Equal Area vs. Log-Tchebycheff

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Equal Area vs. Log-Tchebycheff

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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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Airflow control is important to assure proper air change requirements, space pressurization, and personal safety. However, airflow measurement devices are often misapplied because they are not widely understood. This article seeks to provide the technical understanding needed to select and apply them properly in a pharmaceutical setting.
The second edition of Ventilation Control of the Work Environment incorporates changes in the field of industrial hygiene since the first edition was published in 1982. Integrating feedback from students and professionals, the new edition includes problems sets for each chapter and updated information on the modeling of exhaust ventilation systems, and thus assures the continuation of the book's role as the primary industry textbook. This revised text includes a large amount of material on HVAC systems, and has been updated to reflect the changes in the Ventilation Manual published by ACGIH. It uses both English and metric units, and each chapter concludes with a problem set.
Fan energy use in variable-air-volume (VAV) systems can be reduced by resetting the supply duct pressure. The standard way to reset duct pressure is by con trolling the most open terminal damper to a nearly open position. This strategy is rarely used because of a variety of issues including sensing limitations, network bandwidth, and stability. This paper describes the development of a new method of determining the critical supply duct pressure for VAV systems. The method relies on a short, simple functional test and a data processing technique that is based on a simple model of the system behavior. The method can be implemented during normal system operation, and it could be automated. The system model includes the effect of duct leakage, which offers the potential for dual use as a duct leakage diagnostic. Results from experiments on a laboratory-scale system demonstrate good accuracy for determining critical pressure and moderate accuracy for determining duct leakage. Results from experiments on two commercial air-handling units demonstrate that the method is practical and that it offers the potential for large energy savings.
This work presents the results of a 3D numerical analysis, particularly the effect at the volumetric flow measurements due to a perturbation generated downstream by fitting used in ducts of the air conditioning systems. The numerical simulations were developed using commercial software based on Computational Fluid Dynamics (CFD), where the Reynolds Averaged Navier–Stokes equations (RANS), through an approach of finite volume method (FVM) using several turbulence models, are solved. The turbulence models used are the one-equation Spalart–Allmaras, the two-equation eddy viscosity (EVMs) and Reynolds stress (RSM). The numerical results were compared with velocity profiles obtained from technical literature, where the Spalart–Allmaras turbulence model offers the best numerical predictions, according to measurements taken near and far from the perturbations in the case of 90° elbow with single thickness vanes. Also, the behavior of the mass flow rate measurements taken downstream of the fitting is presented; identifying the zones where the largest deviations are located with respect to the real mass flow rate supplied to the system, which are about 1.61% in the case of 90° elbow CR3-9, 2.6% and 4.6% in the cases of rectangular transition piece ER4-1 with θ = 60° and θ = 180°, respectively. As well as the effect on the flow measurement due of the number of points used in the measurement plane. This study allows knowing the flow behavior downstream of fitting.
The ability to measure the velocity and airflow volume in new and existing local exhaust ventilation systems is of importance for both initial acceptance of the system and periodic testing to ensure that the system continues to meet both design specifications and regulatory standards. The emphasis in this chapter is on those techniques useful for work field in industrial plants and mines not in laboratory settings. Initial treatment covers the design, application, and interpretation of data from pitot tube traverses in exhaust ducts. This coverage reinforces familiarity with the introduction to pressures existing in ducts introduced in Chapter 2. A list of good practice is presented to ensure the information obtained from a pitot tube survey is valid. The current accepted traverse methods for both round and square duct are described. As a part of this discussion a review is presented of a range of fluid manometers and transducers for total, velocity, and static pressure measurements. A group of mechanical devices for velocity measurement at hood stations including rotating vane, deflecting vane, and bridled vane anemometers are then described and the appropriate application realm for each type is defined. A second major field instrument type based on a heated sensor is reviewed and the advantages and limitations of these devices are reviewed. Although quantitative measurement is usually preferred in evaluating hood and system performance, field engineers frequently observe air flow patterns at hood locations with visible tracers. The available tracers are reviewed and evaluated for in plant use. A method of evaluating airflow into a hood by means of a hood static pressure measurement is introduced as a rapid and accurate technique if the coefficient of entry of the hood is known. The chapter closes with a discussion of the methods of calibrating mechanical and heated instruments.
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