Evaporative Condensers

Operating Principles

Evaporative condensers combine heat rejection mechanisms into a single unit where refrigerant condenses inside tubes or coils while water sprays over the exterior surface and air flows across the wetted coil. Heat transfer occurs through two simultaneous processes: sensible heat transfer from the temperature difference between refrigerant and air/water, and latent heat transfer from water evaporation.

The evaporative cooling effect provides significantly lower condensing temperatures compared to air-cooled condensers operating under identical ambient conditions. This results in lower refrigerant head pressure, reduced compressor power consumption, and improved system efficiency.

Heat Transfer Process

Heat rejection in an evaporative condenser occurs in three stages. First, hot refrigerant gas enters the condenser coil at saturated or superheated conditions. Second, sprayed water contacts the external coil surface while air flows through the wetted media. Third, heat transfers from refrigerant to tube wall, then to water film, with a portion of the water evaporating and carrying away latent heat.

The combined heat transfer coefficient is substantially higher than air-cooled designs because evaporation dominates the heat rejection process. Water evaporation accounts for 75-80% of total heat removal, while sensible cooling from air and water provides the remaining 20-25%.

Wet-Bulb Approach Temperature

Evaporative condensers operate based on ambient wet-bulb temperature rather than dry-bulb temperature. The wet-bulb approach is defined as the difference between refrigerant condensing temperature and entering air wet-bulb temperature.

Typical design approaches range from 10-15°F (5.5-8.3°C) for standard efficiency units. High-efficiency designs achieve 7-10°F (3.9-5.5°C) approach temperatures through increased coil surface area, optimized spray patterns, and enhanced air distribution.

For a design condition of 78°F (25.6°C) wet-bulb temperature with a 12°F (6.7°C) approach, the refrigerant condenses at 90°F (32.2°C). An air-cooled condenser operating at 95°F (35°C) dry-bulb would typically condense refrigerant at 110-115°F (43.3-46.1°C), demonstrating the 20-25°F (11.1-13.9°C) condensing temperature advantage.

Water Consumption

Water consumption in evaporative condensers consists of three components: evaporation, blowdown, and drift. Evaporation removes approximately 1.8 gallons per minute per 1 million Btu/hr (0.00027 L/s per kW) of heat rejection at design conditions. This represents the primary water usage and cannot be eliminated as it provides the cooling mechanism.

Blowdown water maintains acceptable dissolved solids concentration in the recirculating water. The required blowdown rate depends on cycles of concentration, makeup water quality, and treatment program requirements.

Total water consumption typically ranges from 2.1-2.4 gallons per minute per million Btu/hr (0.00032-0.00036 L/s per kW) of heat rejection capacity at design conditions.

Spray Water System

The spray water system distributes water uniformly over the condenser coil surface. Spray nozzles or distribution pans deliver water at rates between 1.5-3.0 gpm per square foot (1.0-2.0 L/s per m²) of coil face area. Higher spray rates improve wetting but increase pump power and potential for drift.

Water distribution uniformity directly affects heat transfer performance. Poor distribution creates dry spots on the coil where heat transfer reverts to air-cooled conditions, significantly reducing capacity and increasing condensing pressure.

The sump at the unit base collects water for recirculation. Sump capacity typically provides 1-3 minutes of pump operation at the circulation rate to allow time for makeup water addition and prevent pump cavitation during transient conditions.

Air Flow Configuration

Induced-draft configuration places the fan at the discharge, pulling air through the wetted coil. This arrangement provides uniform air distribution, prevents recirculation of saturated discharge air, and allows the fan to operate in relatively dry air.

Forced-draft designs position the fan at the inlet, pushing air through the unit. This configuration may encounter fan motor overheating in saturated air conditions and requires careful design to prevent short-circuiting of warm, moist discharge air back to the inlet.

Airflow rates typically range from 300-500 cfm per ton (15-25 L/s per kW) of refrigeration capacity. Lower airflow reduces fan power but requires larger coil area to maintain approach temperature.

Scale and Corrosion Control

Scale formation occurs when dissolved minerals in makeup water concentrate through evaporation and precipitate on heat transfer surfaces. Calcium carbonate, calcium sulfate, and silica represent the most common scale-forming compounds. Scale deposits insulate heat transfer surfaces, increasing condensing temperature and reducing system efficiency.

Cycles of concentration describe the ratio of dissolved solids in recirculating water to dissolved solids in makeup water. Operating at 3-5 cycles of concentration balances water conservation against scale formation risk. Higher cycles reduce water consumption but increase scaling potential.

Corrosion control prevents metal loss from condenser coils, basin, and structural components. Corrosion occurs through galvanic action, microbiological activity, and chemical attack. Copper and steel components in contact require dielectric isolation to prevent galvanic corrosion.

Water Treatment Programs

Chemical water treatment controls scale, corrosion, and biological growth. Treatment programs typically include:

• Scale inhibitors (phosphonates, polymers) that prevent crystal formation and growth
• Corrosion inhibitors (azoles, phosphates) forming protective films on metal surfaces
• Biocides (oxidizing and non-oxidizing) controlling bacteria, algae, and fungi
• pH adjustment maintaining water chemistry within acceptable ranges (7.5-8.5 typical)

Treatment chemical feed methods include continuous injection, timer-based feed, or conductivity-controlled feed systems. Conductivity measurement provides indirect indication of dissolved solids concentration, enabling automated blowdown control.

