This paper summarizes my personal experience in ventilation and air conditioning engineering construction for many years, and introduces the manufacturing and installation technology, common quality problems and corresponding countermeasures of air ducts in combination with the specific engineering example of a five-star hotel I participated in in in the past.
Air duct assembly technology
A hotel project is located on the south side of Chang 'an Avenue, which is another landmark building along Chang 'an Avenue undertaken by our unit. It is a comprehensive five-star hotel integrating accommodation, catering, conference and entertainment. Among them, there are twenty-five floors above ground and three floors underground. The second and third floors underground are garages, and the air-conditioning equipment floor is located on the first floor underground. Cold source of central air conditioning system: the cooling load of this project is 6000KW, and three centrifugal water chillers are selected, with a single refrigeration capacity of 600PT. The refrigeration room is located on the first floor underground. The cooling tower is located on the 25th floor of this building. The chilled water temperature is 7/ 12℃, and the cooling water supply and return water temperature is 32/37℃. Heat source: the original heating station of the hotel provides heat, and the heat exchange station is located in the basement. The heat medium of air conditioner is hot water at 60℃-50℃, which is manually switched in winter and summer. The heat load is 4800KW. Form of air conditioning water system: the cold source side of air conditioning water system is a constant flow system, and the load side is a variable flow system. The cold and hot water system of air conditioning is divided into two systems: winter pump and summer two-pipe system, and the expansion water tank device is used for constant pressure. Fan coil units and air conditioners are equipped with dynamic balance valves to control water flow. Air conditioning system form: According to the use function of the building, this project mainly adopts all-air air conditioning system and fan coil plus fresh air system. The fan coil unit adopts horizontal high static pressure type to bring back the bellows, and the indoor thermostat is adjusted in three gears. Add aluminum alloy filter screen to the air return port. Corridor exhaust air is recovered by the total heat recovery fresh air ventilator installed on the roof.
1 Assemble air duct
In this project, the air duct itself is assembled by means of composite connection, and the connection between pipe sections cannot be reconciled.
Flanges can be connected in two ways.
1. 1 Unable to connect to the LAN.
Duct flange connection has the characteristics of tight joint, good quality, light joint weight, material saving, simple construction technology, labor saving, easy realization of full mechanization, automatic construction and low cost, so it is widely popularized and applied. At present, there are dozens of connection forms of flangeless air ducts, and new forms are still emerging, but according to their structural principles, they can be divided into five types: socket type, cutting type, bite type, iron flange type and mixed connection type. Flange-free connection is mainly used for air ducts with small side length, including C-shaped cutting connection and S-shaped cutting connection. After the connection is completed, rivet the end of the slip with the air duct with a hollow rivet, and then apply sealant at the gap to ensure the airtightness of the air duct. The basic measures to improve the construction quality of air duct without flange connection are as follows:
(1) According to the specification requirements, strictly control the application scope of various flanges such as "S" shape and "C" shape.
The application range of drilling cuttings is that the long side of rectangular air duct is not more than 630mm, and the vertical bite is not more than100 mm. Vertical bite 90 degrees
The fillet width should be consistent with the vertical bite height, the 90-degree angle should be accurate, and the joints should be connected one by one when flanging.
Knock and put square iron on the back to make the flanging facade flat and the 90-degree line straight.
(2) In strict accordance with the requirements of duct size tolerance. If the counterpart dislocation is obvious, the chip is offset; Small mouth falls into big mouth.
Unable to fasten or the connector is skewed and twisted. The cutting piece should not be obviously skewed, and the seam should be in the middle, no matter
Cutting or pipe bite flanging should be accurate and tight, and the connecting joints should be neat and tight in the future.
(3) The pipe ends of each side of the flanging should be flush on one surface, and the small tube can be folded and flanged by a folding machine at one time.
It should be the same width on the whole extension line. This is also a necessary condition that the air duct interface must be straight when installing the butt joint.
(4) Except that the metal flange spring clip (including metal Falanca tile) has a gasket on the mounting butt surface, the rest.
