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Solving Installation Problems On Ventilation And Air Conditioning Engineering Iii

  • source:metmac;
  • Time:09/10/2020

Stainless steel duct has poor corrosion resistance

12.1 Forms of manifestation: There are scratches, scratches and other defects on the surface of the air duct and welding slag splash the surface of the weld is black, yellow and variegated. Even the air duct is partially rusted eclipse


12.2 Harmfulness: reduce the corrosion resistance of the stainless steel ventilation system, shorten the use life. At the same time, due to local corrosion of the air duct, the tightness of the ventilation system is reduced.

This can cause harmful gases to diffuse into the environment and affect the health of workers.


12.3 Cause analysis

① The method of blanking and processing the duct plate is improper;

② During the operation, the carbon steel is in contact with the stainless steel, causing corrosion on the surface.

Corrosion center, destroy the passivation film of its oxide layer;

③ The selected welding process is unreasonable. Argon arc welding and DC arc welding should be used. However, oxygen-acetylene welding must not be used.


4. During the welding process, no measures were taken to prevent the welding slag from falling directly onto the duct plate the measures should be coated with white powder on the surface of the C side of the weld.


5. If the surface is not cleaned after welding, the grease, welding slag and flying test objects should be removed first. After pickling, hot water washing and passivation treatment.


6. Opening a hole in the weld and its edge green will deform the same mouth and twice by hand metallographic structure changes caused by welding.


7. The air duct bracket adopts carbon steel bracket without isolation measures


8. The flange connecting bolts and nuts of the air pipe are not fastened by stainless steel parts: If carbon steel fasteners are used, acid-resistant paint should be applied.



Corrosion resistance of aluminum air duct is reduced

13.1 Form of expression: There are scratches on the surface of the air duct, and welding slag and welding are left in the weld. Medicine, local corrosion of air duct.

13.2 Harmfulness: reduce the corrosion resistance of aluminum ventilation pipes and shorten the use life.

13.3. Analysis of the causes

①. In charge of the board marking and cutting, it is not carried out on the workbench covered with rubber sheets. Metal scribing needles cannot be used for lofting and marking, otherwise it will damage the corrosion-resistant alumina film;

②. No measures were taken during welding, that is, the welding port and welding wire were not eliminated during welding oxide skin, etc.;

③. After the duct is welded, the weld is not cleaned with hot water and the welding slag and welding on the weld are removed.

④. The flange and the air pipe are not made of the same material, resulting in electrochemical corrosion.

When the flange is made of steel, the surface of the angle steel flange is not galvanized or sprayed with insulating paint.

Green treatment for electrochemical corrosion;

⑤.The air pipe and flange are connected with carbon steel rivets, and 4-6TLm is not used.

⑥. No anti-corrosion insulation treatment measures are taken for the support;

⑦. The flange connecting bolts and nuts do not match the material of the duct, such as galvanized screws bolts, nuts, and galvanized washers on both ends of the flange to increase the contact surface to prevent the flange is scratched by the nut.



Rigid PVC plastic rectangular duct is twisted and angled

14.1 Form of expression: The surface of the duct is not flat, the diagonals are not equal, and the adjacent surfaces are mutually not perpendicular, the planes of the two pipe ends are not parallel.


14.2. Harmfulness: the air duct is twisted and raised, which makes the air duct and the air duct connected

The receiving force is uneven, the flange gasket is not tight, and the air leakage is increased; the air duct system is due to failure to meet the requirements of straightness and uneven force and damage, reducing the service life.


14.3. Analysis of the causes

①. Rigid polyvinyl chloride plastic board is made by laminating method. In the process of making air duct

After being heated again, due to the anisotropy and residual stress inside the plate, the cold

The shrinkage phenomenon will appear after it has failed. The shrinkage test of each batch of plates is not done before cutting,

After determining the shrinkage value, release the shrinkage part when marking, and then cut the material;

② When the board is scribed and cut, the length and width of the two opposite sides are not matched.

③ The heating folding is not accurate;

④ The welding groove is incorrect, and it is not carried out according to the requirements of the construction and acceptance specifications.



The welding quality of hard PVC plastic duct is poor

15.1 Forms of expression: the strength of the weld is low, the weld is raised, and the weld is combined

Not tight, cracks and other defects appear.


15.2. Harmfulness: The strength of the duct joint is reduced; the tightness is not enough, which affects.


15.3. Analysis of the causes:

① The welding temperature is inappropriate. The temperature of the welding air should be controlled at the range of 210-250℃;

② The diameter of the electrode does not match the diameter of the welding gun. Generally, the nozzle diameter of the welding gun is close to

The welding seam strength is the highest at the electrode diameter; the form of the welding seam must be adapted to the air duct,

The structural characteristics of the sub-sections are not selected according to the requirements of the ``Construction and Acceptance Code'';

③ The welding method is incorrect.