Primary Air
Treated supply air that enters the air terminal unit.
Specifying Pressure
Minimum Operating Pressure: the static or total pressure drop through a terminal at a given airflow rate with the damper/valve placed in its full open position by its actuator while the terminal is operating under steady-state control.
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 obtain- able. 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.
Sound Pressure Level: The ear’s response to sound waves in air and are variations in pressure above and below atmospheric pressure.
NC: A single number for rating the sound of a space. The measured octave bands for a space are compared with the NC curves. These curves were developed with lines that represent constant perceived loudness by the human ear. The NC rating is the value of the highest NC curve touched by measured sound in Octave Bands 1 through 8.
