Overview
Cooling towers reject waste heat from process cooling water, refrigeration condensers, power plant condensers, and HVAC systems to the atmosphere by evaporating a small portion of the water. They are the dominant heat rejection method for industrial cooling and large HVAC. Sizing and selection depend on the required heat rejection, range, approach, wet-bulb temperature, and water quality considerations.
Evaporative Cooling Principle
Cooling relies on latent heat of vaporization of water: when water evaporates, it absorbs ~2,260 kJ/kg (970 BTU/lb) from the remaining water, cooling it.
In a cooling tower:
- Hot water is distributed at the top over fill media (large surface area for air-water contact)
- Air is drawn upward (induced draft) or blown upward (forced draft) through the falling water
- A small fraction (~1-2% of flow) evaporates into the air stream, cooling the remaining water
- Cooled water collects in cold water basin at bottom, pumped back to process
- Warm moist air exits at top (visible plume in cool weather)
The theoretical limit to cooling is the wet-bulb temperature (WBT) — the lowest temperature achievable by evaporative cooling. This is the temperature air would reach if cooled to saturation (100% RH) by evaporating water into it.
Key Performance Terms
| Term | Definition |
|---|---|
| Range | T_hot in - T_cold out (°C or °F) — typically 5-15°C (10-27°F); represents heat load |
| Approach | T_cold out - Wet Bulb Temperature (°C) — typically 3-8°C (5-15°F); represents tower size/efficiency |
| Wet Bulb Temperature (WBT) | Design ambient WBT — site-specific; design values 21-28°C depending on location (26°C typical for US summer) |
| Flow (gpm or m³/h) | Circulating water flow rate |
| Heat load (tons / kW) | Tons of refrigeration or kW of heat to reject |
| Drift loss | Water droplets entrained and carried out with air (0.001-0.2% of flow) |
| Blowdown | Intentionally drained water to limit dissolved solids concentration |
| Makeup water | Water to replace evaporation + drift + blowdown |
Tower Types
By Air Flow Generation
Induced Draft (ID) (most common):
- Fan at top of tower draws air up through the fill
- Air velocity more uniform through fill
- Recirculation of discharged moist air is less than forced draft
- Fan handles moist warm air (more corrosion)
- Lower horsepower per ton than forced draft
- Used for most industrial and HVAC towers
Forced Draft (FD):
- Blower fan at air inlet pushes air into tower
- Fan handles ambient (dry, cool) air — less corrosion, can be simpler
- Poor air distribution (high velocity at fan, low at far end)
- Higher noise at ground level (fan at low elevation)
- Higher exit velocity reduces recirculation but uneven fill velocity
- Used where fan access/maintenance at low level is preferred
By Air-to-Water Flow Arrangement
Counterflow:
- Air flows upward, water falls downward — opposite directions
- Better heat transfer (greatest temperature differential at top, where air is coolest and water hottest)
- Taller tower; pressurized spray distribution system
- Smaller footprint per ton
- More difficult to maintain nozzles and fill
Crossflow:
- Air flows horizontally across falling water
- Fill is accessible from side; gravity water distribution (no pressurized nozzles)
- Lower pumping head; easier maintenance
- Larger footprint per ton
- Slightly less thermal performance at equivalent size
- Popular for large industrial and power plant towers
By Construction
| Type | Description | Application |
|---|---|---|
| Field-erected (FEP) | Large, site-built concrete/wood/FRP structures | Power plants, refineries, large process (>10,000 gpm) |
| Factory-assembled (package) | Built in factory, shipped as complete unit | HVAC, small to medium process (up to ~5,000 gpm) |
| FRP (fiberglass) | Corrosion-resistant, lightweight | Coastal, chemical, water treatment facilities |
| Concrete | Long life (30+ years), durable | Power generation, heavy industry |
| Wood | Traditional, low cost | Older industrial sites; less common now |
Sizing Calculation
Heat Load
Q (kW) = m_dot × Cp × range = m_dot × 4.18 × (T_hot - T_cold)
Q (BTU/h) = 500 × gpm × range (°F)
Q (tons_refrigeration) = gpm × range / 25 (for chiller condenser service, 1 ton = 15,000 BTU/h heat rejection)
For a typical chiller condenser: 1 ton of refrigeration rejects ~15,000 BTU/h + compressor heat (~3,000 BTU/h) = 18,000 BTU/h in cooling tower = 3 gpm at 10°F range.
