How to Test Airtightness of Air Duct

Air Duct With the progress of society and the gradual improvement of human requirements for various infrastructure facilities, air duct is more and more widely used. And the quality of the air duct needs to be tested more and more, especially its airtightness.
General provisions Air Duct
- The strength and tightness of air duct shall be tested according to the requirements of design and this specification.
- The strength of air duct shall meet the requirements of 1.5 times of working pressure for micro pressure and low pressure air duct, 1.2 times of working pressure for medium pressure air duct and no less than 750pa for high pressure air duct, and 1.2 times of working pressure for high pressure air duct, which shall be maintained for 5min or more, with no crack at joint and no permanent deformation and damage of overall structure.
- The tightness test of air duct should be divided into appearance quality test and air leakage test. Appearance quality inspection can be applied to micro pressure air duct, and can also be used as the inspection of process quality of other pressure air ducts. Tight structure and no obvious through gaps and holes should be qualified. The air leakage detection shall be the measurement and verification of the air leakage of the air duct system under the specified working pressure, and it shall be qualified if the air leakage is not greater than the specified value. The detection of air leakage of system air duct should be based on main pipe and main pipe, and the method of subsection detection and comprehensive analysis should be adopted. The test sample air duct should be composed of 3 sections or more, and the total surface area should not be less than 15㎡.
- The testing instrument shall be within the period of validity. The test method shall meet the requirements of this specification.
- When testing the air leakage of the air duct of the purification air conditioning system, the high-pressure air duct and the system with the air cleanliness level of 1-5 shall be tested as the high-pressure air duct, and the system with the working pressure of no more than 1500pa and the system with the air cleanliness level of 6-9 shall be tested as the medium pressure air duct.
Testing device
- The air leakage test shall adopt the qualified special air leakage measuring instrument, or adopt the measuring device set up by the measuring elements specified in the current national standard “measuring the full pipe fluid flow with the differential pressure device installed in the circular section pipeline” GB / T 2624.
- The air leakage test device can be air duct type or air chamber type. Orifice plate should be used as measuring element in duct type test device; The nozzle shall be used as the measuring element in the air chamber type test device.
- The fan, air pressure and air volume of the air leakage test device should be 1.2 times or more of the specified test pressure and the maximum allowable air leakage of the system or equipment to be measured.
- To adjust the test pressure of the air leakage test device, the method of adjusting the fan speed or controlling the opening of the throttle device can be used. The air leakage value should be measured when the system reaches the test pressure and keeps the steady pressure.
- The differential pressure measurement of the air leakage test device should use a micro manometer with a resolution of 1.0pa.
- The duct type air leakage test device shall meet the following requirements:
The duct-type air leakage test device shall be composed of the fan, connecting duct, pressure measuring instrument, rectifier grid, throttle, and standard orifice plate (Fig. 1).
Standard orifice plate with corner connection shall be used. Orifice plate β The distance between the orifice plate and the straight pipe section of the front and rear rectifying grids should be greater than or equal to 10 times and 5 times the diameter of the air duct respectively.
The connecting air duct shall be smooth round. Within the range of orifice plate to 2 times of air duct diameter upstream, the allowable roundness deviation shall be 0.3%, and 2% downstream.
The orifice plate shall be connected with the air duct, and the allowable deviation of perpendicularity between the front end and the pipe axis shall be 1°; The allowable deviation of concentricity between the orifice plate and duct shall be 1.5% of duct diameter.
After the first rectifier gate, all connection parts should be tight.
The air leakage shall be calculated according to the following formula:
Q=3600ε×α×An√(2∆P/ρ)
Where: Q — air leakage (m3/ h);
ε—— Expansion coefficient of air beam;
α—— Discharge coefficient of orifice plate;
An — opening area of orifice plate (m2);
ρ—— Air density (kg / m3);
Δ p – orifice plate differential pressure (PA).
The discharge coefficient of the orifice plate and its application β The relationship between values should be determined according to figure 2, and the following conditions should be met:
Firstly, when 1.0 × 105<Re<2.0 × 106, 0.05< β 49, 50 mm < D ≤ 1000 mm, the influence of pipe roughness on flow coefficient is not considered;
Secondly, when re is less than 1.0 × The flow coefficient should be calculated according to the relevant provisions of the current national standard “measurement of full pipe fluid flow by differential pressure device installed in circular cross-section pipe” GB / T 2624 α.
The expansion coefficient of air beam in orifice plate ε The values can be determined according to figure 3.
β2-P1/P2 | 1.00 | 0.98 | 0.96 | 0.94 | 0.92 | 0.90 | 0.85 | 0.80 | 0.75 |
0.08 | 1.0000 | 0.9930 | 0.9866 | 0.9803 | 0.9742 | 0.9681 | 0.9531 | 0.9381 | 0.9232 |
0.10 | 1.0000 | 0.9924 | 0.9854 | 0.9787 | 0.9720 | 0.9654 | 0.9491 | 0.9328 | 0.9166 |
0.20 | 1.0000 | 0.9918 | 0.9843 | 0.9770 | 0.9689 | 0.9627 | 0.9450 | 0.9275 | 0.9100 |
0.30 | 1.0000 | 0.9912 | 0.9831 | 0.9753 | 0.9676 | 0.9599 | 0.9410 | 0.9222 | 0.9034 |
Note: 1. This figure allows interpolation, not extension.
