Article

Equal Area vs. Log-Tchebycheff

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Equal Area vs. Log-Tchebycheff

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Abstract

Air-flow rates obtained with the Log-Tchebycheff and Equal Area methods were compared, and the influence of transverse-plane location on the measurements was examined. The testing was part of an effort to identify duct-velocity profiles and calibrate air-flow-measuring stations for the general-service air-handling system. It can be drawn from these tests that the uniformity of the velocity profile has a more significant influence on an air-flow measurement than does the method used to determine the measurement grid. The results do suggest that additional research aimed at comparing the accuracy of the Log-Tchebycheff and Equal Area methods is merited.

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... The Log T method has been demonstrated to provide a more realistic measurement of the flow rate in a duct by means of a series of very closely spaced Pitot tube traverse measurements (140 points on a 14 x 10 grid in a 28 inch x 20 inch duct) [10]. The reason that the Log T method is more precise is that the method obtains data points closer to duct walls thereby accounting for increased friction loss (and hence lower air velocities) close to the walls. ...
Conference Paper
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A series of four simultaneous tracer gas and traverse flow rate tests was performed on a specially-fabricated section of square ductwork at the Lagus Applied Technology, Inc. (LAT) facility in Escondido, California. The duct section was essentially the same as that used currently to measure makeup flow rates at ANO. In order to provide a correlation between tracer gas-determined flow rates and those determined using a traverse technique, flow rate measurements using the tracer gas technique and two traverse methods were undertaken over a range of flow rates encompassing those utilized at ANO during operation of the CREVS in the Pressurization Mode. Traverse flow rate measurements were performed with a calibrated hot-wire anemometer using the Equal Area method and the Log Tchebycheff (Log T) method. The particular hot wire anemometer used in the testing provides flow velocity data that are corrected to standard conditions for temperature (70 Deg F). In order to allow traverse measurements to be reduced to standard conditions (14.7 psia and 70 Deg F), a digital barometer was used to measure barometric pressure during the traverse flow measurements. The Log T flow rate data and the Tracer Gas flow rate data are essentially identical when the Log T data are reduced to standard conditions. The flow rate determined by the Equal Area Method (at standard conditions) is approximately 6 % higher than the corresponding Log T flow rate over the approximate range of 400 SCFM to 500 SCFM. These measurements demonstrate that the Log T method is superior to the Equal Area for determining flow rates in a square duct.
... The estimated error appears to be much less than 3%, so validating the hypothesis according to which the flow velocity for the estimation of the flow rate can be directly inferred from a single measurement at a chosen position even in the square cross-section pipes as well as in circular cross-section ones. A better estimation of the error could be pursued by choosing the measuring points in the cross section according to the Log-Tchebycheff method, [16], [17], which minimizes the error due to the failure in accounting for losses in the boundary layers. ...
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