This article originally appeared in the Spiral Duct Manufacturers Association insert in the January 2017 issue of SNIPS magazine.
There are several factors that a designer should consider when designing a duct system, such as ductwork performance, fabrication, installation, economics and acoustics.
This article examines these factors to show how round/spiral duct and associated fittings create a much more energy-efficient duct system over rectangular ductwork with major labor savings.
Based on noise criteria, the American Society of Heating, Refrigerating and Air-Conditioning Engineers has developed a set of guidelines for maximum airflow velocities in a duct system based on the duct location.
Higher velocities lend themselves to better duct designs using static regain. Static regain designs usually result in low operating cost and balanced systems. All systems are initially designed or sized using round dimensions. If the designer desires a rectangular system, or because of space restrictions requires the use of flat oval, the round size must be converted to rectangular or flat oval by either calculating the equivalent size that will give the same pressure loss per 100 feet of ductwork or by keeping the velocity the same as in the round size. This is typically done with a duct calculator, smartphone app or spreadsheet.
However, higher velocities in rectangular systems are much more likely to create pressure loss and noise problems than in round or flat-oval ductwork.
Let’s look at two examples of converting rectangular to round, using the same pressure drop and using the velocity. For the example we will assume a round size that carries 5,000 cubic feet per minute at 2,000 feet per minute.
- Case No. 1: Rectangular-size with the same velocity as round size.
- Case No. 2: Rectangular-size with the same pressure loss as the round.
The most energy efficient rectangular shape is one with an aspect ratio of 1-to-1. Even at that aspect ratio there would be an increase in static pressure of 19 percent, and in a common aspect ratio of 3-to-1, the static pressure loss increases by 46 percent. In contrast, flat oval with an aspect ratio of 1-to-1 is round. Flat oval will also have lower pressure losses than rectangular at the same velocity as round.
The pressure loss is that of the round. Note how the velocity must decrease to have an equivalent pressure drop. These means that the cross-sectional area of the rectangular duct is getting larger causing the perimeter to increase. For a 1-to-1 aspect ratio the perimeter increased 15 percent. At the 3-to-1 aspect ratio, the perimeter increases by 33 percent.
When comparing flat oval versus rectangular when keeping the velocity the same, the static pressure loss will increase from about 3 percent for high aspect-ratio ducts to over 17 percent when the aspect ratio is about 1-to-1. If we keep the static pressure loss the same by aspect ratio, the rectangular perimeter is about 3 percent at high aspect ratios, to over 16 percent greater when the aspect ratio is close to 1-1.
The increases in rectangular perimeters when compared with round or flat oval will cause more heat loss or gain and more insulation wrap required, heavier ducts and more reinforcement.
Round and flat-oval ducts are typically available in 12-foot or longer lengths where rectangular sections are restricted 4-, 5- or 6 feet, requiring more joints, more labor to install, resulting in more leakage sites than round or flat oval.
Fitting losses for flat-oval versus rectangular are similar for similar shapes. The main idea to understand is there are fittings that are more efficient that perform the same function. For example, use a smooth-radius elbow instead of a mitered elbow with turning vanes whether you have a round, flat-oval or rectangular fitting. The smooth-radius elbow will have losses less than half of the mitered elbow. Use a 45-degree entry fitting instead of a straight tap. Again the pressure loss of the 45 degree will be half of the straight tap regardless of being round, flat oval or rectangular.
The round/flat-oval spiral duct system will be more efficient overall, but the designer should use fittings that minimize the pressure losses regardless of the shape of the duct. If a fitting causes an increase in operating cost because it is less efficient, the owner will see that increase for the life of the duct system. You can see the direct relationship in the equation below. Less efficient fittings will call for a higher fan total pressure, which creates a higher cost per year for every year in operation.
The leakage impact of rectangular ductwork can be also significant. If a duct system has no leakage, the operating point is where the system curve intersects the fan curve.
When the system has leakage, the pressure is essentially relieved so the operating pressure is lower, but the correct amount of air is not getting to the desired locations. If the fan speed is increased to compensate to the original operating pressure, the correct amount of air will still not get to the desired locations because of the leakage. To fully compensate for the leakage without additional sealing, the operating pressure and the amount of airflow will have to increase to point D in the graph. This will require more brake horsepower and could affect the construction requirements of the fan housing and impeller.
Suppose you have a 20,000 cfm system operating at 4 inches water gauge of fan total pressure, operating at about 4,000 hours per year with a cost of electricity at 10 cents per kilowatt hour. The overall efficiency is 75 percent. The operating cost will be:
Now assume a 10 percent leakage rate on the original system. If the airflow has to be increased to 22,222 cfm and the fan total pressure has to be increased to 4.5-inch water gauge, the operating cost per year becomes:
This is a 25 percent increase in operating cost to fix the problem without additional sealing and this cost recurs every year. There is additional heating or cooling cost as well. It’s much more economical to seal the system. Round/flat-oval systems are much easier to seal than rectangular.
Probably the key advantage of round and flat-oval ductwork is the capability of being fabricated in longer lengths than standard rectangular ductwork. Sections that are 12 feet long are not uncommon for round and flat oval. Rectangular ductwork is typically limited to 5-foot lengths. This results in less than half the number of joints in round and flat-oval systems as there are in rectangular ductwork.
