1. Field measurement
Measure the size of the ventilation system at the installation site and transform the results into a sketch that will serve as the basis for the production of the air duct.
- Please note the following items during the measurement process:
- Measure the distances between the location of the ventilation system and the column, partitions, reserved holes, external walls and the height from the floor and ground to the roof.
- Measure the thickness of the outer wall, partition wall, size of reserved holes, width and height of doors and windows, column cross-section size, distance between the bottom of the beam and the flat roof, platform height, and other relevant information to the ventilation system.
- Measure the size, position, height and relative position of production equipment, air duct equipment and connection ports for ventilation components connected by the air duct.
- Measure the size, height and distance from the foundation wall or ventilation equipment support.
The specific content of the measurement will depend on the actual situation, and it is important to pay attention to the crossing and spacing of the various pipes and electrical lines.
2. Actual sketch
Through the above work, draw processing facility sketches.
3. Spreadsheet correction
(1) Steel coil leveling machines are commonly used to straighten coils through a series of repeated bends with multiple rollers.
(2) Typically, flat plates are corrected for bending deformation using manual hammering correction methods.
If the sheet material is less than 0.8mm thick, a large, soft, flat-head wooden hammer with fast flattening and high efficiency should be used for hammering and smoothing.
If the thickness is ≥ 0.8mm, a flat-head steel hammer is recommended for smoothing.
Based on the unevenness of the plate, the deformation characteristics such as warping or unevenness must be identified and then the iron platform used for smoothing.
4. Underline
Determine the thickness of the plate according to the size of the air duct design, select the number of tubes bent, and determine the interface mode. Use calculation and unfolding methods to cut the material, define the cutting line and make cutting marks.
5. Unfolding
(1) Select the appropriate model material:
When choosing the material for the model, it is important to choose something that is not too thick, with a thickness of 1 to 3mm being ideal. The material used must also be free from curvature or deformation, with options such as kraft paper, linoleum paper, soft plastic sheet and thin iron sheet being preferred.
(2) Calculate the appropriate sample plate length:
The sample length of the circular tube must be equal to the length of the outer diameter of the tube plus the thickness of the sample material, multiplied by π. However, it is important to note that the actual pipe circumference may differ from the calculated length due to seasonal and material influences.
For example, linoleum paper becomes harder in winter and may not fit well against the outer wall of the tube, requiring the length of the template to be increased. In summer it can become softer and more stretched, requiring a reduction in the unfolded length of the model. It is important to make these adjustments before drawing the expansion curve, as neither growth nor reduction can be carried out later.
(3) Check the actual revision volume:
After creating the model, it is essential to check its shape and size by wrapping it around the outer wall of the duct and checking the volume. The model must be close to the tube wall, with the two ends meeting, without gaps or overlaps. There are three methods to expand the model: parallel line expansion, radiation expansion, and triangle expansion.
6. Suppression
On the sheet material, mark the unfolded drawing and clear outline of the size of the blank. Then the next cutting step can be carried out. Manual cutting is only suitable for steel sheets with a thickness of less than 0.8mm, while thicker ones are generally cut by machines.
(1) Before cutting, it is important to accurately align the marking line on the plate, and there must be a clear tangent mark on the steel plate to be cut. After cutting, carefully check the size of the part before proceeding with processing.
(2) After cutting is completed, the steel plate must be held vertically and cut along the tangent line. During the cutting process, lifting the cut sheet by hand may reduce cutting resistance.
(3) When cutting curves, fold lines and corners, avoid cutting line marks on the sheet. To do this, the tip of the scissors must be aligned with the top of the corner and not too far away.
(4) When drilling a hole, first drill a hole, insert the scissors, and then cut counterclockwise along the line. If cutting a circle, use curved scissors for smaller diameters and cut counterclockwise. For larger circles with a smaller margin, it is allowed to cut clockwise.
(5) After finishing cutting the plate, be sure to use scissors or a chamfering machine to chamfer the end of the plate.
7. Air duct closure
(1) Select the board thickness according to different specifications and sizes of the air duct, and leave room for discharge.
(2) The line drawing process must be precise to ensure right angles, flat lines and accurate measurements. Geometric sizes should be checked frequently and all necessary lines such as cutting lines, chamfer lines, bend lines, flange lines, hole lines and closure lines should be drawn.
(3) Cutting and chamfering must be done accurately to minimize errors. After cutting, the edges must be beveled with a chamfer or iron scissors before closing the edges. Make sure there is no overlapping or flanging during operation.
(4) The plate should be placed on the press brake according to the drawn bend line and bent at the desired angle. During operation, align the bending line with the upper and lower molds of the square press brake.
(5) To create a round air duct, use a clapper to shape the edge into an arc, circle the bite, and adjust the arc to make it even.
(6) After bending or rounding the steel plate, use a sewing machine or manual sewing. Apply even pressure, avoiding applying too much force to prevent the seams from becoming uneven or bursting.
