Indirect Evaporative Cooling

Indirect evaporative cooling (IEC) provides sensible cooling to supply air without adding moisture, overcoming a key limitation of direct evaporative systems. By separating the supply airstream from the evaporative process, IEC extends evaporative cooling applicability to humidity-sensitive applications.

Operating Principles

Two-Airstream Concept

IEC systems use two separate airstreams:

Primary Air (Product/Supply):

• Air delivered to conditioned space
• Cooled by heat exchange
• No moisture added

Secondary Air (Working/Scavenger):

• Evaporatively cooled
• Absorbs heat from primary
• Exhausted outdoors

Heat Exchange Process

Heat transfer occurs across an exchange surface:

$$Q = UA \times LMTD$$

Where LMTD accounts for temperature differences between streams.

Primary Air Cooling: $$T_{primary,out} = T_{primary,in} - \epsilon_{sensible}(T_{primary,in} - T_{secondary,wb})$$

Effectiveness Definition

IEC effectiveness relates outlet temperature to wet-bulb approach:

Wet-Bulb Effectiveness: $$\epsilon_{wb} = \frac{T_{1,in} - T_{1,out}}{T_{1,in} - T_{2,wb,in}}$$

Typical values: 50-80% for conventional IEC

Heat Exchanger Types

Plate-Type Exchangers

Parallel plates separate primary and secondary streams:

Construction:

• Aluminum or polymer plates
• Alternating primary/secondary passages
• Secondary side wetted for evaporation
• Cross-flow or counter-flow arrangement

Performance:

• Effectiveness: 50-70%
• Pressure drop: 0.3-0.5" w.g. each stream
• Compact design
• Moderate cost

Tube-Type Exchangers

Air flows through or around tubes:

Configuration:

• Primary air inside tubes
• Secondary air over wetted tubes
• Vertical tube orientation common
• Enhanced surfaces available

Characteristics:

• Robust construction
• Higher pressure capability
• Easier maintenance
• Lower effectiveness than plate

Heat Pipe Exchangers

Two-phase heat transfer mechanism:

• Evaporator section in hot stream
• Condenser section in cool stream
• Working fluid transfers heat
• No cross-contamination

Regenerative (M-Cycle) Exchangers

Advanced design achieves sub-wet-bulb cooling:

Maisotsenko Cycle:

1. Primary air pre-cools through dry channels
2. Portion diverted to wet channels
3. Evaporative cooling in wet channels
4. Counter-flow heat exchange

Performance:

• Achieves 80-90% dew-point effectiveness
• Supply temperature approaches dew-point
• Highest efficiency IEC technology

Performance Characteristics

Wet-Bulb Approach

Primary air outlet approaches secondary wet-bulb:

Airflow Ratio

Secondary-to-primary flow ratio affects performance:

$$Ratio = \frac{\dot{m}{secondary}}{\dot{m}{primary}}$$

Typical range: 0.3-1.0

Higher ratio increases effectiveness but also:

• Fan energy consumption
• Water consumption
• Equipment size

Cooling Capacity

Sensible cooling delivered:

$$Q_{sensible} = \dot{m}{primary} \times c_p \times (T{in} - T_{out})$$

Example: 10,000 CFM primary, 100°F inlet, 78°F outlet $$Q = 1.08 \times 10,000 \times 22 = 237,600\ Btu/h ≈ 20\ tons$$

System Configurations

Standalone IEC Units

Pre-packaged indirect coolers:

• Self-contained with fans and pumps
• Outdoor air intake
• Supply air delivery
• Capacities: 5,000-50,000 CFM

AHU Integration

IEC section within air handling unit:

• Pre-cooling outdoor air
• Upstream of cooling coil
• Reduces mechanical cooling load
• Energy recovery function

Hybrid Systems

Combined IEC with mechanical cooling:

• IEC provides first-stage cooling
• DX or chilled water for trim cooling
• Optimizes efficiency across conditions
• Automatic staging based on demand

Advantages Over Direct Evaporative

No Moisture Addition

Supply air humidity unchanged:

• Suitable for humidity-sensitive spaces
• Offices, retail, data centers
• Museums and archives
• Healthcare facilities

Wider Climate Applicability

Effective in moderately humid climates:

• Operates when direct evaporative marginal
• Useful to 60% outdoor RH
• More consistent performance

Better Indoor Air Quality

No water contact with supply air:

• Eliminates microbial growth concern
• No carryover of treatment chemicals
• Cleaner supply air

Design Considerations

Airstream Arrangement

Counter-flow: Maximum effectiveness Cross-flow: Lower pressure drop, simpler construction Parallel-flow: Lowest effectiveness (rarely used)

Secondary Air Source

Options for secondary airstream:

1. 100% Outdoor Air: Maximum cooling potential
2. Exhaust Air: Energy recovery mode (ERV function)
3. Return Air: Not recommended (humidity concern)

Water Management

Secondary side wetting requirements:

• Continuous recirculating spray
• Drip-type distribution
• Media-assisted wetting

Water quality and treatment similar to direct evaporative.

Pressure Drop Budget

Both airstreams require fan energy:

$$W_{total} = \frac{\dot{V}_1 \Delta P_1}{\eta_1} + \frac{\dot{V}_2 \Delta P_2}{\eta_2}$$

Balance effectiveness vs. fan energy in design.

Applications

Data Center Cooling

IEC increasingly specified for data centers:

• ASHRAE TC 9.9 expanded temperature envelope
• Significant energy savings
• Partial cooling adequate for much of year
• DX backup for peak conditions

Commercial Buildings

Suitable for many commercial applications:

• Office buildings in dry to moderate climates
• Pre-cooling for conventional systems
• Dedicated outdoor air systems (DOAS)

Industrial Processes

Process cooling applications:

• Cooling without humidity increase
• Make-up air systems
• Equipment heat rejection

Indirect evaporative cooling provides the energy efficiency benefits of evaporative technology while maintaining supply air humidity control, enabling broader application across building types and climates.