Since decades, MEP and HVAC engineers have been “designing” the duct systems; however you dig a bit dipper to realize that they are actually not designing the HVAC duct systems. They are sizing the ducts according to some set guidelines.
These guidelines vary for every engineering firm, and mostly the ducts are sized to be round by a preset friction rate decided by the engineering firm. Then incase if the spiral duct is not found to be a perfect fit, it is converted to rectangular, which eventually makes it a highly non efficient system. If this was not all, the entire ductwork system is “sized” using the duct calculator. No or minimal emphasis is given to the fittings used, and leakages of the system are not considered during the design phase.
How to design HVAC duct systems that are energy efficient?
Of the many aspects to a green HVAC design, the most prominent one is to focus on the duct system design. Considering the size of a building construction project with multiple complexities, architects, contractors in agreement with sheet metal fabricators should first decide on to a HVAC design support service provider.
An HVAC expert, who is equipped with latest technology/software to minimize the use of energy, time and material, and most importantly could design HVAC ducts that meet the acoustical requirements of the duct system. In order to attain all these, you need to decide on the type of duct system, what the duct system would be used for, duct system layout, fitting selection, system leakage, acoustical properties, equipment selection and much more.
Equal Friction, Static regain and Constant velocity, are the three methods used more often for duct designing for commercial and industrial duct systems. Sincerest suggestion is to consider both; the supply side (positive pressure) of the air handling unit aka Fan, and the return side & makeup-air side (negative pressure) of the fan. There are no set rules pertaining to the use of equal-friction method and constant velocity systems.
However; equal friction is mostly used on the supply and return systems, whereas constant velocity is used for exhaust systems that are supposed to convey particulate or fumes. Static regain can certainly to be used for a positive pressure design.
Various other aspects to be paid attention to during the HVAC design phase, includes fan or air handler selection, system effects, leakage, diversity, room-air distribution, equipment layout and commissioning. In this article we will focus on the positive side of the fan air handling unit.
How to choose the appropriate and efficient HVAC duct design?
While designing duct systems for HVAC construction projects, you certainly would want to opt for a design method that minimizes energy, material and time consumption. Any design method can be used to design a duct system for almost any pressure, but static regain does not automatically design systems at a lower total 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 of the scenario, the velocities are supposed to be kept within acceptable limits to avoid noise challenges, whereas the static-regain design goal is to produce a balanced system. It is the one where all paths are design legs, and or require the exact same amount of static pressure for the leg’s respective airflow. In the end for the final balancing of systems, smaller sizes in non-critical paths will use excess pressure.
So for instance if two designs of the same duct systems are created, that have the same operating pressure, but one with equal friction and one with static regain; the later one should use smaller duct sizes because it balances the system. In such cases one can have the benefit of more round sizes and, as smaller sizes in general are used for balancing the non-design paths; the benefit of lower duct and fitting costs as well can be attained. Benefit of a spiral duct is that it has lower breakout noise, resulting in a quieter design. Additional benefit of round duct and resultant smaller size is that they are convenient to install and seal.
Total pressure Design; all paths are critical paths to make the duct system perfectly balanced
Irrespective of the fact that the system is designed with equal friction or with static regain, chances of it being imbalanced cannot be denied. Though probability is less with static regain design, but because you do not have infinite number of duct sizes, some amount of imbalance will certainly prevail. This is why treating all paths as critical is the best way of doing things, in order to make the duct system perfectly balanced.
Basic understanding of imbalance is that some paths have more pressure available than what they require, but why can’t we think that it also means that these paths have sections that can be made even smaller. Less efficient fittings will generate more noise, but usually it is not a challenge until you get close to the final runouts. However; the best option is to design with high-quality fittings that have lower pressure drop than to use smaller sizes.
If you want to save more money, utilize multiple runs of round rather than rectangular or flat-oval ducts. Usually the process of considering a given design, determining the excess pressure available in the non-design legs and reducing their size to use up the excess pressure, is known as total pressure design. Though it is applicable to any design method, it best suits to static regain method. The disruptive technology and software brings us the good news by pointing the critical legs, making it convenient to identify where there will be excess pressure.
If compared to rectangular or flat-oval ductwork, round ones cost less, saves installation time, easier to seal and the static regain and total pressure design methods are much more balanced.
Several other advantages of total-pressure design are that it is a balanced system using smaller sections of duct, with higher attenuation and insertion losses. Hence; using this kind of knowhow, along with keeping the velocities reasonable, total pressure designs should not require extensive noise control.
Leakage; should be no more than 5 % of the total airflow volume
If a high performance duct design is to be defined, it should minimize the leakage. According to American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), system leakage should not be more than 5 percent of the total airflow volume. Is it that important? Yes, it is.
If a system leaks and airflow requirements are not met in locations they were intended, the leaks either need to be sealed or the fan speed must be increased to generate the requisite volume of airflow. Leaks that are not sealed, which is not permitted as per many codes, the additional volume will need to be pushed through with a higher static. It is so because the system will continue to leak and that the airflow volume and the additional leakage airflow volume caused by higher static pressures will have to be taken care of as well.
ASHRAE studies suggest that 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. Now that huge money that you might be losing on due to leakages…...isn’t it?
To sum it up all, in order 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
But the question is that how would you meet these objectives? Utilizing the static regain/total pressure design to determine duct sizes is recommended, as well as widely used. A chart published in the ASHRAE 2011 Applications Handbook, could be of help.
Using static regain to size the duct sections using the most efficient fittings proves to be cost effective in the long run. Upon completion, one can use the total pressure design to further balance the system with help of smaller sizes and less efficient fitting in non-critical paths. This certainly will give you the final result in form of a well-balanced system through the smallest sizes possible for the initial velocity.
What are smaller duct sizes and how are they beneficial?
- Number of round duct sizes will be more.
- 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.
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