404.3.2 Secondary-Recirculation Systems

For secondary-recirculation systems where the supply air or a portion thereof to each ventilation zone is recirculated air (air that has not been directly mixed with outdoor air) from other zones, zone ventilation efficiency (E_vz) shall be determined in accordance with Equation 404.3.2(1). Examples of secondary-recirculation systems include dual-fan dual-duct and fan-powered mixing-box systems, and systems that include transfer fans for conference rooms.

[Equation 404.3.2(1)]

E_vz = (F_a+X_s•F_b—Z_pz•E_p•F_c)/F_a

The system air fractions Fa, Fb, and Fc shall be determined in accordance with Equation 404.3.2(2), Equation 404.3.2(3), and Equation 404.3.2(4). The zone primary air fraction (Ep) shall be determined in accordance with Equation 404.3.2(5). For single-zone and single-supply systems Ep shall equal to 1.0. The zone secondary recirculation fraction (Er) shall be determined by the designer based on system configuration. The zone air distribution effectiveness (Ez) shall be determined in accordance with Section 403.2.2. [ASHRAE 62.1:A1.2.2]

Where:

E_p - Primary air fraction: The fraction of primary air in the discharge air to the ventilation zone.

E_r - Secondary recirculation fraction: In systems with secondary recirculation of return air, the fraction of secondary recirculated air to the zone that is representative of average system return air rather than air directly recirculated from the zone.

E_vz - Zone ventilation efficiency: The efficiency with which the system distributes air from the outdoor air intake to the breathing zone in any particular ventilation zone.

E_z - Zone air distribution effectiveness: A measure of the effectiveness of supply air distribution to the breathing zone. E_z is determined in accordance with Section 403.2.2.

F_a - Supply air fraction: The fraction of supply air to the ventilation zone from sources or air outside the zone.

F_b - Mixed air fraction: The fraction of supply air to the ventilation zone from fully mixed primary air.

F_c - Outdoor air fraction: The fraction of outdoor air to the ventilation zone from sources of air outside the zone.

V_dz - Zone discharge airflow: The expected discharge (supply) airflow to the zone that includes primary airflow and secondary recirculated airflow, cfm (m³/min).

V_pz - Zone primary airflow: Determine in accordance with Section 403.5.1.

X_s - Average outdoor air fraction: At the primary air handler, the fraction of outdoor air intake flow in the system primary airflow.

Z_pz - Primary outdoor air fraction: The outdoor air fraction required in the primary air supplied to the ventilation zone prior to the introduction of secondary recirculation air. [ASHRAE 62.1:A3]