2. P2 / P1 is the ratio of the total pressure behind the orifice to that in front of the orifice
For the air leakage test device under negative pressure, the suction port of the fan should be connected with the throttle and orifice flow measurement section phase by phase, and the front 10d rectifier grid of the orifice should be placed at the windward end to form a complete device. Then it should be connected with the air duct or equipment to be measured through the soft interface (Fig. 4).
- The air leakage test device of air chamber type shall meet the following requirements:
The chamber-type air leakage test device shall be composed of the fan, connecting duct, pressure measuring instrument, flow equalizing plate, throttle, chamber, diaphragm, and nozzle (Fig. 5).
In order to use the nozzle to measure the air volume, the diaphragm should divide the air chamber into two cavities, and the measuring nozzle should be installed on the diaphragm. According to the needs of the test air volume, nozzles with different diameters and numbers can be used. In order to ensure the stability of the airflow at the nozzle inlet and the correctness of the flow rate, the center distance between the two nozzles should not be less than 3 times the throat diameter of the large-diameter nozzle;
The distance between the center of any nozzle and the nearest sidewall of the air chamber shall not be less than 1.5 times of the throat diameter of the nozzle. The distance between the installation position of the equalizing plate at the inlet end of the metering nozzle and the diaphragm shall not be less than 1.5 times of the large diameter nozzle, and the distance between the installation position of the equalizing plate at the outlet end and the diaphragm shall not be less than 2.5 times of the large diameter nozzle. The air outlet of the fan shall be connected with the test device (Fig. 5).
When the standard long diameter nozzle is selected as the metering element type after the diameter is determined, the neck length should be 0.6 times of the diameter, the nozzle big mouth should not be less than 2 times of the diameter, and the length of the expansion part should be equal to the diameter; The nozzle end shall be planed and 1 / 3 thick and 10 mm thick ° Tilt (Fig. 6).
The air chamber is a sealed box with connecting joints at both ends. The cross-sectional area of the air passage should be according to the maximum test air volume. When passing, the average air velocity should be less than or equal to 0.75m/s. The air outlet of the fan shall be connected with the interface of the throttle and nozzle inlet direction, and the other end shall be connected with the air pipe or equipment to be measured through the soft interface (Fig. 5).
The static pressure taps at both ends of the nozzle in the air chamber should be multiple and evenly distributed on the four walls. The distance between the static pressure tap and the nozzle baffle shall not be greater than 1.5 times the minimum nozzle throat diameter. Multiple static pressure interfaces should be connected in parallel to form a static pressure ring, and then connected with the pressure measuring instrument.
When the device is used to measure the air leakage, the flow rate through the nozzle throat should be controlled in the range of 15m / s ~ 35m / s.
All the connecting parts behind the nozzle clapboard in the air chamber shall be tight without leakage.
The air volume of a single nozzle shall be calculated as follows:
Q=3600Cd×Ad√(2∆P/ρ)
Where: Q — air leakage of the single nozzle (m3/ h);
Cd — flow coefficient of the nozzle (0.99 for diameter 127 mm and above, and 0.99 for diameter less than 127 mm can be obtained according to table c.2.7 or figure c.2.7-3);
Ad — throat area of the nozzle (m2);
Δ P — static pressure difference before and after the nozzle (PA).
Re | Cd | Re | Cd | Re | Cd | Re | Cd |
12000 | 0.950 | 40000 | 0.9730 | 80000 | 0.9830 | 200000 | 0.9910 |
16000 | 0.956 | 50000 | 0.9770 | 90000 | 0.9840 | 250000 | 0.9930 |
20000 | 0.961 | 60000 | 0.9790 | 100000 | 0.9850 | 300000 | 0.9940 |
30000 | 0.969 | 70000 | 0.9810 | 150000 | 0.9890 | 350000 | 0.9940 |
Note: temperature coefficient is not considered.
The air volume of multiple nozzles shall be calculated according to the following formula:
Q=∑Qn
For the air leakage test device under negative pressure, the suction port of the fan shall be connected with the opposite interface of the throttle and the nozzle inlet of the air chamber box, and the other end shall be connected with the air pipe or equipment to be measured through the soft interface (Fig. 9).
Note: first use the diameter and temperature scale to calculate the point on the index scale (x), and then connect the index with the pressure scale point to calculate the flow coefficient.
Air leakage test
The air leakage test of the system air duct and equipment should be divided into positive pressure test and negative pressure test. It should be determined according to the working state of the tested air duct, and it can also be checked by a positive pressure test.
The air leakage test of the system air duct can be carried out in whole or in sections, and all openings of the tested system shall be closed without air leakage.
When the air leakage of the air duct of the tested system exceeds the requirements of the design and this specification, the air leakage position (which can be detected by listening, touching, ribbon, water film, or smoke) shall be found out and marked; After the repair is completed, it shall be retested until it is qualified.
The measurement of air leakage should generally be the measured value under the specified working pressure (maximum operating pressure) of the system. Under special conditions, it can also be replaced by the test under the pressure close to or greater than the specified pressure. The air leakage can be calculated according to the following formula:
Q0=(P0/P)0.65
Where: Q0 — air leakage under specified pressure [m3/( h·㎡)];
Q — air leakage volume tested [m3/( h·㎡)];
P0 — the specified working pressure (PA) of air duct system test;
P — test pressure (PA).
In a word, measuring the airtightness of the air duct has a positive effect on the installation and final commissioning of the air duct.
Nice article of air duct, can you please introduce much more knowledge of HVAC air duct.