11 Steps for Making Metal Air Duct: Ductwork Installation

Production process of metal air duct:

Production process of metal air duct

1. Field measurement

Measure the size at the installation site of the ventilation system, and draw the measured results into a sketch as the basis for the system to process the air duct.

The following items should be noted during the measurement process:

(1) Measure the distance between the location where the ventilation system is installed and the column, between the partition wall and the partition wall, between the reserved holes, between the partition wall and the outer wall, as well as the height of the floor and the height from the ground to the roof.

(2) Measure the thickness of the external wall, partition wall, the size of the reserved holes, the width and height of the doors and windows, the cross-sectional size of the column, the distance between the bottom of the beam and the flat roof, the height of the platform, etc. related to the ventilation system.

(3) Measure the size, position, height and relative position of the production equipment, air duct equipment, and ventilation component connection ports connected by the air duct.

(4) Measure the size, height and distance from the wall of the foundation or support of the ventilation equipment.

The specific content of the measurement is determined according to the actual situation, and attention should be paid to the crossing and spacing of various pipes and electrical lines.

2. Actual sketching

Through the above work, draw processing installation sketches.


3. Sheet correction

(1) Steel coil leveling machines are often used for coils, and the steel plates are corrected by repeated bending with multiple rollers.

(2) Generally, the bending deformation of flat plate is corrected by the manual correction method of hammering.

When the sheet material is less than 0.8mm, a flat-head wooden hammer with large area, soft nature, fast flattening and high efficiency must be used for hammering and smoothing;

If the thickness is ≥ 0.8mm, a steel flat-head hammer should be used for smoothing.

According to the unevenness of the sheet, find out the deformation characteristics, warpage or unevenness, and then place the iron platform for smoothing.

4. Underlining

Determine the thickness of the plate according to the design size of the air pipe, select the number of bent pipes, and the interface mode.

Use the calculation and unfolding methods to cut the material, define the cutting line, and make the cutting mark.

5. Unfolding

(1) Select the appropriate model material.

The material used to make the model should not be too thick, preferably 1 to 3mm.

No curling or deformation, it is best to use kraft paper, linoleum paper, soft plastic sheet, thin iron sheet, etc.

(2) Calculate the appropriate length of the sample plate.

The length of the circular tube sample is equal to the length of the pipe outer diameter plus the thickness of the sample material and multiplied by π.

Due to the influence of the season and the material of the model, it is easy to make the drawn length differ from the actual pipe circumference.

For example, a model such as linoleum paper becomes hard in winter, and the phenomenon that the outer wall of the tube is not tight can appear, so the model is not long enough.

In the summer, it becomes soft, and the model is easily stretched.

In this way, corresponding measures must be taken to appropriately increase or decrease the unfolded length of the model.

It should be noted that neither growth nor reduction can be carried out after the expansion curve is drawn.

(3)Check the actual volume of the review.

After the model is completed, the shape and the size of the review must be checked and wrapped on the outer wall of the pipeline for the volume identification.

The model is close to the wall of the pipe.

After the two ends meet, it is better to have no gaps and no overlap.

There are 3 ways to expand: parallel line expansion method, radiation expansion method, and triangle expansion method.

6. Blanking


On the sheet material, the unfolding drawing and clear imprint of the marginal line of the blanking size are made.

The next shearing process is possible.

The thickness of the steel sheet cut by hand shear is less than 0.8mm.

The rest are generally cut with machinery.

(1) Before cutting, you need to strictly calibrate the size of the scribe line on the plate, there must be a clear tangent mark on the steel plate being cut.

After cutting, the blank size must be carefully checked before processing.

(2) After the cut is opened, the steel plate should be clamped vertically and cut in line with the tangent.

During the cutting process, lifting the cut sheet upwards by hand can reduce the resistance during the cutting process.

(3) When cutting curves, fold lines, and corners, never cut the line marks on the sheet.

For this reason, the end of the scissors must coincide with the top of the corner, not too far away.

(4) When cutting a hole, cut a hole first, put in scissors, and cut in a counterclockwise direction along the line.

When cutting a circle, if the diameter is small, use a curved scissors to cut counterclockwise.

