Cooling Tower Fundamentals | Engineering Guide

1. Cooling Tower Fundamentals

Cooling towers reject heat from water to the atmosphere through evaporative cooling. A small portion of the circulating water evaporates, absorbing latent heat and cooling the remaining water. This makes towers far more effective than dry air-cooled exchangers for achieving cold water temperatures near the ambient wet bulb.

Evaporative cooling

Latent heat transfer

~80% of heat rejected via evaporation of ~1% of flow. Sensible heat transfer contributes ~20%.

Driving force

Wet bulb temperature

Cooling is limited by T_wb, not dry bulb. Lower humidity means better performance.

Water loss

Makeup required

Makeup = evaporation + blowdown + drift. Typically 2-5% of circulation rate.

Key Terminology

Range: Temperature drop of the water through the tower Range = T_hot - T_cold (°F) Approach: How close the cold water gets to wet bulb Approach = T_cold - T_wb (°F) Heat Duty: Total heat rejected Q = GPM × 500 × Range (Btu/hr) Where 500 = 8.33 lb/gal × 60 min/hr

Approach Temperature Design Guidelines

Approach (°F)	Feasibility	Tower Size	Relative Cost
> 10	Easy	Small / standard	1.0x (baseline)
7-10	Normal design	Standard	1.0-1.3x
5-7	Achievable	Large / premium fill	1.5-2.0x
3-5	Difficult	Very large	2.5-4.0x
< 3	Impractical	Economically prohibitive	> 5x

Design rule: Target 7-10°F approach for economic designs. Every 1°F reduction in approach below 7°F significantly increases tower size and cost. The absolute minimum practical approach is about 5°F.

2. Tower Types & Configurations

Mechanical draft cooling towers use fans to move air through the tower. They are classified by airflow direction relative to the falling water and by fan placement.

Draft Types

Induced draft

Fan at top (most common)

Fan pulls air through fill. Even air distribution. Higher efficiency. Reduces recirculation of hot, moist exhaust air.

Forced draft

Fan at base

Fan pushes air upward. Easier maintenance (fan at grade). Higher recirculation risk. Used in smaller towers and confined spaces.

Natural draft

Hyperbolic shell

Buoyancy-driven airflow through tall concrete shell. Used for large power plants (>100 MW). Very high capital, low operating cost.

Airflow Configuration

Feature	Counterflow	Crossflow
Air direction	Vertically upward, against falling water	Horizontally through falling water
Thermal efficiency	Higher (more contact time)	Lower (shorter contact path)
Tower height	Taller (air must travel through fill)	Shorter and wider
Pressure drop	Higher (air vs. water flow)	Lower
Maintenance access	More difficult (enclosed fill)	Easier (open sides)
Freeze protection	Better (enclosed structure)	Worse (exposed fill)
Best for	Demanding duties, tight approach	General service, easy maintenance

Fill Media Types

Fill Type	Description	Application
Film fill	Thin PVC sheets creating large wetted surface	Clean water; highest efficiency; most common
Splash fill	Horizontal bars or grids breaking water into drops	Dirty water; debris tolerant; lower efficiency
Trickle fill	Modular blocks with channels	Moderate water quality; good balance

Fill selection: Film fill provides highest KaV/L per unit volume but clogs with dirty water. For process cooling with potential fouling, splash fill is more reliable despite requiring a larger tower.

3. Merkel Equation & Thermal Design

The Merkel equation (1925) is the fundamental theory for cooling tower thermal analysis. It relates the tower's ability to cool water (demand) to the tower's physical size and mass transfer capability (supply).

Merkel Equation

Tower Characteristic (KaV/L): KaV/L = integral from T_cold to T_hot of dT / (h_s - h_a) Where: K = Mass transfer coefficient (lb/hr·ft²) a = Contact area per unit volume (ft²/ft³) V = Active tower volume (ft³) L = Water mass flow rate (lb/hr) h_s = Enthalpy of saturated air at water temperature (Btu/lb) h_a = Enthalpy of air stream at that elevation (Btu/lb) KaV/L is also called the "Merkel number" or "NTU" (number of transfer units).

Chebyshev 4-Point Integration

The Merkel integral is evaluated numerically using the Chebyshev 4-point method, which samples at 0.1, 0.4, 0.6, and 0.9 of the cooling range:

Chebyshev Approximation: KaV/L = Range/4 × [1/Delta(h)₁ + 1/Delta(h)₂ + 1/Delta(h)₃ + 1/Delta(h)₄] At water temperatures: T₁ = T_cold + 0.1 × Range T₂ = T_cold + 0.4 × Range T₃ = T_cold + 0.6 × Range T₄ = T_cold + 0.9 × Range Delta(h)_i = h_s(T_i) - h_a(T_i)

L/G Ratio

The liquid-to-gas ratio (L/G) is a critical design parameter that defines the relationship between water and air flows through the tower.

Energy Balance: L × Cp × (T_hot - T_cold) = G × (h_a,out - h_a,in) Therefore: L/G = (h_a,out - h_a,in) / [Cp × (T_hot - T_cold)] Typical L/G: 0.75-1.50 for mechanical draft towers Where: L = Water mass flow (lb/hr) G = Dry air mass flow (lb/hr) Cp = Specific heat of water = 1.0 Btu/(lb·°F) h_a,in = Entering air enthalpy at wet bulb h_a,out = Leaving air enthalpy

Tower Demand vs. Supply

Factor	Increases KaV/L (demand)	Decreases KaV/L (demand)
Approach	Smaller approach	Larger approach
Range	Larger range	Smaller range
Wet bulb	Higher wet bulb	Lower wet bulb
L/G ratio	Higher L/G	Lower L/G

Design verification: A tower manufacturer provides a "supply" KaV/L curve based on L/G and fill characteristics. The design point is where the demand curve (from process conditions) intersects the supply curve.

