Evaporative Cooler Selection

Proper evaporative cooler selection ensures systems deliver required cooling capacity efficiently across expected operating conditions. This guide covers system sizing, equipment type selection, and performance verification for successful installations.

Climate Analysis

Wet-Bulb Frequency Analysis

Before selecting evaporative cooling, analyze local climate data:

Bin Data Analysis:

• Compile hourly wet-bulb temperatures
• Determine hours in each temperature bin
• Calculate cooling degree-hours
• Identify percentage hours evaporative cooling viable

Selection Criteria:

Design Point Selection

Select design conditions for sizing:

Conservative Approach: 1% annual wet-bulb Standard Approach: 2.5% annual wet-bulb Economy Approach: 5% annual wet-bulb

More conservative selection provides more hours of comfort but increases equipment size.

Capacity Calculation

Sensible Cooling Load

Determine space sensible cooling requirement:

$$Q_{sensible} = Q_{transmission} + Q_{solar} + Q_{internal} + Q_{infiltration}$$

Note: Evaporative cooling primarily addresses sensible loads; latent removal is limited.

Airflow Calculation

Required airflow for sensible cooling:

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

Where:

• $T_{room}$ = design indoor temperature
• $T_{supply}$ = achievable supply temperature

Supply Temperature Estimation

Estimate achievable supply temperature:

Direct Evaporative: $$T_{supply} = T_{db,OA} - \epsilon_{DEC}(T_{db,OA} - T_{wb,OA})$$

Two-Stage: $$T_{supply} = T_{db,OA} - [\epsilon_{IEC} + \epsilon_{DEC}(1-\epsilon_{IEC})](T_{db,OA} - T_{wb,OA})$$

Equipment Type Selection

Direct Evaporative Coolers

Select When:

• Design wet-bulb <65°F
• Humidity addition acceptable
• Lowest first cost required
• Simple installation preferred

Typical Effectiveness: 70-90%

Indirect Evaporative Coolers

Select When:

• Supply humidity must be controlled
• Moderate humidity climates
• Humidity-sensitive applications
• Higher first cost acceptable

Typical Effectiveness: 50-75% (wet-bulb)

Two-Stage Systems

Select When:

• Maximum cooling required
• Moderate humidity acceptable
• Climate supports both stages
• Energy savings justify cost

Typical Effectiveness: 90-115% (wet-bulb)

Hybrid Systems

Select When:

• Evaporative cooling marginal
• Backup cooling required
• Variable climate conditions
• Peak load beyond evaporative capacity

Equipment Sizing

Face Area Sizing

Media face area determines capacity:

$$A_{face} = \frac{CFM}{V_{face}}$$

Recommended Face Velocities:

Media Depth Selection

Deeper media increases effectiveness and pressure drop:

Fan Sizing

Fan must overcome total static pressure:

$$\Delta P_{total} = \Delta P_{media} + \Delta P_{intake} + \Delta P_{duct} + \Delta P_{outlet}$$

Select fan for design CFM at total static, with margin for loaded media.

Water System Sizing

Evaporation Rate

Water consumption for cooling:

$$\dot{m}{evap} = \frac{Q{sensible}}{h_{fg}} ≈ 3\ gal/ton \cdot hr$$

Bleed-Off Requirements

Prevent mineral concentration:

$$Bleed\ Rate = \frac{Evap\ Rate}{Cycles - 1}$$

Where cycles of concentration = 3-5 typical

Total Water: 4-5 gal/ton·hr including bleed

Sump Sizing

Sump capacity for adequate reserve:

• Minimum: 5-10 gallons per ft² face area
• Or: 3-5 minutes pump flow capacity

Performance Verification

Commissioning Tests

Verify installed performance:

1. Airflow measurement: Compare to design CFM
2. Temperature drop: Verify effectiveness
3. Pressure drop: Check fan operating point
4. Water distribution: Confirm media wetting

Effectiveness Verification

Calculate actual effectiveness:

$$\epsilon_{actual} = \frac{T_{in} - T_{out}}{T_{in} - T_{wb,in}}$$

Compare to published performance curves.

Energy Performance

Measure EER or kW/ton:

$$EER = \frac{Q_{cooling}}{W_{total}}$$

Compare to design values and alternatives.

Selection Documentation

Equipment Schedule

Document selections:

Operating Limits

Specify operating parameters:

• Maximum wet-bulb for evaporative mode
• Changeover points for hybrid systems
• Maintenance schedules
• Media replacement criteria

Careful evaporative cooler selection, matched to climate conditions and application requirements, ensures systems deliver expected performance while maximizing energy savings.