When the diameter of the circle is larger and the margin is smaller, it can be cut clockwise.

(5) After shearing the sheet, the end of the sheet must be chamfered with scissors or a chamfering machine.

7. Closing of the air duct

Closing of the air duct

(1) Select the plate thickness according to the different specifications and sizes of the air duct.

Then leave a closing edge to make allowances for unloading.

(2) The line drawing method should be strict, so that the angle is straight, the line is flat, and the aliquots are accurate, the geometric size should be checked frequently.

The lines that should be drawn must be drawn, such as: cutting line, chamfering line, folding line, flanging line, hole line, closing line, etc.

(3) The cutting and chamfering should be accurate to minimize errors.

After the sheet is cut, it must be chamfered with a chamfering machine or iron scissors before edge closing.

Avoid overlapping of seams and flanging during operation.

(4) The plate after edge closing is placed on the folding machine according to the drawn folding line, and the folding operation is carried out.

During the operation, the folding line should be aligned with the upper and lower molds of the square folding machine, and folded to the desired angle.

(5) When making a round air duct, press the edge close to the bite into an arc shape with a clapper, and then circle the bite, and adjust the arc at any time to make it uniform.

(6) After folding or rounding the steel plate, use a seaming machine or manual seaming.

During the operation, it is necessary to apply even force, not to be too heavy, to avoid the fact that the bite is not real, causing the phenomenon of bursting and half bite.

(7) The bite seams of the splicing of air duct plates should be staggered, and no cross-shaped seams are allowed.

(8) Common bite forms:

  • A single bite can be used for the splicing bite of the plate and the closed bite of the circular duct.
  • For rectangular air ducts or accessories, corner bite, joint angle bite, snap button bite can be used.
  • Round elbows can use vertical bite.

Steel plate duct bite joint:

  • The thickness of the steel plate for making air ducts and accessories δ≤1.2mm can be connected by bite.
  • δ> 1.2mm should be welded.
  • Flange butt welding should adopt gas welding.
  • Galvanized mesh panels for making air ducts and accessories should be bite-joined or riveted.
  • Plastic composite panel air ducts can generally only use bite and riveting methods.To avoid burning the plastic layer by gas welding and electric welding, the bite machine must not have sharp edges to avoid scratches. If the plastic layer is damaged, it should be painted and protected in time.

Stainless steel plate duct bite joint:

  • The bite connection can be used when the wall thickness of the stainless steel plate air duct δ≤1mm.
  • δ> 1mm can use arc welding, argon arc welding, gas welding is not allowed.

The electrode should be of the same type as the base material, and the mechanical strength should not be lower than the minimum value of the base material.

Aluminum plate air duct bite joint:

  • When the wall thickness δ of aluminum duct and accessories is less than or equal to 1.5mm, it can be connected by bite.
  • δ> 1.5mm adopts gas welding or argon arc welding.
  • There should be no scratches on the surface of the aluminum air duct and accessories. When setting out, color pencils or colored pens should be used to set out.The bite or shaping of the air duct should be made of wooden hammer or wooden square ruler to avoid deformation of the bite seam.

(9) Width and amount of bite: The bite width is determined by the thickness of the air duct.

Generally speaking, for a single flat bite, a single vertical bite, and a single angle bite, the width of the bite on the first plate is equal.

On the second plate, it is twice as wide, so that the bite allowance is equal to 3 times the bite width.

The amount of bite should be left on both sides according to the needs of the bite.

(10) Processing of bite:

Mechanical bite processing is mainly the operation of various bite machines.

Except for the steel hand hammer used to extend the edge of the board, wood square feet and wooden hammers should be used for curved lines or solid bites, so as not to cause obvious marks.

The joint of the bite joint should be tight, and there should be no half bite or cracking.

Straight pipe joints should be staggered in longitudinal bite seam, because the meaning of air duct generally includes elbow, tee and so on.

The round elbow is composed of several short inclined tubes, and the single bite is in one direction when forming the elbow.

That is, the bite seam of each section is opposite. This is the need for the production of elbows, so it is not limited by this regulation.

The width of the bite seam should be uniform to avoid the phenomenon that the bite seam is wide at one end and narrow at the other end.

