All variable air volume (VAV) terminal units—including single duct, bypass, system powered induction, fan-powered or dual duct—have a common function: regulate the amount of primary air entering the terminal unit.
This control of the primary air volume can be used for the following purposes:
The primary air valve assembly contains an inlet tube, damper and air flow sensor. When sizing a terminal unit inlet, HVAC designers aim to minimize the sound generation of the terminal unit, as well accurately resolve the flow signal at both full and minimum cooling.
Air Flow Sensor
One of the most important components of a terminal unit is the air flow sensor, of which the most commonly supplied is a velocity pressure sensor, also known as a differential pressure sensor. This sensor provides the terminal unit controller with a velocity pressure (Pv) signal (difference between total pressure (Pt) and static pressure (Ps)). Regardless of upstream static pressure fluctuations, the velocity pressure air flow sensor will generate a velocity pressure signal dependent on the speed of the air as it passes the sensor. The terminal controller uses this velocity pressure signal to adjust the air flow to match a control scheme and/or operate as a pressure independent device.
There are two basic types of velocity pressure air flow sensors:
Not all multiple point sensors are equal. The center averaging type provides the most stable and consistent velocity pressure signal, even with poor inlet conditions, due to the way the measurement points are distributed throughout the inlet duct (see Figure 1). The Price SP-300 flow sensor is an example of a center averaging, multiple point sensor.
The Price SP-300 has 12 total pressure ports and four static pressure ports. In general, the higher the number of measurement points (total and static), the higher the expected accuracy of the velocity pressure signal at both minimum turn-down and full-open air flow volumes. Simply having several total pressure ports and static pressure ports will not ensure a stable and accurate velocity pressure signal.
When the SP-300 was developed, there was a significant amount of CFD (Computational Fluid Dynamics) analysis to determine the optimal position and quantity of the total pressure ports along each of the four sensor arms, as well as the positioning and quantity of the static pressure ports.
The aerodynamic profile of the SP-300 is designed to maximize the signal while minimizing the turbulence of the air flowing around the sensor. Many center point sensors simply use a blunt face to maximize the velocity pressure signal, which unfortunately generates a significant volume of turbulence that results in a significant amount of sound generation. The turbulent air leaving the flow sensor will also generate additional sound as it passes the damper blade due to the turbulence imparted to the air flow stream by the flow sensor. It is very important to select a flow sensor with a good aerodynamic profile to present sound issues!
Precise control at minimum settings is critical in maintaining the air-change rates in the occupied zone. Since minimum flows occur at the minimum turn-down for cooling, it is necessary to adequately resolve the flow signal at the lowest possible flow volume. The ability to control the flow to the lowest possible minimum will potentially lead to a reduced requirement for reheat due to the lowered cool air volume. Due to the profile shape, a typical center averaging sensor can operate at a minimum air velocity of 400 to 450 fpm in the primary air inlet. The Price SP-300 can provide a consistent velocity pressure signal down to around 200 fpm (see Figure 2). However, this low of a flow signal requires a careful analysis of the pressure sensor on the terminal controller to determine if the pressure sensor can accurately resolve the pressure signal.
The HVAC designer must balance the flow signals and the overall size of the inlet. A common misconception with terminal units is that an inlet with a larger diameter will generate less sound. This, however, is not true. Figures 3 and 4 show the discharge and radiated sound for different inlet sizes of single duct terminals, all operating at the same air volume and static pressure drop across the damper. As you can see, there is no significant difference in sound generation between the inlet sizes until the unit size is very large.
Figure 4: Radiated sound generation for different size inlets
If the single duct terminal was only operating in constant volume mode, the difference between the inlet sizes would not be of concern. However, since most terminals are variable air volume (VAV) devices, the unit will also have a minimum air flow volume it must control. This minimum air flow volume is often around 25% of the maximum air flow volume. In a large inlet, at the minimum turn-down flow, the flow sensor may not be able to accurately resolve the velocity pressure signal. It is recommended that the terminal unit valve be selected so that the maximum air volume needed is around 75 to 85% of the inlet rated air volume capacity—this is typically the air volume at 2000 fpm through the inlet. The designer should then verify that the minimum turn-down air volume is going to provide an adequate air velocity pressure signal—a safe neck velocity for minimum turn-down is 400 fpm.
Flow sensors are often used as handles when the contractor is lifting the terminals during installation. In no small part, they are used due to their lack of sharp metal edges and the desire of the contractors to not inadvertently cut themselves. No worries when the Price SP-300 is the flow sensor! This sensor is capable of being used to pick up a single duct without damage due to the robust construction and the use of non-brittle polymers.
For more information on this topic, please see the Price Engineer’s HVAC Handbook.
Jerry Sipes, Ph.D., P.E.