By Benjamin Brits
Ducting, and its related componentry, may be something that seems oh so simple, but this aspect of a HVAC system involves many technical elements to be revered that may never even be seen.
Most of the time, when projects are marvelled for their great end-results, certain functional aspects are easily forgotten. Ducting, as an example, falls into that category because generally this is a component hidden away in ceiling voids, within service passages, under the floor or mounted openly or discreetly outside the facility – so well out of sight. Some projects do of course showcase ducting in the industrial or open-style spaces seen in modern architecture today.
Although not a particularly pleasing component to the eye for some, the critical supply and extraction of air cannot be provided without the necessary main trunk, branches, returns or vent sections that form part of a ducting system – as well as associated components such as grilles, louvres, dampers, diffusers and speciality nozzles. No matter if you are working with low, medium or high-pressures and you require channelling or guiding of air with any sort of precision, ducting will form part of your solution.
It may be interesting to know that several historical accounts point to the concept of ducting being adapted from what was then a central heating system using fire and smoke to alleviate cold conditions – the “ducting” was put in place to eliminate smoke filling a room. Additionally, the concept of heating reticulation progressed then to underfloor type system configurations in certain regions and only from the early 1900s did ducting evolve to the format still used today. The expansion of function was enhanced by the development of technology such as fans, filtration, air conditioning systems, and then the improvement of raw material production.
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Today ducting comes in a variety of shapes and sizes, as well as materials to suit a particular application. So, from galvanised steel, to aluminium, stainless steel, plastic, and various fabrics – the ways to carry your air from one place to the next can be served simply or in style with custom designed graphics or branding.
Airflow however also involves quite a few technical engineering principles and you would encounter terminology such as velocity, friction, pressure, tumble, leakage, force, resistance, throw, roll, and capacity that are directly linked to the function of ducting collectively.
One logical thought on the subject is that no matter how much is spent on a high-spec HVAC system, the equipment installed won’t necessarily perform optimally without properly designed and installed ductwork. Ducts that are not well designed ultimately result in discomfort, high energy costs, possible bad air quality, and even increased levels of noise.
It then indicates that a good design and installation would tick these boxes while also taking into account other factors, such as investment costs for the clients, duct sizing versus space restrictions, capacity for future expansions, rates of air to specific spaces with varying requirements, impact on operational costs, and partnering with specific system components.
Managing airflow
The primary function of a duct system is essentially the conveyance of air – be this supply or return form. The flow of air is a result of pressure differential between two points. Air flow will originate from an area of high energy (or pressure) and proceed to area(s) of lower energy. Air thus moves according to three fundamental laws of physics:
- conservation of mass
- conservation of energy, and
- conservation of momentum.
Conservation of mass: an air mass is neither created nor destroyed. From this principle it follows that the amount of air mass coming into a junction in a ductwork system is equal to the amount of air mass leaving the junction, or the sum of air masses at each junction is equal to zero. In most cases the air in a duct is assumed to be incompressible, an assumption that overlooks the change of air density that occurs as a result of pressure loss and flow in the ductwork. In ductwork, the law of conservation of mass means a duct size can be recalculated for a new air velocity using a simple equation.
The law of energy conservation states that energy cannot disappear – it is only converted from one form to another. This is the basis of one of the main expressions of aerodynamics, the Bernoulli equation. Bernoulli’s equation in its simple form shows that, for an elemental flow stream, the difference in total pressures between any two points in a duct is equal to the pressure loss between these points.
Conservation of momentum is based on Newton’s law that a body will maintain its state of rest or uniform motion unless compelled by another force to change that state. This law is useful when one needs to understand air flow behaviour in a duct system.
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System components
Any air distribution structure will include components based on the required function of the overall HVAC system. In a broad and general consideration, there are five typical functions of ductwork:
Supply air ductwork supplies conditioned air from an air handling unit to a conditioned space.
Return air ductwork removes air from a conditioned space and returns that air to an air handling unit, which then re-conditions the air. In some applications, part of the return air is required to be exhausted to the exterior of a space too.
