Air Leakage from Ductwork
It is of prime importance to keep air and gas leakage from ductwork at a minimum, as it represents increased fan running cost and the waste of treated air. Leakage into extract ductwork reduces the efficiency of the collection system.
The leakage factor is the rate of air leakage at a given static pressure per m2 of duct surface area:
FKi = ^ , (9.157)
Fre( is the leakage factor at a reference pressure Apcef, m"’ s’1 m~- qvj is the leakage volume flow rate, m’s-1 A is the duct surface area, m2
The leakage factor depends on the pressure Aptef at which the leakage airflow rate is measured. In this case, it is the arithmetical mean value of the maximum and minimum values of the static pressure difference throughout the ductwork (Pa),
TABLE 9.10 Airtightness Classes as Delined in the EUROVENT Guidelines 111 Air Leakage Rate in Sheet Metal Air Distribution Systems
Note that for laboratory duct testing, these values ate divided by 2, which gives the upper limits of this quantity for the three different classes.
Three classes of airtightness (A, B, and C) are considered.
The classification is based on the following equation:
K = (9.158)
Where K is the leakage coefficient per m2 of duct surface area, m3 s-1 nr 2 Pa-0-6-5. This quantity provides a measure of the ductwork leakage. This should be independent of the ductwork static pressure test.
Table 9.10 gives the upper limits of this quantity for the three different classes.
Leakage is determined by means of the fan pressurization method.
The test pressure for class A ductwork should not exceed 1000 Pa or the maximum design static pressure, whichever is smaller.
For class C ductwork, the pressure can be increased to 2000 Pa.
The test pressure should not be less than the design operating (static) pressure.
Table 9.11 gives the upper limits of the leakage volume flow rate for the three ductwork classes at typical test pressures.
High-quality constructed and installed duct systems may perform better than class C. A reduction to 1/3 of the class C values is possible.
TABLE 9.11 Maximum Leakage Factor for the Three Classes and for Typical Test Pressures
TABLE 9.12 Fan Power Increase Due to Ductwork Leakage
Leakage may require an increase in the fan power in order to ensure that the required design air distribution to all distribution points is achieved (See Table 9.12).
For circular ducts, at least 10% of the total surface shall be tested, and for rectangular ducts, at least 20% shall be tested. In either case the area to be tested shall normally be at least 10 m2. Note that there is no specific information regarding the determination of the duct surface area.
If the air leakage rate does not comply with the class requirement, the test shall be extended to include an additional equal percentage of the total surface area.
If the system is still leaking excessively, the total surface area shall be tested.
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