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Pressure loss of corrugated spiral duct.

Article Excerpt
INTRODUCTION

Corrugated spiral ducts typically are used underground because the corrugations increase the limits of imposed load applied by backfill. Recently they have also been used for above ground HVAC systems, due to their reduced shipping and storage costs. Corrugated ducts can withstand greater loads, thus allowing them to be stacked higher for transportation and storage. However, there is a lack of corrugated round duct information in the Duct Design chapter of the ASHRAE Handbook (2005). As a result, the cognizant Technical Committee of ASHRAE (TC 5.2) sponsored this research to determine the resistance to flow of corrugated galvanized spiral ducts.

The original research by Wright (1945) resulted in the ASHRAE Friction Chart which was used prior to 1989. The friction chart was based on rolled round duct with longitudinal seams and joints on 760 mm (2.5 ft) centers. Such ducts had an absolute roughness of 0.15 mm (0.0005 ft). They were placed in the "average" roughness category when ASHRAE developed the Duct Roughness Factor table in the Handbook.

All other duct materials were categorized as either "smooth", "medium smooth", "medium rough" or "rough." The new Friction Chart published in 1989 is based on galvanized steel spiral duct with joints on 3000 mm (10 ft) centers per Jones (1979), and rolled duct with longitudinal seams and joints on 1200 mm (4 ft) centers as reported in Griggs et al. (1987). Such ducts are in the "medium smooth" category. The purpose of the present research was to determine the absolute roughness, roughness category of corrugated galvanized spiral ducts, and the percent increase of corrugated duct friction factor compared to spiral ducts without corrugations.

EXPERIMENTAL PROGRAM

Three round, spiral, 24 gage, galvanized spiral steel ducts with two corrugations between circumferential seams were tested in this project. Duct diameters were 203 mm (8 in.), 356 mm (14 in.), and 508 mm (20 in.). In each case the duct sections were 3.05 m (10 ft) in length, and were connected by beaded slip couplings sealed by duct tape. The geometric details of the ducts are shown in Figure 1.

[FIGURE 1 OMITTED]

The test apparatus shown in Figure 2 was in compliance with ASHRAE Standard 120-1999. The test setup included a 30-hp centrifugal fan, followed by a cylindrical nozzle chamber which was used for flow measurement. A variable frequency drive (VFD) was used to control the fan speed (and hence the air flow through the system). Screens mounted upstream and downstream of the nozzle board inside the chamber were used to settle the flow. The system was blow through in nature. The nozzle board contained four long-radius spun aluminum flow nozzles having throat diameters of 51-mm (2-in.), 102-mm (4-in.), 152-mm (6-in.) and 203-mm (8-in.). The nozzles were mounted on a 25-mm (1-in.) thick plywood board....

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