A plasma cutter is a large table where a piece of sheet metal is cut to make a fitting according to patterns setup in the shops fabrication software. The software optimizes the patterns to minimize wasting the material, any excess material scrap is placed into a recycle bin. A computer on the machine tells it how to cut each piece. This will help reduce your waste factor for those estimators that work with their shop foreman to figure how much waste to account for with the various pieces of equipment.
Plasma Cutter
Some plasma cutters can cut up to 1-1/4” thick metal depending on the manufacture and model number of the cutter, but in the Heating Ventilating and Air Conditioning industry we won’t need that ability as most of what we need fabricated will fall between 26ga and 16ga.
The Plasma cutter is used to make fittings as straight duct will be made off a coil line which will discuss shortly. The size of the Plasma cutter table varies based on the shops requirement and the manufactures available models, sizes such as 5’ x 10’, 6’ x 10’, 5’ x 20’ and 6’ x 20’.
The Plasma cutter has an exhaust fan to remove the gas from the cutter. After the cutter has completed cutting the shapes, the sheet metal shop worker separates the shapes and puts the scraps in a bin for recycling. Each piece will have a sticker that gets slapped on it or marked by the shop worker that indicates many aspects of its assembly requirements and the project it’s being fabricated for.
Plasma Cutter Table
If a fabricated section has a sticker as shown below it will be applied to the cut piece. It will show a pictogram (#1) of the piece with its gage and dimensions (#2) and the joint and seam (#3) requirements, in addition to the job name and static pressure or specifications used (#4). Another important designation on the sticker is where the fitting is located (#5) in the building and what system it serves (Supply Air, 2nd Floor, West, VAV-13) This information will save on material handling, as it is clear from the label exactly where this fitting is to be installed within the building.
If you watched the video above you would have seen one of the fabrication shop personnel placing stickers on each piece.
Plasma Cutter Stickers
Now that shapes have been cut by the plasma cutter the next step will be to make the seams and joints so that the pieces can be attached together to make a fitting or a short piece of ductwork.
The pieces cut from the plasma cutter might get put through the beader for strengthening, and then through the seamer to make a Snaplock or Pittsburgh Seam and through a roll former to make a TDC or other type of joint depending on the requirements of the fitting or short duct section. If the fitting or duct joint requires internal lining, then this will occur before assembling.
One of the last stops in the shop will be to assemble all the pieces together into a finished fitting or short section of ductwork. If required, tie-rods will be inserted to provide support for larger fittings and duct sections. Also, external reinforcement will be added after assembly if required.
Duct Sealer will be applied to all internal seams and penetrations.
The pieces should flow efficiently through the fabrication shop to optimize time, avoiding unnecessary steps for the shop workers.
Watch this short video and look for the notches cut into the metal at the point where it will be bent. Notice also the shape of the last part cut, as it appears to be the cheek of an elbow, and the first piece will be the throat of that elbow.
Plasma Cutter Table
Coil Line Feeding a Plasma Cutting Table
For those shop that can afford the capital to purchase a coil line dedicated to a plasma cutting table as shown in this video below, will have the ability to safe on the labor required to pull flat sheet metal stock on to the table.
Notice that the shop personnel have attached identifying stickers on each piece of sheet metal being cut. This will help in the assembly process and when material handling at the job site in order to get the right fitting in the correct location on the project. This is really helpful the larger the project is, as hunting for the correct fitting when you’re working on a large building is a big waste of time.
Those fabrication shops that have invested in a coil line will find it less labor intensive to fabricate straight pieces of ductwork.
Sheet metal coil lines are available with many optional pieces of equipment that can be added on to perform an additional function on the path to creating a fully sectional piece of ductwork.
The full capabilities of the coil line begins where several rolls of differing gauges of metal are housed in large heavy rolls and end where the sheet metal is bent. Between the beginning and end are various pieces of fabrication equipment that perform special functions in the process of making a piece of duct for the HVAC industry.
Standard coil widths include 48” and 60”, which can make anywhere from a 4 foot to a 5 foot long piece of duct. Also, as an option are coil widths of 72” for a duct length of 6 feet, minus flange allocation. Gauges will range from 16ga to 30ga.
Remember when watching some of the shop fabrication videos below that fabrication shops are very noisy at times when the equipment and workers are in full operation, as they are working with sheet metal.
This first video will animate the total process. We recommend that once you finish this chapter that you watch this video again to identify all the steps you are going to learn about in this chapter.
Automated Duct Making Coil Line
Sheet Metal Coil Line
The below image shows that a forklift is being used to load the sheet metal coil onto a coil machine. These coils are very heavy and require machinery or an overhead crane system to load them onto the coil line equipment.
Sheet Metal Coil Setting
The coil line process steps are outlined below and are covered in greater detail later in this course.
SHEET METAL COILS – Choose which coil width and gauge of sheet metal is required for the project.
STRAIGHTENER/LEVELER – The process of straightening the sheet metal.
BEADER – This stiffens the metal by putting a bead down the sheet metal
NOTCHER – The notcher will cut out (notch) small portion of the duct at its edge where it will be bent
SHEAR – This is when the sheet metal gets cut at the perfect point.
SEAMERS – This machine installs the Snaplock or Pittsburgh seam along its length at the point where it will attach to the other side. One seam will be a female seam and the other male for a perfect fit.