This phenomenon not only affects the appearance, but also affects the firmness and tightness of the bite seam.

8. Welding form of air duct

Welding form of air duct

(1) Butt welding:

It is used for the splicing of plates or horizontal seams and vertical closed seams.

(2) Lap welding: 

Used for longitudinal closed seam of rectangular duct or pipe fitting or the elbow of rectangular duct, corner joint of tee, etc.

The general lap joint amount is 10mm, and the lap wiring should be marked before welding.

Spot welding is performed according to the line during welding, and then the weld is smoothed with a small hammer before continuous welding.

(3) Flange welding: 

Used for closing joints of flangeless connections and round pipes and elbows. Gas welding can be used when the sheet is thin.

(4) Fillet welding: 

It is used for the longitudinal closed seam of rectangular air ducts or pipe fittings or the turning joints of rectangular elbows and tees, and the closed seam of round rectangular air duct heads.

(5) Welding of carbon steel duct:

Carbon steel ducts should be welded with a DC welding machine. Before welding, the dirt, oil marks and rust at the welding port should be removed.

When spot welding or continuous welding is used, oxides must also be removed.

The gap should be kept to a minimum, and the weld nodules at the manual spot welding positioning should be removed in time.

After welding, remove the electrode slag and residual welding wire in the welding seam and nearby areas in time.

(6) Welding of stainless steel duct: 

Before welding, the grease and dirt in the welding seam area should be cleaned to prevent the welding seam from having air holes and sand holes.

Cleaning can be done with gasoline, acetone, etc.

When welding stainless steel plates by arc welding, white powder should generally be applied to both surfaces of the weld to prevent welding spatter from adhering to the surface of the plate.

After welding, the slag at the welding seam should be removed, and the metallic luster should be brushed with a copper wire brush, then pickled with 10% hydrochloric acid solution, and then washed with hot water.

(7) Welding of aluminum air duct: 

Before welding, the welding port must be degreased and the oxide film removed, a stainless steel wire brush can be used.

Welding must be performed within 2 to 3 hours after removal, and degreasing treatment must also be performed after removal.

Welding must be performed within 2 to 3 hours after removal, and degreasing treatment must also be performed after removal.

Degreasing can use aviation gasoline, industrial alcohol, carbon tetrachloride and other cleaning agents and wood chips for cleaning.

(8) The welding direction of the gas welding of the thin steel plate duct is generally suitable from left to right.

You should master the direction of the flame so that the heat on both sides of the weld remains balanced.

The flame should move forward smoothly and evenly, and the speed of the welding wire into the molten pool must be uniform.

(9) Quality requirements of welds:

There should be no defects such as cracks, burn-through, or missing welding on the surface of the weld.

The longitudinal welds must be staggered.

The welding seam should be smooth, and the spot welding should be alternately symmetrical during welding to prevent deformation and the width of the welding seam should be uniform.

After welding, the weld should be cleaned to remove welding slag.

9. Flange production

Flange production

(1) The distance between the bolts and rivet holes of the air duct flange of the low-pressure system of the metal air duct flange should not be greater than 150mm.

The air duct of the high-pressure system shall not be larger than 100mm.

The four corners of the rectangular duct flange should be provided with screw holes.

(2) The distance between the bolts and rivets on the flange, the air duct of the low-pressure and medium-pressure system is less than or equal to 150mm.

High-pressure system air duct is less than or equal to 100mm.

The four corners of the rectangular flange must be provided with reinforcement bolts or rivets.

(3) Production of round flanges:

During processing, the whole angle iron or flat iron is rolled into a spiral shape on a steel coiling machine, and the rolled steel strip is cut and drawn one by one on the platform for leveling and correction.

After adjustment, welding and drilling are performed. The holes should be evenly distributed along the circumference, so that the flanges can be used interchangeably.

(4) Rectangular flange production:

The rectangular flange is made of four pieces of angle iron. When marking and blanking, it should be noted that the inner edge of the flange after welding cannot be smaller than the outer dimension of the air pipe, and should be within the allowable deviation value.

The cutting and punching of angle steel are strictly prohibited to use oxygen and acetylene cutting, and can be cut by a material cutting machine or hand saw.

