Air pressure drop APD

[edit] Introduction

In the context of heating, ventilation and air conditioning (HVAC) filtration systems, air pressure drop (APD) describes the change in static pressure that occurs when air is drawn through a filter. APD is sometimes referred to as airflow pressure drop or simply as pressure drop.

Static pressure in HVAC systems is defined as the resistance to airflow that is created by the filters and other components within the system. In most cases, lower static pressure is preferable, since high static pressure may result in lower airflow. Lower airflow means the system will take more time to condition less air, which means it may have to work for longer periods.

[edit] APD and airflow

APD is associated with the level of restriction an air filter has on airflow. The higher the pressure drop the more restrictive the filter is to airflow. The lower the pressure drop, the easier it is for air to flow through the filter. Excessive APD indicates that an HVAC may be struggling to process the air.

When evaluating filters, the APD is first measured when the filter is new. This records its initial resistance level or initial APD. The filter is then put into use so a high level of particles can be trapped. This will cause the filter’s APD level to increase. A second measurement is taken once the filter has reached its dust holding capacity. When at capacity, it will be more difficult for air to pass through the filter. This measurement is the final resistance level or final APD.

APD may increase as the minimum efficiency reporting value (MERV) number of a filter increases. MERV is a rating system used in the US for comparing air filters made by different manufacturers.

For more information, see Minimum efficiency reporting value MERV.

[edit] Changes in ADPs

A filter’s APD changes throughout its life. As a filter traps and gathers airborne contaminants, it becomes more effective at capturing more particles. However, the filter eventually becomes overloaded, causing the APD to increase while the airflow decreases to the point where it may become extremely restrictive.

Filters that are dirty or blocked in any way may be associated with excessive ADP and should not be used beyond their final APD point. Changing filters before they reach this state can help prevent operational inefficiencies and possible repairs to HVAC units.

ADP can be affected by the type of material used in the filter. Materials with better electrostatic properties may be more efficient filters since they inherently attract particles. The ADP of a filter can also be influenced by its construction based on the available surface area of the material used.

[edit] Related articles on Designing Buildings

• Air filtration and clean indoor air quality standards.
• Clean indoor air for healthy living - New air filter standards.
• Designing HVAC to resist harmful microorganisms.
• Growing focus on IAQ challenges for specifiers and HVAC manufacturers.
• HVAC.
• Mechanical draft system.
• Mechanical ventilation of buildings.
• Minimum efficiency reporting value MERV.