After the connection is completed, apply sealant outside the joint and clean the joint before gluing. Sealant can't use putty,
Lime paste, etc. Instead, the bag should be sealed with special glue for air duct. The sealing of air duct should be based on the sealing of plate connection, and sealant can be used for sealing. The performance of the sealant should meet the requirements of the use environment, and the sealing surface should be located at the positive pressure side of the air duct.
(5) The supports and hangers for air duct installation shall be set according to the specification requirements. After the air duct is connected, the air leakage test of the air duct should be carried out according to the requirements of the specification grade.
1.2 has flange connection.
Angle steel flanges have been used to connect two sections of air ducts in China for many years. Combined with engineering practice, TDF and TDC are generally used for connection.
(1)TDF connection is the flange connection at both ends of the air duct itself, and then the two sections of air duct are fastened with flange angle steel and flange clamps.
A. Insert the four corners of the air duct into the flange corner;
B, evenly filling sealant around the flange surface of the self-provided flange of the air duct;
C, assembling the flange, and inserting flange clamps from the four corners of the flange;
D. Tighten the bolts of the four flange corners;
E. clamp the flange clamp and the two flanges together with a vise;
F. If the distance between the flange clamp and the flange angle is 1500mm, four flange clamps shall be used; 3 flange clamps with flange side length of 900- 1200mm; Flange side length is 600mm, with 2 flange clips; If the flange edge length is less than 450 mm, use 1 flange clamp in the middle.
(2) TDC connection is plug-in duct flange connection. This connection method is suitable for the connection of large side length of air duct between 1500-2500mm. Generally, it is divided into the following types: according to the length of the four sides of the air duct, four flange strips are prepared respectively; Four flange strips and four flange corners are respectively inserted into four sides of the air duct; Check and adjust the flatness of the flange opening; The flange strip and the air duct are riveted by hollow rivets; The combination of two air ducts. Fill the flange surface with sealant evenly, insert the two flange combinations into the flange clamp, and tighten the bolts at the four flange corners. Finally, clamp the flange clamp and the two flanges together with a vise. For air ducts with large public floors, when the length of the large side of the air duct exceeds 2500mm, the angle steel flange connection method will continue to be adopted, and the angle steel flange connection will be adopted in the design of smoke control and exhaust ducts.
Air duct air leakage detection
In order to test the air leakage of new technologies and new processes whose flanges can't be connected with TDF and TDC, and to verify whether they meet the requirements of national standards and codes, air leakage tests were carried out on C-shaped sliding connection air duct, TDF flange connection air duct, TDC flange connection air duct, C-shaped or S-shaped air duct and mixed connection air duct of TDF and TDC.
1. detection method
The test section of the air duct to be tested is closed and tested with 1 Q89 air duct air leakage tester. Firstly, connect the air supply hose of the fan to be tested with the air duct test section, then lead a small hose from the air duct test section to connect with the inclinometer on the tester, and finally start the fan of the tester, so that the speed of the fan can be continuously adjusted from slow to fast, and the pressure in the air duct test section will increase accordingly. When the pressure rises to 500Pa, it will be stable. At this time, the air leakage of the test section is equal to the supplementary air volume of the fan, and the negative pressure reading will be displayed on the inclinometer.
Air leakage in test section: q = f * a * p * p
Where: f-cross-sectional area of air supply pipeline;
A- flow coefficient, generally 0.97 ~ 0.98;
P- negative pressure displayed by inclinometer;
P- density of air, generally 1.293.
Then according to the area of the air duct in the test section, the air leakage per unit area is calculated.
2. Test results
C-chip, with sealant, with air leakage of 4.5m3/(m2? h)； For the joints with vertical S-shaped and C-shaped chips, the air leakage is 4.8m3/(m2? h)； For TDF flange connection, the air leakage is 1.86m3/(m2? h)； C-shaped slip, vertical S-shaped slip, TDF flange and TDC flange are mixed, with air leakage 1.95m3/(m2? h)； Apply sealant to the bite, and the air leakage is 1.83m3/(m2? h).