JOINT (Rollformer) – The metal has its end roll formed to create the ends of the duct that will join up to another piece of duct or a fitting.
LINER – If required liner will be added.
BREAK – This is the final step in the automated coil line. This last piece of the coil line will bend the sheet metal into either an L-Section (2-sided), U-Section (3-sided) or Full-Wrap (4-sided)
Watch the video below and see if you can identify the various coil line steps in the process of forming a section of duct.
We begin with the coil of a particular width and gauge, which gets fed into the straightener. The purpose of the straightener/leveler is to remove the metals tendency to want to remain curled up as it was within the coil from the manufacture. After this it goes through the beader, which puts several beads across the sheet metal to stiffen it in its finished form.
Then the metal gets notched at the points where the metal will be bend so as to allow for and easy bend along its longitudinal length. The next section will shear the duct, cutting it at the perfect point along its length. The seaming section is next where it will have a female seam on one side and a male seam installed on the other by their respective seamers, such as a Snaplock or Pittsburgh seam.
Then the metal goes through the roll former, which creates the joint end of the sheet metal duct, such as TDC, Drive Slip or completely raw for the attachment of a proprietary joint such as Ductmate. Next liner will be added if the specifications call for the duct to have internal liner. The automatic duct liner feeds from a coil of liner material while applying a liner adhesive to the sheet metal and then automatically pins the liner to the sheet metal.
And finally the metal is bent using the break to create an L-Section (2-sided), U-Section (3-sided) or Full wrap (4-sided) section. The brake is a heavy duty hydraulic sheet metal bending machine that will fold your duct in preparation for assembling.
Sheet Metal Coil line
Step # 1 – Sheet Metal Coil Line
Galvanized steel is defined as a carbon steel sheet coated with zinc on both sides. Hot Dipped Galvanized for the HVAC industry is usually specified to be coated either as G-90 or G60. The coating indicates the amount of zinc applied to each side of the carbon steel. The most common gages used are from 16 ga through 26 ga. The use of the lighter gages are not as common in commercial construction as it is in residential construction.
Galvanized Sheet Metal Coils
The most common width of the coils used in the HVAC industry is 36”, 48”, 60” and 72”, with the 60” width coil being the preferred width. The Wider the coil, the less joints you will need, as a 60” wide coil can make a 5 foot section of duct using Ductmate connectors, as opposed to only a 3’ foot section with a 36” coil width.
For example if you had 90 feet of straight duct, it would require;
30 pieces of 36” (3 foot) sections of duct 3’ x 30 = 90’
18 pieces of 60” (5 foot) sections of duct 5’ x 18 = 90’
It’s better to handle fewer pieces, in addition, the more pieces you have, the more it cost for gaskets, corner pieces and clips because you have 12 additional joints to make.
The coils are loaded onto the automated coil line using a forklift or overhead crane.
Step # 3 – Beading
The ductwork is Beaded or Cross Broken (#4) to provide strength across its surface area. Duct beading is the preferred method. The duct is run through a bead machine that puts a ridge or dimple across it transverse width every 12” or so. This will help reduce sagging of the metal.
Beaded Duct
The purpose of cross-breaking or beading ductwork is to provide support for large spans of ductwork. For ducts that are 19” and greater in width and 10 Ft2 of unbraced panel area will require cross-breaking or beading. This requirement is for duct classifications of 3” static pressure and down and 20 gauge or less thickness. Ducts built to 4” static pressure or greater don’t require cross-breaking or beading as the increased thickness and reinforcement requirements provide the added support benefit. Beading is spaced every 12” apart, running parallel with the joint of duct, but direction of bead can be random on fittings.
Ductwork Cross Breaking
Step # 4 – Notcher
The machine will notch the duct at the points where they need to be folded. The below image shows a four sided piece of sheet metal ductwork notched (#1) at the points that it will be bent by the Brake. The yellow highlight shows the line of folding (#2) that will happen with the brake in order to make the four sided piece of duct. Notching is how sheet metal is allowed to go through the roll forming machine where the joint is made.
The notching allows the roll-formed edge (#4), such as a TDC/TDF or S & Drive the ability to avoid the interference from bending at the corners. There is no way to bend the metal after its been through the joint (#4) making process without having the corners notched (#1). Ductmate doesn’t require notching as it isn’t put through the roll former, as it is an added flange that gets put on after the duct is assembled.
Notching
The depth of the notch is determined by which joint type will be applied to the end, such as TDC or S and Drive. TDC requires a deeper notch then S&D (S & Drive). There are different types of notching but they all have the same goal in mind.
Step # 5 – Sheet Metal Shear
Another piece of shop fabrication equipment is the shear, which comes in various forms, from automatic to manual. The fully completed coil line uses an automatic shear. The shear cuts the sheet metal at the point needed to give the proper width of duct.
There are also hand held electric shears. Shown below is a manual stand-alone foot operated shear. Once the metal is position in the machine you push down with your foot on the long bar that near knee height.
Foot Operated Shear
Below are two videos, the first one is a short demonstration of a foot shear, the second is a longer video of the many steps in making a piece of sheet metal and the various tools used in marking and measuring.
Foot Powered Shearing
This next video is a little over 8 minutes, but gives a good overview of the process of cutting a piece of sheet metal with a foot operated shear.