(7) The seams of air duct plates must be staggered and cross-shaped seams are not allowed.
(8) Common shapes of seams:
- A single seam can be used to splice and close circular ducts.
- Corner seams, joint angle seams and snap button seams can be used for rectangular air ducts or accessories.
- Vertical seams can be used for round elbows.
Steel sheet duct bite joint:
- The thickness of the steel plate for making air ducts and accessories δ≤1.2mm can be bite connected.
- δ> 1.2 mm must be welded.
- Flange butt welding must adopt gas welding.
- Galvanized mesh panels for making air ducts and accessories must be joined or riveted.
- Plastic composite panel air ducts generally can only use biting and riveting methods. To avoid burning the plastic layer by gas and electric welding, the bite machine should not have sharp edges to avoid scratches. If the plastic layer is damaged, it must be painted and protected in time.
Stainless steel plate duct bite joint:
- Bite connection can be used when the wall thickness of the stainless steel plate air duct is δ≤1mm.
- δ>1mm can use arc welding, argon arc welding, gas welding is not allowed.
The electrode must be of the same type as the base material and the mechanical resistance must not be lower than the minimum value of the base material.
Aluminum plate air duct bite joint :
- When the wall thickness δ of the aluminum duct and accessories is less than or equal to 1.5mm, it can be bite connected.
- 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 laying out, you must use colored pencils or colored pens to define. The bite or shaping of the air duct should be done with a wooden hammer or wooden square ruler to avoid deformation of the bite seam.
(9) Bite width and amount: Bite width is determined by the thickness of the air duct. Generally, for single flat bite, single vertical bite and single angle bite, the bite width on the first plate should be equal. On the second plate, it should be twice as wide, so that the bite allowance is equal to three times the bite width. Bite amount should be left on both sides as needed.
(10) Bite processing: Mechanical bite processing mainly involves the operation of various bite machines. For curved lines or solid bites, wooden square feet and wooden hammers should be used instead of a steel hand hammer to extend the edge of the board to avoid visible marks. The bite joint should be firm and there should be no half bites or cracks. Straight pipe joints should be staggered in the longitudinal seam, since the purpose of the air duct usually involves elbows, tees, and so on. The round elbow consists of several inclined short tubes and the single bite is formed in one direction when making the elbow, which means the bite seam of each section is in opposition. This is necessary for the production of elbows and is not limited by this regulation. The width of the bite seam should be uniform to avoid the phenomenon of a wide bite seam at one end and a narrow bite seam at the other, as this affects both the appearance and the firmness and tightness of the bite seam.
8. Welding shape of air duct
(1) Butt welding:
It is used to join plates or for closed horizontal and vertical seams.
(2) Lap welding:
It is used for closed longitudinal seams of rectangular ducts or pipe fittings, elbows of rectangular ducts, corner joints of tees, etc.
The general overlap is 10mm, and the overlap area must be marked before welding. Spot welding should be done along the marked line and then the weld should be smoothed with a small hammer before continuous welding.
(3) Flange welding:
It is used to close joints without flanges, round tubes and elbows. When the sheet is thin, gas welding can be used.
(4) Fillet welding:
It is used for longitudinal closed seams of rectangular air ducts or pipe fittings, swivel joints of rectangular elbows and tees, and closed seams of round rectangular air duct heads.
(5) Welding of carbon steel pipelines:
Carbon steel ducts must be welded with a DC welding machine. Before welding, the welding area must be cleaned of dirt, oil marks and rust. When spot welding or continuous welding is used, oxides must also be removed. Gap should be kept to a minimum and nodules in the manual spot welding position should be removed immediately. After welding, electrode slag and residual welding wire in the welding seam and nearby areas must be removed immediately.
(6) Stainless steel pipeline welding:
Before welding, the grease and dirt in the welding seam area must be cleaned to avoid air holes and sand in the welding seam. Cleaning can be done with gasoline or acetone. When arc welding stainless steel plates, white powder should be applied to both sides of the weld to prevent welding spatter from adhering to the surface of the plate. After welding, the slag on the welding seam should be removed and the metallic luster brushed off with a copper wire brush, then pickled with 10% hydrochloric acid solution and finally washed with hot water.
(7) Aluminum air duct welding:
Before welding, the welding area must be degreased and the oxide film removed. A stainless steel brush can be used for this. Welding should be done within 2 to 3 hours after cleaning and degreasing treatment should also be done after welding. Degreasing can be done with aviation gasoline, industrial alcohol, carbon tetrachloride or other cleaning agents and wood chips.
(8) Gas welding of thin steel sheet pipelines:
The direction of gas welding is generally left to right. The direction of the flame must be controlled to ensure that the heat on both sides of the weld remains balanced. The flame must advance smoothly and evenly, and the speed of the welding wire through the molten pool must be uniform.
