Wetted Media Evaporative Humidifiers

Technical Overview

Wetted media evaporative humidifiers pass airstreams through continuously moistened media surfaces, enabling water evaporation through direct air-water contact. Media materials provide extensive surface area for heat and mass transfer, facilitating moisture addition without atomization or steam generation. Recirculation pumps distribute water over media while sumps collect drainage for reuse. This adiabatic process absorbs sensible heat, cooling air while adding moisture. Applications include commercial buildings, industrial facilities, and installations where low operating costs and evaporative cooling benefits justify maintenance requirements.

Rigid Media Pad Construction

Rigid media employs corrugated cellulose, plastic, or composite materials forming structured passages for airflow. Corrugation patterns optimize pressure drop versus heat transfer effectiveness, typically achieving 70-90% saturation efficiency. Media thickness ranges from 4 to 12 inches (100 to 300 mm), with greater thickness improving effectiveness at the expense of pressure drop. Modular panel construction enables field replacement of fouled or damaged sections. Media mounting frames seal against air bypassing preventing effectiveness degradation.

Expanded Metal Mesh Systems

Expanded metal mesh stacks create tortuous airflow paths promoting air-water contact. Mesh density and stack depth determine effectiveness and pressure drop characteristics. Stainless steel construction resists corrosion from minerals and water treatment chemicals. Media replacement costs less than rigid pad alternatives but may offer lower saturation effectiveness. Some designs incorporate wicking materials within mesh assemblies improving distribution uniformity.

Ceramic Coated Media Technology

Ceramic coatings on media substrates enhance durability and cleanability compared to uncoated materials. The coatings resist biological growth, scale formation, and degradation from water treatment chemicals or minerals. Ceramic surfaces clean more thoroughly during maintenance, extending media life. Higher initial costs offset against extended replacement intervals and improved performance retention. Applications with challenging water quality particularly benefit from ceramic-coated options.

Water Distribution Systems

Distribution headers deliver water uniformly across media surfaces ensuring complete wetting without channeling or dry spots. Perforated pipes, spray nozzles, or porous distribution pads spread water over media width. Flow rates balance distribution uniformity against pump energy consumption, typically 0.5-2.0 GPM per linear foot (6.3-25 L/min per meter) of media width. Proper distribution proves critical for effectiveness and preventing biological growth in stagnant zones.

Recirculation Pump and Hydraulics

Centrifugal recirculation pumps circulate water from sumps through distribution systems, returning excess drainage to sumps for reuse. Pump capacity exceeds distribution demand ensuring positive pressure throughout headers. Typical pump heads range from 10-30 feet (3-9 m) overcoming piping friction and distribution nozzle requirements. Continuous recirculation during operation flushes media, controls biological growth, and maintains uniform moisture distribution.

Sump Reservoir Design

Sumps collect drainage water, provide settling volume for particulate separation, and maintain system water inventory. Sump capacity accommodates several minutes of recirculation at full flow enabling startup and brief supply interruptions. Sloped sump floors direct sediment toward drain connections. Overflow provisions prevent flooding from control malfunctions. Sumps incorporate float switches controlling makeup water addition maintaining proper levels.

Overflow and Drain Systems

Overflow drains prevent sump flooding from failed fill valves or excessive makeup flow. Drain connections enable complete sump drainage during maintenance or system shutdown. Bleed-off drains remove concentrated water maintaining acceptable total dissolved solids limits, typically implementing continuous or timed bleed cycles removing 10-30% of recirculation flow. Proper drainage slopes prevent standing water during shutdown periods.

Bleed-Off Water Quality Management

Mineral concentration in recirculation water increases as evaporation removes pure water leaving minerals behind. Bleed-off systems periodically drain concentrated water replacing it with fresh makeup, controlling TDS within acceptable limits. Bleed-off rates depend on makeup water quality and target concentration factors, typically maintaining 3-6 times makeup TDS. Insufficient bleed-off accelerates scale formation while excessive bleed-off wastes water and increases sewerage costs.

