ASHRAE Journal - September 2014 - 27

TECHNICAL FEATURE

Primary Air: Treated supply air that enters the air terminal unit.
External-Static Pressure Loss: for forced air systems
the static pressure loss resulting from airflow through
the ductwork and other elements external to the unit.
Maximum Allowable Pressure: maximum gage pressure permitted on a completed system.
Operating Pressure: the pressure occurring at a reference point in a system when the system is in operation.
There are two ways to specify the allowable pressure
drop of an air terminal unit. The first is to schedule the
inlet or primary pressure, and the discharge or external
static pressure associated with each air terminal unit.
Although this method clearly defines how each air terminal should be selected to meet the required pressure
requirements, it can be time consuming. Using current
available air terminal software selection tools can make
this an effective method.
The second method of specifying the allowable pressure drop for an air terminal is to specify the operating
pressure and the maximum allowable pressure drop for
the terminal. This method is commonly used by specifying engineers and, in most cases, produces selections
with the required accuracy for a project. This method
also only requires the engineer to give these two parameters for all the air terminal units on a project.
It is fairly common to see specifications that do not
define one of the two methods of specifying allowable
pressure drop for air terminal units. The purpose of
specifying the maximum allowable pressure for an air
terminal unit is to make sure there is a high enough
pressure at the inlet of the air terminal to deliver the
required flow by overcoming the pressure of the air
terminal, and the downstream external static pressure
where applicable. If this pressure is not clearly shown
in a specification or schedule, smaller units could be
selected that may not be able to deliver the scheduled
flow.
If the terminal units require a hot water coil, make
sure the pressure drop through the air terminal unit
includes the pressure drop through the hot water coil.
The pressure drop through the hot water coil can be significant and without accounting for this, the required
flow through the air terminal unit may not be obtainable. Regarding electric heat, typically the pressure drop
through an electric heater is very low and insignificant
when specifying the allowable pressure drop through

an air terminal unit. However, there are applications
where the pressure drop through the heater should be
considered.
Finally, to select the sound produced by a unit at a
given capacity requires the operating pressure to be
stated. The operating pressure may or may not be the
same as the primary inlet pressure. In the case of single
duct units, dual duct units and parallel fan-powered
units, it is the pressure required to pass through the
terminal and through the downstream ductwork. On a
series fan-powered air terminal, it is the pressure that is
needed to deliver the required primary air to the inside
of the unit. Operating pressure is given by the design
engineer and is the pressure used to determine sound
performance. It may not be the actual pressure differential the installed air terminal will see. Providing the
operating pressure to determine sound performance
ensures selections for sound are based on the same
criteria.
The sound produced by an air terminal unit will
increase as the inlet pressure increases. A terminal
selected at an operating pressure of 0.5 in. w.g. (125
Pa) will produce lower sound levels compared to the
same terminal, at the same flow at 1 in. w.g. (249 Pa).
Depending on the type of unit, radiated or discharge or
both may increase.

Specifying Noise Criteria (NC)
It is typical for an air terminal unit specification to
include a maximum NC level. An example is a typical
specification may indicate a maximum air terminal unit
NC level of 35. The NC level, without giving the allowable
attenuation factors is a meaningless number. There are
two sources of sound generated by an air terminal unit.
These are:
Radiated Sound Power Level: sound power that radiates from the terminal casing (plus the induction port if
present).
Discharge Sound Power Level: sound power that is
transmitted from the terminal outlet.
If noise criteria is used by the designer, the following
terms need to be understood:
Sound: A physical disturbance, vibration, or frequency
transmitted by a solid, liquid, or gas that is capable of
being detected by the human ear.
Sound Power Level: The sound power of a source is its
rate of emission of acoustical energy.
SEPTEM BER 2014

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