2.3 standard requirements
According to the national standard Code for Construction and Acceptance of Ventilation and Air Conditioning Engineering (GB50243-2002), the allowable air leakage of low-pressure air duct is 6m3/(m2? H) The following; European standard "Building Standard of European Air Conditioning Contractors Association" (DW/ 143), the allowable air leakage of low-pressure air duct is 5.5m3/(m2? H) below.
Common quality problems of risk control safety and their prevention
1. The material does not meet the quality requirements.
(1) phenomenon: the surface of the plate is uneven, with obvious indentation, crack, sand hole, scar and corrosion; The plane of the air duct sinks, and the side faces protrude outward, with obvious deformation;
(2) Harmfulness: when the system is running, the air duct leaks air, causing undue air conditioning load loss and affecting the service life of the air duct; The surface of the air duct vibrates, resulting in noise;
(3) Cause analysis: Before making the air duct, the material quality inspection was lax; The thickness of steel plate is not enough;
(4) Preventive measures: firstly, check the factory certificate and quality certificate of materials, and then check the appearance of materials; Measure the thickness of the steel plate. The main materials used, such as plates and section steels, should have factory certificates or quality appraisal documents. The material, variety, specification, performance and thickness of metal air duct shall conform to the design and current national product standards. The thickness of steel plate or galvanized steel plate shall not be less than relevant regulations, and the surface of galvanized steel plate shall not have defects such as cracks, scars, watermarks, etc., and there shall be crystal patterns of galvanized layer.
2. The air duct is warped and twisted, and the elbow angle is inaccurate.
(1) phenomenon: the air duct surface is uneven; Diagonal lines are not equal; Adjacent surfaces are not perpendicular to each other; The two opposite surfaces are not parallel, and the plane of the two pipe ends is not parallel;
(2) Harmfulness: it will cause uneven stress on the air duct, the installed air duct is not straight, the flange gasket is not tight, and the system leaks air, resulting in air conditioning load loss and shortened service life; Affect the accuracy of the installation position of air duct and tuyere;
(3) Cause analysis: the blanking layout of plates is inaccurate; The air ducts are parallel in pairs, and the length and width of the plates on the opposite sides are not equal; The bite width of the four corners of the air duct is not equal; The setting position of biting seam is wrong, and the strength of manual biting seam is different; No corresponding reinforcement measures have been taken; Angle steel duct flange angle is less than 90;
(4) Precautions: The square of the plate material should be strictly controlled during blanking, and the length, width and inspection diagonal of each plate material should be controlled within the allowable range; The bite width and allowance are determined by the thickness of the plate and shall meet the requirements of relevant regulations; After blanking, the two plates on the opposite side of the air duct should overlap to check the accuracy of the size; The reserved size of sheet metal bite must be correct to ensure the same bite width; Occlusal seams are located at four corners. When manually occluding and sewing, firstly, tighten the central parts of both ends of occlusion with a wooden hammer, and then evenly compact along the whole length; Folding or circular steel plates are sewn by sewing machine or by hand. When operating, the force should be even and not too heavy. Single and double mouths do bite, not half-bite. The relevant provisions of the national standard "Code for Construction and Acceptance of Ventilation and Air Conditioning Engineering" shall be implemented. Strictly control the entry of angle steel to ensure the angle. At the same time, when welding, spot welding should be carried out first, and then full welding should be carried out after adjustment, so as to ensure that the dimensions and angles of angle steel flanges meet the specification requirements. Duct reinforcement: the diameter of circular duct is greater than or equal to 80mm, and its section length is greater than 1250mm or its total surface area is greater than; When the length of rectangular air duct is greater than 630mm, the length of thermal insulation air duct is greater than 800mm, the length of pipe section is greater than 1250mm, or the flat area of one side of low pressure air duct is greater than that of medium and high pressure air duct, reinforcement measures should be taken.
