For the past 50 years, engineers have been “designing” duct systems. Quite often, they were not actually designing the systems, but were sizing duct the duct according to some rule of thumb.
The rules varied by different engineering firms and often ducts were sized to be round by a predetermined friction rate that was chosen by the engineering firm. Then if the spiral duct did not fit and sometimes when it does, it was often converted to rectangular, making it a lot less efficient system. Often, the whole ductwork system is “sized” that way using a duct calculator. Not much thought is given to the fittings used and often leakage of the system is not considered during the design phase.
So how do you go about designing a duct system to be energy efficient? There are many aspects to green HVAC design, but focusing on the duct system design, you want a system that minimizes the use of energy, time and material and meets the acoustical requirement of the application. To accomplish these goals, you have to consider what the duct system will be used for, duct system layout, fitting selection, system leakage, acoustical properties and equipment selection.
There are three common methods of duct sizing or design for commercial and industrial duct systems: equal friction, static regain and constant velocity. Both the supply side (positive pressure) of the fan or air-handling unit and the return side and makeup-air side (negative pressure) of the fan have to be considered. The equal-friction method and constant-velocity systems can be used for either the supply or return side. Most often equal friction is used on the supply and the return systems while constant velocity is used for exhaust systems that have to convey particulate or fumes. Static regain, however, is strictly used for a positive-pressure design.
This article will focus on the positive side of the fan or air-handling unit. Other important aspects that that should be looked at during the design phase may include fan or air handler selection, system effects, leakage, diversity, room-air distribution, equipment layout and commissioning.
Choosing a duct design
When you design duct systems for HVAC construction projects, you want the design method that minimizes energy, materials and time. But static regain does not automatically design systems at a lower total pressure. Any design method can be used to design a duct system for almost any pressure. A 6-inch-water-gain system can be designed either by equal friction — just increase the design friction rate — or by static regain — just increase the initial velocity.
In either case, the velocities should be kept within acceptable limits to avoid noise problems. But a static-regain design goal is to produce a balanced system. That is, one in which all paths are design legs, or require the exact same amount of static pressure for the legs’ respective airflow. For final balancing of systems, smaller sizes in non-critical paths will use excess pressures. So if two designs of the same system are created that have the same operating pressure, one with equal friction and one with static regain, the static-regain method should use smaller duct sizes, because it is balancing the system. When this is the case you will likely have the benefit of more round sizes and because smaller sizes in general are used for balancing the non-design paths, the benefit of lower duct and fitting cost as well. A benefit of spiral duct is it has lower breakout noise, resulting in a quieter design. Another benefit is that round duct and the resultant smaller sizes are easier to install and seal.
Total pressure
Whether a system is designed with equal friction or with static regain, there is still likely to be imbalance. There should be less with the static regain design, but because you don’t have an infinite number of duct sizes, there is always some imbalance. Using static regain helps to minimize this imbalance. Ideally you want all paths to be critical paths. That would mean the system is perfectly balanced. Imbalance means some paths will have more pressure available than they need, which means they likely have sections that can be made even smaller. Less efficient fittings will generate more noise, but that is generally not a problem until you get close to the final runouts. It’s best to design with high-quality fittings that have lower pressure drop than to use smaller sizes.
Using multiple runs of round rather than rectangular or flat-oval duct could save even more money. The process of taking a given design, determining the excess pressures available in the non-design legs and making them smaller to use up the excess pressure, is often referred to as a total-pressure design. It can be applied to any design method, but is most suited to be applied to static-regain method, since it should already be fairly balanced. The good news is some of the duct-design programs pinpoint the critical legs making it easy to identify where there will be excess pressure.
In an article published by the Air Conditioning, Heating and Refrigeration News (like Snips, owned by BNP Media) in their June 18, 1990, edition, a system was designed with the equal-friction sizing method with three different friction rates (0.05 inches water gain/100 feet, 0.10 inches water gain/100 feet and 0.50 inches water gain/100 feet). Then, each of these three systems was designed with static regain, then the total pressure design. The system design airflow is 26,800 cubic feet per minute and the designs were such that the static regain and total pressure methods had about the same operating pressure as the respective equal-friction designs. The first section was the same size in all design methods.
The system had a 12-inch height restriction. For the 0.05-inch water gain/100-foot equal friction design, the percent of sections that became round went from 32 percent to 61 percent by using the static-regain design method and 71 percent for the total pressure design. Similar results can be seen for the two higher-pressure designs as well.
Round ductwork costs much less than rectangular or flat-oval, saves installation time, is easier to seal and the static regain and total pressure design methods are much more balanced.
The other advantage of total-pressure design is that it is a balanced system using smaller sections of duct, which have higher attenuation and insertion losses. So using that knowledge, and keeping the velocities reasonable, total pressure designs should not require as much noise control.
Leakage
A high-performance duct design should minimizing leakage. The American Society of Heating, Refrigeration and Air-Conditioning Engineers recommend system leakage be no more than 5 percent of the total airflow volume. Why is that so important? If a system leaks and airflow requirements are not met in the locations they were intended, the leaks either have to be sealed or the fan speed must be increased to generate more volume. If the leaks are not sealed — a practice which is no longer allowed by many codes — that additional volume will need to be pushed through with a higher static, because the system will continue to leak and that the airflow volume, plus the additional leakage airflow volume caused by higher static pressures, will need to be overcome as well.
Essentially you are changing the system curve from what was designed to what is actually happening. The design point for a fan is where the system curve crosses the fan curve, as shown in Figure 1. It assumes no leakage is occurring. If there is 10 percent system leakage, the duct system is essentially relieved and the system curve moves down. That is because the same volume of air does not have to move through the entire system as some has leaked out. The airflow increases while the fan total pressure decreases, as shown in Figure 2. You can’t just speed up the fan, as shown in Figure 3 so it produces the same original fan total pressure as leakage is still occurring. You will have to move to higher rotations per minute and increase the fan total pressure and the airflow so we get the correct airflow to the outlets, as shown in Figure 4.
ASHRAE studies have shown the cost of leakage could be $0.00050 per cfm per hour. So a 50,000 cfm system operating 2,600 hours per year with 10 percent leakage of 5,000 cfm, could cost an additional $6,500 per year.
To design high-performance duct systems that do not have acoustic sound problems you need to:
- Minimize the use of energy
- Minimizes the use of construction/manufacturing labor and material
- Make sure it does not add noise to the environment
- Design balanced systems
To best meet these objects, the static regain/total pressure design should be used to determine duct sizes. To use the static regain method, you have to choose an initial velocity. It is recommended you use the chart published in the 2011 ASHRAE Applications Handbook as Table 8 on page 48.14.
Use static regain to size the duct sections, using the most efficient fittings. When finished, use the total pressure design to further balance the system using smaller sizes and less efficient fittings in non-critical paths. The final result will be a well-balanced system using the smallest sizes possible for the initial velocity.
Smaller duct sizes mean:
More of the duct sizes will be round.
- Round spiral duct is much easier to install and has fewer joints.
- Many sizes will be smaller than those in other design methods, making even them easier to install and use fewer materials.
- Smaller sizes will be easier and less costly to seal making very low leakage duct systems possible.
- The duct system will be balanced assuring everyone gets enough air and testing and balancing time will be minimized.
- Done right, round ductwork results in quieter system with less risk of noise problems.
You will likely need a computer to do the proper designs, but if you do; less time, material and energy will be expended. With the proper controls and other components, the static regain/total pressure duct system designs are used in true high-performance air systems. What more could you ask for in a duct design?
For reprints of this article, contact Jill DeVries at (248) 244-1726 or email devriesj@bnpmedia.com.
