Effect of fittings on volumetric airflow measurements (RP-1245): Multiple-path (tee) duct disturbances

Abstract

Current methods of making volumetric airflow measurements in the field are prone to a number of known inaccuracies because field technicians are required to take measurements in nonideal circumstances close to fitting disturbances. This is especially problematic when measurements are made in close proximity downstream of tee fittings as part of test and balancing procedures for HVAC systems designed to meet comfort and air quality requirements. The overall objectives of this research were (1) to quantify the influence of several duct disturbances on volumetric flow rate measurements and (2) to develop guidelines for field technicians to assist them in making more accurate volumetric airflow measurements in rectangular ducts during test and balance operations. Previous work has addressed the measurement issues associated with single-path duct disturbances. This article focusses on the error caused by multiple-path (diverging tee) duct disturbances. Measurements are presented of the error caused by the distance from tee fitting disturbances to a given airflow measurement (traverse) location. It is shown that in many situations, traverse measurements taken in the branch downstream of the tee are subject to significant regions of reverse flow, which cannot be sensed correctly by either pitot-static probes or hot-wire probes. In such situations, large airflow rate errors will be the result, and it is therefore recommended that alternative traverse positions be utilized in these circumstances.