There are projects that will require large plenums or casings, such as in built up system where components of the air conditioning system are bought in separate pieces and then assembled. In order to build these larger sheet metal plenums you will need to use additional methods of construction than those for traditional duct fabrication. These larger sheet metal plenums are often built the size of a small or large room and house fans, coils, filter banks and dampers. These large plenums will have doors to provide access to the various sections.
Gas Station Panel – Built-up Plenum
The purpose is to provide an area for supply, return or exhaust air to accumulate before or after the fan section, so they will need to be built to the appropriate pressure class. In some cases walls and floors of the building will make up one or more of the sides of the plenum, requiring that the plenum be sealed tight at this connection. Any penetration of the casing or plenum will require proper sealing to prevent leakage or infiltration.
SMACNA states that all ductwork fabricated on the suction side of the fan are built to 2 in. wg (500 Pa) pressure classification, while the discharge matches the design pressure classification of the system. If the casing or plenum exceeds negative 3 in wg (750 Pa) then you may build according to SMACNA’s Rectangular Industrial Duct Construction Standards.
As the height or width (#1) of the Casing/Plenum increases, the thickness (#2) of the material, Steel Angles (#3) and Standing Seams (#4) will increase as shown in SMACNA Figure 9-1 below.
Built Up Casings
Construction methods include the use of Standing Seams, Gas Station Panels or TDC/TDF Flanges. As can be seen from the chart above, as the height increases so does the requirement for a thicker gage material, larger angle and standing seam. Openings in the plenum or casing will be framed with angle to provide reinforcement.
Gas Station Panel Roll Forming Machine
There are not many fabrication shops that own their own Gas Station Panel Roll Forming Machine because the cost of these machine are high compared to the volume of this type of sheet metal required by the average commercial construction company. Unless your company builds a lot of large built up systems, then chances are they won’t own one of these, but will use other methods that are less productive but yet provide a similar product.
The below chart shows what determines the requirements for building a large Plenum or Casing. The columns are as follows: Span (#1) defines the Panel Height of the casing, Panel Gage (#2) defines the thickness of the metal, Depth (#3) is the Panel Thickness (2”, 3” or 4”), Load Class (#4) is determined by the pressure class (Static Pressure wg.) of the system.
Maximum Gas Station Panel Size
For example in the chart above, if you had a Panel Span (#5) of 8 feet and a system Static Pressure (#6) of 4” wg., and you wanted to use a Panel Depth (#7) of 3” with a Panel Width (#8) of 16”, this will inform you to use a Panel Gage (#9) of 20 Ga. The above chart includes additional Panel Span Heights: 10’, 12’, 14’, 16’ and 18’, (See SMACNA Manual for Additional Chart Data)
Sheet Metal Fabrication shops that have a roll forming machine will want to make efficient use of the coil sizes in inventory without waste. The below image shows a panel that was made from a standard 24” coil without any waste. If you add up all the dimensions of the below panel you will come up with 24”. Although the panel width in the below example is only 15-1/2”, you would use the 16” panel width and 3” wide column for all your panel requirements.
Gas Station Panel Section
Many of these roll forming machines are used to fabricate metal roofing panels on large industrial projects.
The casing can be lined with insulation and a solid (Double Wall) or perforated sheet metal interior skin. There are vendors, who sell prefabricated insulated plenum wall panels, but they are usually more expansive then those that your shop can fabricate or that the shop you purchase your duct from can fabricate for you.
Estimating Built Up Casings / Plenums
In order to determine how much sheet metal is included in the design you will need to measure the linear feet of the Panel walls and ceilings across its Face Area. As shown below a typical Panel Width of 15-1/2” when stretched out or flatten, the actual width is 24”, which includes all the sides and parts of the metal that was rolled formed. The stretch-out of the panel above would look like the panel below. This would allow the fabrication shop to use a standard 24 inch wide panel or coil.
Gas Station Panel Stretch-out
Example:
20 linear feet of a 16” (15-1/2”) Panel Width would be equivalent to much more because an additional 8” or 50% of the metal is consumed in the roll forming process using a 24” sheet metal coil.
