How to Select an Air Handling Unit for an Industrial or Commercial Project

AHU Function

An air handling unit (AHU) conditions and distributes air within a building or process facility. It typically contains a fan section, cooling and heating coils, filtration stages, a mixing box for fresh and return air, and a casing that contains and connects these components. The AHU draws in return air from the served space, mixes it with outdoor fresh air in the proportions required by the ventilation design, conditions the mixed air to the required temperature and humidity, filters it, and delivers it to the supply ductwork.

Application and Operating Conditions

The application determines many of the AHU's design requirements. A comfort air conditioning system for an office building has different requirements from a cleanroom AHU, a pharmaceutical manufacturing AHU or an industrial process ventilation unit. The operating environment — indoor or outdoor installation, ambient temperature range, humidity, corrosive atmosphere, altitude — must be defined before the AHU is specified.

Air Quantity

The supply air quantity is determined by the cooling or heating load, the ventilation requirement and, for cleanrooms, the air-change rate required to maintain the specified cleanliness classification. The design air quantity must account for duct leakage, filter pressure drop and any future expansion allowance. The AHU fan must be selected to deliver the required air quantity against the total system resistance at the design operating point.

External Static Pressure

The external static pressure is the resistance that the fan must overcome to deliver air through the ductwork system. It is determined by the duct layout, fittings, terminal units, dampers and any downstream equipment. An underestimated static pressure will result in insufficient airflow; an overestimated value will result in a fan that operates away from its design point, with reduced efficiency and increased noise. The static pressure must be calculated from the approved duct design.

Fresh-Air and Return-Air Requirements

The proportion of fresh outdoor air to recirculated return air is determined by the ventilation standard applicable to the space, the occupancy, the process requirements and any regulatory requirements. For cleanrooms and pharmaceutical facilities, the fresh-air fraction may be specified in the project design basis. The mixing box must be sized and controlled to maintain the required fresh-air fraction across the full range of operating conditions.

Cooling and Heating Loads

The cooling coil must be sized to handle the sensible and latent heat loads at the design conditions. The heating coil must be sized for the maximum heating demand, which typically occurs at the minimum outdoor temperature. The coil selection inputs — entering and leaving air conditions, coil face velocity, chilled water or refrigerant conditions, hot water or steam conditions — must be provided by the project mechanical engineer.

Filtration Stages

The filtration arrangement depends on the application. A typical comfort system may use a pre-filter and a fine filter. A cleanroom AHU may require pre-filtration, intermediate filtration and terminal HEPA filtration. The filter efficiency classes, face velocities and replacement intervals must be specified. The AHU casing must accommodate the filter frames and allow filter replacement without contaminating the downstream sections.

Fan and Motor Selection

The fan type — forward-curved, backward-curved or plug fan — affects efficiency, noise and the shape of the fan curve. Backward-curved and plug fans are generally preferred for their stable operating characteristics and higher efficiency. The motor must be sized for the maximum power demand, including the effect of filter loading over the service interval. Variable-speed drives allow the fan speed to be adjusted to match varying system conditions and can significantly reduce energy consumption.

Casing and Insulation Options

The AHU casing must be thermally insulated to prevent condensation on the external surface and to limit heat gain or loss. The casing panel construction — single-skin, double-skin or sandwich panel — determines the thermal and acoustic performance. For cleanroom and pharmaceutical applications, the internal surface must be smooth, cleanable and resistant to the cleaning agents used. The casing must be airtight to the leakage class specified in the project design.

Indoor versus Outdoor Installation

An AHU installed outdoors must be weatherproofed to prevent rain ingress, UV degradation and corrosion. The casing, fasteners and coil headers must be suitable for the outdoor environment. Access doors must be sealed and lockable. Outdoor units may require additional thermal insulation to prevent freezing of coils and condensate traps in cold climates.

Noise and Vibration

The AHU is a source of noise and vibration that can be transmitted to the served space through the ductwork and the building structure. Anti-vibration mounts under the AHU base frame isolate structural vibration. Flexible duct connections at the supply and return connections isolate duct-borne vibration. Acoustic attenuators in the ductwork reduce fan noise transmitted to the served space. The noise criteria for the served space must be defined in the project specification.

Condensate Management

The cooling coil produces condensate that must be collected and drained. The drain pan must be sized to collect the maximum condensate rate and must slope to the drain connection. The condensate trap must be deep enough to overcome the static pressure in the AHU casing. Blocked or shallow traps are a common cause of water carry-over into the supply ductwork.

Controls and BMS Coordination

The AHU control system must interface with the building management system (BMS) to allow remote monitoring and control of supply air temperature, humidity, fan speed, filter pressure differential and fault alarms. The control philosophy — constant air volume, variable air volume, demand-controlled ventilation — must be defined in the project design basis. The control panel and sensor locations must be agreed with the BMS engineer.

Service and Maintenance Access

Access doors must be provided on both sides of each component — coils, filters, fans, dampers — to allow inspection, cleaning and replacement. The access doors must be large enough for the maintenance task and must be positioned so that they can be opened without obstruction. The AHU must be positioned so that there is adequate clearance around it for maintenance activities.