90 degree angle in air conditioning ducts, should I avoid them?

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The reason that 90 degree turns (or any tight radius bends) in duct systems are discouraged are because they reduce air-flow. The friction that is encountered by the moving air as it hits the wall of the turn slows it down decreasing the distance it can travel.

There are equations that can be used to calculate the number of bends before air performance is affected. Duct size and shape, motor wattage, register opening, etc. will need to be considered. If you can orient the duct system and configure the shape with "gentler" turns such as using (2) 45 degree bends in place of (1) 90 degree bend. This would restrict air-flow less. Installing a motorized fan duct in-line with the new duct branch would allow for longer runs if needed and account for sharp turns.

Is it feasible, when viewing the drawing you posted, to branch off the main duct sooner with a 45 degree duct and circumvent the restroom with 45's? Or consider using flexible insulated duct which is much more...

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EDIT: I forgot about one case where 90 degree corners ARE bad: high voltage PCBs. This is nothing to do with reflection or radiation, but how any sort of sharp shape will concentrate the high electric field and make a dielectric breakdown and arc over more likely. This can be exploited for PCB spark gaps, but otherwise, one should avoid right angle corners on a high voltage PCB, 1kV+. And one should use rounded pads for everything, even SMD resistor/capacitor pads. Avoid sharp copper shapes as much as possible.

No, there is no reason to prefer 45 angles over right angles. I will say this definitively: The other answers claiming that right angles cause more EMI are demonstrably false. This is not some sort of theoretical unknown that can be debated. We can measure EMI radiated from various trace shapes, and we have, and right angles do not radiate more than than 45 degree angles. Bring up as many theoretical reasons why right angles should be bad for EMI, but they don't...

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It seems like there are about a million things that need to be considered when you’re planning a renovation of your commercial space. And the problem is, unless you’ve done this many times before, you may not know about all of them. That is, until you’ve made a mistake and it’s too late to correct it.

If you’re like most business owners, you probably didn’t realize that your HVAC system and its ductwork design is something you need to be thinking about even before the architect has completed the plans.

Do you want comfortable, consistent temperatures in your space? How about good air quality? Energy efficiency? If you do, then it pays to plan your ductwork design early in the renovation process to avoid mistakes.

Consequences of Poor Ductwork Design

According to the US Department of Energy, the average HVAC duct system is only about 60 percent efficient. That means air is not flowing through your space and your HVAC system as it should be, which leads...

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In-line or centrifugal fans are the ones to consider when pushing air through longer ducting installations. Axial fans are OK for short distances like straight through the wall

As noted elsewhere, use rigid type ducting, round or square, as this is more effecient than "flexible" type ducting.

Your other consideration may be noise. If you have a choice of fans lined up, check out the sound levels they produce (dB) lower figures are better. You will hear one running at 45dB whilst one at around 25bB should be very quiet. Ratings are often shown as dBA at 3m. The distance should be the same when making comparisons

For a cloakroom you will not need a heavy duty fan. The screwfix 53730 noted elsewhere in the thread is way over the top!

I did notice that screwfix seemed to have good supporting information on many of their extract fans. You can download their manual on the fan you are looking at. Some of their small bathroom/wc fans indicate in excess of 9m duct runs...

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Process duct work conveys large volumes of hot, dusty air from processing equipment to mills, baghouses to other process equipment. Process duct work may be round or rectangular. Although round duct work costs more to fabricate than rectangular duct work, it requires fewer stiffeners and is favored in many applications over rectangular ductwork.

The air in process duct work may be at ambient conditions or may operate at up to 900 °F (482 °C). Process ductwork varies in size from 2 ft diameter to 20 ft diameter or to perhaps 20 ft by 40 ft rectangular.

Large process ductwork may fill with dust, depending on slope, to up to 30% of cross section, which can weight 2 to 4 tons per linear foot.

Round ductwork is subject to duct suction collapse, and requires stiffeners to minimize this. but is more efficient on material than rectangular duct work.

There are no comprehensive, design references for process duct work design. The ASCE reference for the design of...

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Air distribution is the weak link in many heating and cooling systems. Here's how the industry's top pros detail their ductwork — for results you can take to the bank.

by Ted Cushman

Experts agree that sealing ducts is vital to good HVAC performance, especially in a vented crawlspace or attic, where leaky return ducts will continually draw moist unconditioned air directly into the air handler. At around $6 a bucket, high-performance duct mastic is cheap, but the labor to seal the ductwork properly is tedious and painstaking.

Good ductwork is key to HVAC performance. Yet hidden away in the dank crawlspace or the hot, humid attic, a home's ductwork goes unappreciated and underexamined. And too often, sad to say, this vital part of the HVAC system brings the energy perfor-mance of the home to surprising lows.

In one study, for example, researchers with the Florida Solar Energy Center (FSEC) replaced several oversized air conditioning units with...

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HVAC for BEGINNERS: DUCTWORK BASICS
Rectangular Ells, Tees, and Reducers

Here in our Hvac for Beginners duct fittings section we will look at some common duct fittings and configurations. On a previous page we discussed plenums and return air drops. On this page we will look at common hvac system ells, tees, and reducers.

As discussed in our Hvac for Beginners main page, each component of a forced air system, from the filter to each branch duct, adds a certain amount of restriction to the airflow of that system. Every duct fitting adds a certain amount of restriction. This restriction (friction loss) adds to the TEL (total equivalent duct length) of the system.

Therefore, the TEL of the ducting must be minimized for the best performing comfort system.

90 degree turns in forced air ducting are to be avoided whenever possible. However, if structurally there is no choice, there are ways to minimize impact. Example 1, the square ell is...
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Whether it's a freezing winter morning in Minneapolis or a torrid summer afternoon in Phoenix, air leaking from ducts and air handlers can bring a little welcome relief to a home inspector braving the elements in an unconditioned attic. For clients, however, the leak that brings relief to the home inspector can represent a costly defect. Poorly installed HVAC ducts cost our clients money and reduce their comfort every minute the HVAC system runs. Installation errors can also contribute to moisture problems and related fungal growth.

This article focuses on flexible HVAC duct (flex duct), the most common material used in modern residential HVAC duct systems. While other materials such as sheet metal and duct board are still used, the cost advantages of flex duct make it a popular choice for many builders. Unfortunately, the factors that give it a cost advantage--mainly less skilled and less expensive labor--also make installation errors more likely when compared to other...

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