What is the approach temperature in a cooling tower?

Approach temperature is the difference between the cold water temperature leaving the tower and the ambient wet bulb temperature. A typical approach is 7-10°F. Smaller approach (5-7°F) requires a larger, more expensive tower. Below 5°F approach is generally impractical and uneconomical.

How do you calculate cooling tower makeup water?

Makeup water equals evaporation loss plus blowdown plus drift loss. Evaporation is approximately 1% of circulation rate per 10°F of cooling range. Blowdown equals evaporation divided by (cycles of concentration minus 1). Drift is typically 0.005% of circulation rate for modern towers with drift eliminators.

What is the L/G ratio in cooling tower design?

L/G ratio is the liquid-to-gas mass flow ratio (lb water / lb dry air). It typically ranges from 0.75 to 1.5 for mechanical draft towers. Lower L/G means more air per unit of water, requiring larger fans but a shorter tower. Higher L/G means less air, requiring a taller tower with more fill volume.

What is KaV/L (tower characteristic)?

KaV/L is the Merkel number or tower characteristic that quantifies cooling tower thermal performance. It represents the number of transfer units (NTU) and is calculated by integrating dT/(h_s - h_a) over the cooling range using the Chebyshev 4-point numerical method. Higher KaV/L means more difficult cooling duty.

Cooling Tower Sizing Calculator | Mechanical Draft Tower Design

Thermal Design

Input Mode

Heat Duty

MMBtu/hr

Water Flow Rate

GPM

Hot Water Temperature (inlet)

°F

Cold Water Temperature (outlet)

°F

Ambient Conditions

Wet Bulb Temperature

°F

Design wet bulb: use 1% or 2.5% ASHRAE summer design value

Site Altitude

Tower Configuration

Draft Type

Number of Cells

cells

Cycles of Concentration

Typical: 3-7. Higher = less blowdown but more scaling risk.

Understanding Cooling Tower Design

Approach Temperature:
Approach = T_cold - T_wb. The minimum practical approach is ~5°F. Smaller approach requires larger, costlier towers. Typical design: 7-10°F.

Cooling Range:
Range = T_hot - T_cold. The temperature drop across the tower. Range affects evaporation rate and tower size.

Water Balance:
Makeup = Evaporation + Blowdown + Drift. Evaporation is ~1% of flow per 10°F range. Blowdown depends on cycles of concentration to control dissolved solids.

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Results

Makeup Water

GPM

Water Flow Rate -

Heat Rejection -

Range (Hot - Cold) -

Approach (Cold - WB) -

Evaporation Rate -

Blowdown Rate -

Drift Loss -

L/G Ratio -

Tower Characteristic (KaV/L) -

Fan HP per Cell -

Total Fan HP -

Annual Water Use -

Formula

Q = GPM × 500 × Range

Q = Heat duty (Btu/hr)

GPM = Water circulation rate

500 = 8.33 lb/gal × 60 min/hr

Range = T_hot - T_cold (°F)

Approach = T_cold - T_wb (°F)

Makeup = Evap + BD + Drift

Standards & References

CTI STD-201
Cooling Tower Performance Testing
ASHRAE Handbook
HVAC Systems - Cooling Towers Chapter
API 661
Air-Cooled Heat Exchangers (related)
Merkel (1925)
Evaporative cooling tower theory
CTI Toolkit
Tower characteristic curves and design
GPSA Engineering Data Book
Section 10: Cooling Systems

Engineering Notes

Approach: Minimum practical ~5°F; typical design 7-10°F for economy
Wet bulb: Use ASHRAE 1% or 2.5% design wet bulb for location
Cycles: 3-5 typical; higher cycles reduce blowdown but increase scaling
Drift: Modern eliminators reduce drift to 0.001-0.005% of flow
Counterflow vs Crossflow: Counterflow more efficient but taller; crossflow easier to maintain
Fan HP: Induced draft is more common; fans at top pull air through fill

Quick Reference — Typical Values

Evaporation: ~1% of GPM per 10°F range
Drift loss: 0.005% of circulation rate
L/G ratio: 0.75-1.50 (mechanical draft)
KaV/L: 0.5-2.5 typical range
Fan HP: 30-100 HP per cell typical
Tower approach: 5-15°F range

Cooling Tower Sizing Calculator

Thermal Design

Ambient Conditions

Tower Configuration

Understanding Cooling Tower Design

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — Typical Values

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