Pressure Losses across Multiple Fittings in Ventilation Ducts

Z T Ai,C M Mak

doi:10.1155/2013/195763

Abstract

The accurate prediction of pressure losses across in-duct fittings is of significance in relation to the accurate sizing and good energy efficiency of air-delivery systems. Current design guides provide design methods and data for the prediction of pressure losses only for a single and isolated fitting. This study presents an investigation of pressure losses across multiple interactive in-duct fittings in a ventilation duct. A laboratory measurement of pressure losses across one fitting and multiple fittings in a ventilation duct is carried out. The pressure loss across multiple interactive fittings is lower than that across multiple similar individual fittings, while the percentage decrease is dependent on the configuration and combination of the fittings. This implies that the pressure loss across multiple closely mounted fittings calculated by summing the pressure losses across individual fittings, as provided in the ASHRAE handbook and the CIBSE guide, is overpredicted. The numerical prediction of the pressure losses across multiple fittings using the large-eddy simulation (LES) model shows good agreement with the measured data, suggesting that this model is a useful tool in ductwork design and can help to save experimental resources and improve experimental accuracy and reliability.

Highlights

In air-delivery ductworks of HVAC systems, pressure losses across duct fittings such as dampers, sensors, bends, transition pieces, duct corners, branches, and even splitter attenuators are important in counteracting the pressure difference created by fans
It is found that the pressure loss across a fitting almost has a linear relationship with the duct Reynolds Number (Re). This implies that any factors increasing the duct Re, such as an increase in velocity and cross-sectional dimensions, can result in an increase in pressure loss across an in-duct fitting
The results demonstrate that the effect of a neighboring fitting is complex; that is, different fitting type, location, obstruction ratio, and duct Re can result in very distinctive pressure losses

Summary

Introduction

In air-delivery ductworks of HVAC systems, pressure losses across duct fittings such as dampers, sensors, bends, transition pieces, duct corners, branches, and even splitter attenuators are important in counteracting the pressure difference created by fans. The commonly adopted data of pressure losses across HVAC duct fittings are those provided in the well-known design guides, such as the ASHRAE handbook [1], the CIBSE guide [2], and the handbook by Idelchik [3]. Atkin and Shao [7] applied computational fluid dynamics (CFD) modeling to analyze the effects of the separation and orientation of two closely connected bends on total pressure loss They found that at a separation distance of 8 to 10 hydraulic diameters, the pressure drop across the two bends is highly dependent on their relative orientation.

Methods

Results

Conclusion