Air Handler Unit

HVAC equipment manufacturers may specialize in “pharma grade” air handlers, particularly in large capacity

• GMP air-handlers should be constructed to meet stringent performance, improved reliability, and maintenance requirements for critical areas

• Air handler components, such as coils, humidifiers, dehumidifiers, dampers, fans, motors, and filters should be designed and constructed to provide 115% of design capacity to accommodate potential increased demand or future expansion

5.1.2.1 Cabinet Construction

In geographic regions of moderate to high ambient humidity levels, condensation on the exterior of the AHU casing may be an issue. Where high humidity is relatively persistent, this condensation may lead to exterior rust, mildew, and mold growth. Consideration should be given to no through metal (a thermal break) on wall, floor, doorframe, ceiling sections, and doors. If thermal breaks are not correctly designed and implemented, there is a potential for exterior condensation.

To minimize leakage of expensive conditioned air, total air leakage rate from the casing may be specified at no greater than 0.5% of rated flow at 150% of the design pressure or 50 CFM (1.42 m³/min), whichever is greater, or the requirements stated in EN 1886 standard (Reference 6, Appendix 12) for the most severe “leakage class” operation.

As eventual deterioration of seals on doors, dampers, and other components will lead to increased air leak rates, the initial measured air tightness can be used as a gauge to identify future loss of air flow.

This Document is licensed to

Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

Downloaded on: 10/5/11 2:26 PM

5.1.2.2 Insulated Wall Panel Construction

For larger air handlers, the roof, floor, and ceiling may be constructed of “sandwich” panels that are insulated with foam (polyisocyanurate) that is approved by the insurers and meets local fire ratings. Foam should not be exposed to the air stream and should be covered to isolate it from the surrounding area. Other insulating materials may be used, but foam panels are:

• structurally superior

• eliminate migration of moisture and air • less subjected to deflection due to air pressure • have better insulating properties

The interior panel surfaces and joints should be smooth and continuous, constructed of a material such as aluminum,

5.1.2.3 Removable Wall Panels

Removable panels in large AHUs provide a method to remove large components, such as fan assemblies and coils that would not fit through the AHU access door. The panel should be removable with simple hand tools to avoid cutting or sawing and creating a leakage problem after reassembly.

5.1.2.4 Flooring

Flooring in large AHUs should be of a sufficient thickness to prevent “oil canning” (deformation) when walked upon and damage from dropped tools or equipment. The floor may be designed to have a capacity of 100 pounds per square foot (psf) live load to accommodate a service mechanic working inside the unit. Flooring should have a non-slip texture for the safety of personnel standing within the unit. Floor seams should be sealed to the wall for a watertight floor system.

5.1.2.5 Condensate Pan

The cooling coil condensate drain pan (upstream and downstream) should be of 304L SS to maximize its life. The

This Document is licensed to

Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

Downloaded on: 10/5/11 2:26 PM

5.1.2.6 Wash Down Capability

In specific applications, the interior of the air handler may require cleaning and wash down. In these applications,

5.1.2.8 Hardware

Hardware (i.e., screws, nuts, washers, etc.) should be corrosion resistant (e.g., 300 series SS) with flexible washers to minimize air leakage in the exterior. Materials that oxidize or promote rust should not be used in the construction of equipment. Adequate materials of construction along with painting of components (valves, fittings, etc.) should help protect against deterioration (i.e., corrosion) dependent on the environment the unit will encounter.

5.1.2.9 Doors

Access doors should be installed on each section of the AHU (i.e., coils, filters, fan, humidifier, etc.), sufficiently wide

5.1.2.10 Mixing Plenum

The mixing plenum is where outdoor air is mixed with return air. Outdoor air louvers in mixing plenums should be aligned to promote mixing with return air to avoid stratification.

5.1.2.11 Duct Connections

Cabinet duct connections can significantly reduce the system’s delivery capacity if sized to match the size of ductwork

This Document is licensed to

Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

Downloaded on: 10/5/11 2:26 PM

• dampers

• coils

• filters

• silencers

• ductwork

When selecting fans to operate smoothly over their intended life, aspects to consider include:

• materials of construction (rigidity, weight, corrosion, cleanability) determined for the type of operation (clean/

contaminated air, humidity, temperature, severity)

• bearing

• lubrication

• direct versus belt driven • static pressure flow sensing • safety guards

Air handlers are configured as either a draw-through or blow-through operation; draw-through typically are used.

Draw-through units have the fan located downstream of the pre-filters, coils, and humidifier. Their advantages include a shorter unit length, negative pressure on all access doors except the fan discharge section, and reheating of air leaving the fan section, which will reduce reheat coil requirements.

Fan pressure performance and construction are identified as Class I, II, III, or IV by AMCA, based on certain minimum operating criteria. A Class I fan offered by any particular manufacturer has a lower allowable minimum operating range than its Class II counterpart. As a result, a Class I fan has less mechanical design strength and with less first cost than a Class II fan. Typically, Class II and Class III fans are sufficient to handle pharmaceutical applications.

Fans should not be sized too small, such that they operate above 1800 RPM, shortening bearing life. High fan RPM also has a risk of dangerous vibration (operating too near a fan’s critical speed) and has more noise.

Fans typically used in air handlers on the supply side are either plug/plenum fans or centrifugal fans fitted with a drain plug and cleanout panel. Fans can be direct driven or belt driven. Exhaust/extract operations typically use direct or belt driven vane axial or centrifugal fans.

Plenum fans should be selected for high efficiency with non-overloading airfoil aluminum wheels. They should include inlet cones matched to the wheel intake rim to ensure efficient and quiet operation.

Vane axial fans are used where large volumes of air need to be moved at low to moderate pressures. The tubular design, high efficiency rotor, and integral straightening vanes provide high performance using minimal space. These fans are considered a suitable choice for HVAC systems using variable air volumes, high airflow to cleanrooms, and exhaust/extract. They are efficient as return fans to air handlers and for exhaust/extract applications (fume hoods, bio-safety cabinets.) These units should be configured for direct drive (motor in the air stream) although a belt drive could be used.

This Document is licensed to

Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

Downloaded on: 10/5/11 2:26 PM

Another variation of the direct drive fan configuration is an array of smaller plug fans (commonly called a “fan wall”) to

This Document is licensed to

Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

Downloaded on: 10/5/11 2:26 PM

Fans and motors for critical applications should be provided with vibration sensors to provide early warning and trending of bearing performance with signal wiring brought out to a vibration interface enclosure mounted on the outside of the AHU.

One method of reliably measuring fan airflow without impeding air movement in or near the fan inlet is to install a combination piezometer ring and static pressure tap integrated into the fan inlet cone. The inlet cone of the fan is then used as the flow nozzle. The flow sensor should be provided with the fan.