The angle steel fractures should be smooth, remove burrs at both ends, and weld on the platform.

The angle of the flange should be measured and adjusted after spot welding so that the lengths of the two diagonals are equal.

The location of the flange screw holes must be accurate to ensure the smooth installation of the air duct.

The drilling method is the same as the circular air pipe flange.

(5) Aluminum plate flange production:

The aluminum plate flange is made of flat aluminum or angle aluminum.

If you want to use angle steel instead of aluminum plate flange, you need to do insulation and anti-corrosion treatment to prevent the electrochemical corrosion of the aluminum plate air pipe after contact with the carbon steel flange.

Generally, the surface of the angle steel flange is galvanized or sprayed with insulating paint.

(6) Flange and air pipe connection:

When the air pipe and flange are connected by riveting, the riveting should be firm, and there should be no riveting or leakage riveting.

The flanging should be smooth and close to the flange, and its width should be the same, not less than 6mm, and there should be no cracks or holes in the bite seam and the four corners.

When the air pipe and flange are connected by welding, the end face of the air pipe should not be higher than the plane of the flange interface.

The air duct of the dust removal system should be full inside welding and intermittent outside welding.

The end face of the air duct should not be less than 5mm from the flange interface plane.

When the flange of the stainless steel plate or aluminum plate air pipe is made of carbon steel, anti-corrosion treatment should be done according to the design requirements.

The rivet should be made of the same material as the air duct or without electrochemical corrosion.

(7) Quality acceptance of flange production:

The welding seam of the air duct flange should be well fused and full without false welding and holes.

The allowable deviation of flange flatness is 2mm.

The arrangement of screw holes of flanges of the same specification processed in the same batch should be consistent and interchangeable.

10. The air duct no-flange connected production

(1) Most of the circular air pipes adopt direct socket connection or core tube connection.

Core tube connection, using the core tube as an intermediate connector.

Two air pipes are inserted at both ends of the core pipe to achieve connection, and the insertion depth is not less than 20mm.

Then you have to use pull rivets or self-tapping screws to fix the connection section of the air pipe and the core pipe, and seal the seam tightly with sealant.

(2) The interface of the rectangular air pipe mostly uses inserts, bites, metal spring clips, and mixed connections.

Its size should be accurate, the shape should be regular, and the interface should be tight.

11. Duct reinforcement

Air Duct reinforcement

(1) The reinforcement methods are:

  • Reinforcement method of joint height (that is, using a standing bite).
  • Angle steel reinforcement ring around the air duct.
  • The large side of the duct is reinforced with angle steel.
  • The inner wall of the air duct is longitudinally reinforced with ribs, and the steel plate of the air duct is reinforced by rolling grooves or crimping ribs.

Quality requirements for air duct reinforcement:

At least the reinforcement of the air duct must be firm. If it is to be excellent, it needs to be neat.

The spacing of each reinforcement should be appropriate, uniform and parallel to each other.

(2) The form and requirements of air duct reinforcement:

The reinforcement of the air duct can be in the form of corrugated bars, standing bars, angle steel (internal and external reinforcement), flat steel (using vertical reinforcement), reinforcing bars and tube internal support.

As shown in Figure

Reinforcement form of air duct

Figure Reinforcement form of air duct

(3) For the reinforcement of corrugated bars or corrugated wires, the arrangement should be regular, the interval should be uniform, and there should be no obvious deformation on the board surface.

(4) The reinforcement of angle steel and reinforcement ribs should be neatly arranged and evenly symmetrical, and the height should be less than or equal to the flange width of the air pipe.

The riveting of angle steel, reinforcement ribs and air duct should be firm, the spacing should be even, and the spacing should not be greater than 220mm.

And the two intersections should be connected into one.

(5) The support and the air pipe should be firmly fixed, and the spacing between each support point or the edge or flange of the air pipe should be uniform, not be greater than 950mm.

(6) When the length of the duct section of the medium-pressure and high-pressure system air duct is greater than 1250mm, there should also be reinforcement bars.

The single bite seam of the metal air duct of the high-pressure system should also have reinforcement or reinforcement measures to prevent the bite seam from bursting.

2 thoughts on “11 Steps for Making Metal Air Duct: Ductwork Installation”

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