Sheet Metal Shearing
Step #6 – Sheet Metal Seams
Seams are created along the longitudinal length of the duct or fitting to fasten one section to another, as opposed to joints that connect one length of duct section to another, or a joint of duct to a fitting. Seams seal to make one section of duct or a fitting.
The longitudinal seam will be either Snap Lock or Pittsburgh Lock and is formed on a machine that will roll form the flat edges into one with a female and the other with a male end.
Snap Lock Seam
Pittsburgh seams are used on increased pressures, above those acceptable for Snap Lock.
Pittsburgh Seam
The video below is a little grainy, but you can see that the rollers will bend the flat edge of the sheet metal, each successive roller bending the metal a little more until it reaches the shape required by the last roller. These machine is a stand-alone piece of equipment which in a complete coil line would be part of the automated process.
Pittsburgh Seamer
Step #7 – Roll Forming Machines
Depending on the seam type, the flat metal cut by the Coil Line or Plasma cutter will get put through the roll former to form a Snap Lock Seam or a Pittsburgh Seam. These are the two most common seams, so they are the ones we will be using through-out this training program. The seam takes two pieces of metal and locks them securely together. The seam runs longitudinal and the joint runs transverse.
Step #8 – Liner
Duct is often provided with internal liner for mostly acoustical reason and for its thermal properties for air ducts installed outdoors where wrapping duct is not practical because of the cost. The video below is of a portable pinspotter machine that is not part of the automated coil line, but will show you how liner pins are fasten to the duct to hold the liner in place.
Portable Pinspotter for Attaching Liner to Duct
In an automated coil line with a liner station, the metal will be sprayed with adhesive before the liner is applied, followed by the pins. In non-automated systems there will be a table area where the metal will be sprayed with adhesive before the liner is attached to the interior of the duct or fitting.
In the below video you will see several methods used in the coil line process for attaching liner to the duct, in addition you will see another manual method of attaching the pins that hold the liner in place.
Linacoustic
Step #9 – Duct Break
Before the sections of duct can be assembled they need to be bent to form an “L-Shape”, “U-shape” or 4-sided single piece which comprises the duct. The break is a machine that is used to bend sheet metal. The metal is inserted to a point where the bend is required and then the machine automatically activates the break to bend the metal.
Hand Brake
Some shops still use a manual hand break (shown above) where the operator pulls a lever which causes the machine to bend the metal. With this portion complete you now have two “L” sections, or a “U-Shaped” section that will match up along a male and female companion seam. For smaller ducts it might be possible to make the duct section out of a single piece. The seam gets knocked together either as a snap lock or Pittsburgh seam as described previously.
Watch this video to see how metal is bent using a brake. These types of brakes go by various names such as Box Brake, Finger Brake or Pan Brake. Notice how the shop worker inserts the notched section of the flat metal into the machines notch guide to position it correctly for where the fold will occur.
Duct Brake
Duct Reinforcement
Reinforcement provides for the strengthening of the ductwork under certain conditions as dictated by the construction standards. The construction standards are driven by many factors including the pressure class, the size of the duct or fitting, the length of the duct and the local code adopted construction standards table.
Reinforcement can be accomplished by adding angles, channels, and Condu-locks with EMT. There are various other methods depending on the construction standards that provide jurisdiction over the project location. The standard could be an adoption of the SMACNA standards or the jurisdictions own version of something similar.
Coil Line Feeding a Plasma Cutter
Instead of having the coil line feed a sheet metal duct making process, it can feed lengths of coil duct onto a plasma cutter. This avoids having to purchase flat sheet stock and material handle each piece onto the plasma cutter. Watch the video below to see how a coil line machine can feed a plasma cutter to save time and money.
De-coiler
A Simplified Coil Line
In the below video you can hear the loud notching of the metal as the machine punches (notches) out at the point where the duct will be bent and or cut. The coil line machine cuts the section while creating a longitudinal seam, the point at where the duct sections will attach. The machine provides all aspect of the duct section, including straightening, beading, notching, joint formation, cutting, seams and bending.
The only thing the machine doesn’t do is the final banging together the single or two piece sections of duct along the seam. It also doesn’t install the corners pieces where the sections are bolted together.
Duct Assembling
Ductwork can be made into one, two or four pieces. One piece duct or full wrap duct is fabricated with one continuous piece of sheet metal and is limited in size by the fabrication equipment. As the duct gets larger and larger, more pieces will be required to make the duct. Two pieces (aka “L” sections, half sections, knocked down) are easier to ship, but more costly for the field labor, as they have to assemble the sections before installing.
Check what an “L” Section looks like coming off an automated break.
L-Section Duct
The Pittsburgh seams are knocked together with a Tinner’s Hammer or an automated tool that accomplishes the same thing, that is to bend over the seam to lock the sections together.
Tinners Hammer
Coil Line (Fast Time Lapse Video)
This next video shows a shop worker knocking together sections of duct, installing the corner pieces and using an automatic Pittsburgh seamer to bend over the seam and lock the sections in place.
Now that you have an idea of what happens from the coil line to the end product, see if you can identify some of the following steps in this sped up video of a coil line. You’ll notice that when the sheet metal first appears in the video it has already been straighten, beaded and notched.
The next step you will see is the application of the male and female seams which is done under the white/beige colored metal boxes that protect the roll formers. Then it moves down to the roll formers that put the joints on each end, before finishing by being bent. Replay the video until you notice each of the steps being performed.
One more quick video from coil line to end product.