Fresh air ductwork supplies outdoor air directly to a space or to an air handling unit. Outdoor air is used for ventilating, among others, any occupied spaces of a building.
Exhaust (also termed relief) air ductwork carries and discharges air to a designated area outside of a building or space. Exhaust air is typically taken from toilets, kitchen, laboratories and other areas requiring air extract for the particular application.
Mixed air ductwork mixes air from outside air and return air.
Diffusers, grilles and louvres
These components of a typical system are generally terminal devices that force or manipulate air flow in various directions through the use of their deflecting vanes or profiled blades. They are simply designed to mix supplied conditioned air with air already in a space. Grilles are defined as air devices that are typically used to supply or return air to or from a space without any deflection. Grilles are generally not used in supply distribution due to their inability to control air flow.
Louvres are similar to grilles but are comprised of one-way or two-way adjustable air stream deflectors and dampers to restrict the amount of air flow required to be returned, supplied or exhausted.
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Dampers
An air damper is a mechanical device used to stop or regulate the flow of air within a duct, chimney, variable-air-volume (VAV) box, air-handling unit or other similar equipment. Dampers can also be used to stop airflow into unoccupied or unused rooms where air conditioning is not required. Dampers can further be operated manually or by means of electrical control.
Fire/smoke dampers
A fire/smoke damper is a device installed in ducts and air transfer channels intended to interrupt the passage of flame or smoke, and therefore maintain the integrity of any fire rated break or separation sections of a facility. Fire dampers are different from regular air flow dampers as they are equipped with a “fusible link” (rated for a particular temperature) that holds the unit’s blades open until the rated temperature is reached. Upon the melting point, the blades then shut closed and stop any flame or smoke from moving into adjoining spaces. Fire dampers are generally installed in or near the wall or floor, at the point of duct penetration into a space or zone to retain integrity and fire rating of a wall or floor.
Attenuators and noise
Noise criteria (NC) values are established in the HVAC sector for evaluating the acceptability of various sound levels. NC values for different types of buildings form part of a particular range measured in decibels. A decibel is a unit of comparative sound measurement. A whispered conversation, for example, has a sound level of 30 decibels. This is similar to the target range of most fans. However, fans can contribute upwards of 80 decibels onto a duct system. For comparison, the banging of a hammer equals approximately 130 decibels.
Various documents exist for designers and installers to understand the impact of noise on air dispersion systems. The major source of noise in a HVAC systems is due to air velocity as well as the noises originating from any fans or components that can generate vibrations. Sound attenuators, also known as silencers, sound traps, or mufflers, are generally standalone components fitted to duct sections as a noise control treatment that are designed to reduce the transmission of noises through the ductwork system. Attenuators can be designed to use various insulation or sound absorbing materials such as foams, boards, fibre or rubber.
Ducting material options
Today’s market sees various options available to clients and the choice of selection is reliant on several considerations above the requirements of simply managing air movement from one area to the next. Specific applications may be better suited to the use of a particular material type, or even a section of the dispersion system could make use of any type.
In a corrosive environment as an example, the use of certain steel or fabric would not be suitable and so as you would see in certain applications, the use of stainless steel or copper required as the appropriate material.
Galvanised steel
This material is the traditional standard and most common material used in fabricating ductwork for HVAC systems. The specifications for galvanised steel sheet ducts are, for the most part, guided by the US Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) standards as well as the local South African National Standard: SANS 1238: Air-conditioning ductwork. Thickness of material as well as required supports are found in these documents.
Aluminium
This material is widely used in clean room applications. These are also preferred systems for moisture laden air, special exhaust systems and decorative duct systems.
Stainless steel
Generally used in duct systems for kitchen exhaust, moisture laden air, and fume/chemical exhaust.
Mild/black steel
Widely used in applications involving flues, stacks, hoods and other high temperature and special coating requirements for industrial or processing use.
Copper
Although vary rare in South Africa, this material would mainly be used when certain chemicals exist in air that need to be extracted that would otherwise result in unfavourable reactions with other material types.
