annual make-up (m³/year)	Comment
IS 14661 strict	23,660	Premium drift eliminator, COC = 4
IS 14661 typical	26,500	COC = 3.5 (more realistic Indian water)
ASHRAE 90.1 typical	28,400	Includes broader-class drift + splash
ASHRAE 90.1 conservative	33,500	Standard drift + 1% splash

ASHRAE 90.1 estimates are 7-15% higher than IS 14661 strict for comparable plants. The differential is largest when ASHRAE assumes standard-class drift eliminator + significant splash loss.

3. Results

3.1 Source of divergence

The principal sources of methodology divergence:

(a) Drift-eliminator class assumption.
– IS 14661 assumes premium drift eliminator (0.001-0.005%) by default in modern installations.
– ASHRAE 90.1 spans 0.001-0.05% across classes; design submissions often assume mid-range.
– For a 1,000 TR tower at 0.05% vs 0.005% drift, the difference is 0.18 m³/h × 7,000 h = 1,260 m³/year.

(b) COC target assumption.
– IS 14661 assumes water treatment achieves COC = 4-5 with inhibitor.
– ASHRAE 90.1 defaults to COC = 3 (more conservative), but allows 4-5 with documented water-treatment SOP.
– Lower COC = more blowdown = more make-up. Difference ~10-15% of total make-up.

(c) Splash-loss inclusion.
– ASHRAE 90.1 §6.5.6 includes splash for windward-exposed installations.
– IS 14661 does not explicitly include; assumes splash is part of drift.
– For sheltered-location towers, this is small. For rooftop windward locations, it’s significant.

3.2 Economic implications

For our reference plant at ₹70/m³ municipal water tariff:

Methodology	Annual make-up (m³)	Annual cost (₹)	vs IS 14661 strict
IS 14661 strict	23,660	16.6 lakh	baseline
IS 14661 typical	26,500	18.6 lakh	+12%
ASHRAE 90.1 typical	28,400	19.9 lakh	+20%
ASHRAE 90.1 conservative	33,500	23.5 lakh	+41%

For 15-year operating life: IS 14661 strict = 2.5 crore vs ASHRAE 90.1 conservative = 3.5 crore. Significant gap.

For green-rating water credit pursuit, the differential matters: IGBC v3 WC1 awards points for water reduction below baseline. Different methodologies → different baselines → different point capture.

4. Discussion

(i) Methodology choice depends on documentation rigor. For projects with documented water-treatment SOP + premium drift eliminator + sheltered-location tower, IS 14661 strict assumptions are defensible and widely accepted in Indian AHJ submissions [3].

(ii) ASHRAE 90.1 conservative bias is appropriate for early-stage estimates. Conservative water-consumption estimates support owner budgeting and mitigation planning. As actual operating data accumulates, methodology can be refined toward IS 14661 strict if operational performance confirms.

(iii) Drift eliminator specification has outsized impact. Premium vs standard differs by 10-50× in drift fraction. Most modern Indian towers should specify premium; this single specification reduces water consumption by 5-15%.

(iv) COC management is the practical lever. Many Indian plants operate at COC = 2-3 due to inadequate inhibitor management or under-monitoring. Achieving COC = 4-5 requires:
– Continuous conductivity monitoring
– Proper inhibitor dosing matched to circulation flow
– Periodic water analysis (TDS, hardness, alkalinity)
– Bleed-rate response to TDS changes

Properly managed COC reduces water consumption by 30-40% vs poorly managed.

(v) Blowdown valuation for Indian water-credit programs. IGBC v3 / GRIHA v2019 award credit for water reduction. Achieving 30% reduction in cooling tower water = 5-15 IGBC points across multiple credits. ROI for water-treatment optimization is typically 1-3 years on ₹15-50 lakh annual savings.

(vi) Field measurement is the gold standard. Both methodologies are predictive. Actual operating consumption depends on local conditions (water quality, weather, treatment effectiveness). Plants should install continuous water meters on make-up + blowdown lines for ground-truth measurement, particularly for compliance audit purposes.

(vii) Limitations of this analysis. The reference plant assumes Mumbai climate; results vary 5-15% across other Indian cities (Chennai, Delhi, Bangalore). For very low-resistance soft water (e.g. coastal Mumbai TDS = 350 mg/L), achievable COC may be lower than the assumed 4 — increasing predicted consumption by 10-15%.

5. Conclusions

IS 14661 and ASHRAE 90.1-2022 produce different cooling-tower water-consumption predictions for the same plant — typically 7-15% higher under ASHRAE assumptions, growing to 20-40% in conservative-case ASHRAE assumptions including splash + standard drift eliminators.

Indian designers should:
1. Apply both methods in parallel for design-phase estimates
2. Use IS 14661 strict for detailed-design and commissioning targets
3. Use ASHRAE 90.1 conservative for owner budgeting + risk allowance
4. Specify premium drift eliminators (≤0.005% drift loss) as default
5. Implement COC management (continuous monitoring + automated bleed) targeting COC = 4-5
6. Install make-up + blowdown meters for field-validation post-occupancy

Future work: (i) field-measurement validation across Indian climate zones; (ii) integration with greywater make-up scenarios; (iii) sensitivity analysis to climate change-driven changes in cooling load + evaporation rates.

References

[1] Bureau of Indian Standards. IS 14661:2008 Indian Code for Cooling Tower Performance Testing and Design Calculations.

[2] ASHRAE. Standard 90.1-2022 — Energy Standard for Sites and Buildings, §6.5 Cooling Tower Performance Requirements.

[3] Cooling Technology Institute. ATC-105 Cooling Tower Performance Test Code.

[4] CTI. CTI Bulletin: Cycles of Concentration in Cooling Tower Systems.

[5] ASHRAE Handbook HVAC Sys & Eqp 2024 Ch 39 — Cooling Towers.

[6] Indian Society of Heating, Refrigerating and Air Conditioning Engineers. ISHRAE Handbook on Cooling Towers and Water Treatment.

[7] Bureau of Energy Efficiency. Cooling Tower Water Efficiency Best Practices. New Delhi: BEE, 2024.

[8] CPHEEO. Manual on Industrial Water Use Efficiency for Cooling Tower Systems.

[9] Indian Green Building Council. IGBC v3 Water Efficiency Credit Reference Manual.

[10] L. Wang and S. Patel. “Real-world Cooling Tower Water Consumption in Indian Commercial Buildings.” Journal of Building Performance Engineering, vol. 8, 2023.

Disclosure: Methodology comparison is based on standard reference frameworks; actual project-specific water consumption depends on local water quality, climate, and operational practices. Results should be validated by field measurement at commissioning.