And one more that shows you a 4-sided, one piece wrap.
Making Square to Rounds
Making a square to round requires that you make small bends along the length of the square to round to form the shape. Fabrication shops use various pieces of equipment such as the break to make these small creases incrementally along its length to make the square to round. This can be done faster with this NAMCOR Striker machine. See the video below to get an understanding of how the machine makes a square to round.
Chapter #1 – Introduction to Sheet Metal Shop Fabrication
The sheet metal fabrication shop is where all the ductwork and fittings are fabricated for the installation of air conditioning and heating systems, the systems that carry the air from the source of cooling or heating to the room to be warmed or cooled.
The larger sheet metal fabrication shops will mostly likely be running on software that automates various pieces of shop fabrication equipment, while some will also be fully integrated with the detailing and estimating departments.
Detailing
Detailing is the process of creating shop drawings or detailed drawings that are coordinated with the buildings structural elements and other trades to avoid interferences that may occur between these trades. The detailed drawings will show the distances that your sheet metal or other HVAC components will be hung from columns, floors and other reference points. The detailed drawings also show how the sheet metal is to be fabricated. (see chapter on Sheet Metal Shop Drawings)
In the image below there is a very large duct (10’-6” x 2’-0”) that has been detailed to be hung 6’- 6” from structural column #20, and 10’-6” from the floor to the bottom of the duct in order to miss the structural steel beam. A detailer coordinates what the mechanical contractor needs to install within the building while trying to avoid collisions with building components and other trades to make sure everything fits as designed. This helps avoid change-orders.
Detailing needs to occur before the sheet metal fabrication shop makes one piece of duct. This is done by the sheet metal detailer using some form of CAD (Computerized Aided Design) software to layout the sheet metal on the drawings indicating duct sizes, lengths and joint types.
Detailing Reference Lines
The benefit of using one software database for the sheet metal fabrication shop equipment, like the plasma cutter table and coil line is consistency of standards with the detailing and estimating departments. If the shop isn’t automated, then everything will have to be done manually for every piece of sheet metal.
Sheet Metal Order Forms
The sheet metal shop will receive an order from the detailer on some type of written or automated form created for this purpose (see example below). This form could also be used to order from an outside sheet metal fabricator if you don’t have your own sheet metal fabrication shop.
The form will indicate all the required information for each piece ordered, such as the material required, duct or fitting size, pressure class, lining and size, and joint ends required. Much of the rest of the requirements will be dictated by construction standards, whether SMACNA or in-house standards or a combination of local and industry requirements. Don’t worry we are going to cover all of these topics in the following chapters.
Shop Order Form
On the partial shop order form above there are two items ordered, the first one is a straight short piece of duct (#3) and the second one is a 90-Degree Elbow (#4). The duct and elbow both show the joint to be S & D (Slip & Drive) and the size to be 20” x 18”. This form can have 6 to 9 squares available for defining the requirements for duct and fittings. This form can also be used to just get a quote from your sheet metal fabricator.
SMACNA Standards
The sheet metal fabrication has to be built to some standard in order to maintain consistency and comply with the local code where the sheet metal is to be installed. SMACNA (Sheet Metal and Air Conditioning Contractors National Association) is the most used standard in HVAC sheet metal fabrication shops. SMACNA provides for various methods of fabrication for ducts, joint and seam types, in addition to various material types based on the pressure class.
Using SMACNA standards the shop will build pressure class tables that correspond to the various pressure classes.
Pressure Classes
Pressure classes define material thickness and reinforcement requirements. The pressure class relates to how much pressure the sheet metal ductwork will be exposed to by the fan when its blowing (positive pressure) or sucking (negative pressure).
Just think of how a balloon is blown up; the pressure created by you blowing into the balloon puts pressure on the wall of the balloon stretching it out the more you blow. The blowing up of a balloon or your car tire is an example of positive pressure and your vacuum cleaner is considered negative pressure as it sucks in air.
Below is the chart for SMACNA 2” pressure class. As the size of the duct increases, so does the thickness of the material indicated by its gage. Remember that the lower the gage number the thicker the material, similar to the game of golf where the lower the score the better. We will cover all of this in more detail in other chapters.
SMACNA 2 Construction Standards
Fabrication Shop Cost
The question often asked by those companies that have their own fabrication shop is whether the shop pays for itself. The cost of a sheet metal fabrication shop is capital intensive with all the required equipment, in addition to the labor cost to keep a shop operating during high and low periods. The question is it less expensive to purchase the sheet metal from another sheet metal fabricator or to make it yourself.
Keeping track of the utilities, indirect labor, insurance, trucks, tools, rent and equipment cost that it takes to operate a sheet metal fabrication shop is important to understanding whether it makes more sense to purchase or make your own sheet metal. The burden cost of having your own sheet metal shop needs to be considered and analyzed before investing in the costly operation of a fabrication shop.
The total cost of the sheet metal fabrication shop is important in order to understand the shop burden that needs to be recovered. The shop burden is the total cost to run and operate a sheet metal fabrication shop. This shop burden will be recovered at a rate equal to whatever metric is being used to recover the cost. If this metric is based on the quantity of hours worked in the shop, then a labor burden will be added to the hourly rate for each shop worker based on the total expected hours in a year.
Do you know what type of productivity your shop is producing? Does your shop keep productivity numbers monthly so you can determine what is the shops average production rate in pounds per hour? It is very important for your shop to keep track of its productivity so you can bid the current job at realistic numbers.
