TECHNICAL FEATURE FIGURE 2: Amplification of a flow sensor signal is created by reducing the static pressure on the back side of the sensor. TABLE 1 Comparison of obtainable minimum flows (cfm) at 0.03 in. w.g. Vp INLET SIZE which the airflow through the air terminal is independent of system pressure. The majority of systems designed today are pressureindependent systems. In a pressure-independent control system, the flow range for an air terminal unit is determined by two factors. The first is the differential velocity pressure as determined by the difference between the total and static pressure values sent by a flow sensor to the controller. The second is the operating range of the transducer that is built into the controller. Typically, a pressure independent VAV controller has a built-in transducer that receives the signal from the air terminal unit sensor and senses the differential across a diaphragm. The DDC VAV controller uses the pressure differential to compare to a lookup table and reports airflow based on the unit size and the K factor. K factors vary between manufacturers (Equation 2). The minimum and maximum capacity for flow in a terminal unit is set by the operating range of the transducer in the controller and the amplification of the VAV sensor. VAV controllers generally have an operating range of velocity pressures from 0.03 in. w.g. to 1.0 in. w.g. (7.47 Pa to 249 Pa) and it is this range that sets the minimum and maximum capacities of an air terminal unit's primary air capacity. The minimum velocity pressure of 0.03 in. w.g. (7.47 Pa) has been an industry standard for 32 ASHRAE JOURNAL ashrae.org SEPTEM BER 2014 6 IN. 8 IN. 10 IN. 12 IN. 14 IN. 16 IN. MANUFACTURER A 81 154 318 433 576 805 MANUFACTURER B FIGURE 1: AHRI Standard 885-2008 Appendix E (courtesy of AHRI). 94 171 284 407 563 710 MANUFACTURER C 78 157 249 328 522 665 PERCENT SPREAD ON MINIMUM FLOW (0.03 IN. W.G. V P) 17.0% 10.1% 21.8% 24.3% 9.4% 17.4% many years. However, advancements in the accuracy of the transducers in VAV controllers has reduced the minimum for some controllers to a velocity pressure between 0.01 in. w.g. and 0.015 in. w.g. (2.49 Pa and 3.74 Pa), lowering the controllable flow for air terminal units. One of the major differences in performance between different air terminal unit manufacturers is the amplification that is produced by the flow sensor installed in the unit and the resulting operating range published for that air terminal unit. The amplification of the velocity pressure produced by a sensor is created by directing the airflow around the sensor to create a lower static pressure at the point where the static pressure is measured (Figure 2). Amplification factor (F): the ratio of sensor output to true velocity pressure. For example, a pressure sensor with a reading of 1.0 in. w.g. (249 Pa) of pressure at a true velocity pressure of 0.43 in. w.g. (107 Pa) would have an amplification factor of 1.0/0.43 = 2.3. F may be calculated from K with the following formula, where A is the nominal duct area in square feet. The nominal duct area is calculated based on the geometry of the duct, not on the actual free area.