Pressure calculation for HVAC

Static pressure describes the resistance experienced by air as it travels through an HVAC system. In other words, it is the pressure a fan must overcome to move air through ducts, as needed for heating, ventilation and cooling. Static pressure and airflow are the two main aspects that determine the operating point of a fan, as well as power consumption.
Various types of HVAC equipment establish an airflow through ducts, and two common examples are air-handling units (AHU) and packaged rooftop units (RTU). These pieces of equipment are designed to deliver a specific performance in terms of airflow and velocity.
For an HVAC system to operate normally, equipment capabilities must match the needs of the air duct system. In particular, air handling systems must be capable of overcoming the static pressure. Other parameters considered by HVAC designers include the installed cost, operating expenses, maintenance, noise and vibration.

Friction Losses Calculated
For design purposes, losses are classified by their source: those caused by the ducts themselves, and those caused by fittings.

•     Duct friction losses depend on duct size, length and material roughness, as well as air velocity. The path with the highest pressure loss is called the critical path - a major factor to consider during duct design.
•     Fitting losses account for the highest fraction of total losses. They occur as air moves through filters, offsets, elbows, dampers, coils, and other fittings and accessories. ASHRAE provides fitting loss coefficients to simplify their selection - using the right fittings in the right locations can lead to significant cost reductions and energy savings.

Once all the duct losses have been accounted for, HVAC designers can easily select a fan that will provide the required static pressure and airflow.

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