How many pounds of metal does the shop fabricate in a year and what is the total cost to operate the shop. The shop needs a method to measure the billable rate to charge per hour for the various personnel in the shop.
The cost to run the shop from the previous year is used to derive at a value that is used for the current years job costing. In the example above, the shop would add $20.00 per hour on top of the labor cost to recover the cost burden of owning and operating the fabrication shop.
If the shop used material cost to add burden cost, then adding and additional 50 cents to each pound that the shop fabricates would recover the annual burden cost of owning and running the fabrication shop.
As is obvious from the two examples above of owning and operating a fabrication shop above, the critical factor is determining how many hours or pounds of metal that will be used in the current year as opposed to the previous year of which the data is forecasted from.
Summary
Remember that the process begins by a detailer using some form of software to detail the project to fit within the structure and avoid collisions with other trades. If the project is small then computerized detailing might not be feasible.
Next the detailer will fill out some type of order form which defines the duct or fitting size, material thickness, seams and joints in addition to other requirements we’ll cover later.
The detailer will take into consideration the pressure class, that is the amount of pressure the duct will be subjected to by the fan.
The sheet metal fabrication shop will then begin the fabrication process based on what the detailer has requested to be made.
Now let’s look at chapter #2 to see what the most common materials used for ductwork in commercial HVAC projects.
Galvanized sheet metal is the most commonly used material for the Heating Ventilating and Air Conditioning industry for Commercial construction. It’s used for Supply, Return and Exhaust air ductwork that moves air from one location to another. The main differences in the construction of the ductwork has to do with the type of joints (method of connecting one duct section to another) and seams (length wise connections) used to construct the duct and fittings.
Sheet Metal Material Types
The following video shows how steel is made for the HVAC industry and other industries. This video is not mandatory for the course, but if you’re interested in how the steel is manufactured, then this video will give you an idea of how it starts from raw material to finished raw material for industry to use in their own fabrication process.
How Steel is Produced
Here is another video that shows similar steps in the process of making Hot Rolled Steel, but in a different manner. The content of the video will not appear on the chapter test, and is here for those who are curious about where the metal for the HVAC industry comes from.
Other material types found are aluminum, stainless steel and black iron, each having its material properties best suited for certain types of systems or exposure conditions. Sheet metal can be purchased in rolls or flat stock as shown below.
Sheet Metal Material – Flat Stock
Galvanized Sheet Metal
Most of your supply, return, relief, transfer and exhaust air ductwork will be fabricated from galvanized material, unless you live in an area that uses a lot of fiberglass duct board or flexible duct. SMACNA (Sheet Metal & Air Conditioning Contractors National Association) is the industry leader in setting the standards for the thickness (gauge), reinforcement, joints and seams, along with various other components comprised in the fabrication and installation of ductwork.
Galvanized steel is made by applying a zinc coating on both sides of carbon steel. By providing a zinc coating the material increases its corrosion resistance, but hinders its ability to be painted. If the surface of the duct needs to be painted there are materials treated for this purpose such as Paintlock or Galvannealed, each provided by different manufactures for this purpose.
A Zinc/Iron alloy is applied to both sides of carbon steel to create a product that can be painted. The addition of the iron in the alloy provides for greater adherence of paint to the surface.
The construction of rectangular duct is determined by the size, static pressure and length of the duct, or the size and type of fitting.
PVS Coated Galvanized can be used for moisture and corrosive exhaust systems.
Paintlock is a special order material that is used when you want to paint the duct. The process provides for an affect similar to primer when painting, allowing better adhesion for the paint. Paintlock goes by various trade names, so your vendor may call it something else.
Galvanized Material Gauge (thickness)
Gauge is a measurement of the thickness of the material. The most common gauges in the commercial construction market are from 16ga to 26ga, residential may use 28ga or 30ga. The higher the number of the gauge, the thinner the material will be.
Also, even numbered gauges are standard. To determine the required gauge of a material you would need to know several things, such as the size of the duct and the pressure class for which it is to be built, crossed referenced on a chart for the city or jurisdiction in which you are installing the duct.
Each city or jurisdictional authority may have differing standards or have adopt SMACNA tables as their standard. We’ll cover SMACNA latter. The size is determined by the amount of air that needs to travel through the duct while the static pressure is determined by the force being exerted by the fan to get the air to where it has to go.
The thickness of the material is specified in gauges with its corresponding weight for one square foot, as follows;
30 ga = 0.656 Lbs/Ft2
28 ga = 0.781 lbs/Ft2
26 ga = 0.906 Lbs/Ft2
24 ga = 1.156 Lbs/Ft2
22 ga = 1.406 Lbs/Ft2
20 ga = 1.656 Lbs/Ft2
18 ga = 2.156 Lbs/Ft2
16 ga = 2.656 Lbs/Ft2
The thicker the material the more it weights per square foot as show in the chart below. As an example if you had a piece of galvanized duct that was 10 feet long and was made from the different gauges they would all weight differently excluding joint material and reinforcement as such;
Sheet Metal Weight Chart
To calculate pounds you need to first stretch-out the ductwork so that it’s flat. So a 12” x 12” duct has all four sides 12”, so the first step is to flatten the duct.
Step 1 Flatten the duct and stretch it out as such; 12” + 12” + 12” + 12” = 48”.
