Direct Evaporative Cooling

Direct evaporative cooling (DEC) passes air directly through wetted media, achieving sensible cooling through the adiabatic saturation process. This fundamental cooling technology offers exceptional energy efficiency in appropriate climates.

Operating Principles

Adiabatic Saturation Process

In direct evaporative cooling, air contacts water at the wet-bulb temperature:

$$T_{out} = T_{in} - \epsilon (T_{in} - T_{wb,in})$$

Where:

• $T_{out}$ = leaving dry-bulb temperature
• $T_{in}$ = entering dry-bulb temperature
• $T_{wb,in}$ = entering wet-bulb temperature
• $\epsilon$ = saturation effectiveness (0.7-0.95)

Humidity Addition

Moisture content increases as sensible heat converts to latent:

$$W_{out} = W_{in} + \frac{c_p (T_{in} - T_{out})}{h_{fg}}$$

The leaving relative humidity approaches saturation (85-95% RH).

Heat and Mass Transfer

Combined heat and mass transfer governs performance:

$$\dot{Q} = h_c A (T_{air} - T_{water}) + h_m A h_{fg} (W_{sat,water} - W_{air})$$

The Lewis relation connects heat and mass transfer coefficients: $$\frac{h_c}{h_m c_p} \approx 1$$ (Lewis number ≈ 1 for air-water)

Media Types

Rigid Media (Cellulose)

Cross-corrugated cellulose pads dominate commercial applications:

Construction:

• Kraft paper impregnated with anti-rot compounds
• Cross-fluted design creates turbulent flow
• Standard thicknesses: 6", 12", 18"
• Available widths: 12-60 inches

Performance:

Service Life: 5-10 years with proper maintenance

Aspen Pads

Traditional residential and small commercial media:

• Random fiber packing (aspen wood excelsior)
• Lower cost than rigid media
• Effectiveness: 60-70%
• Shorter life: 1-3 seasons
• Requires more frequent replacement

Synthetic Media

Engineered plastic or fiberglass media:

• Washable and reusable
• Consistent performance over time
• Higher initial cost
• Suitable for harsh water conditions

System Components

Water Distribution

Uniform wetting ensures maximum effectiveness:

Distribution Methods:

• Gravity overflow troughs
• Spray headers with nozzles
• Recirculating pump systems

Design Requirements:

• Flow rate: 1.5-3 GPM per linear foot of media
• Coverage: 100% media saturation
• Even distribution prevents dry spots

Sump and Pump

Recirculating water system includes:

• Sump capacity: 5-10 gallons per face ft²
• Pump sizing: 3-5 GPM/ft² face area
• Float valve for makeup water
• Overflow/bleed connection

Air Moving

Fan selection for evaporative coolers:

$$CFM = \frac{Q_{sensible}}{1.08 \times (T_{room} - T_{supply})}$$

Face Velocity: 400-600 fpm through media (500 typical)

Performance Characteristics

Effectiveness Factors

Saturation effectiveness depends on:

1. Media depth: Deeper = higher effectiveness
2. Face velocity: Lower velocity = higher effectiveness
3. Media condition: Clean, properly wetted
4. Water temperature: Approach to wet-bulb

Performance Curves

Typical direct evaporative cooler performance:

Capacity

Cooling capacity:

$$Q = 1.08 \times CFM \times (T_{in} - T_{out})$$

Example: 10,000 CFM, 100°F inlet, 71°F outlet $$Q = 1.08 \times 10,000 \times 29 = 313,200\ Btu/h = 26\ tons$$

System Configurations

Packaged Rooftop Units

Complete factory-assembled coolers:

• Capacities: 1,000-30,000+ CFM
• Horizontal or downdraft discharge
• Pre-piped water connections
• Integrated controls

Side-Draft Units

Ground or wall-mounted:

• Horizontal airflow through media
• Suitable for residential/light commercial
• Gravity water distribution
• Capacities: 2,000-10,000 CFM

Built-Up Systems

Custom installations for large applications:

• Multiple media banks
• Plenum construction
• Central station AHU integration
• Capacities: 10,000-200,000+ CFM

Control Strategies

Basic Control

Simple on/off control:

• Thermostat starts fan and pump
• Pump starts before fan (pre-wet cycle)
• Timer delay between stages

Variable Speed

Modulating capacity:

• Variable frequency drive on fan
• Speed varies with cooling demand
• Maintains leaving air temperature
• Improved part-load efficiency

Economizer Integration

Outdoor air economizer mode:

1. Free cooling when outdoor < indoor
2. DEC when outdoor dry-bulb elevated
3. Mechanical backup when DEC insufficient

Limitations

Climate Constraints

Direct evaporative cooling limited by:

• Outdoor wet-bulb temperature
• Required supply air temperature
• Indoor humidity tolerance

Rule of Thumb: Effective when outdoor RH < 40%

Indoor Humidity

Moisture addition may cause:

• Occupant discomfort at high supply RH
• Material damage in humidity-sensitive areas
• Mold growth potential in conditioned space

Mitigation: Increased ventilation rate flushes moisture

Water Quality Effects

Hard water causes:

• Scale buildup on media
• Reduced effectiveness
• Shortened media life
• Increased maintenance

Treatment options: Bleed-off, softening, chemical treatment

Direct evaporative cooling delivers outstanding energy efficiency when climate conditions and application requirements align, providing sustainable cooling with minimal environmental impact.