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Energy and acoustic performance effects due to VAV duct design and installation practice variations.

Article Excerpt
INTRODUCTION

Research on updating friction loss coefficients for duct fittings in rectangular, round, and flat oval ducts and for improving design methodologies for HVAC air-distribution systems in buildings has helped to improve the design of HVAC air-distribution systems to the point where branch ducts supply air to variable volume terminal units. However, design information on duct design from variable volume terminal units to room air terminal devices is predicated on ideal system installations, which seldom if ever occur in field installations. There is little measured data concerning the effects on building air distribution, energy, and sound due to variations in duct installations between air terminal units and room diffusers.

The installation of supply duct and variable air volume (VAV) systems raises a number of issues that impact overall air-distribution system efficiency and performance. Although research information on specific details is limited, there are established guidelines addressing common installation issues. The primary information sources for VAV duct design are the Sheet Metal and Air Conditioning Contractors National Association, Inc.'s (SMACNA) HVAC Systems--Duct Design (SMACNA 1990), the California Energy Commission's (CEC) Advanced Variable Air Volume System Design Guide: Design Guidelines (Hydeman et al. 2003), and the 2001 ASHRAE Handbook--Fundamentals (ASHRAE 2001). Information available covering the performance of the air distribution system section from the VAV box to the diffuser includes duct design issues, performance issues, and installation problems.

Two predominant duct design issues deal with sound level and performance. Issues with flexible duct connections at the inlet of the diffuser include increased pressure drop, increased sound levels, and nonuniform air distribution from the diffuser (Int-Hout and Ratz 1996). Sound level is also an important consideration in HVAC duct design, since ducts serve as transmitters of break-in noise, while flexible ducts are effective attenuators of upstream sound sources (Int-Hout and Ratz 1996). One example of guidance is that VAV units above acoustical ceilings should have radiated noise criteria (NC) levels no more than ~5 NC above the desired room NC rating. (Hydeman et al. 2003).

Variable airflow, proper unit sizing, and minimum airflow settings of VAV systems are the main performance issues. The variability of the flow rates common to VAV systems appears in the literature as a common issue. Diffusers are designed to optimally distribute the air at some particular load condition and air volume. Thus, the performance of outlets with regard to throw, room velocity, and sound levels will vary greatly with the discharge volume (SMACNA 1990). However, diffusers are often selected without regard for the effect of VAV turn-down (Int-Hout 2001), which results in the degraded performance of diffusers at very low flow rates (Hydeman et al. 2003). HVAC systems may operate at only one-half of the design airflow for the bulk of the time. When considering proper room air distribution, SMACNA suggests designing with overthrow at maximum design volumes to achieve acceptable throw at part load volumes (SMACNA 1990).

Installation problems predominantly concern the length and type of the duct branch, how duct turns are accomplished, and how the duct approaches the diffuser. Without sufficient length to develop a uniform profile, flow in duct branches too close to the VAV terminal or a previous branch is nonuniform, and, hence, often causes an increase in pressure loss. SMACNA suggests a sufficiently long duct section before elbows. Without turning vanes, especially in fittings used for avoiding obstructions, good airflow in the duct system can be totally destroyed (SMACNA 1990).

The duct approach to the diffuser is also very important, since detrimental effects of improper duct approach cannot be corrected by the diffuser itself. Both SMACNA and ASHRAE agree that for proper diffusion, the velocity of the air stream must be as uniform as possible over the entire connection to the duct and must be perpendicular to the outlet face. However, few outlets are installed in this manner. Most ceiling outlets are attached either directly to the bottom of horizontal ducts or to vertical take-off ducts that connect the outlet with the horizontal duct (SMACNA 1990; ASHRAE 2001).

With respect to sound level, SMACNA suggests placing the diffusers as far as possible from duct elbows and branch take-offs to minimize sound transmission. Direct duct connections to the diffuser can result in sound levels as much as 12 NC higher than catalog levels (Int-Hout and Ratz 1996). An offset of the flexible duct connection between the diffuser and the supply duct equal to the diffuser diameter over a connection length equal to two diameters can increase the sound power level as much as 12 dB (SMACNA 1990).

PROJECT OBJECTIVE

This project had a specific goal of providing useful information to the HVAC industry on duct design and installation, from the VAV unit to the diffuser. Therefore, collecting industry needs was essential to determining the best direction and scope of the investigation. Specific guidance was sought regarding the following: obtaining a consensus on a best or industry-standard installation; determining what are the typical variations from that standard seen in the field, including predominance and magnitude; obtaining an estimate of the performance expectations from an industry standard installation and how important that performance variation is. The information collected was used to determine the different installation parameters and to prioritize the list based on the likelihood of the variation and the anticipated performance effect of variation in each parameter.

EXPERIMENT FORMATION AND METHOD

To meet the project objectives, a detailed investigation methodology was developed and followed. The test laboratory was configured, and test protocols were developed to conduct experiments called for in the experimental design.

Aligning Project Scope with the Knowledge Gap

Valuable assistance from outside sources was used to obtain the voice of the customer with regard to the knowledge gap in duct design and installation. The literature review, the project expert panel, and a Department of Energy review panel made particularly significant contributions toward identifying the knowledge gap.

To ensure relevance with up-to-date installation practices and problems, an expert committee was formed and consulted during the initial project methodology development to identify the state-of-the-art VAV-to-diffuser duct design and common construction variations found in the field. The committee recommended testing at isothermal conditions so that airflow pattern variations would be due to installation variations and not disturbed by heat-source-generated airflow. Several possible test setups were identified along with the variations that should be tested in this investigation.

To cover a reasonable range of field...

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