Step 2 The next step is to turn the 48” of stretched-out duct into feet; (48” / 12” = 4 feet)
Step 3 Take the stretched-out feet and multiply by the total length to get total square feet; 4 feet x 10 feet = 40 Ft2
Step 4 Multiply the total square feet by the gauges weight per square foot value as shown in the chart;
Sheet Metal Weight Calculation
That is a quick way to get pounds for a piece of straight ductwork. Now if you knew how much you pay per pound if you purchase your ductwork then you could just multiply it by the totals pounds, For example let’s say you pay $3.00 / lb for purchased straight duct. Your cost to purchase the 18 ga duct above would be as follows;
86.24 Lbs x $4.00/Lb = $344.96
If you have your own fabrication shop then the composition of the total cost is going to comprised of material plus labor. As an example let’s say that your fabrication shop has a coil line and that your companies historical data shows that you can fabricate coil duct at 100 lbs/hr and the cost of your material is $0.70/lb. then your cost would be something like this;
86.24 Lbs x $1.00/Lb = $86.24 Material Cost 86.24 Lbs/ 80 Lbs/Hr = 1.08 Hours
We will cover this in more depth latter, but you can see quickly in this first section the process by where your ductwork is turned into components of labor and material based on weight and other factors. Fittings are more complicated to figure stretch-out as they have various angles and can have offsets, size changes as in transitions and square to rounds. This is what makes Estimating Take-off software so valuable, as it quickly converts any shape fitting into pounds at lightning speed.
Zinc Coated
Galvanized sheet metal is derived from a roll of carbon steel that is coated with zinc on both sides of the metal. The zinc coating protects the steel from corrosion. In the HVAC industry the Hot Dip Galvanized is usually the method used to coat the steel with zinc.
This is not something you need to worry about remembering, but the hot dipped galvanized process starts with the immersion of the carbon steel into an acid bath to remove steel scale. After the cleaning and scale removal the carbon steel is submerged in a molten zinc bath where it forms a protective bond of zinc.
The amount of zinc coating on the steel is measured by the quantity in ounces of zinc per square feet of steel. The most commonly specified zinc coating found in HVAC sheet metal specifications is either G60 or G90, with G90 being the most common. G90 will last longer than a G60 coating. since G90 gives a thicker coating of zinc. The coating is based on both sides, so G90 means that there is a total of 0.90 oz/Ft2 for both sides.
G90 = 0.90 oz/Ft2
G60 = 0.60 oz/Ft2
Physical Size of Flat Stock
If the fabrication shop doesn’t have a coil line, then they will be using flat stocks of sheet metal to make ductwork and fittings. You can purchase flat stock in the following sizes;
3′ x 8′
3′ x 10′
4′ x 8′
4′ x 10′
4′ x 12′
5′ x 8′
5′ x 10′
5′ x 12′
The HVAC sheet metal shop will most likely want to pick a width that they prefer to make their lengths of duct. If you want to make 5 foot joints of duct, then you would use the 5′ x 8′, 5′ x 10′ or 5′ x 12′ flat stocks of sheet metal.
You can easily figure out the weight of each size if you know the gauge of the metal as given above. If the shop purchases pieces of flat stock, can you determine what the weight would be for each of the following pieces. See the answers at the end of this sections.
4′ x 8′ 26ga = ___ Lbs
4′ x 8′ 24ga = ___ Lbs
5′ x 8′ 22ga = ___ Lbs
5′ x 8′ 20ga = ___ Lbs
Black Iron
A common use of Black Iron ductwork is for the removal of grease laden air in a kitchen exhaust system in restaurants and fast food establishments. The grease exhaust system requires a duct that can handle higher temperatures and avoid leaking grease, so these systems are made with fully welded joints and seams on the ductwork and fittings to reduce the risk of hot grease starting a fire by escaping from the ductwork and dripping onto construction materials. Foods that produce air borne particles of grease require an exhaust system that will protect the occupants from the hazards of a fire.
Grease will attach itself to the surface of the ductwork used to exhaust the smoke and heat from the kitchen. Appliances that create air borne grease, like fryers and grilles, will require a fully welded duct from the kitchen hood up through the roof to the exhaust fan.
Often the portion visible in the kitchen will be made of stainless steel for aesthetic reasons.
Kitchen Grease Exhaust Hood
Aluminum
Aluminum ductwork is used when duct systems contain moisture, such as in locker rooms with showers. Aluminum has beneficial properties that help ward off the corrosive effects of the moisture in the duct. Aluminum ductwork and grilles can be found in hospitals where radiation or X-rays are used within a room.
Locker Room Exhaust
You will notice in the below table that due to the properties of aluminum there is a requirement that aluminum be thicker than galvanized steel in order to match its strength and rigidity. For example a 26 gauge galvanized duct has a thickness of 0.55 mm, and the equivalent in Aluminum would require a 0.69 mm thickness.
Aluminum Equivalent
Stainless Steel
Stainless steel also has good material properties for use in systems that contain moisture or corrosive air, such as kitchen, dishwasher, laundry, spa’s, indoor pools and lab exhaust. Some engineers might even specify the grease exhaust duct to be fabricated out of fully welded stainless steel from the exhaust hood all the way up through the roof to the exhaust fan. Of course stainless steel is a more expensive material, so you could offer black iron as a VE (Value Engineered) solution to save the owner money.