3. The air duct fittings are of poor quality, with large air leakage and ugly appearance.
(1) Phenomenon: When making tees, elbows, reducers and front and rear bends in rectangular air ducts, the gaps at intersections and joints are too large and not dense;
(2) Harmfulness: it will make the air duct not tight, the system leaks air, resulting in the loss of air conditioning load, and some even increase the airflow noise in the pipeline;
(3) Cause analysis: The cutting and layout of plates are complicated, and the staff can't accurately master this technology; The blanking error of air duct is large, and the bite of air duct is not dense; Rough processing technology, not strictly in accordance with the construction process;
(4) Preventive measures: Construction personnel are required to be familiar with the construction technology of air duct fittings, and mass production can only be carried out after the samples are qualified. Inspection of intersections and interfaces should be strengthened, and special sealant for air ducts should be applied. Bite seam between air duct and fittings should be tight and uniform in width; Angle should be straight, radian should be uniform; Both ends are parallel. Duct without obvious distortion and warping; The surface shall be smooth, and the roughness shall not be greater than10mm; ;
4. The duct passing through the firewall is not handled correctly.
(1) phenomenon: when the ventilation and air conditioning system duct passes through the fire partition firewall, the fireproof materials are not used according to the specification requirements;
(2) Harmfulness: unqualified acceptance and non-compliance with fire protection regulations. When a fire breaks out, it will pass through the fire zone to expand the fire area, making the fire zone lose its function;
(3) Cause analysis: The gasket of ventilation and air conditioning system only needs flame retardant B 1 sealing material, and incombustible materials must be selected within 2M on both sides when crossing the fire zone. During the construction, the requirements of fire partition were not considered, and the construction was carried out according to the requirements of ventilation and air conditioning ducts;
(4) Prevention and control measures: Construction management personnel should pay attention to the different requirements of air ducts passing through fire zones and give timely guidance to prevent unnecessary rework. Combined with a specific project, several commonly used air duct connection methods in ventilation and air conditioning systems are compared, and quantitative analysis is made through air leakage test. This paper puts forward the quality problems, prevention methods and matters needing attention in the construction of air duct. In the manufacture of air duct, we should pay attention to the quality management of details, strictly control the pass, and prevent unnecessary rework to reduce production costs and improve comprehensive benefits.
Four, duct leak test after installation
After the air duct system is installed, it must be checked for tightness, and the next working procedure can be delivered only after it is qualified. The air tightness inspection of the air duct system is mainly based on the main pipeline. On the premise of ensuring the processing technology, the low-pressure air duct system can be detected by light leakage method. The air leakage test of medium-pressure air duct should be carried out in strict accordance with the requirements of construction specifications.
After the installation of the air duct system, the tightness inspection shall be carried out according to the system category, and the tightness inspection of the air duct system shall meet the following requirements:
(1) sampling inspection shall be adopted for the tightness inspection of air ducts in low-pressure systems, with a sampling rate of 5% and no less than 1 system. On the premise of ensuring the processing technology, the light leakage method is used for detection. When the test is unqualified, the air tightness test shall be conducted at the specified sampling rate.
(2) The tightness inspection of the air duct of the medium-pressure system shall be carried out after the light leakage test is qualified, with a sampling rate of 20% and no less than 1 system.
(3) The sampling system of air duct leak test of the system is all qualified, which is deemed as qualified; If there is anything unqualified, you should double the sampling again until all of them are qualified.
Light leakage detection: detect the detected air ducts in sections, with a sampling rate of 50%. The air duct of low-pressure system should not exceed two light leakage points per 10m, and the average light leakage point per 100m should not exceed 16; The light leakage points of the air duct shall not exceed 100 m, and the average number shall not exceed 8 per 1 00 m. If there is light leakage in a gap during inspection, it shall be sealed with glue.
Concluding remarks
In many air conditioning and ventilation projects, there are problems in the production and installation quality of air ducts, which lead to insufficient air supply, air leakage exceeding the specification requirements, energy waste, insufficient heat source and unstable operation of air conditioning and ventilation projects, all of which affect the normal operation of air conditioning. Therefore, the manufacture and installation technology of air duct and the corresponding preventive measures are extremely important for the quality control of ventilation and air conditioning.
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