Ducted or non ducted air systems

Designing High Performance Duct Systems


Ducted or non ducted air systemsFor the past 50+ years, engineers have been “designing” duct systems. Quite often, they were not actually designing the duct system, but were sizing duct the duct according to some rules of thumbs. The rules of thumbs 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 round duct did not fit and sometimes when it does, it’s often converted to rectangular, making it a lot less efficient system. Often, the whole ductwork system is “sized” that way using a Ductulator. Not much thought is given to the fittings used and often leakage of the system is not considered during the design phase.

Duct Design General Concepts

So how do we go about designing duct system to be green? There are many aspects to design, but focusing on the duct system design we want a system that minimizes the use of energy, time and material and make sure it meets the acoustical requirement of the application. To accomplish these goals we have to consider what the duct system will be used for (standard air conditioning, exhaust, or other), 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 (AHU) 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 design of the duct system on the positive side of the fan or Air Handling Unit (AHU). Other important aspects that that should be looked at during the design phase may include fan or AHU selection, system effects, leakage, diversity, room air distribution, equipment layout and commissioning. Those are topics for other papers.

spiral duct


Choosing a Duct Design Method

When we design duct systems we want the design method that minimizes energy, material and time. But first to dispel a myth, 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-in wg 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 though 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 round 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. Some pros and cons of the two design methods are shown below:



Total Pressure Design

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 in the real world we don’t have an infinite number of duct sizes, there is always some imbalance. Using the Static Regain design though helps to minimize this imbalance. Ideally we want all paths to be critical paths. That would mean the system is perfectly balanced. With imbalance means some paths will have more pressure (excess) available than they need, which means they likely have sections that can be made even smaller. Note to designers less efficient fittings will generate more noise, but that is generally not a problems till you get close to the final runouts. It’s best to design with high quality fittings that have lower pressure drop, then use smaller sizes. Using multiple runs of round rather than rectangular or flat oval duct could save even more money on the job. 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 the Static Regain Design 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.

Example of Using Equal Friction vs Static Regain vs Total Pressure Design

In an article published by the Air Conditioning, Heating and Refrigeration News in their June 18 1990 edition, a system was designed with the Equal Friction sizing method with three different friction rates (0.05 in wg/100 ft., 0.10 in wg/100 ft. and 0.50 in wg/100 ft.). Then each of these three systems was designed with Static Regain, then the Total Pressure design. The system design airflow is 26,800 cfm 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 results of the three designs are given in the Table 2:


The system had a 12-inch height restriction. For the 0.05 in wg/100 ft Equal Friction design, the percent of sections that became round went from 32% to 61% by using the Static Regain design method and 71% for the Total Pressure design. Similar results can be seen for the two higher pressure designs as well.

Round ductwork cost 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 since 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.


A high performance duct design should minimizing leakage. ASHRAE recommends the 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 remember that 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% 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 TP decreases as shown in Figure 2. We can’t just speed up the fan as shown in Figure 3 so it produces the same original Fan TP as leakage is still occurring. We will actually have to move to a higher RPM and increase both the Fan TP 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-hr. So a 50,000 cfm system operating 2600 hrs per year with 10% leakage of 5000 cfm, could cost an additional $6500 per year.



To design high performance duct systems that do not have acoustic sound problems we need to

  • minimizes the use of energy
  • minimizes the use of construction/manufacturing labor and material
  • make sure it does not contribute noise to the environment
  • design balanced systems

To best meet these objects, the Static Regain/Total Pressure design should be used to determine the duct sizes in a system. To use the Static Regain method, you have to choose an initial velocity. It is recommended you use the follow chart published in the ASHRAE Applications Handbook 2011, Table 8, page 48.14.



Then use Static Regain to size the duct sections, utilizing the most efficient fittings. When finished designing, 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 less 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 the Testing and Balancing peoples 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.

Guest Blog Writer:

Pat Brooks, General Manager


Data Center HVAC Design Considerations

Data Center #HVACData centers today not only require protection from the elements, but also need to be designed to save energy as it is estimated they consume about 1.5 percent of all total demand. According to the Natural Resources Defense Council, data centers are one of the largest and fastest growing consumers of electricity in the United States. In the U.S in 2013 three million computer rooms used enough electricity to match the annual output of 34 large coal-fired power plants. Annual consumption is projected to increase by roughly 47 billion kilowatt-hours by 2020. The NRDC recommends that best-practice efficiency behaviours across the data center industry need to be adopted as demand rises to unprecedented levels.

Energy Savings in Data Centers through HVAC Equipment

Traditionally when a building needs cooling, compressors engage and fans start to move air over cooling coils. This cooled air is used to condition the internal environment where the temperature is required to be lowered. This process is extremely effective but requires costly compressor and fan energy, adding avoidable cost considering external building temperature is lower than the temperature inside. When the outdoor enthalpy (a combination of temperature and humidity) is preferred over the indoor enthalpy, conditions are suitable for “free cooling”.  Depending on the geographic location of the facility, economizer cooling can represents a dramatic reduction in overall energy consumption.