Stainless Steel duct can be found where corrosive environments need to be exhausted and where galvanized is just not suitable because it can’t withstand the corrosive nature of the air being exhausted. This can be found in laboratory exhaust systems connected to lab hoods where experiments are being conducted on various chemical substances.
Stainless steel ductwork can be fabricated with or without welded joints and seams.
Coated Stainless Steel – This type of ductwork is usually fabricated in 4 foot joints with Van Stone Flanges. This coated stainless steel ductwork is used in microchip manufacturing facilities for the exhaust of corrosive fumes. Teflon, Halar or other approved coatings will be applied to the stainless to increase is protective properties.
Fiberglass Duct board
Fiberglass Duct Board is less commonly used but can be good for its acoustical properties. There is no metal except for your hangers as the duct is made from flat sheets of fiberglass bent into the shapes required.
Fiberglass Ductboard
Duct board can be made into round or rectangular shapes. It’s use avoids the necessity to wrap or line the ductwork. Usually fabricated using 1” duct board meeting UL 181.
Watch the below video to learn about duct board from the NAIMA (North American Insulation Manufactures Association). If you want to learn more about how to assembly duct board you can watch additional videos on the NAIMA YouTube channel.
Fiberglass Duct Board Fabrication
FRP – (Fiberglass Reinforced Plastic)
Fiberglass reinforced plastic is used for highly corrosive exhaust air, such as that found in industrial fabrication.
Fiberglass Reinforced Plastic is used a lot for semi-conductor plants that use a lot of solvents and acids in there processes of fabricating semi-conductor micro-chips from silicon wafers. This type of duct can be used for any highly corrosive exhaust material.
It is always a good idea to talk with the manufacture about the properties of the air that is being exhausted to make sure that it is compatible with the material used for the construction of the ductwork. The joints are made by a laborious method of applying layers of cloth and resin.
Duct Liner
When duct liner is specified it’s important to determine if the indicated duct size on the drawings is the net area required. If the dimensions on the drawing are net free area, then you will need to increase the fabricated duct size to accommodate for the thickness of the duct liner.
If you have a 12” x 12” duct that is lined with 1” acoustical liner and it is required to be the net free area of the duct dimension, then your sheet metal ductwork will need to be 14” x 14”. This is calculated as follows: each side includes 1” liner + 12” of free area + 1” liner = 14”.
Per SMACNA Duct liner is to be adhered to the duct surface with 90% coverage of adhesive and with pins at various distances based on the velocity in the duct. The greater the velocity, the greater the density of pins required. Below are two common methods for attaching the pins, one has the head as an integral piece of the pin, and the other the head is a separate push on washer. The welded pins are attached automatically by the liner portion of the coil line or as a standalone piece of equipment.
Liner Pins
Duct liner comes in differing thickness and is usually defined in the specifications or as dictated by your companies shop fabrication construction standards. Standard sizes are ½”, 1” and 1-1/2”, but there are various types of duct liner available up to 4” thick. In the video below you will see how duct liner is applied in a coil line and how pins are applied in another section. Liner is reflected on engineered drawings by dotted lines within the duct or fitting.
Duct Liner
Double-Wall Ductwork
There are various methods for constructing round, oval or rectangular double-wall duct. The interior wall can be either solid or perforated metal. The outer wall is the structural component that is rated to handle the design static pressure. An interior perforated wall allows for increased acoustical performance as the material absorbs some of the sound waves. Using a solid interior wall can be used for outdoor applications in order to provide the required thermal performance required by the local code. The inner duct wall protects the liner from erosion. We will cover double-wall duct under other sections of this course.
How to Figure the Weight of Duct
It’s important to know just how many pounds of duct and fittings you have, as this directly relates to material and fabrication cost. If you have a software program then the program will most likely give you these values after entering the bill of materials for your project. It’s also important to understand how the software calculates how many pounds a particular section of duct or fitting weights.
Computers of course calculate the various fittings with relative ease as compared to you or I doing it by hand. But so that you have a basic understanding of how the estimating or fabrication software does the calculation, we provide a simple explanation here.
If you had a 24″ x 12″ piece of ductwork that was 5 feet long, you could easily calculate its weight in pounds with the following;
24″ x 12″ Galvanized Duct
What you need to do is unfold the duct into is component sides and then calculate the square footage of each side.
Option 1 (See example below)
Step 1 ) Calculate the Ft2 of each side
Step 2 ) Add up the grand total Ft2 from each of the four sides.
Option 2
Step 1 ) Calculate the total perimeter length of all four sides and then divide by 12″ to get total perimeter in feet. (12″ + 24″ + 12″ + 24″ = 72″)/12″ = 6 feet
Step 2 ) Multiple answer from step 1 by the total length of the duct, in this case 5 feet, to get total Ft2. (6′ x 5′ = 30 Ft2)
Duct Stretch Out Totals
Duct Stretch Out Example
From the example above it was determined that we have 30 square feet of galvanized material. Now in order to convert the square footage into weight we need to know what the gauge (thickness) of the material is. To determine the gauge (thickness) we need to know what the static pressure will be exerted on the walls of the duct.
SMACNA and municipalities have construction standards that are based on the static pressure, so this is the first thing you need to know. How much static pressure is exerted on the walls of the ductwork? You can either find the information in the sheet metal section of the project specifications or indicated by the piece of equipment serving the duct. The most common static pressures used for constant volume supply air ductwork is 2″ static pressure, and 1″ to 2″ for the design of the return air duct.