Water Losses
- Evaporation: ~1% of flow per 10°F (5.6°C) range = 1.8 m³/h per 1000 gpm per 10°F
- Drift: 0.005-0.02% of flow (modern drift eliminators achieve 0.001% — very low)
- Blowdown: calculated based on cycles of concentration
Cycles of Concentration (COC)
As water evaporates, dissolved solids stay behind — concentration increases. Blowdown limits concentration ratio:
COC = dissolved solids in recirculating water / dissolved solids in makeup water
COC = makeup conductivity / blowdown conductivity
Blowdown = evaporation / (COC - 1)
Makeup = evaporation + blowdown + drift
Typical COC targets:
- 3-5 cycles: good water quality, easy treatment
- 5-7 cycles: excellent treatment, significant water savings
- 7-10 cycles: high-efficiency; requires very good water treatment, may need acid or softening
- Higher COC = less makeup water + less blowdown + less chemical cost, but higher scaling risk
Water Treatment
Untreated cooling water causes:
- Scale formation (calcium carbonate, calcium sulfate): insulates heat transfer surfaces, reduces tower/chiller efficiency, blocks piping
- Corrosion: electrochemical attack on steel/copper components, leading to leaks
- Biological growth (algae, bacteria, Legionella): health hazard, slime formation, fouling
- Fouling: suspended solids depositing on surfaces
Treatment Program
- Scale inhibition: phosphonates, polyacrylates, sulfuric acid for pH control
- Corrosion inhibition: zinc, molybdate, orthophosphate, filming inhibitors
- Biocides: chlorine/bromine oxidizing biocides; isothiazolin or glutaraldehyde non-oxidizing for Legionella
- Filtration: side-stream filtration (1-5% of flow) to remove suspended solids
- Blowdown control: conductivity-based automated blowdown valve
- Regular testing: daily pH/conductivity, weekly bacteria counts, monthly corrosion coupons
Fan and Drive Selection
- Propeller fans (axial): large diameter, low pressure, high volume, efficient — standard for induced draft
- Tip speed < 60 m/s for noise control; < 50 m/s for residential areas
- Drive: gear reduction (long life, industrial) or belt (lower cost, HVAC)
- Variable frequency drives (VFDs): vary fan speed with load/wet bulb — save 30-50% energy vs constant speed (fan laws: power ∝ rpm³ — half speed uses 1/8 power)
- Motors: totally enclosed air-over (TEAO); severe-duty for outdoor/coastal
Cold Water Basin and Distribution
- Basin sized to contain system volume when pump shuts down (water drains down) plus operating level
- Basin level control: makeup valve (float or electronic) with overflow drain
- Basin sweepers / debris screens to prevent clogging
- Hot water distribution: spray nozzles (counterflow) or gravity distribution deck (crossflow) — must be kept clean
Performance Optimization
- VFDs on fans modulate capacity — huge energy savings at part load
- Proper water treatment maintains heat transfer efficiency (scale adds 30% to energy use)
- Clean fill regularly (fouled fill can reduce cooling by 20-40%)
- Maintain proper drift eliminators (saves water, prevents Legionella)
- Operate at highest possible COC safely achievable (water savings)
- Use free cooling: when wet bulb is very low, water may be cooled enough without chiller (plate heat exchanger bypass)
Common Problems
| Problem | Cause | Solution |
|---|---|---|
| High cold water temp | Fan off; fill fouled; high approach vs design; under-sized | Check fan; clean fill; check WBT vs design |
| Scale deposition | Hard makeup water; high COC; no treatment | Soften makeup; acid feed; scale inhibitor |
| Corrosion of steel | Low pH; oxygen; galvanic; treatment inadequate | pH control; corrosion inhibitor; material upgrade |
| Algae/biofilm | Sunlight on open basin; no biocide; poor housekeeping | Biocide shock + maintenance; cover basin |
| Legionella | Stagnant water; temperatures 25-50°C; no biocide | Regular biocide; cleaning; periodic testing; record-keeping |
| Excessive drift | Damaged/missing drift eliminators; high air velocity | Repair/replace drift eliminators |
| White rust (galvanized steel) | New galvanized steel exposed to high pH water (>8.3) | Passivate new towers (controlled pH 7-8 for first 6 weeks) |
Summary
Cooling towers reject heat by evaporating water; performance is defined by range (heat load) and approach (tower efficiency relative to wet-bulb temperature). The standard design is induced-draft counterflow or crossflow with axial fans. Approach is the key sizing parameter — a 5°C (10°F) approach to design wet-bulb is the standard. Water treatment (scale, corrosion, biological control) is essential for both efficiency and Legionella safety. Cycles of concentration (target 4-6) minimize makeup water and blowdown. VFDs on fans deliver dramatic energy savings at part load. Cooling towers are selected on heat load (tons), flow, range, and approach to design wet-bulb — not just tonnage.