Economizer Data CenterWhat is an Economizer and how can it reduce energy usage?

An economizer is like a window that automatically opens itself – with the added advantage of going through the rooftop AC’s filtration system. An airside economizer simply recognizes the preferred enthalpy of the outside air. When enthalpy conditions are suitable for “free cooling”, the economizer controls position outdoor air, return air, and relief dampers to facilitate free cooling through the first and sometimes second stages of cooling.

Economizers can contribute to a reduction in data center power consumption by utilizing the cooler external building temperatures to assist in cooling the facility and equipment when required. In maximizing energy savings and reducing HVAC cooling load, the cooling system’s product life can be extended.

A study on building control systems by Battelle Laboratories found that, on average, the normalized heating and cooling Energy Use Intensity (EUI) of buildings with economizers was 13 percent lower than those without economizers. When an airside economizer works properly, the savings are significant. Whether your company is looking to burnish its environmental credentials, to lower the cost of operating its data center, or both, a properly designed system integrating an airside economizer is a cornerstone of achieving both goals.

Economizers and Indoor Air Quality (IAQ)

A confined, un-aerated indoor space within a building allows gaseous fumes, odors, germs, and even fungi to grow in concentration to the point that the indoor air is qualitatively different from the ambient air. IAQ is important because the health and the comfort of people working indoors are an important factor in sustainable productivity. Poor IAQ in a working environment can cause discomfort or health problems sometimes resulting in a loss of productivity, increased errors, and even litigation. With the added benefit of reducing cost in power consumption, introducing outside air into a building via economizers can also contribute to improving indoor air quality. Following the relevant ASHRAE standards that apply to ventilation, air movement and exhausting of contaminants ensures that IAQ requirements will be met. To meet the requirements of ASHRAE 62 the outside air entering a building should be measured and controlled.

The most important part of an airside economizer are the damper blades that allow the control and supply of a fixed amount of outside air into the building. Parallel bladed economizers do a better job of mixing the outside and return air to provide optimal benefit to the system.  The sealing ability of the damper is essential to the system as a whole, when contending with extreme temperatures external to the building. AMCA certified dampers can ensure leakage rates meet the appropriate standards.

It has to be recognized that during different seasons and in different climates the benefits from economizers may vary.

Relevant Codes and Standards applicable to Data Center HVAC

Ruskin EconomizerFeaturing Ruskin’s exclusive one-piece galvanized airfoil blade and stainless steel jamb, the Economizers provide low-leakage performance as described in ASHRAE Standard 90.1.  Each unit also features Ruskin’s “SUREFLOW” sensing tubes and blade position indicator to help determine minimum airflow.  This also helps assist in mixed air temperature verses blade position field adjustments.

Data Center Protection

The Natural Resources Defense Council states that Data centers can be regarded as the back bone of a modern economy serving businesses and communications. Defending data means not only protecting it from Mother Nature but also giving back to her with sustainable designs. A question that must be considered during the design of a data center, is ‘How likely could the facility be compromised in extreme weather conditions such as tornadoes and hurricanes?’

When evaluating potential HVAC equipment it is advisable to use FEMA rated louvers and grilles. FEMA rated grilles and hurricane-resistant louvers have been tested against high windloads and large missile impacts. Outside air control dampers can seal up the center when necessary to reduce humidity and heat.

XP500 FEMA GRILLERuskin’s XP500S Extreme Weather Grille protects wall penetrations from flying debris caused by tornadoes, hurricanes, and severe storms.  This type of protection is critical in the design of Community Shelters (ICC-500) and Safe Rooms (FEMA 361). It offers designers a ventilation solution for their near-absolute life safety requirements. The heavy duty grille can be mounted internally, externally, or in conjunction with other louvers providing protection and certified performance. Rated for an industry leading 266 psf windload, the XP500S Grille meets or exceeds the building envelope protection requirements while complementing the construction of data centers.

Relevant Certification

  • FEMA P-361, Safe Rooms for Tornadoes and Hurricanes
  • ICC-500 – ICC/NSSA Standard for the Design and Construction of Storm Shelters

For more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative nearest you or Contact Ruskin directly at (816) 761-7476.

The Importance of Ventilation in Schools

Ventilation in schools IAQVentilation is perhaps the single most important element of an HVAC system. It influences air quality and energy efficiency, and proper ventilation controls odors, dilutes gases (such as carbon dioxide), and inhibits the spread of respiratory diseases. Ventilation air is critical in educational facilities.