Legionella Prevention

Legionella bacteria proliferate in water systems operating between 68-122°F (20-50°C), with optimal growth at 95-115°F (35-46°C). Evaporative condensers fall within this temperature range and can aerosolize water droplets containing bacteria, creating potential health risks.

Prevention strategies include:

1. Temperature management: Maintain water temperature below 68°F (20°C) when possible through system operation scheduling
2. Biocide programs: Implement continuous oxidizing biocide (chlorine, bromine) maintaining 1-2 ppm free halogen residual
3. System cleaning: Perform periodic mechanical cleaning and disinfection, removing biofilm and sediment
4. Drift elimination: Install high-efficiency drift eliminators reducing aerosol release to <0.001% of recirculation rate
5. Monitoring: Conduct regular water testing for Legionella bacteria presence and supporting conditions

ASHRAE Standard 188 and Guideline 12 provide comprehensive Legionella risk management frameworks for evaporative cooling equipment.

Freeze Protection

Freeze protection prevents ice formation during cold weather operation or shutdown. Water freezing in the sump, piping, or on the coil can cause equipment damage requiring costly repairs.

Operating freeze protection methods include:

• Basin heaters: Electric immersion heaters maintain sump water above 40°F (4.4°C)
• Water circulation: Continuous pump operation prevents freezing through water movement
• Coil spray during operation: Water film on coil surface provides freeze protection through latent heat release
• Reduced airflow: Damper modulation or fan cycling maintains discharge air above freezing

Shutdown freeze protection requires complete water drainage from all components. Sloped piping, drain valves at low points, and compressed air purging ensure complete water removal. Some installations use glycol solutions in the spray water system for winter operation, though this reduces heat transfer efficiency.

Capacity Control

Evaporative condenser capacity modulation maintains acceptable condensing pressure across varying load conditions. Control methods include:

1. Fan cycling: On/off operation of single or multiple fans in steps
2. Fan speed control: Variable frequency drives providing continuous capacity modulation
3. Water flow modulation: Reducing spray water flow rate, though this risks dry spots and reduced efficiency
4. Air dampers: Modulating inlet or discharge dampers to restrict airflow

Two-speed or variable-speed fan control provides superior performance compared to on/off cycling by maintaining more stable condensing pressure and reducing power consumption at part-load conditions.

Efficiency Comparison with Remote Systems

Comparing evaporative condensers with separate cooling tower and water-cooled condenser systems reveals trade-offs in efficiency, space requirements, and installation complexity.

Evaporative condensers typically achieve 5-10°F (2.8-5.6°C) lower condensing temperature than cooling tower systems because they eliminate the approach temperature loss in the water-cooled condenser. This translates to 8-15% reduction in compressor power consumption.

However, cooling tower systems offer advantages in large installations where multiple compressors share common cooling water, in applications requiring very low condensing temperatures, or where water quality challenges make direct spray on refrigerant coils problematic.

Maintenance Requirements

Regular maintenance sustains performance and prevents premature failure. Critical maintenance tasks include:

Monthly:

• Inspect water distribution uniformity across coil
• Check spray nozzles for plugging or misalignment
• Monitor water treatment chemical residuals
• Verify makeup water and blowdown operation

Quarterly:

• Clean or replace inlet screens and filters
• Inspect drift eliminators for damage or buildup
• Check fan operation and vibration levels
• Test water quality for pH, conductivity, and biological activity

Annually:

• Inspect coil for scale, corrosion, or fouling
• Clean sump and remove sediment accumulation
• Verify freeze protection systems operation
• Perform complete water system disinfection

Coil cleaning restores heat transfer capacity degraded by scale or fouling. Chemical cleaning using descaling agents followed by neutralization and passivation proves most effective. Mechanical cleaning risks coil damage and should be performed carefully.

Design Considerations

Proper evaporative condenser selection requires analyzing multiple factors:

• Capacity: Size for peak load conditions with appropriate safety factor (10-15% typical)
• Wet-bulb temperature: Use ASHRAE design wet-bulb for geographic location, typically 1% or 2% value
• Approach temperature: Balance first cost (larger unit, lower approach) against operating cost (higher efficiency)
• Water quality: Analyze makeup water chemistry to specify appropriate materials and treatment
• Location: Consider drift concerns near parking areas, building air intakes, or pedestrian zones
• Noise: Evaluate sound levels and specify sound attenuation if required for sensitive locations
• Access: Provide adequate clearance for maintenance, coil cleaning, and component replacement

Material selection addresses corrosive water conditions. Copper coils provide excellent heat transfer but may corrode in aggressive water. Stainless steel coils offer superior corrosion resistance at higher first cost. Galvanized steel basins and structures require protective coatings in many water conditions.

Performance Degradation

Operating efficiency degrades over time without proper maintenance. Scale accumulation of 0.015 inches (0.4 mm) thickness increases condensing temperature by approximately 3-5°F (1.7-2.8°C). Biological fouling creates similar insulating effects while accelerating corrosion through microbiologically influenced attack.

Monitoring condensing temperature versus wet-bulb temperature tracks performance degradation. Increasing approach temperature beyond design values indicates fouling, scaling, poor water distribution, or inadequate airflow requiring corrective action.