lindab | we simplify construction Flow measuring unit-rectangular FRA Technical data Settings V nom indicates the measuring range for the actuator. A stand- ard FRA is calibrated to a V nom of 7 m/s according to the table 3. In special cases the FRA can be set to a higher V nom , e.g. 10 m/s. The output signal from VRD3 is linear between 2 -10 V which corresponds to a flow between 0 and V nom . Air flows corresponding to air velocities below 1.2 m/s will result in a 2 V signal. Technical data Volume flow measurement The accuracy of volume flow measurement depends on the flow conditions in front of the measuring cross. It is prefera ble to have a long straight duct section in front of the meas uring point, according to the table below. If these recommendations are not followed, it will cause an unstable flow measurement and therefore higher inaccuracy in the regulation of the required air flow. Components Bend Tee-piece Damper Recommended straight duct before unit 3 x d h * 4 x d h * 6 x d h * Table 2. FRA and recommended straight duct before unit. d h * is the hydralical diameter for a rectangular duct (and FRA), d h can be calculated by using FRA dimension a and b: d h * = 2 x a x b / (a + b) Air flow direction 3 x d h Bend Duct FRA Duct Example above showing top view of recommended straight duct distance between duct bend and a FRA. b a Front view of FRA and dimension a and b. 2 Lindab reserves the right to make changes without prior notice 2020-10-30
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