Building codes dictate the amount of outside air that must be supplied to various spaces in such facilities. In the ASHRAE Handbook – HVAC Applications, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers recommends that design considerations for education facilities must take into account that today’s schools are used for functions year-round. Adult education, night classes, and community functions put ever-increasing demands on these facilities. ASHRAE Standard 62 gives recommended ventilation rates for various spaces within educational facilities.

The environmental control system must be designed carefully to meet these needs. Many schools use gymnasiums as auditoriums and community meeting rooms, resulting in wide variations in use and occupancy rates. Flexibility is important in these multi-use rooms.

Ventilation Requirements

The code requirements are varied, but the new International Building Code (IBC) has been adopted by 44 states and will be the code most used in the future. The IBC gives data to determine the minimum outdoor airflow rate based on occupancy of the space and the occupancy load.

The occupancy load used for the design of the ventilation system shall not be less than the number determined from the estimated maximum occupant load rate indicated in the table. The ventilation system shall be designed to supply the required rate of ventilation air continuously during the period the building is occupied.

The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during the operation of the system is based upon the rate per person indicated in the table. The IBC also addresses the special needs of variable air volume (VAV) systems and requires controls to regulate the flow of outdoor air over the entire range of supply air operating rates.

Measuring Air Quality

Ruskin Air Measuring Equipment

Ruskin has the most comprehensive line of air measuring and control solutions in the industry.  Products include differential pressure probes for high velocity applications, combination units that measure and maintain flow, and highly sophisticated, intelligent solutions that incorporate thermal dispersion technology with microprocessor based controls that communicate with any building automation system.


For more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative nearest you or Contact Ruskin directly at (816) 761-7476.

Ventilation Rate Procedure for HVAC Systems in the Healing Environment

Ventilation Rate IAQ Procedure - Air MeasuringThe IBC/IMC, the AIA, and Chapter 7 of the ASHRAE Handbook—HVAC Applications all reference ANSI / ASHRAE Standard 62 for determining ventilation. ANSI / ASHRAE Standard 62 provides two methods for engineers to follow to determine the required minimum ventilation rate to achieve acceptable indoor air quality.

To obtain compliance for ventilation design, there are two procedures in the ASHRAE standard. The ventilation rate procedure states that acceptable IAQ is achieved by providing ventilation air of the specified quality and quantity to the space. The indoor air quality procedure identifies a method for achieving acceptable IAQ within the space by controlling known and specifiable contaminants.

Ventilation Rate Procedure

The ventilation rate procedure provides a more definitive, prescriptive procedure based on physiological needs and subjective evaluations. The indoor air quality procedure uses guidelines for the specification of acceptable concentrations of certain contaminants in indoor air —with no prescriptive formula for ventilation rates.

The Indoor Air Quality (IAQ) Procedure

The indoor air quality procedure typically does not result in lower outside air in healing environment applications since the various potential sources of contamination require ventilation rates to be equal to that of the ventilation rate procedure. Additionally, the indoor air quality procedure requires the designer to specifically identify how each of the known contaminants is to be dealt with. The problem facing system designers with this procedure is in determining the possible contaminants that may result from the eventual use/occupancy of the healing environment. For example, controlling formaldehydes, aldehydes, nitrogen dioxides, organics, etc., all need to be understood and accomplished with “engineering reason” when tackling this method. As a result, most designers opt for the more common ventilation rate procedure (VRP ) to determine outdoor airflow needs at the space and system levels. This method prescribes the outdoor air quality acceptable for ventilation, outdoor air treatment where necessary, ventilation rates, and the criteria for reductions of outside air where recirculation treatments occur.

ASHRAE also addressed maintenance of outside ventilation systems. ANSI /ASHRAE Standard 62-2001 identifies minimum ongoing operation and maintenance criteria. The requirements of this section apply to buildings and their ventilation systems and their components constructed or renovated. In section 8.4 of ASHRAE 62, it states:

At a minimum of once every three months or as specified in the Operations and Maintenance Manual, the outdoor air dampers and actuators shall be visually inspected or remotely monitored and determined to verify that they are functioning in accordance with the Operations and Maintenance Manual.

ANSI /ASHRAE Standard 62’s definition of a maintenance manual is as follows:

An operations and maintenance manual either written or electronic shall be developed and maintained on site or in a centrally accessible location for the working life of the applicable ventilation system equipment or components.

This manual shall be updated as necessary. The manual shall consist, at a minimum, of the operation and maintenance procedures, final design drawings, operation and maintenance schedules, and any changes made thereto and the maintenance requirements.

Measuring Air Quality

Ruskin Air Measuring Equipment

Ruskin has the most comprehensive line of air measuring and control solutions in the industry.  Products include differential pressure probes for high velocity applications, combination units that measure and maintain flow, and highly sophisticated, intelligent solutions that incorporate thermal dispersion technology with microprocessor based controls that communicate with any building automation system.


For more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative nearest you or Contact Ruskin directly at (816) 761-7476.