Media Replacement Schedule

Media replacement intervals depend on water quality, operating hours, and maintenance diligence. Rigid media typically requires replacement every 3-7 years as minerals, biological growth, and degradation reduce effectiveness and increase pressure drop. Ceramic-coated media may extend to 5-10 years. Expanded metal mesh cleaned properly endures 10-15 years before replacement. Plugged, scaled, or biologically fouled media requires premature replacement, emphasizing maintenance importance.

Evaporative Cooling Effect

Adiabatic humidification follows constant enthalpy processes, exchanging sensible heat for latent heat as evaporation occurs. Air temperature decreases while moisture content increases, with the process approaching wet-bulb temperature. Temperature depression ranges from 5-20°F (2.8-11°C) depending on inlet conditions and effectiveness. This free cooling reduces mechanical cooling loads during warm weather but may require reheat during cold periods when humidification occurs.

Adiabatic Humidification Process

The evaporation process follows adiabatic saturation principles where air moisture content increases toward saturation while temperature decreases toward wet-bulb temperature. Effectiveness equals the ratio of actual moisture gain to theoretical moisture gain at full saturation. Typical wetted media effectiveness ranges from 70-90% depending on media type, thickness, and air velocity. Higher effectiveness requires greater media depth or surface area but provides superior humidification capacity.

Legionella Prevention Measures

Legionella bacteria proliferate in stagnant water at temperatures between 68-122°F (20-50°C), creating health risks. Prevention strategies include UV sterilization in recirculation loops killing bacteria, biocide treatment suppressing growth, system drainage during extended shutdown periods eliminating breeding environments, regular cleaning removing biofilm and organic matter, and maintaining water temperatures below 68°F (20°C) or implementing thermal disinfection above 140°F (60°C). ASHRAE Standard 188 provides comprehensive Legionella risk management protocols.

Pressure Drop Characteristics

Pressure drop across wetted media ranges from 0.2 to 1.0 inches w.g. (50 to 250 Pa) depending on media type, thickness, face velocity, and fouling condition. Face velocities typically range from 400-700 fpm (2-3.5 m/s) balancing effectiveness against pressure drop and moisture carryover. Fouled media exhibits increasing pressure drop signaling cleaning or replacement needs. Excessive pressure drop increases fan energy consumption and may reduce airflow affecting building ventilation.

Maintenance Requirements

Routine maintenance includes monthly visual inspection for proper water distribution and media condition, quarterly sump cleaning removing sediment and biofilm, semi-annual media cleaning through flushing or chemical treatment, annual media replacement assessment, and biannual water treatment system service. Proper maintenance sustains effectiveness and prevents biological growth. Deferred maintenance accelerates media degradation and performance loss.

Advantages and Limitations

Advantages include lowest operating energy cost of humidification technologies (pump power only), beneficial evaporative cooling during warm weather, no combustion products or compressed air requirements, and simple mechanical systems. Limitations encompass biological growth risks requiring water treatment, media replacement costs and maintenance requirements, pressure drop impacts on fan energy, adiabatic cooling undesirable during heating seasons, and water quality demands including bleed-off treatment.

Application Selection Criteria

Wetted media evaporative humidifiers suit applications with extended operating hours justifying maintenance requirements, conditions where evaporative cooling provides value, acceptable water quality or treatment capacity, adequate maintenance staffing and capabilities, and space accommodating equipment size. They match poorly with intermittent operation, heating-dominated climates where cooling proves detrimental, very hard water without treatment, minimal maintenance capabilities, or tight space constraints.

Installation and Integration

Installation requires adequate structural support for equipment weight including water inventory, makeup water supply with appropriate pressure (20-80 psig) and quality, drainage for bleed-off and overflow, electrical service for pumps and controls (typically 0.5-5 HP), and proper airflow direction through media. Mounting locations downstream of heating coils and upstream of cooling coils optimize performance. Integration with economizer controls prevents simultaneous operation during free cooling cycles.

Control System Design

Controls modulate makeup water addition maintaining sump levels while managing bleed-off cycles limiting TDS concentration. Humidity sensors provide feedback for system enable/disable control. Freeze protection drains systems during cold weather shutdown. Maintenance lockout prevents operation with failed pumps or water supply. Building automation integration enables centralized monitoring, scheduling, and alarming. Performance trending identifies degradation requiring maintenance intervention.