Two-Stage Evaporative Cooling

Two-stage evaporative cooling combines indirect and direct stages to achieve supply air temperatures below the outdoor wet-bulb temperature—a thermodynamic impossibility with single-stage systems. This configuration maximizes evaporative cooling effectiveness while managing humidity addition.

System Configuration

Indirect-Direct Arrangement

The standard two-stage configuration:

Stage 1 - Indirect Evaporative Cooling (IEC):

Pre-cools outdoor air sensibly
No moisture addition to supply air
Reduces dry-bulb without affecting wet-bulb
Sets up favorable conditions for Stage 2

Stage 2 - Direct Evaporative Cooling (DEC):

Further cools pre-cooled air
Adds moisture through adiabatic saturation
Achieves lowest possible supply temperature

Process Path on Psychrometric Chart

Outdoor Air → [IEC: horizontal left] → [DEC: diagonal toward saturation] → Supply

The two-stage process reaches lower temperatures because Stage 1 reduces the dry-bulb while the wet-bulb remains unchanged, increasing the wet-bulb depression available for Stage 2.

Thermodynamic Analysis

Stage 1: Indirect Cooling

$$T_{1,out} = T_{OA} - \epsilon_{IEC}(T_{OA} - T_{wb,OA})$$

The wet-bulb temperature remains constant: $$T_{wb,1} = T_{wb,OA}$$

Stage 2: Direct Cooling

Using pre-cooled air with unchanged wet-bulb: $$T_{supply} = T_{1,out} - \epsilon_{DEC}(T_{1,out} - T_{wb,1})$$

Combined Effectiveness

Overall wet-bulb effectiveness exceeds either stage alone:

$$\epsilon_{total} = \epsilon_{IEC} + \epsilon_{DEC}(1 - \epsilon_{IEC})$$

Example: εIEC = 70%, εDEC = 85% $$\epsilon_{total} = 0.70 + 0.85(1 - 0.70) = 0.70 + 0.255 = 0.955$$

Sub-Wet-Bulb Achievement

Two-stage systems achieve supply temperatures below outdoor wet-bulb when: $$T_{supply} < T_{wb,OA}$$

This occurs when combined effectiveness exceeds 100%: $$T_{supply} = T_{OA} - \epsilon_{total}(T_{OA} - T_{wb,OA})$$

Performance Example

Design Conditions: 105°F DB / 70°F WB outdoor

Stage 1 (IEC, 70% effective):

Temperature drop: 0.70 × (105 - 70) = 24.5°F
After Stage 1: 105 - 24.5 = 80.5°F DB / 70°F WB

Stage 2 (DEC, 85% effective):

Temperature drop: 0.85 × (80.5 - 70) = 8.9°F
Supply air: 80.5 - 8.9 = 71.6°F DB

Result: Supply air (71.6°F) is below outdoor wet-bulb (70°F)!

Performance Comparison

Condition	DEC Only	Two-Stage
105°F/70°F WB	75°F	71.6°F
100°F/66°F WB	71°F	68.3°F
95°F/65°F WB	70°F	67.6°F

System Components

Integrated Packaged Units

Factory-assembled two-stage coolers:

Components:

Indirect heat exchanger (plate or tube)
Direct media section (rigid cellulose)
Two water systems (may share sump)
Separate or common fans
Integrated controls

Capacities: 3,000-30,000+ CFM

Built-Up Systems

Custom configurations for large installations:

Separate IEC and DEC modules
Central station AHU integration
Multiple stages for ultra-low temperatures
Process cooling applications

Water Systems

Each stage requires water management:

IEC Water System:

Secondary airstream wetting
Recirculating pump
Lower evaporation rate
Bleed-off for concentration control

DEC Water System:

Media wetting distribution
Higher evaporation rate
Combined or separate sump options

Control Strategies

Staged Operation

Sequential control based on conditions:

Mild conditions: IEC only
Warm conditions: IEC + DEC
Hot conditions: Maximum staging

Variable Speed Control

Modulating fan speeds optimize efficiency:

Primary fan: Matches supply CFM demand
Secondary fan: Balances effectiveness vs. energy
Pumps: Ensure adequate wetting

Leaving Air Temperature Control

Supply temperature setpoint control:

$$T_{setpoint} = f(outdoor\ conditions,\ load)$$

Controller modulates stages to maintain setpoint.

Energy Performance

Comparison with Mechanical Cooling

Two-stage evaporative vs. DX cooling:

Metric	Two-Stage Evap	DX Cooling
EER	30-50	10-14
Peak kW/ton	0.3-0.5	0.9-1.2
Water use	3-4 gal/ton·hr	0
Peak demand	60-75% reduction	Baseline

Annual Energy Analysis

Bin analysis determines annual performance:

$$Energy_{annual} = \sum_{bins} hours_i \times \frac{Q_i}{EER_i}$$

Compare two-stage evaporative, single-stage, and mechanical systems for lifecycle cost.

Applications

Commercial Buildings

Excellent fit for:

Office buildings in dry climates
Retail and big-box stores
Schools and universities
Gymnasiums and recreation

Industrial Facilities

Process and comfort cooling:

Manufacturing plants
Warehouses
Aircraft hangars
Assembly facilities

Data Centers

Growing application:

Free cooling hours maximized
PUE improvement
Backup mechanical for peak
ASHRAE climate zones 1-5

Design Considerations

Climate Suitability

Two-stage most effective when:

Outdoor RH < 50% design
Wet-bulb depression > 20°F
Hot-dry summer climate
Extended cooling season

Humidity Management

Stage 2 adds moisture—consider:

Leaving humidity acceptable for application
Ventilation rate dilutes indoor humidity
Bypass damper for humidity-sensitive periods

First Cost vs. Operating Cost

Higher equipment cost offset by:

Lower operating energy
Reduced peak demand charges
Lower maintenance than mechanical

Water Availability

Total consumption both stages: $$\dot{m}{water} ≈ \frac{Q{total}}{h_{fg}} + Bleed$$

Typically 4-6 gal/ton·hr total

Two-stage evaporative cooling provides the ultimate evaporative cooling performance, achieving supply temperatures impossible with single-stage systems while maintaining exceptional energy efficiency in appropriate climates.