worlds in motion AIRFLOW MEASUREMENT USING INSTRUMENTED INLET RINGS ebm-papst offers a simple airfl ow measurement technique for backward-curved centrifugal fans using modifi ed versions of the inlet rings (IR) typically used with these fans to maximize performance. The technique is based on the Bernoulli and Continuity equations, which allow calculation of fl ow through a converging nozzle based upon measurement of the static pressure drop across the nozzle. The applicable equation is of the form Q = 9.36k  /  ∆ P , where: s Q = airfl ow in CFM k = nozzle calibration factor ∆ P = static pressure drop across the nozzle in inches H 0 2 3 = standard air density of .075 lbs/ft s 3 = actual air density in lbs/ft 3 The calibration factor “k” has been empirically determined for each IR at standard air density of .075 lbs/ft . Large variations from this air density can effect the measurement and can be taken into account using the / ratio, otherwise this term can be dropped s from the equation. Two types of instrumented inlet rings are available for each impeller size. One type uses a single pressure tap positioned near the discharge end of the IR. The 2nd type uses four pressure taps spaced along the circumference of the IR, connected with tubing to form a piezometer ring. The piezometer ring averages the static pressure measurements around the circumference of the IR to provide a more accurate fl ow measurement in case of uneven pressure distribution around the inlet. In either case, the customer connection on the inlet ring is to a single pre-mounted tube connector, suitable for pneumatic tubing with an internal diameter of 4mm. This connection should run to the low pressure side of a differential pressure transducer (DPT), while the pressure tap in front of the IR is connected to the high pressure side of the DPT. If the DPT is mounted near the front of the IR but out of the airstream, then the high pressure tap on the DPT is suitable and no additional pressure tap is needed. 100 Hyde Road, Farmington, CT 06034 USA  Internet: http://www.ebmpapst.us     Tel: 860 674 1515  E-Mail: sales@us.ebmpapst.com  Fax: 860 674 8536 ©ebm-papst Inc. 2006. ebm-papst Inc. reserves the right to change any specifi cations or data without notice 10/2006 INSTRUMENTED INLET RINGS Find Instrumented Inlet Ring Part Number Using Table 1, fi nd the proper instrumented inlet ring P/N based on the impeller size and type (e.g., 3D EC). Choose either the P/N for the IR with a single pressure tap or for the version with multiple taps connected in a piezometer ring. In cases where there are mutiple listings for a single impeller size, please discuss the choice of the correct inlet ring with an ebm- papst applications engineer. Table 1: Inlet rings with measuring device to determine air flow for backward curved centrifugal fans Part no. with 1 pressure tap Part no. with piezometer ring Size (mm) k-value Dimensions see Impeller & fan type 25002-2-4013 25001-2-4013 250 69 96359-2-4013 2D AC/EC 25040-2-4013 25030-2-4013 250 67 25020-2-4013 2D EC 28002-2-4013 28001-2-4013 280 88 96360-2-4013 2D AC/EC 28040-2-4013 28030-2-4013 280 87 28020-2-4013 2D EC 31100-2-4013 31080-2-4013 310 120 31050-2-4013 3D AC/EC (1) 31110-2-4013 31090-2-4013 310 103 31051-2-4013 3D AC/EC 31060-2-4013 31040-2-4013 310 108 31020-2-4013 2D EC 35600-2-4013 35580-2-4013 355 155 35560-2-4013 3D AC/EC (1) (1) 35610-2-4013 35590-2-4013 355 138 35561-2-4013 3D AC/EC 35540-2-4013 35530-2-4013 355 141 35520-2-4013 2D EC 54515-2-4013 54500-2-4013 400 183 54476-2-4013 3D AC/EC + 2D AC 40040-2-4013 40030-2-4013 400 161 40020-2-4013 2D EC 54514-2-4013 54510-2-4013 450 217 54478-2-4013 2D AC/EC 64015-2-4013 64005-2-4013 450 214 63045-2-4013 3D AC/EC 45040-2-4013 45030-2-4013 450 225 45020-2-4013 2D EC 54518-2-4013 54516-2-4013 500 265 54480-2-4013 2D EC 64025-2-4013 64002-2-4013 500 283 63072-2-4013 3D AC/EC 64030-2-4013 64001-2-4013 560 350 63071-2-4013 3D AC/EC 64040-2-4013 64000-2-4013 630 480 63070-2-4013 3D AC/EC (1) subject to alterations short inlet ring Select Pressure Transducer Range It’s important to the accuracy of the fl ow measurement to select a differential pressure transducer (DPT) with a range appropriate to the expected pressure differential. It’s best to choose a DPT with a range that only exceeds the maximum expected pressure differential by a small safety margin. Excessive range will impact the fl ow measurement accuracy because the DPT accuracy is typically specifi ed as a percent of full scale. To fi nd the proper pressure transducer range: 1. Determine the maximum CFM fl ow rate expected from the fan over its operating range as installed in the application. If this info isn’t available, use the free airfl ow as provided in the fan specifi cation. 2. Use Chart 1 to fi nd the pressure differential in inches w.g. for the given fl ow and impeller size. 3. Select a differential pressure transducer with an appropriate range based on the pressure differential determined in step 2. INSTRUMENTED INLET RINGS Chart 1 - Pressure Diff erential vs Air Flow for Available Impeller Sizes 100.000 10.000 1.000 impeller size (mm) 0.100 250 280 310 0.010 355 400 450 0.001 500 560 630 0.000 10 100 1000 10000 flow [cfm] delta p [in. H2O]