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Designing an inflatable duct system
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Polythene air conditioning ducts
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designing a duct systemduct system design --------------

air flow conversion Air flow conversions.

m3/s = cfm x 0.00047
cfm = m3/s x 2119

static pressure conversion Static pressure conversions.

Pa = In. wg. x 0.004
In. wg. = Pa x 250

length conversions Length conversions.

m = ft. x 0.30479
ft. = m x 3.28084

- - -

Designing an inflatable duct system

Designing an Activair inflatable duct system is straight forward providing the following basic rules are adhered to:

Perforated ducts should be as short as possible- ideally 50 to 80 feet (15 - 24 meters) and never longer than 100 feet (30 meters).

Note: we do supply perforated ducts up to 150ft. BUT we cannot accept any responsibility for their performance.

Ducts should be straight - Avoid unnecessary elbows etc.

Keep the design simple - Use as few ‘tee-pieces’ , ‘y-pieces’, splitters etc. as possible

Use flow straighteners - after fans, elbows, etc.
See installing air ducts for examples of practical duct system layouts.

Maximum inflatable duct lengths

Plain ducts - To convey air from point to point - maximum 300ft. (91m)

Perforated ducts - To distribute air.
Optimum 50 to 80 feet (15 - 24 meters)

Maximum 100ft. (30.5m).

Ducts from 100ft (30.5m). up to 150ft. (45.7m), can be manufactured, however the lateral temperature control suffers. They are supplied strictly on the basis, that W. Tombling Ltd does not accept responsibility for their performance.

Air temperature inside inflatable duct systems

Maximum air temperature - 150 degrees Fahrenheit (65 degrees centigrade)
Minimum air temperature - 41 degrees Fahrenheit ( 5 degrees centigrade)

Selecting a fan - static pressure

Static pressure is a measure of a fan’s ability to push air against resistance. It is the most important characteristic of a fan used in inflatable duct systems. The fan should be chosen so that it produces the required air flow at the optimum static pressure.
Minimum static pressure - 75pa (0.3in. or 6mm water gauge)

Optimum static pressure - 125pa (0.5in. or 12mm water gauge)

Maximum static pressure - 150pa (0.6in. or 14mm water gauge)

Note. Fan manufacturers often quote the airflow of their fans at ‘free air’ meaning no resistance. Always ask for the airflow at 125pa.

Selecting a fan - size for single duct systems

The Activair inflatable duct system is compatible with most propeller, centrifugal, and axial fans. The fan size should be chosen as follows:

Propeller fan diameter = duct diameter x 0.8

Axial fan diameter = duct diameter

Centrifugal fan area = duct area

Where an exact match is not available fit a reducer to the fan outlet, and use the next smallest duct size. Make sure the airflow is within the range given in table 1. below

Selecting duct size - multiple duct systems

For systems with multiple ducts, use the following table to determine the duct size required.

Recommended duct sizes at 0.5" w.g. (table 1)
10 6.36 240 310 350
13 8.25 470 600 680
14 8.875 470 600 680
16 10.25 680 870 975
20 12.75 975 1250 1400
24 15.25 1640 2100 2350
27 17.12 2100 2700 3000
30 19.12 2600 3300 3700
36 22.87 3150 4600 5150

Fan speed control

The use of fan speed control should be avoided. Where possible replace large fans with two or more smaller fans each with their own ducts, and switch them in steps.

Where speed control is used, design the duct system for maximum speed (airflow). Assuming the duct is sized in accordance with table 1 above, a reduction in speed of upto 50% should produce acceptable results.

Note. we are unable to guarantee the performance of inflatable duct systems used with speed controllers.

Air throw from inflatable ducts

Air throw increases with the hole size - see table 2 below. By using different sized holes, ducts can be adapted to suit different applications:

Small holes - 0.5in. diameter - create gentle diffused air distribution, avoiding drafts

Large holes - 1 to 2in. diameter - strong jets of air, used in cooling systems.
Air throw from duct perforations at the recommended
airflow and 0.5" w.g. (table 2)
Perforation Diameter
Air throw
0.5 4
1 8
2 16

The size and number of holes in a duct depend on the airflow. As the duct diameter and airflow increases, so will the number and size of the holes. For example, 8in diameter ducts with 600CFM airflow typically have 0.5in. holes, while 30in. diameter ducts with 8600CFM airflow have 2in. holes.

When designing the ducts we will calculate the number and size of the holes most suitable for the duct diameter and airflow. If a customer specifies a particular hole size we will try to accommodate the request, however it may not be possible because of the above. In such cases we will notify the customer before proceeding with the order.

Hole position in the ducts

Activair inflatable ducts have two rows of holes running the length of the duct, when manufacturing the duct we will space the holes as follows:

Ducts shorter than 120ft. (36.5m) - The holes will be closer together at the air inlet than the far end of the duct.

Ducts longer than 120ft. (36.5m) - The holes will be spaced equidistantly.
The holes will be at 8 and 4 o’clock (same as 10 and 2 o’clock) on the circumference of the duct, which is suitable where the ducts are used as heating ducts or air conditioning ducts.

When ordering the customer may specify the following alternative positions:

Acceptable perforation positioning for Activair air distribution ducts

Air straighteners in Activair duct systems

Fitting an air straightener upstream of the fan and after elbows or ‘tee-pieces’, reduces air turbulence inside the duct, improving air distribution and reducing duct fatigue caused by flapping.

adding an air straightener to the air distribution system reduces flapping and fatigue of the ducts

A simple cross arrangement is sufficient for most applications. Its length should be twice the diameter of the duct.

Air filters for Activair duct systems

Activair duct systems do not require air filters. However they are used in dusty environments to provide clean air for occupants. They have no effect on the design of the inflatable ducting, but will place additional resistance to air flow on the fan which will need to be allowed for.

Sound data

Activair duct systems absorb sound, and are quieter than equivalent metal ductwork. However noise levels do increase with airflow. Where quiet operation is important use the minimum airflow’s given in table (1).

Slow start for fans

The lifespan of Activair duct systems can be increased by using soft starters with the fan motors. They reduce the initial strong surge of air, which prevents the ducts ‘popping’ and splitting.

Activair air duct system lifespan

In a properly designed and installed system Activair inflatable ducts will last for approximately 3 to 5 years.

Their lifespan is shortened by:
High air pressure - in excess of 0.5in. w.g.

Excessive air volume - greater than given table 1.

Extremes in air temperature - above or below the recommended levels

Inadequate duct support - see the installation guide for recommendations

Prolonged exposure to strong sunlight - Activair inflatable ducts are formulated to resist breakdown from ultra-violet rays but eventually they will harden and split.

Further information

Activair inflatable duct systems are a reliable and efficient method of distributing air. Thousands of systems are in use worldwide in commercial heating, ventilation and air conditioning systems.

See installing air ducts for further information or layflat ducting for a quotation.

Click on the icon, for a printer friendly data sheet. pdf version

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