(9) Welding quality requirements:
The weld surface must not have defects such as cracks, burns or lack of welding. Longitudinal welds must be staggered. The welding seam must be smooth, and the spot welding must alternate symmetrically during welding to avoid deformation. The width of the welding seam must be uniform. After welding, the weld must be cleaned to remove welding slag.
9. Flange production
(1) The distance between the screws and the rivet holes on the air duct flange of the metal air duct low pressure system should not exceed 150 mm. The air duct of the high pressure system should not be longer than 100 mm. The four corners of the rectangular duct flange must have screw holes.
(2) The distance between the screws and rivets on the flange of the low and medium pressure system air ducts must be less than or equal to 150 mm. The air duct of the high pressure system must also be less than or equal to 100 mm. The four corners of the rectangular flange must be reinforced with screws or rivets.
(3) Round flange production:
The entire angle iron or flat iron is processed by winding it into a spiral on a steel winding machine. Then, the rolled steel strips are cut and stretched one by one on a platform for leveling and correction. After adjustment, welding and drilling take place. The holes must be evenly distributed along the circumference to ensure that the flanges can be changed.
(4) Rectangular flange production:
The rectangular flange is made up of four pieces of angle iron. During marking and blanking, it is crucial to note that the inner edge of the flange after welding should not be smaller than the outer dimension of the air pipe and should be within the acceptable deviation value. Cutting and punching of angle steel must not be done using oxygen and acetylene cutting and can only be cut with a material cutting machine or manual saw. The angle fractures must be smooth and the burrs at both ends must be removed.
Then welding takes place on the platform. The flange angle must be measured and adjusted after spot welding to ensure that the lengths of the two diagonals are equal. The location of the screw holes must be accurate to ensure 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 angled aluminum. If angle steel is used instead of the aluminum plate flange, insulation and anti-corrosion treatment must be done to prevent electrochemical corrosion of the aluminum plate air pipe after contact with the carbon steel flange. Typically, the surface of the steel angle flange is galvanized or sprayed with insulating paint.
(6) Flange and air pipe connection:
When the air pipe and flange are connected by rivets, the riveting must be tight and leak-free. The flanging must be smooth and close to the flange, with a width of not less than 6mm and no cracks or holes at the bite seam and 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 must have complete internal welding and intermittent external welding. The end face of the air duct should not be less than 5 mm from the flange interface plane. If the flange of the stainless steel plate or the air pipe of the aluminum plate is made of carbon steel, anti-corrosion treatment should be done in accordance with the design requirements. The rivet must be made of the same material as the air duct or without electrochemical corrosion.
Quality acceptance of flange production:
- The welding seam of the air duct flange must be well fused and without any false welds or holes.
- The allowable deviation for flange flatness is 2 mm.
- Bolt hole arrangements for batch processed flanges of the same specification must be consistent and interchangeable.
10. Production connected without air duct flange
(1) Most circular air pipes use direct socket connection or center pipe connection. Center tube connection involves using the center tube as an intermediate connector, where two air tubes are inserted into both ends of the center tube for connection.
The insertion depth must not be less than 20 mm. Then, rivets or self-tapping screws should be used to fix the connection between the air tube and the central tube and tightly seal the joint with a sealant.
(2) The rectangular air pipe interface generally uses inserts, bites, metal spring clips or mixed connections. The size must be accurate, the shape must be regular, and the interface must be firm.
11. Duct reinforcement
(1) Reinforcement Techniques:
Joint height reinforcement technique (using standing bite). Reinforce the air duct with an angled steel ring around the entire circumference. Reinforcement of the longer side of the duct with steel angle iron. Longitudinal reinforcement of the inner wall of the air duct with ribs and reinforcement of the air duct steel plate with rolled grooves or crimped ribs.
Requirements for the quality of air duct reinforcement:
The air duct must be well reinforced and, to be considered excellent, it must be well maintained.
The spacing between each reinforcement must be adequate, uniform and parallel.
(2) Forms and Requirements for Reinforcement of Air Ducts:
The air duct can be reinforced in the form of corrugated bars, vertical bars, angle steel (for internal and external reinforcement), flat steel (using vertical reinforcement), reinforcing bars and internal pipe support.
See Figure 4.3.1.11.
Figure 4.3.1.11 Way to reinforce the air duct
(3) Reinforcement with bars or corrugated wires must be arranged in a regular pattern at uniform intervals and there must be no obvious deformations on the surface of the duct.
(4) Steel angles and reinforcing ribs must be arranged in an orderly and symmetrical manner, with a height not exceeding the width of the air duct flange. The riveting of the steel angles, reinforcing ribs and air duct must be secure, with uniform spacing not exceeding 220 mm, and the two intersections must be joined as one.
(5) Brackets and air ducts must be securely fixed, with uniform spacing between each support point, or edge or flange of the air duct, not exceeding 950mm.
(6) For medium and high pressure system air duct sections with a length of more than 1250 mm, reinforcing bars must also be used. The metal air duct of the high-pressure system must have reinforcement or reinforcement measures to prevent rupture of the single-bite seam.