Plastic/foam board
A relatively new product in the local region and typically sandwiched between an aluminium layer, this material is suitable for certain exhaust systems for chemicals and underground duct systems. Its advantages include resistance to corrosion, the system is light weight, and easy to modify while its major limiting characteristics include cost, acceptance in standards, weight, and fire impact risk.
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Fabric
Fabric ducting, also known as textile ducts, are made of special permeable polyester material and can be used in new buildings or retrofit projects. Fabric ducts are used to disperse air in open ceilings and underfloor applications. Condensation is not a concern with permeable fabric as permeable fabric prevents the formation of condensation on the duct surface.
Systems that utilise fabrics provide solutions for various applications and it has been proven by some academic studies that fabric ducting reduces energy costs significantly.
Flexible Ducting
This product consists of a duct inner liner supported on the inside by a helix-type wire coil and covered by an insulation with a flexible vapor barrier jacket on the outside (most often a type of foil). Flex ducts are used primarily to connect the main ducting trunk or branches to supply points as they allow easy adaptation and flexibility in installations. This product does have significantly more friction loss than other ducting materials and so “runs” should be as short as possible (1.5metres maximum) and be tensioned or stretched as tightly as practicable.
SMACNA manufacturing guidelines indicate recommended specifications based on duct size – and have several similarities to the South African standard for low pressure systems:
Duct shape
Ducting can be manufactured in round, square, rectangular or oval shapes and each style has advantages and disadvantages. Round duct shape is the most efficient as they offer the least resistance in conveying moving air and also require less material compared to square or rectangular ducts for the same volume of air handled. Although this style also has several other advantages, the biggest challenge is that they require more space for installation.
Square and rectangular ducts generally have a better suitability to the various conditions of a building design. They are able to be installed in ceiling voids, into walls, under or between floors, and they are notably easier to install.
Design considerations and good engineering practices
Air distribution to any space is required to primarily maintain space conditions and minimise noise factors. Ducting can be manufactured to meet almost any dimensions, so is essentially limited by each manufacturer’s capacity. In South Africa, producers are able to create products from as small as 70mm x 70mm to as large as 5000mm x 2000mm.
Low pressure duct metal thickness. Extracted from SANS 1238:2005
Some guidelines when designing ducting systems that can be considered in order to optimise objectives follow.
- Design should provide proper control of air flow as well as appropriate stability.
- The design will ultimately be determined by each site as well as the site conditions.
- Availability of space to work with and well as aesthetics if applicable will be high up on the list.
- Trunk-&-branch configurations with shorter runs generally work best.
- Long “meandering” sections of ductwork can lead to trouble, such as ducts collapsing over time or becoming twisted and blocked.
- Air wants to go straight and will lose energy if you make it bend but from a cost perspective, straight runs costs less than “fittings” or connector pieces. Try to reduce the number of bends and turns.
- Ducts should be the correct size – too small and you won’t be able to carry enough air to heat or cool – too large ducting can lose both air and waste energy.
- Make sure you include enough return ducts – every system also requires enough return ducts to bring expended air back to the HVAC unit to be conditioned again. Each room that receives conditioning should have at least one return duct or provision.
- You may want to consider thermal zoning, a practice of dividing a building into distinct thermal zones which have similar heating and/or cooling requirements
- Balancing air is important in ductwork design layout. Actual air flow can exceed design flow if the fan pressure is higher than any pressure loss. Therefore the function of adjusting the volume control dampers equalises potential friction losses. Balancing may be required in both supply and return ducting.
- Operating pressure in the duct is also a significant consideration. If the pressure is minimal, materials with a great deal of strength are not needed. The thickness of the duct material thus depends on pressure as well as the dimensions, the length of the individual sections, and branch requirements.
- Once you have determined the thickness of material required, the next step is deciding on what flange connections will be suitable and the type of duct section joining techniques will be used. Here various industry standards may be applicable such as S/Drive, TDC, mezz flanges & angle flanges.
- Consideration of other services on a site as well as potential conflicts when installation is done could affect the overall system performance. For example if wet services are installed because of an easier route, this may affect duct installation and thus airflow delivery.
