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Lstiburek talks about ductwork in attics in hot-humid and mixed-humid climates. He also offers tips on how to insulate ducts and not to worry about condensation or sweating ducts.

We have performed direct numerical simulations of turbulent flows in a square duct considering a range of Reynolds numbers spanning from a marginal state up to fully developed turbulent states at low Reynolds numbers. The main motivation stems from the relatively poor knowledge about the basic physical mechanisms that are responsible for one of the most outstanding features of this class of turbulent flows: Prandtl's secondary motion of the second kind. In particular, the focus is upon the role of flow structures in its generation and characterization when increasing the Reynolds number. We present a two-fold scenario. On the one hand, buffer layer structures determine the distribution of mean streamwise vorticity. On the other hand, the shape and the quantitative character of the mean secondary flow, defined through the mean cross-stream function, are influenced by motions taking place at larger scales. It is shown that high velocity streaks are preferentially located in the corner region (e.g. less than 50 wall units apart from a sidewall), flanked by low velocity ones. These locations are determined by the positioning of quasi-streamwise vortices with a preferential sign of rotation in agreement with the above described velocity streaks' positions. This preferential arrangement of the classical buffer layer structures determines the pattern of the mean streamwise vorticity that approaches the corners with increasing Reynolds number. On the other hand, the centre of the mean secondary flow, defined as the position of the extrema of the mean cross-stream function (computed using the mean streamwise vorticity), remains at a constant location departing from the mean streamwise vorticity field for larger Reynolds numbers, i.e. it scales in outer units. This paper also presents a detailed validation of the numerical technique including a comparison of the numerical results with data obtained from a companion experiment.

John discusses how much time is spent selecting air-distribution devices. In conversations with design engineers, they estimate spending weeks and months laying out chillers, air handlers, controls and ductwork, but estimate they spend only minutes selecting and laying out air-distribution devices. Most of the HVAC consulting engineer's time is spent designing a system to meet the required ventilation rate and space temperature, but occupant discomfort can still occur if the space air velocity is too high.

A previous investigation showed that fire in circular, mechanically-ventilated, horizontal 0.30-m and 0.61-m ducts could be extinguished effectively by discharging water mist in a co-flow manner with the exhaust flow. However, besides horizontal ducts, an exhaust system is likely to also have ductwork in vertical and other orientations. Since the water mist dispersion and transport to the fire could be affected by the duct orientation due to fire buoyancy, there was a need to determine if the established horizontal duct protections would also be applicable to other duct orientations. Considering the fact that the fire buoyancy effect in ducts is bracketed between horizontal and vertical duct orientations, a series of fire tests was therefore conducted to evaluate if the above-mentioned horizontal duct protections would also be applicable when the ducts were to be in the vertical orientation. This paper presents the evaluation of vertical duct protection employing the same nominal spacing of 0.31 m for the water mist application as for the horizontal duct protection. The evaluation included the two protections established previously for the horizontal 0.30-m duct: one provided a minimum water mist concentration of 300 g/m3 with a median droplet size of 77 μm, while the other provided 399 g/m3 with a median droplet size of 88 μm. On the other hand, the protection for the horizontal 0.61-m duct yielded a minimum water mist concentration of 270 g/m3 with a median droplet size of 115 μm. The vertical duct tests showed that, by discharging water mist upward in the co-flow manner with the exhaust flow, both the protections for the horizontal 0.30-m duct could successfully extinguish the fire in the vertical duct, but the protection for the horizontal 0.61-m duct could not completely extinguish the fire in the vertical duct when water mist was discharged upward in the co-flow manner. However, by discharging water mist downward against the exhaust flow, the fire in the vertical 0.61-m duct could be extinguished rapidly. The vertical duct tests showed that, to ensure the applicability of a water mist protection for different duct orientations, the protection should be verified for both the horizontal and vertical duct orientations, with appropriate adjustments if required.