#1 Panel Height 8 Feet
#2 Casing Wall Width 25.83 Feet (310 Inches)
#3 Casing Wall Width 5 Feet
Gas Station Panel Estimate
A rough way to estimate how much panel material there is in the above casing walls is as follows;
Step #1 (Calculate Quantity of Panels in Casing Wall #2)
The first step is to determine how many Full Height Panels (#1) are required in Casing Wall Width #2 by turning the total footage into Inches and then dividing by the Panels Face Width.
Step #3 (Calculate the Total Square Feet of Material)
Sum the total quantity of Casing Panels.
20 Panels + 5 Panels = 25 Casing Panels
Figure the Total Stretch-out Square Footage based on Panel Face Width and Material Width used to create the panel. The width of the material is determined by the wall width (2”, 3” or 4”). In the example above we were using a 24” coil width with a 3” Casing Panel Width, so we will use 24” for a stretch-out material width. Remember that the visible Face Panel Width is only a portion of the total material that you see or use in creating a panel, the rest is used in making the roll formed panel width (2”, 3” or 4”).
Using Panel Height (8 Feet) we can now calculate the total Square Feet of the material required to make this plenum or casing.
25 Panels x 24” width/Panel = 600 Inches / (12 Inches/Foot) = 50 Feet total width of material stretched out.
50 Feet (Width) x 8 Feet (Panel Height) = 400 Ft2
Answer 400 Ft2 of Material
Step #4 (Determine Total Poundage of Material)
Using the Maximum Allowable Panel Width chart above, we can see that with a Panel Height of 8 feet (#5) and a width of 3” (#7), and a static pressure of 4” wg, we find that using a panel width of 16 inches (#8) we get the required sheet metal gage of 20 ga. Note that we are using a panel width of 15-1/2”, and not 16”. The chart allows any size smaller then that listed, but not one greater than the listed panel width. The reason we are using 15-1/2 inches is that allows for you to use a perfect 24” sheet metal coil are flat stock without any waste.
Maximum Gas Station Panel Gauge
Now that we know the Gage and the square footage of the material required to make the panels we can determine the total poundage.
400 Ft2 x 20 ga (1.656 Lbs/Ft2) = 400 x 1.656 = 662.4 Lbs
Galvanized Material Thickness
Whenever you have a bunch of calculations to perform its best to use a computer program or application for this like Microsoft’s Excel program.
There are additional materials when figuring out the cost of a Casing or Large Plenum, such as Angles, anchors, gaskets, sheet metal screws, Access Doors, etc . There are also other methods of fabricating casings, but these are the ones illustrated in the SMACNA manual.
Access Doors
With large built up HVAC systems there will be a need for access doors in the plenum walls to provide a means for maintenance staff to replace the filters, clean the coils or to replace fan motors. SMACNA prefers that you limit the amount of doors so as to maintain the integrity of the plenum.
This traverse duct connector is fabricated from the same piece of metal as the duct (#3), and has a gasket (#10) inserted between the two joints of duct or fitting and is locked in place by a cleat (#9). Each corner will be bolted together. Watch the videos below to get a better sense of how this joint is assembled.
TDC Joint
Ductmate Duct Connectors (Slip on Flange)
The Ductmate duct connector is a proprietary connector manufactured by Ductmate Industries. One of the differences between this joint and the TDC joint above is that this is a slip on flange (#11) that is not fabricated from the piece of ductwork (#3) but is attached (slipped on) and secured to the raw end of the duct or fitting. This joint also requires a gasket (#10) between the flanges and cleats (#9) to fasten the two joints together. Watch the video below to get a better understanding of this proprietary joint.
Slip On Flanges like Ductmate
Watch the following video for a better understanding of what Ductmate offers.
Duct Flange Rollformer Machine
This machine will make a slip-on flange to make duct joints. A fabrication shop that has this machine will be able to make their own flange instead of having to buy a proprietary flange like that manufactured by Ductmate.
Rollformer
The following video shows a demonstration of how the attached duct connector is assembled onto the end of a piece of ductwork.
