By R. V. Simha
Air Conditioning Consultant
R. V. Simha is a graduate engineer in both mechanical and electrical engineering, with over 40 years of experience in HVAC. He has been a practising consultant for the last 26 years. He is an active member of ISHRAE and ASHRAE South India chapter.
We now have in place the necessary tools to plan “leak testing”. The first step is to determine the permissible system leakage. It is the responsibility of the design engineer to fix this all-important parameter. The consideration is essentially the nature and requirement of the project. Once this parameter is fixed, it will be necessary to apportion it amongst the various elements of the air distribution system. As we have seen, the leakage depends on the static pressure in the duct and that is not the same for all sections of the ducting. Leakage also depends on the "leakage class", i.e., the class or quality of duct construction, which will not – and need not – necessarily be the same for all sections of the ducting. In addition, there are various elements of the air distribution system, which are also sources of leakage. It should be remembered in this context that the maximum system leakage referred to for ductwork is the system leakage, less leakage through other elements of the system – which can be called the duct leakage. The design engineer will accordingly find that he has to formu-late a “leakage budget”, which, will look somewhat like this:
The duct leakage will be the system leakage less the sum of leakages of items b to j.
An important component of leakage is obviously the air-handling unit. It becomes a major player in the budget particularly where the system static pressure is high, say, 75 to 150 mm. It is therefore, necessary to specify and calculate the AHU leakage.
An applicable AHU leakage standard is the European Standard EN 1886. Extracts from this standard are furnished in Table 6 and 7 below:
|Table 6 : Leakage limit for casing at 400 Pa negative pressure|
|Leakage class|| Max. leakage
rate L/s per m2
efficiency Em %
||Em < 40|
||1.32||F5-7||40 ≤ Em < 90|
|B||0.44||F8-9||90 ≤ Em|
The appearance of "filter class" and "filter efficiency" may be explained in terms of the need to relate leakage rates to quality of air i.e., the higher the air quality, the tighter should be the ducts.
|Table 7 : Leakage limit for casing at 700 Pa positive pressure|
|Leakage class|| Max. leakage
rate L/s per m2
The following equation can be used to determine maximum allowable leakage in case the units are tested at pressures different from 700 Pa.
fm = maximum allowable leakage rate at the actual test pressure (L/s per m2)
f700 = maximum allowable leakage rate at 700 Pa (L/s per m2)
It is recommended that the specified leakage class for AHUs be lower than that specified for the ductwork, because of the larger number / length of joints per m2 of casing area than in ductwork and the greater difficulties encountered in sealing air handling units, due to penetrations, access doors etc.
A standard AHU from a "high quality" manufacturer can be expected to be within the limits of DW 142 Class A and Class B. Class C may be targeted where project requirements so stipulate – but then, the specifier must accept that there is a price penalty to be paid for specialized unit construction and testing. Figure 3 shows AHU leakage calculations.
Having seen how AHU leakages can be calculated, the way is now clear for arriving at the duct leakage. To be sure, there are other items of leakage too, but focus has been on the AHU, since it happens to account for a significant fraction of the system leakage.
The method of calculation of duct leakage is best understood by studying a worked example (see box on following page). The procedure described therein is also shown graphically in Figure 4.
Let us suppose that it is required to test the 5 nos. branch ducts of the worked example. The approach is shown below:
|No. of branch duct
Mean perimeter (70% of max.)
Leakage cfm / branch duct
5 ft x 2 ft.
490 x 0.044 =
1.5 m x 0.6 m
46 x 0.214 =
It is seen that with a test rig capable of handling air flow rates down to 21 cfm, an entire branch can be tested in a single run (in practice, it would be designed to handle a some what lower flow rate, say 15 cfm – to keep a safety margin). If however, it is not possible to put together all the pieces of an entire branch in one go, the flow rate will be less than 21 cfm. In that case, the rig must be able to measure the smaller flow rate called for – perhaps, around 10 cfm. This fixes the lowest flow rate that the rig must be able to handle (the ducting being tight – as it will be, if it is constructed to higher pressure classes, can also call for low flow rates). The difference between the maximum and minimum flow rates, at the lower end of the range of the rig, might some times warrant the use of two different orifice plate assemblies – one for low flow rates and another for larger flow rates – like say 5 cfm and 20 cfm. If on the other hand, all the 5 branches are to be tested at the same time, the rig should be able to handle 21x5=105 cfm; the rig would then have a rating of 130 cfm approx. Thus the maximum and minimum flow rates for the rig would be 130 cfm and 5-10 cfm respectively.
| The design procedure is best understood
by a worked example
(See Figures 3, 4 & 6 for clarifications)
|Conditioned area (say 160 x 50ft.)
