Chapter 1: Outdoor Air Intake

1.1. Theory and Applications


1.1.  Theory and Applications

The outdoor air intake is the portion of the system that brings outdoor air from the exterior of the building to the air handling unit location.  For some systems, such as packaged rooftop equipment, this section is simply a screen and an outdoor air damper incorporated into the unit housing.  For other systems with mechanical rooms deep inside a building, the outdoor air intake can be more extensive, including:

·       Louvers

·       Outdoor air dampers

·       Fire dampers, smoke dampers, or combination fire and smoke dampers

·       Ductwork and related accessories

For small systems or packaged air handling units, there is usually little functional testing required for the outdoor air intake section.  However, if the system is large, complex, or capable of generating significant static pressures, then functional testing may be required. Some of the tests may focus on verification of the performance of individual components such as acoustical louvers or smoke and fire dampers.   Other tests may ensure that the individual components work in harmony with the rest of the air handling system.  For instance, verification of the interlocks associated with the intake damper or an operable intake louver on a large, 100% outdoor air system can be crucial to prevent duct implosion if the intake damper or louver failed to open during start up.

Louvers are typically found in HVAC systems at the point where air enters and exits the air handling equipment.  In the outdoor air intake section, louvers generally have fixed blades, but operable louvers are installed when the system does not include outdoor air dampers.  While louvers are often thought of as relatively insignificant and passive elements in the system, they perform critical functions and, when located in a moving air stream, have a dynamic impact on the performance of the system, especially operable louvers.

Louvers typically provide one or more of the following functions:

·       Contaminant Protection The louver blades themselves and any screening provided behind the blades acts as the first stage of very rough filtration for the air handling equipment.  Some manufacturers have developed designs that will filter out sand and dust in the 140-200 micron size (a human hair is about 100 microns in diameter).

·       Visual Screening Louvers can screen equipment such as cooling towers or rooftop air handlers from view when they are located on the exterior of a building.  Typically, this function is achieved by a combination of blade orientation (vertical or horizontal) and blade design. 

·       Vandal Protection Louvers located in accessible areas are sometimes designed to be vandal proof.  These louvers can include special vandal resistant hardware, security bars, and blade designs aimed at preventing penetration of the louver by foreign objects.

·       Acoustical Protection - Occasionally, the equipment located on the system side of a louver can have a noise signature that would be objectionable on the exterior of the building without acoustic treatment.  Vane-axial fans are one example of equipment that may have acoustical problems.  Some louver designs utilize blades constructed and arranged to decrease sound transmission.

An important louver parameter that is common to many of the louver performance specifications is louver free area. The free area of a louver is the area actually available for the passage of air. This should not be confused with louver face area, which is the overall cross sectional area of the louver perpendicular to the direction of airflow.  The referenced Air Movement and Control Association (AMCA) standards indicate how free area can be calculated based on dimensions taken from the louver. Some manufacturers state performance data in terms of free area velocity while others state performance in terms of face area velocity, so it is important to determine which velocity is being used when documenting louver performance criteria.

As indicated previously, one of the primary functions of a louver is to minimize the entry of water into an air handling system.  In addition to being designed to resist penetration by water and channel water away from the interior of the system, louvers must be applied properly to achieve success in this area.   As a general rule, face velocities should be 700 fpm or less to prevent water induction.  It is also important to understand that a louver that is designed and tested to AMCA standards may still allow some water penetration.  The test associated with the AMCA standard is designed to define the ‘point of beginning water penetration’, the louver face velocity at which water begins to make its way through the louver. This provides a consistent basis for comparison of different louver designs, but does not provide quantitative data on how much water can be expected to penetrate a louver under actual service conditions. For instance, the AMCA standard does not address driving rain, and real rainfall rates can exceed test standards.  The design of the intake compartment should assume some rain penetration will occur, so the use moisture tolerant materials and including drainage is advisable.

The moisture droplets associated with heavy fog can penetrate louvers even when they have been properly sized and selected to minimize rain penetration.  This moisture often becomes trapped in the first filter section in the system and can cause problems there unless the filters are selected and installed to minimize this complication.  Systems that handle 100% outdoor air are particularly prone to this problem. 

Additional information regarding louvers can be obtained a number of publications, which are referenced in Appendix B: Resources.