30 November 1998
e. Early in the system design process, the designer must give consideration to the space
requirements necessary to successfully implement minimum outside air flow control. The OA ductwork
must be long enough to permit accurate measurement by the AFMA. The minimum functional length of
the ductwork section between the upstream weather louver and the AFMA is two times the equivalent
duct diameter. Transition angles leading to the AFMA should not exceed 15 degrees. The minimum
functional length of the ductwork section between the AFMA and the downstream control damper is one
half the equivalent duct diameter. Do not locate control dampers upstream. These distances assume that
an AFMA with multiple sensors and an air straightener is used. It is recommended that an access door
be located immediately upstream of the air flow station.
f. The minimum OA duct section should be sized for the condition where the main OA damper is
closed and the return air damper is open with the fan speed at minimum load turn down. Under this
condition the flow through the minimum OA duct is at maximum. The recommended duct sizing range is
between 1.5 and 2 m/s (300 and 400 fpm) at the location of the AFMA. Louver openings should be sized
in accordance with ASHRAE recommendations such that they limit louver face velocity to 2 m/s (400
fpm) or less. Pressure drop across the air flow measurement array plus air flow straightener can be
estimated to be 1 Pa (0.003" wc) at 2 m/s (400 fpm).
g. Pitot tube type AFMAs are not recommended for use in outside air measurement applications, as
their minimum measurable air flow velocity is approximately 3.5 m/s (700 fpm). Pitot tube AFMAS are
also more inclined to be adversely affected by airborne contaminants.
h. Electronic AFMAs, by design, are resistant to airborne contaminants, but the station is to include a
honeycombed airflow straightening device located immediately upstream of the AFMA. The airflow
straightener helps to remove turbulent and rotational air flow. The honeycombs are specified to be 3 mm
(1/8 inch) diameter, presenting the potential for clogging by bugs and other particulate matter. Because
of this, filter media are to be used, located no less than 1 m (39) inches upstream of the AFMA.
I. Exhaust fans impact building ventilation because they can remove significant volumes of air from a
building. This exhaust must be considered in determining the quantity of air to be returned to the air
handling unit. To help ensure a small positive building pressure, slightly more air must be supplied than
is returned. If a return duct is used, size it to account for exhaust fan and exfiltration losses.
j. When a return fan is used, determine the differential volume flow rate (bias) between the supply
and return ducts by taking into account the exhaust from the supplied spaces. The bias setting will be
slightly larger than the total building exhaust. Making the bias slightly larger than the maximum exhaust
will help to ensure positive building pressure and adequate ventilation. Refer to section 8 - RETURN
FAN VOLUME CONTROL LOOP for additional guidance on selection of the return fan bias setting.
7. SUPPLY DUCT STATIC PRESSURE CONTROL LOOP. The supply duct static pressure control loop
is shown in figure 3-11. A differential pressure sensing element and transmitter (DPT) sends a signal to
static pressure controller PC, which operates IP to control DA, which in turn operates fan inlet vane IV
provided that the fan is on. DPT must have a relatively low range, such as 0 to 500 Pa (0.0 to 2.0 inches
of water column). The supply fan may have been selected for a much larger static pressure, but the
static pressure at the location of the DPT's sensor is typically 250 to 375 Pa (1.0 to 1.5 inches of water
column). The recommended sensing location of DPT is approximately 75 to 100 percent of the distance
from the first to the last air terminal unit along the duct calculated to have the greatest pressure drop.
This sensing location insures that the static pressure will be controlled at the value required to enable all