4. Water Balance & Treatment

Cooling towers consume water through evaporation, blowdown, and drift. Understanding the water balance is essential for makeup water sizing, chemical treatment, and environmental compliance.

Water Balance Equations

Evaporation Rate: E = Q / h_fg (thermodynamic) E approximately = GPM × Range × 0.001 (rule of thumb: 1% per 10°F) Blowdown Rate: BD = E / (CoC - 1) Drift Loss: D = GPM × 0.00005 (0.005% with modern eliminators) Makeup Water: MU = E + BD + D Where: CoC = Cycles of concentration h_fg = Latent heat of vaporization (~1,000 Btu/lb)

Cycles of Concentration

Cycles of concentration (CoC) represents how many times the dissolved solids in the circulating water are concentrated compared to the makeup water. Higher cycles reduce water waste but increase scaling and corrosion risk.

Cycles (CoC)	Water Savings	Scaling Risk	Treatment Need
2	Baseline	Low	Minimal
3	33% less blowdown	Moderate	Standard
5	50% less blowdown	Moderate-High	Active program
7	58% less blowdown	High	Aggressive treatment
10	63% less blowdown	Very high	Specialized treatment

Water Treatment Requirements

Scale control

Hardness & pH

Calcium carbonate scale forms when Langelier Saturation Index (LSI) > 0. Control pH, hardness, and alkalinity. Use scale inhibitors (phosphonates, polymers).

Corrosion control

Inhibitors

Dissolved oxygen and chlorides cause corrosion. Use molybdate, phosphate, or azole-based inhibitors. Target corrosion rates < 3 mpy for carbon steel.

Biological control

Biocides

Warm, aerated water promotes algae and Legionella. Use oxidizing (chlorine/bromine) and non-oxidizing biocides. Monitor microbial counts regularly.

Legionella Prevention

Cooling towers are a known risk for Legionella bacteria growth. ASHRAE Standard 188 requires a water management plan addressing:

Maintain circulating water temperature above 70°F or below 68°F when practical
Prevent stagnation in dead legs and bypass lines
Maintain adequate biocide residual (free chlorine 0.5-1.0 ppm)
Minimize drift with high-efficiency drift eliminators
Regular monitoring for Legionella (quarterly minimum)

ASHRAE 188: All building cooling towers should have a documented Water Management Plan (WMP) for Legionella prevention. This is now a code requirement in many jurisdictions.

5. Selection & Sizing Guidelines

Design Input Checklist

Heat duty: Total heat to be rejected (MMBtu/hr or tons)
Water temperatures: Required hot and cold water temperatures
Wet bulb: Design wet bulb temperature (ASHRAE 1% or 2.5% value)
Water quality: Makeup water analysis (hardness, TDS, pH)
Site constraints: Footprint, height limits, noise restrictions
Plume visibility: Aesthetics or fog concerns may require plume-abated towers

Tower Selection Guidelines

Application	Recommended Type	Notes
Process cooling (< 5,000 GPM)	Induced draft, counterflow	Most efficient for tight approach
Process cooling (> 5,000 GPM)	Induced draft, crossflow	Lower maintenance, easier inspection
HVAC / comfort cooling	Induced draft, crossflow	Standard package units available
Confined spaces	Forced draft	Lower profile; fan at grade level
Large power plants	Natural draft (hyperbolic)	No fan energy; very high capital
Dirty water / high fouling	Splash fill, crossflow	Debris tolerant; easy cleaning

Fan Horsepower Estimation

Fan HP: HP = CFM × Delta(P) / (6,356 × eta_fan × eta_motor) Where: CFM = Air volume flow (ft³/min) Delta(P) = Tower pressure drop (inches water gauge) eta_fan = Fan efficiency (~0.75) eta_motor = Motor efficiency (~0.93) 6,356 = Conversion constant Typical tower pressure drop: Counterflow: 0.5-0.6 in. WG Crossflow: 0.3-0.5 in. WG Forced draft: 0.5-0.7 in. WG

Cooling Tower vs. Air-Cooled Exchanger

Factor	Cooling Tower	Air-Cooled Exchanger
Cold fluid temperature	Near wet bulb (lower)	15-20°F above dry bulb (higher)
Water consumption	2-5% of circulation	None
Operating cost	Higher (water + chemicals)	Lower (electricity only)
Capital cost	Lower (per ton cooling)	Higher for equivalent duty
Footprint	Smaller per ton	Larger (more surface area)
Maintenance	Water treatment, fill replacement	Fan maintenance, fin cleaning
Environmental	Water use, plume, Legionella risk	Noise, no water use

Typical Performance Ranges

Parameter	Typical Range	Notes
L/G ratio	0.75-1.50	Mechanical draft towers
KaV/L	0.5-2.5	Higher = more demanding duty
Fan HP per cell	30-100 HP	Depends on cell size and airflow
GPM per cell	1,000-10,000	Package to field-erected
Drift loss	0.001-0.005%	Of circulation rate
Fill height	4-12 ft	Counterflow typically taller

Economics: For sites with available water, cooling towers provide lower cold water temperatures at lower capital cost than air coolers. Where water is scarce or expensive, hybrid (wet/dry) or air-cooled systems become competitive despite higher capital cost.

Cooling Tower Design

7-10°F design

~1% per 10°F range

Counterflow & Crossflow

Contents

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