Four Bolted Corner Flange
The pieces that make up the TDC or Ductmate joint is comprised of the Flange (#4) on the end of the duct (#5) or fitting, the Gasket (#1) that provides an air tight seal, the Clip or Cleat (#2) that locks the two pieces together and the corner pieces (#3) that bolt together at each corner.
Duct Joint Assembly
To ensure a proper sealing of flanged joints the use of gaskets with bolted corner pieces will make a tight fit that reduces the chances of leakage. The gasket is a continuous length of approved material with a minimum of 16 gauge corner pieces and 3/8” bolts. The clips are used to lock together the two adjoining pieces of duct and are a minimum of 6 inches in length.
In lieu of clips, screws can be installed at 6” maximum intervals starting at a maximum of 1” inch from the corners. Clips are to be spaced at a maximum of 15” inches on center for 3” static pressure and less, and at 12” centers for 4” static pressure and above.
Watch this video on how to install Cleats/Clips.
Cleat Tool
The sheet metal fabrication shop has several options when making a rectangular flanged connection. One is to use have the flange an integral piece make from the duct, or to add a proprietary flange (#1) on to the raw end of the duct such as the below from Hardcast, a Carlisle Company.
The flange (#1) has a piece for each side of the duct or fitting, and four corner pieces (#2). Cleats (#3) locks two adjoining sections of ductwork with each other or a fitting by using the Cleat Installing Tool (#4). Watch the video above to see how the cleat is attached.
Hardcast Joints
TDC Corner Pieces
The corner of each section of duct will require a corner piece that allows separate sections of duct to be bolted together at their corners. The machine in the video below can do this automatically, otherwise they will be installed manually.
Vertical Duct Seam Closing Machine
S & Drive Joint
Here is another method of attaching ductwork and fittings together using an Slip & Drive connector. This requires two (2) Slips and two (2) Drives for each connection.
S and Drive
#1 in the below image is the Drive portion that gets driven at the joint.
This joint can be used at any length when exposed to 2 inch static pressure and less, and up to 36 inches in length for 3 inch static pressure and less, or 30 inch maximum length for ducts exposed to 4 inches of static pressure. Not approved for any duct static pressure over 4 inches. In the image below you have the Drive Slip (#1) that gets driven down at the joint ends (#2) of the two pieces being attached together.
Drive Slip Joint
And the S-Slip (#3) shown below holds the two pieces of duct (#2) in place on the other ends of the duct, so that you end up with two Drive-Slips and two S-Slips.
S-Slip
There is another version of the S-Slip called a Hemmed S-Slip because as you can see circled in red below the Hemmed S-Slip (#4) has a hem at each end.
Hemmed S Slip Joint
Watch the video below to see how the Slip and Drive is attached to the end of a piece of ductwork.
Slip and Drive JointSlip and DriveSlip PlacementInstalling Drive
This next video shows how easy this joint really is. As you will see, the sheet metal fabrication shop worker attaches two “S” slips to the end of two sections of ducts before pushing them together, then hammering the drives on each side to lock the two pieces together.
Watch the video below to see how a manual Drive Cleat/Slip is made in the shop. There are automated drive cleat machines that would produce these faster.
Below is an automatic cleat bender that is powered by a pneumatic operator.
Standing Drive Slip
Standing Drive Slip is similar to S and Drive except for the additional material and strength gained by a perpendicular extension of the Drive joint. This joint can also be reinforced with a bar for additional strength. This joint can be used at any length when exposed to 2 inch static pressure and less, and up to 36 inches in length for 3 inch static pressure and less, or 30 inch maximum length for ducts exposed to 4 inches of static pressure. Not approved for any duct static pressure over 4 inches.
Standing Drive Slip
Reinforced S-Slip
This joint uses a reinforced drive slip (#1) by using a 16 gauge angle (#2) that stands 1 inch in height. The angle slips within the drive section and is fasten in place with screws. This reinforced S-Slip is good on ductwork fabricated up to a maximum of 2” static pressure. The two sections of adjoining ductwork is shown by reference in the image below as (#3).
S Slip Reinforced
Standing S
The Standing S joint is similar to the reinforced S-Slip, except that the standing portion (#4) is fabricated from the same metal piece as the S connector. Below you can see that the two adjoining duct sections (#3) shown in red are inserted into the “S” section.