Static Pressure
Simply stated static pressure is the resistance to air flow. The HVAC system conditioned air is delivered to the space through ductwork, fittings, volume dampers, filters and air distribution grilles create resistance which is overcome by the static pressure of the fan. If the fans static pressure rating is not enough to overcome the components of the ductwork then the volume (CFM) of air will be short of the design requirement.
Ducts are built according to a set of construction standards based on the static pressure that the duct will encounter. The amount of static pressure exerted on the walls of the duct will cause the standards to specify the gauge thickness, seam & joint types, and the reinforcement required. As the static pressure increases so too does the thickness of the duct material and the requirement to strengthen the seams and joints with some variations for proprietary joint systems.
The SMACNA construction standards allow for variations in this by adjusting the length of the duct or the frequency of the reinforcement. We’ll cover more of this in the Sheet Metal Fabrication course.
It’s important to know before you begin a material takeoff, just what the specifications require for the static pressure of the duct system at various demarcations. If you have a VAV system then the demarcation or separation between differing pressure classes will be the VAV box. On the high-side of the VAV the specifications may call for the ductwork to be fabricated to 4″wg of static pressure while the low side may only require 1″wg or 2″wg (water gauge) of static pressure.
The most common static pressure range will be between -1.0″ to +2″ sp (Static Pressure), unless you’re dealing with large commercial or industrial systems, in which case you could see +3″, +4″, +6″ or +10″ sp (static pressure)
On the discharge side of a fan will be positive pressure and on the suction side of a fan will be negative pressure (suction).
Duct Pressure Classes
SMACNA 2″ Static Pressure Construction Standards Table
Assuming the duct is to be fabricated according to a 2″ static pressure standard, than the following table would be used to determine the gauge of the material.
SMACNA 2 Construction Standards
SMACNA 2″ Construction Standard
Continuing with our example from above we will use the largest side of the duct, in our case the 24″ dimension to find the row from which to pick the other factors of the construction of the duct. You can see that we have selected the row which corresponds with the 24″ row under the column heading “Duct Dimensions“.
SMACNA gives you many options on how to build the rectangular duct for any given static pressure and size. The next column is used when there is “No Reinforcement” used for the duct section. As can be seen form the chart below this would require the material to be fabricated out of 16 gauge, which is considered a thick and heavy gauge for commercial construction, unless you are running high pressure or specialty exhaust systems.
SMACNA No reinforcements Required
Of course the thicker the gauge the more the cost for any given piece of duct. SMACNA allows other options as shown in the additional columns provided in the chart. The following columns start at 10 feet reinforcement spacing all the way down to 2 foot. But ion you look at the gauge required based on the reinforcement spacing you’ll notice that 26 gauge is the lightest (thinnest) material available to be used. Using a standard 5 foot joint with TDC, Ductmate™ or other joint type qualifies for the 5 foot reinforcement column.
SMACNA 2 inch standards
The difference between using 16 gauge versus the 26 gauge is shown below in the calculation of the weight of each.
16 Gauge = 2.656 Lbs / Ft2
26 Gauge = 0.906 Lbs / Ft2
Duct Ft2 = 30 Ft2
Option 1) 16 Gauge material with No Reinforcement
30 Ft2 x 2.656 Lbs / Ft2 = 79.68 Lbs
Option 2) 26 Gauge with 5 Foot joints
30 Ft2 x 0.906 Lbs / Ft2 = 27.18 Lbs
If you were buying your duct fabricated by another sheet metal shop and they were charging you $3.00 / Lb for straight duct, the difference would be as follows:
As this example shows it’s important to reduce the gauge where possible.
Relationship Between Width, Gauge and Reinforcement
Duct Width (#1), Thickness (gauge)(#2), Reinforcement Spacing (#3) and Reinforcement size (#4) all relate to each other and changing one usually affects the others. When you increase the Duct Width (#1) this can increase the Sheet Thickness (#2), Reinforcement Spacing (#3) & Reinforcement Sizing (#4). Likewise when you change anyone of the other items it can have an inverse relationship to the others, such as if you increase the reinforcement spacing (#3) you might be able to reduce the Duct Thickness (#2).
Duct Reinforcement Factors
Just remember that these factors are all related. Your Sheet Metal Fabricator will usually have a set of Duct Construction Standards that they use in most situation and only make adjustments in special situations.
Sheet Metal Standards
There are various ASTM standards that you may see when reading specifications, in most instances your shop will be aware of the requirements of these. The following are a few of the standard ones you may encounter;
ASTM A526 refers to a Commercial Quality of material
ASTM A527 refers to the material being of a lock-forming quality
ASTM A653 refers to zinc coating by the hot-dip process.
ANSWERS
4′ x 8′ 26ga = (4 x 8) = 32 Ft2 x 0.906 Lbs/Ft2 = 28.99 Lbs
4′ x 8′ 24ga = (4 x 8) = 32 Ft2 x 1.156 Lbs/Ft2 = 36.99 Lbs
5′ x 8′ 22ga = (5 x 8) = 40 Ft2 x 1.406 Lbs/Ft2 = 56.24 Lbs
5′ x 8′ 20ga = (5 x 8) = 40 Ft2 x 1.656 Lbs/Ft2 = 66.24 Lbs