High humidity inside ductworks could be a potential risk for microbial growth and there is also a hypothesis that lower night-time ventilation increases the risk of growth. This study investigates the possibility of microbial growth in ventilation ductwork exposed to humid and cold conditions. Two different typical night-time ventilation strategies for public buildings were investigated: ventilation rate was either continuously the same (0.15 L/s, m2) or no airflow during the night-time. Experimental data were collected over a four-month period. In the experiment, microbial media was released inside the ductwork initially. During the test period, air temperature and relative humidity inside the ductwork were controlled between 11–14 °C and 70–90%. Wipe, swab and air samples were taken at the beginning, monthly and at the end of the test period. The study results showed the extinction of colonies by the end of the experiment regardless of the chosen night-time ventilation strategy. The colony count in the air was low throughout the study period. Therefore, the results indicate that the long-term growth on the walls of air ducts is unlikely and the risk of microbial transfer from the air ductworks to room space is low.

In precast concrete systems, connections are often made by grouting bars that project from one member into ducts embedded in another. For bridge bents, bar-duct systems can be assembled rapidly if a few large bars and ducts are used to connect the column and cap beam. In some cases, the required anchorage lengths for the large bars exceed the length available. To evaluate the anchorage requirements for this situation, 14 pullout tests were performed on bars with sizes up to No. 18. The tests and a nonlinear finite element model showed that, under conditions similar to those tested, large bars can develop their yield and fracture strengths in as few as six and 10 bar diameters, respectively. The effects of the bar size were found to be small compared to the scatter among the test results. Parallel tests with polypropylene fibers showed that fibers generally decreased pullout resistance, although this is likely the result of reduced grout strength. [PUBLICATION ABSTRACT]

•The RCAC index is a new method for economic analysis for evaluating air ducts.•The PID duct is economical, with a high RCAC index of each cost.•Alternative air ductwork is an option for replacing the conventional air ductwork.•The PID duct is the best option to reduce condensation and energy losses. This research studies the numerical modelling using computational fluid dynamics with a k-Epsilon turbulence model to analyze airflow behaviors inside the alternative compared with conventional air ductwork combined with a comprehensive analysis of the economic parameter created by the author called Reveal Comparative Advantage Cost index. All alternative air ductwork has lower pressure drop and a higher ability to maintain the temperature inside, showing that alternative air duct systems have a significant advantage over conventional air ductwork. The fabric ductwork has a lower pressure drop than galvanized steel perforated ductwork, which is 69.15 %. The pre-insulated ductwork can keep the best temperature inside, with a different temperature value between the air inlet and air outlet of 0.06°C. The Reveal Comparative Advantage Cost of construction costs of the galvanized steel ductwork and the galvanized steel perforated ductwork are 1.02 and 1.06, which means the conventional air ductwork is initially cost-effective in terms of investment and operating costs. However, alternative air ductwork is the better option for long-term use. They have cost-effective electrical energy and maintenance costs with a high Reveal Comparative Advantage Cost index. Significantly, the pre-insulated ductwork has the Reveal Comparative Advantage Cost index of maintenance, electrical energy, and construction costs of 1.38, 1.09, and 0.95, respectively. The results of the airflow behaviors study combined with the Reveal Comparative Advantage Cost index study show that the galvanized steel perforated ductwork is not a selectivity that qualifies either for airflow behaviors or is unsuitable for investment. Pre-insulated ductwork is the appropriate option due to the high airflow behaviors and cost, which makes it suitable for investment. However, the Thailand Authority should develop alternative air ductwork Thai standards for engineering design and safety building. [Display omitted]

Ivanovich provides summarizy of the status of US fan efficiency codes, standards, and regulations.

VAV systems are the most common HVAC system for commercial buildings, but design practices vary widely around the country and even among design firms in a given area. Some of the variation is due to local construction practices and labor costs, but most of the variation is due simply to how engineers are taught by their mentors in their early years of practice. Here, Taylor compares various VAV box inlet and outlet duct design options including their impact on first costs and pressure drop.