Standing S Joint
Watch this video to see how a standing “S” lock is made.
Standing “S”
Companion Angles
This is another way to reinforce the strength of the joint by adding companion angles (#6) on each end of a duct or fitting with a gasket (#7) between them. The companion angles can be attached to the duct or fitting by rivets (#8), screws or by tack welding, while the companion angles are secured together with bolts.
Companion Flange
Welded Joints
Welded joints (#5) are attached by methods of welding various material types together. Most commonly welded in the HVAC industry would be Galvanized, Stainless Steel or Black Iron. This will necessitate having heavier gauge fittings and ductwork, as lighter gauges can’t handle the temperature of the welding process.
Black iron grease ductwork is a typically used for grease ducts. Two sections of ducts or a duct and a fitting are brought together and welded where their flanges meet. There are other methods of joining in welding, but the flanged joint is commonly used.
Welded Joints
Bell Type Joint
The Bell type joint is also approved for use by the IMC (International Mechanical Code). The choice to use a particular joint is based on your sheet metal shop fabrication standards within the code approved methods. (See chapter on Grease Exhaust Systems for additional information)
Grease Exhaust Duct – Bell Type Joint
Joint Length
The length of the duct will vary depending on which joint you use to fabricate your ductwork. As shown in the image below, the use of Ductmate (#1) a proprietary duct joint doesn’t require roll-forming as opposed to the other two joint types.
Remember Ductmate (#1) gets slipped onto the raw end of the duct, while the TDC/TDF (#3) flange is roll-formed from the same piece of sheet metal that the duct is made from. Since Slip & Drive (#2) and the TDC/TDF Flange (#3) are made from the same material as the duct, this cause the overall length to be shorter than one that made for the use of Ductmate.
Ductmate Joint Duct Length = 60”
Slip & Drive Joint Duct Length = 59”
TDC/TDF Joint Duct Length = 56”
Joint Length Differences
What this means is that for every 100 feet for each type of joint there will be varying amounts of duct sections as shown below. There will be a quantity of 20 Ductmate pieces, 21 sections of Slip & Drive, and 22 sections of TDC/TDF joints. This is a great difference, but we want to make sure you understand that the roll-formers use some of the end of the duct to make the joints for TDC/TDF and the Slip and Drive joints.
duct sections
Round Duct Joints
Flanged Round Duct Joints
There are various methods of attaching round duct and fittings together. Here is a proprietary system by Ductmate® called “Spiralmate”. The duct can come with the flanges already attached from the fabrication shop or can be field installed. Care must be taken not to damage the flange in transit or disturb the sealant that sits in the groove of the flange. If the sealant becomes damaged then more will need to be applied to ensure a proper seal when assembled onto the end of a duct or fitting.
Spiralmate
Here is another manufacture of a flanged round joint; the Spiral Pipe E-Z Flange System.
Spiral Pipe EZ Flange System
And with this next video you can see how the roll former turns the end of a round duct until it is 90 perpendicular to its length thereby preventing the flange from slipping off, and allowing the two mating pieces to be clamped together.
Vanstone Flanges
Sheet Metal Combination Rotary
Preparing sheet metal for a wired edge, turning a burr, beading, and crimping are probably the most difficult of sheet metal forming operations to perform. Crimping the ends of round duct and fittings reduces the circumferences enough to allow the end to slip into its opposing duct or fitting.
Sheet Metal Rotary
This combination rotary is used for beading and crimping which in larger fabrication shops are produced by separate pieces of equipment.
Most low pressure round fittings will either have a crimped end or be coupling sized as shown below.
Crimped Ends
Crimped ends reduce the overall diameter just slightly so that the fitting or duct can slip into the next piece.
Round Crimped Joint
Round Couplings
Couplings slip inside the ends of two round ducts to be joined together and then sheet metal screws are fastened to hold it in place. Couplings could be required at a certain size such as connections 18” and larger. The shaded area in the image below is the coupling which has a beaded center so that each section of round ductwork slips over the coupling the same distance.
round coupling
Slip/Coupling Joint
Spiral duct can slip over the end of a fitting that is made coupling size, so that crimping isn’t required. The spiral duct will slip over the fitting until it hits a beaded section preventing it from going any further. The fitting has a bead similar to the one shown at the center of the coupling above.
