AHU refined

Air Handling Units (AHUs) - Definition and Purpose

  • AHUs filter indoor air. They cool or heat it to a comfortable temperature.

  • They also replace some indoor air with fresh outdoor air to improve quality.

  • AHUs manage conditioned air in commercial buildings.

Characteristics of AHUs

  • Physical Description

    • AHUs are large, rectangular metal boxes connected to ductwork.

    • They contain: Air filters, Blowers, Heating and cooling components.

    • AHUs cool or heat and circulate large volumes of air.

  • Location

    • Placed on rooftops, in basements, or mechanical rooms.

  • Types of AHUs

    • Floor Mounted AHUs

    • Installed on the floor.

    • Advantages: Low initial cost, long lifespan, easy maintenance.

    • Disadvantages: Occupy valuable floor space.

    • Ceiling Suspended AHUs

    • Hang from the ceiling; saves floor space.

    • Advantages: Space-saving.

    • Disadvantages: Higher initial cost, shorter lifespan, difficult maintenance.

Components of Air Handling Units

  • Housing

    • Insulates AHU components; prevents heat transfer.

    • Made of metal, often painted to prevent corrosion.

    • Insulated with 1−21-2 inches of polyurethane foam in key areas.

  • Drain Pan

    • Collects condensation from cooling coils.

  • Blowers

    • Fans that force air through ductwork.

    • Centrifugal fans are common for air circulation.

    • Types: Backward inclined, Backward curved, Forward curved, Airfoil.

    • Fan selection depends on required air volume and static pressure.

    • Blower and motor mount with vibration isolators.

  • Cooling Coils

    • Cool and dehumidify air.

    • Can be: Cold water cooling coils, Refrigerant cooling coils.

    • Chillers generate cold water for circulation through coils.

  • Heating Coils

    • Heat air, using hot water or refrigerant.

    • Electrical heaters can be installed but use more electricity.

  • Air Filters

    • Clean indoor air.

    • Types:

    • Panel filters: Flat, rectangular; low efficiency, for general air conditioning.

    • HEPA filters: High-efficiency (99.97%99.97\%), for clean environments (e.g., hospitals).

    • Electrostatic filters: Use charged electrodes to ionize air and trap particles.

    • Bag filters: Discarded after use to remove dust.

    • Roll filters: For high-velocity applications; rolled up for disposal.

    • Filter efficiency depends on system application.

  • Dampers

    • Plates that regulate airflow in AHU ducts.

  • Condensate Drains

    • Drain pan and drain line collect and remove condensation.

  • Additional Components:

    • Humidifiers: Increase air moisture during winter.

    • Types: Spray type, Steam pan type, Steam grid type.

    • Dehumidifiers: Made of silica gel to absorb excess moisture.

    • Sound Attenuators: Reduce blower noise, improving workspace productivity.

    • Vibration Isolators: Absorb vibrations; mounted between blowers and casings.

Operational Aspects of AHUs

  • Air Intake and Distribution

    • Fans draw unconditioned air, mixing it with outside air.

    • This prevents high CO2 and stale air.

    • Mixing Plenum: Compartment where return air and outside air mix before filtration.

  • Filtration Process

    • Air passes through filters (pre-filter then final filter).

    • Differential pressure monitoring measures pressure drop across filters.

    • More dirt means increased pressure drop and reduced airflow.

  • Heat Exchanging Process

    • Air passes through heat exchanging coils for conditioning.

    • Hydronic systems: Hot or cold water flows through the coil.

    • Direct expansion systems: Refrigerant flows through the coil, exchanging heat with air.

  • Exhaust of Conditioned Air

    • Clean, conditioned air pushes into desired spaces.

Types of Air Handling Units

  • Blow Through AHUs

    • Fan upstream of the cooling coil; blower pushes air through the coil.

    • Advantages: Positive pressure downstream prevents odors; avoids adding motor heat to supply air.

    • Disadvantages: Uneven airflow across the coil can lead to moisture issues.

  • Draw Through AHUs

    • Fan downstream of the cooling coil; blower pulls air through the coil.

    • Advantages: Uniform airflow for consistent heat transfer; good for moisture removal.

    • Disadvantages: Negative pressure can cause odor issues if the trap dries out.

  • Selection of AHU Types

    • Draw through AHUs preferred in space-constrained areas; shorter designs.

    • Blow through AHUs may cost more due to longer design requirements for better airflow.

Summary of Air Handling Processes

  • Air handling filters indoor air. Air distribution sends hot or cold air to rooms.

  • AHUs do not produce hot or cold water; they direct conditioned air.