1 air change per hour
|No. of Air Handling Units (AHU)||2||2|
|Surface Area of each AHU :
a. Positive Pressure side
b. Negative Pressure side
|Leakage per AHU : See Figure
a. Positive Pressure side
b. Negative Pressure side
Total per AHU
Total for 2 nos. AHUs
a. Total permissible leakage
|b. AHU Leakages||187 cfm||85 L/s|
|c. Others :
HEPA Filter, damper,
apparatus casing etc
|d. Sub Total (item b + item c)||277 cfm||126 L/s|
|Net permissible duct leakage
(item a item d)
Duct surface area -
SA duct @ 1250 fpm
RA duct @ 1000 fpm
Permissible leakage rate
Duct No. / size
Duct test pressure
converted to (at 250 Pa)
5 Nos./5ft x 2ft.
2.36 in. wg
5 Nos./1.5m x 0.6m
= 0.121 L/s m2
|Duct pressure class selected||Class C- DW
Class 3- SMACNA
As far as DW is concerned, the leakage rate for Class C is only 0.109 L/s per m2; hence, it will suffice. In any case, DW has a higher duct pressure class (Class D) also.
The rig consists essentially of a fan (centrifugal), fan motor, flow measuring arrangement, flow adjusting arrangement, a manometer or similar pressure indicating device (for displaying the test pressure). See Figure 5.
Typically, the fan duty required would be about 200- 300 cfm against a static pressure of about 6" to 8" wg. The flow rate thru the test duct work is varied by operating the bleed (control) valve.
An orifice plate assembly together with a manometer can be provided for flow measurements. Air flow results in a pressure drop across the orifice plate. This pressure drop is measured by a manometer.
The Orifice Plate has to be made to precise tolerances – to the applicable ASME Standard. The orifice is sized to yield a pressure drop that is large enough – even at the lowest stipulated flow rate – to be read easily under site conditions. (If the pressure drop is too large, the static pressure for which the fan has to be selected becomes correspondingly large – this is why the fan static pressure required turns out to be some what high – at 6” to 8” wg).
Click to view the clear picture
The orifice plate needs to be calibrated in laboratories equipped with instruments of appropriate accuracy levels; also, their calibration should be traceable. It is good practice to calibrate (the orifice plate) with more than one instrument. The calibration results in a pressuredrop vs flow rate curve which can be used to provide a table from which flow rates can be read off pressure drop measurements.
When an orifice plate assembly is used, a pressure drop as high as about 10 mm can be obtained for flows as small as 10 cfm. Such pressure drops can be read easily on manometers in the field – sometimes even U-tube manometers will do – free from hassles of level adjustments. Sophisticated instruments, it may be noted, are required only for calibration, and instruments commonly used by contractors will suffice for field measurements with the rig. The U-tube manometers can, if required, be put together in a matter of few minutes or an hour or two, at the most, in the field.
The fan-motor unit, the bleed control valve and the manometer can be assembled together as a portable unit, while it is convenient to mount the Orifice Plate Assembly close to the ductwork under test.
Test rigs as described above have been fabricated in our country and used successfully during the past 5 years. They have been applied for duct work leakage in the field as well as for AHU leakage measurements. See Photo 1(in AHU manufacturing plant). They can be built to meet specific requirements. In Photo 2 a "Low Velocity Air Leakage Tester" as manufactured by Air Flow Developments Ltd., England is shown.
A typical hook-up of the rig at site can be seen in Figure 7, while Figure 8 furnishes information about orifice plate assembly, orifice sizes, and location of vena contracta taps.
Once the leakage testing rig is in place, the following test procedure may be adopted:
The likely leakage sites are shown in Figure 2 in Part 1. Leaks may be located:
For rectifying leaks:
This article attempts to heighten the awareness of ductwork air leakage; besides it covers duct construction standards, permissible leakages for various ductwork classes and interprets the two well-known standards – SMACNA and DW, which are widely in use all over the world.
A procedure for establishing targets for total system leaks and ductwork leaks (which form a part of system leaks) and a method for planning and staying within system leakage flow rates so established, has been indicated. A worked example illustrates the procedures.
An understanding of the subject of ductwork leakage goes a long way towards aiming and securing superior quality of ductwork and achieving better performance. Considering, the poor standards of workmanship and the generally lax approach to quality of workmanship in our country, it is advisable to over-emphasize the importance of leak testing ductwork rather than looking for opportunities to waive testing requirements on some ground or other.