Welded Connections
The requirement for a welded duct connection can be found for kitchen grease exhaust systems or some industrial processes and laboratory exhaust systems. Some of the more commonly welded duct materials are galvanized steel, stainless steel and black iron. Joints are often butt welded or flanged.
Often welded kitchen exhaust systems will use stainless steel where exposed to view and black iron for the hidden duct to the grease exhaust fan. Grease exhaust ducts need cleanout doors every so often as required by code for access for cleaning. (See chapter on Grease Exhaust Systems)
DuraFlange This is a proprietary system that uses light weight flanges that are inserted into the ends of ducts and fittings in order to join two pieces together with a neoprene or butyl gasket between them. Screws are then placed every 6 inches around the perimeter to fasten the DuraFlange to the end of the duct or fitting. This is a three piece joint, with two flanges (#1 & #3) and a gasket (#2).
DuraFlange
Watch the video below to see how this flange works.
Dura-Flange Ring
Rectangular Duct reinforcement
Intermediate reinforcement goes between the joints to strengthen the section of duct of fitting. The construction standards, like those established by SMACNA will dictate where and when reinforcement is required. The reinforcement could be top and bottom or on all four sides.
Intermediate Duct Reinforcement 2-Sided
Also, reinforcement can be around all four sides as shown below.
Duct Reinforcement 4 sided
Tie Rods
Another method of reinforcing ductwork and fittings is with the use of Tie-Rods. Reinforcement can be at the joint (JTR) or mid-way (MPT) between the joints. You can see by looking at the image below that you can have Joint Tie Rods (#1) or Mid Panel Tie Rods (#2) for additional strengthening of the duct section. The Tie-Rods are shown in yellow, with one at each end (#1) of the duct section and two mid-way (#2) between the joint ends.
EMT or solid rods are used for Tie-rods. Steel rods in sizes 1/4” & 5/16” are used or 1/2” to 1-1/4” EMT based on static pressure and distance to Mid Panel Tie Rod from joint. If the steel rod exceeds 3 feet in length, then a 3/8” steel rod must be used for added stability and strength.
Reinforcement Tie Rods
Mid Panel Tie Rods
The Static Press (#1), Sheet Metal Gauge (#2) and Mid Panel Tie Rod Distance (#4) will effect the Maximum Duct Width (#3) that can use a single toe rod for reinforcement when using a Mid Panel Tie Rod distance of 2-1/2’ (#4). As the table shows with the Red Arrow (#5) as the gauge of the duct gets thinner the maximum duct width (#3) decreases, and as the Static Pressure (#1) increases, the Maximum Duct Width (#3) decreases as shown by the red arrow (#6) although not as dramatically as with the reduction in sheet thickness.
Mid Panel Tie Rods Reinforcement
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.
Unreinforced Ductwork
Ductwork can be built without any additional reinforcement if it remains within SMACNA’s Table 2-47 “Unreinforced Duct (Wall Thickness)”. As can be seen in the image below, as the pressure class increases (#2), so does the thickness or gauge (#3) of the duct. The same applies to the dimension of the duct, so that as the duct gets larger, the thickness of the metal increases. In summary, if the pressure or the duct dimension increases, then the thickness of the material will also increase according to the below chart.
Unreinforced Ducts
Aluminum and Reinforcement
Aluminum doesn’t have the same strength and rigidity as Galvanized steel for the same gauge or thickness. Any construction using aluminum would require an increase in the thickness of the metal and reinforcement in order to match the equivalent in strength and rigidity of that of galvanized steel. For example a 1” x 1” x 16ga Steel Angle would require a 1-1/4” x 1-1/4” x 1/8” aluminum angle to be similar in strength and rigidity (see reinforcement table below). Aluminum is 6061-T Strength.
