Air Handling Unit Product

Overview

An air handling unit is the central machine of a ducted HVAC system: a long sheet-metal cabinet, divided into functional sections, through which all supply air for a building zone passes. In one pass through the unit, return air mixes with outdoor air, is filtered, cooled and dehumidified or heated, and is then pushed into the supply ductwork by a fan. AHUs are built as standard catalog products from about 1,000 m³/h up to custom units handling more than 100,000 m³/h for hospitals, airports, and factories.

The unit is organized along the airflow path: the Mixing Box & Dampers at the inlet, the Filter Bank, the Hot-Water Heating Coil and Chilled-Water Cooling Coil, and the Supply Fan Section section, all enclosed by the Casing & Frame. Systems that actively manage return air add an Return Fan Section. The DDC Control System tie the unit to the building automation system.

Airflow path

Return air from the building enters the mixing box, where it meets outdoor air drawn through a Intake Bird Screen. Three damper sets — outdoor, return, and relief — built from airfoil Damper Blade sections with Blade Edge Seal edges, proportion the two streams. Each set is driven through a Damper Linkage by a spring-return Damper Actuator. At minimum position the dampers admit just the ventilation air required by ASHRAE 62.1; in economizer mode, when outdoor air is cooler than return air, they modulate up to 100% outdoor air for free cooling, one of the largest energy-saving functions of the unit.

The mixed air then passes the filter wall. A 50 mm Pleated Prefilter takes the coarse load; the Bag Final Filter bag stage, MERV 13 or better, protects the coils and the occupants. Both stages sit in a gasketed Filter Holding Frame, and a pair of Pressure Sensor transmitters report pressure drop so filters are changed on condition — at roughly 250 Pa for bag filters — rather than on schedule.

Heating and cooling happen at the coils. The Hot-Water Heating Coil is a shallow one- or two-row exchanger fed with 60–82 °C hot water; the Chilled-Water Cooling Coil is a deep 4–8 row exchanger fed with 6–7 °C chilled water. Both are built the same way: serpentine Coil Tube Circuits mechanically expanded into an aluminum Coil Fin Pack pack, fed through Coil Header manifolds and carried in a slide-in Coil Casing. Below the cooling coil, the stainless Condensate Drain Pan catches condensate — a large AHU can remove hundreds of litres of water per hour from humid air — and an Moisture Eliminator stops droplet carryover at high face velocities. A capillary Freeze Stat across the coil faces shuts the unit down before a winter failure can freeze and burst the tubes.

Fans

Modern AHUs use direct-drive plenum fans: an unhoused backward-curved Plenum Fan Wheel mounted straight on the shaft of the Fan Motor, drawing air through a spun Inlet Cone and discharging into the plenum that feeds the duct connection. This layout eliminated the belts, sheaves, and housing losses of older forward-curved blower sections. The wheel-and-motor module sits on a Fan Frame carried by Vibration Isolator mounts so vibration never reaches the casing. Large units gang several smaller fans into a fan array, which gives redundancy and a flatter, more efficient operating map. Each motor is fed by an Variable-Frequency Drive; because fan power scales with the cube of speed, slowing a fan 20% cuts its power draw by half.

Casing and controls

The Structural Frame is built from thermally broken aluminum extrusions skinned with insulated double-wall Sheet Metal Panel construction; Panel Insulation of 25–60 mm controls both heat gain and radiated fan noise. Gasketed Access Door sections at the fan, coil, and filter bays allow service, and Base Rail channels carry the unit on its pad or roof curb. Casing air leakage is a rated property — EN 1886 class L1 units leak under 0.15 l/s per m² of casing at 400 Pa.

The DDC Controller Board runs the sequence of operations: a duct static-pressure loop commands fan speed, a discharge-air-temperature loop sequences the Coil Valve & Actuator on each coil and the economizer dampers, and Duct Temperature Sensor and Humidity Sensor points at the mixed, supply, return, and outdoor positions feed the logic. The controller exposes every point over BACnet, so the AHU operates as one coordinated element of the building's chilled-water and ventilation plant.