- Design needs to take into account if duct sweating will be a problem for site – this happens particularly when conditioned air inside the duct reacts with any outside air or particular conditions. An example here would be if ducting runs through a plant room area that is hot while conditioning is cold. This may require the addition of insulation, fibreglass or cloth materials at the manufacturing level.
- The amount of angles and guiding vanes in the dispersion system needs careful consideration. For example, 45 degree bends rather than 90 degree bends eliminate several challenges in airflow.
- Air leakage rate or potential leakage rates need to be accommodated in design. Air leakage typically occurs at joints and is the one component where several different connection types and ranges have been developed through the manufacturing process. These joining techniques are mentioned above.
Common systems in South Africa
In South Africa, the most widely used systems include the traditional galvanised sheet metal and fabric duct systems that have gained ground in the region over the past couple of years while a little known fact about fabric material ducting is that is has been around commercially for around 35 years already. The following information is intended to give designers some points to consider when completing their project evaluations.
One would always need to consider total lifetime ownership cost of each solution and the aforementioned points around customisability, as well as provision for future expansion. One particular solution may well have clear and considerable advantages over another.
- Metal systems use duct, dampers and localised diffusers that are spaced out over the length of a run. This approach results in air drafts near the outlets and possibly hot and cold spots in the occupied space. Fabric ductwork is designed to provide uniform air dispersion through a combination of air-porous fabrics, linear vents, nozzles and orifices. This method has a positive impact on energy costs as these systems heat or cool spaces faster and more uniformly to satisfy temperature set points. The result is reduced mechanical equipment runtime.
- Fabric dispersion systems are engineered to distribute and disperse air in open and finished ceiling architecture, critical environments and underfloor applications. They are an innovative, and an aesthetically pleasing alternative for indoor spaces.
- Technology advancements in fabric have created an opportunity for this solution to be used in applications where there are specific performance requirements. Anti-static fabrics are ideal for data centres and anti-microbial treated fabrics for laboratories, food processing and growing facilities.
- The difference in weight of a fabric system versus a metal system can be significant. This difference can impact logistics, installation time and cost, as well as the overall load on the structure.
- A properly designed fabric system will deliver air quietly without the resonation found in metal. Fabric can also absorb noise in certain applications. When there is mechanical noise coming from the air handling unit, a fabric sound attenuator can be installed for additional noise reduction.
- Non-insulated metal ductwork has the tendency to drip condensation and potentially rust in humid applications or climates. Based on the nature of the material, a porous fabric system is not prone to condensation or rust issues. However, steel ducting systems are designed or specified to have specific insulation be this internally or externally.
- Fabric ducts are available in a variety of standard colours, patterns and custom branding.
- Metal systems require minimal upkeep, such as repainting over time, periodic tightening of louvers/registers/grills and resealing of transitions. This remains true even when they are scratched or dented. The durability of the metal makes it that steel ductwork can last for many, many years before replacement is required.
- Fabric systems on the other hand can be commercially laundered multiple times, which is important in applications that have hygiene standards.
- Metal ductwork systems can be exposed to damage over time that will not necessarily affect function, while fabric has the risk for tearing affecting function and has a lower pressure tolerance than steel.
- The price of metal, particularly over the last few years, has been unstable and has become very high over the years along with several supply chain availability problems. The price of fabric on the other hand, has been relatively stable and has therefore not changed substantially over the years.
- In the current economic environment, transportation has escalated to a major cost component for projects. Metal ducts historically have “taken up a lot of space during transportation” while fabric systems can be rolled up and transported by courier or a freight company. To increase efficiency of delivery on metal ducts, they can now be transported in panel-forms to significantly reduce space on transport vehicles.
- Labour costs are another major increase and one needs to consider the time consuming elements that pertain to each solution.
- In an ever-advancing environment of health and hygiene, this factor must be taken into account as well as the function of cleaning and maintenance requirements.
Article Sources:
- ASHRAE Handbook/guidelines
- Continuing Education and Development
- DuctShop
- DuctSox
- FabricAir
- Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA)
- South African National Standards (SANS): SANS 1238:2005 – Air-conditioning ductwork
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