Rectangular ductwork is known to leak far more than round and flat-oval duct. First, rectangular ducts are generally manufactured with two Pittsburgh seams, since two L-sections of rectangular duct are typically assembled to make the single duct piece. Pittsburgh longitudinal seams require mastic inserted into the pocket, stretching from end to end. Often the last inch or so is missed creating leakage sites and if the mastic is not properly applied, even more leakage sites can be created. Then if the corners are not properly sealed, there can be even more leakage sites at the joint. In general, a well-sealed rectangular system may still leak twice as much as a well-sealed round and flat oval system. The Sheet Metal and Air-Conditioning
Contractors’ National Association says unsealed rectangular systems can have a leakage class of 48 compared with a sealed round system that may be leakage class 6. Even a well-sealed rectangular system may have a leakage class of 12. Poorly sealed rectangular systems could leak as much as 15 or more times than a well-sealed round or flat-oval system. You have already seen how much leakage can increase operating cost.
When flat surfaces are subjected to static pressure in the duct system, the tendency is for sides to bulge and the ductwork wants to become round. Reinforcement of the sides of rectangular duct is often required. Joint and intermediate reinforcement are required to keep this deflection reasonable. This is not required for round supply systems. Flat oval ducts may need to be reinforced, but at a much lower rate than rectangular ductwork.
Because of the many patterns required to fabricate flat oval fittings, there have been concerned that flat oval fittings cost more when compared to rectangular. But with computer-numeric-controlled machines (plasma arc or laser tables), the fabrication cost of flat oval fittings has be greatly reduced in the past 20 years.
Besides reducing leakage, the fewer number of joints makes round and flat-oval duct work easier to install than rectangular. Round also is a very strong shape, so can be made much lighter. Slip joints with a pre-applied gasket install very quickly and when properly installed, do not require a messy sealant.
Even traditional slip joints with a sealant applied will install much faster than 5-foot joints of rectangular. Contractors may use gasket fittings for larger sizes. Others limit the use of gaskets to 24-inch diameters and larger sizes are installed with flanged connectors.
In deciding how large of a diameter to use gasket fittings or when to start using flanges, HVAC construction contractors have to consider ease of installation, especially when the ductwork is installed high in the air making handling the ductwork more difficult.
Rectangular duct can be manufactured with pre-formed flanges on some rectangular coil lines. If not, applied flanges may have to be installed. Round and flat-oval flanges can be installed in the shop so they can be mated and screwed together. Or a hoop can be applied around the flanges to hold them in place. Whether rectangular or round and flat-oval duct, a gasket must be applied at the mating surface so the joint does not leak. Round and flat oval duct will have much fewer joints so the overall system will be quicker to install compared with rectangular ductwork.
Also the reinforcement bracing for rectangular will have to be installed adding even more time. Round ductwork will not require any reinforcing for supply systems and flat oval should require much less.
After the previous discussions on performance, fabrication and installation, it becomes obvious that round systems are much more cost effective than rectangular. In SMACNA’s duct system design manual, the association says that round ductwork’s cost is about 67 percent of the cost of square or rectangular ductwork. As the rectangular duct aspect ratio increases, the relative cost of rectangular over round also increases. At a 3-1 ratio, rectangular duct cost about twice as much as round.
Although we were comparing installed round to rectangular, you might extrapolate it a little to compare flat oval as well. Flat-oval ductwork with a 1-to-1 aspect is round. So starting at the round point, a similar curve might be drawn. It could be steeper than the rectangular curve, but likely would be below it until at least a 4-to-1 aspect ratio. Also note that a rectangular duct with a 4-1 aspect ratio cost 30percent more than a rectangular duct with a 1-1 aspect ratio. It’s important to keep the aspect ratio as low as possible, rather using rectangular or flat-oval ductwork.
Noise (unwanted sound) from a duct system is a common problem. Metal duct systems are conduits that transmit sound very efficiently. The fan is generally by far the source of most noise in a duct system but it is not the only one. As fittings and duct create drops in pressure (pressure loss) noise is generated. A rectangular duct may “thunder” as variable-air-volume systems cycle, or in the on-off system of a constant velocity system. A noisy environment that the duct runs through may be picked up by the duct.
All of these sounds may be propagated through the duct system and manifest themselves in offices, school rooms, churches and other sensitive area where noise is not wanted.
The ability to contain sound in a duct system is its transmission loss. A high transmission loss means that the duct system in containing the noise. A low transmission loss means more sound is being transmitted through the walls of the ductwork system. This is called breakout noise. If the duct runs through a noisy environment, you may experience break-in noise that is carried to the next outlet.
Rectangular duct does not contain sound as well as round duct does in lower frequencies. HVAC contractors want the duct to contain the noise as much as possible so it can be treated. Treatments include purchasing a quieter fan, adding duct silencers into the system or using dual-wall insulated ductwork. It’s common to use silencers to quiet noise in duct systems. Often a bank of rectangular silencers is placed in the outlet plenum of air-handling unit to absorb the fan noise.
Designers should compare using silencers versus using dual-wall insulated ductwork. Pressure drop and cost should be included in the evaluation. With a perforated inner liner in dual-wall ductwork, the additional pressure loss and generated noise should be minimal. Remember though that the sound has to see the insulation in order to be absorbed. If the ductwork has an inner wall it should be no less than 23 percent open area. Solid inner-wall or perforated walls wrapped with a polyethylene sheet could destroy most of the noise control of the dual-wall duct.
Round or flat-oval duct systems are much more economical than rectangular systems. The installed cost spiral duct will much lower than rectangular and it will perform better. Installation will be easier and leakage will be much lower and easier to control. They will contain noise better and with the proper acoustical treatment, quieter systems can be designed. Which would you rather have?