Galv Aluminum Duct Reinforcement
Round Duct Reinforcement
Just like rectangular duct, round duct will require reinforcement under certain conditions. As can be seen in SMACNA Table 3-10 below, the requirements for stiffeners is based on duct size (#1) and stiffener spacing (#2). This table is for 2” Negative Pressure Spiral duct. The process would be that you would look up the size (#1) of the spiral duct required per the projects engineered drawings and then decide which stiffener spacing (#2) the duct will be fabricated at. This will determine the required gage or thickness of the metal and the reinforcement or stiffener type.
As example if we look at 54” spiral duct as shown on row #3 we can see our options are as follows starting with the Unstiffened Column #4. Here are your options;
54” Spiral
Unstiffened Gage = 16 gage, Stiffener = NR (Not Required)
As you can see from the example above, as you increase the stiffener spacing (#2), you can reduce the thickness of the material, this is because the stiffener adds strength and rigidity to the duct allowing for a thinner material. Using an Unreinforced duct requires 16 gage material, while installing a stiffener every 5 feet allows you to use a much thinner and cheaper 28 gage duct.
The fabrication shop has to determine what their standard fabrication preference is and the cost for the various options based on stiffener spacing.
Seams run longitudinal along the length of the duct run, while joints run transverse (across). Seams run parallel to the air flow, while joints are perpendicular to the air flow. The seams hold together the duct or section of a single fitting as opposed to a joint that connects two separate pieces together. The seam is highlighted in the below image.
Each section of duct can have 1 to 4 seams depending on how large the duct is. Really large ducts and fittings can have more than 4 seams as required to provide strength and span wide areas, but in most cases there will be 1 or 2 seams.
Seams
Snap Lock and the Pittsburgh seam are the most typical seams used or specified in the HVAC commercial construction industry that we have witnessed. We show them here with a few others.
Three types of Seams
Standing Seams
When ducts get too wide they could require an additional seam to add strength and span the greater distances. The use of a standing seam provides strengthening for the larger ducts. Depending on the size of the duct, the standing seam will extend upward either by 1 inch or 1-1/2 inches.
Standing Seam
Pittsburgh Seams
Pittsburgh Seam
The Pittsburgh seam is used for pressure classes and duct sizes larger than the Snap Lock seam can handle. The Pittsburgh seam comes in small and large sizes depending on the pressure requirements of the ductwork.
Assembly of a Pittsburgh Seams
To lock the two ends of the Pittsburgh seam together it is run through a Pittsburgh Lock machine that folds over the standing lip, effectively locking the two pieces together. If the fabrication shop doesn’t have an automatic Pittsburgh seam machine then there are hand held models or it can be hammered over by a shop worker the old fashion way.
Assembling a Pittsburgh SeamClosing a Pittsburgh Seam
Pittsburgh Seam on Ductwork
Here is a video of a hand held Pittsburgh seam closer.
Pittsburgh Seam Closer
Pittsburgh Seam on a Fitting
This next video shows a sheet metal fabrication shop worker using Pittsburg seams on an elbow.
Button Punch Snap Lock
A Snap-lock roll forming machine will be one of the shop fabrication machines that gets highly used, as this makes the seam for the smaller, low pressure ductwork and fittings. Each piece gets one of its ends rolled to form basically a male and female longitudinal seam connection that gets knocked together and locked in place. The pieces are then bent to form an “L” shape in the case of a two piece fitting. Larger pieces may require four pieces to make the fitting.
Snap Lock Seam
These machine vary on their production. Somewhere around 60 feet per minute for the seam.
Watch the video below to see how quickly this seam is made and assembled.
Welded Seams
For duct systems requiring a heavier duty seam as can be found in some lab exhaust systems, the seam can be fully welded longitudinally. A piece of sheet metal will be cut to the proper width based on the circumference of the round size required and then put through a roller to shape the metal into a circular pattern. The longitudinal seams will be held together as the seam is welded. The ends can have a flange welded on so section of straight round duct can be fasten together or to a fitting with a flange.
Fully Welded Round Fittings
Grooved Seams
Slip Roll Machine
As shown below the metal is slipped into the roller and then it’s rotated around the three rollers. To make a greater or small circumference you adjust the position of the back roller.
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.
