Cooling Tower Performance for HVAC Engineers

Cooling towers reject heat via evaporative cooling. Performance depends on ambient wet bulb temperature, water flow, and airflow. Proper sizing and control optimize chiller efficiency and minimize water/energy consumption.

Performance Parameters

Range:

$$Range = T_{in} - T_{out}$$

Typically 10°F for HVAC applications

Approach:

$$Approach = T_{out} - T_{wb}$$

Typically 7-10°F (lower approach requires larger/more expensive tower)

Effectiveness:

$$\epsilon = \frac{Range}{Range + Approach} = \frac{T_{in} - T_{out}}{T_{in} - T_{wb}}$$

Typical: 0.65-0.75

Heat Rejection

$$Q = \dot{m}_{water} \times c_p \times Range$$

$$Q = GPM \times 500 \times \Delta T$$

Where 500 = 8.33 lb/gal × 60 min/h × 1.0 Btu/(lb·°F)

Water evaporation rate:

$$GPM_{evap} = \frac{Q}{8.33 \times 1,000}$$

Approximately 0.3% of water circulation rate per °F range

Fan Power

Mechanical draft towers:

$$hp_{fan} = \frac{CFM_{air} \times \Delta P_{tower}}{6,356 \times \eta}$$

Energy tradeoff:

More airflow: Lower leaving water temp, better chiller efficiency
Less airflow: Lower fan power, higher leaving water temp

Optimal: Balance chiller and tower fan energy (typically leaving water temp 70-80°F)

Water Treatment

Makeup water:

$$GPM_{makeup} = GPM_{evap} + GPM_{blowdown} + GPM_{drift}$$

Cycles of concentration:

$$COC = \frac{TDS_{circulating}}{TDS_{makeup}}$$

Higher COC reduces water consumption but requires better treatment

Typical COC: 3-6 (limited by scaling, corrosion, biological growth)

Practical Applications

Chiller condenser: 85°F entering, 95°F leaving, 78°F wet bulb design
Free cooling: Tower alone when wet bulb < 45-50°F
VFD fans: Modulate to maintain leaving water setpoint

Related Technical Guides:

References:

ASHRAE Handbook of HVAC Systems and Equipment, Chapter 40: Cooling Towers
CTI Code Tower STD-201: Standard for Certification of Water Cooling Towers