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W. Tombling Ltd.
Wembley House
Dozens Bank
West Pinchbeck
Spalding
Lincolnshire
PE11 3ND
U.K.
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Air flow conversions.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).See installing air ducts for examples of practical duct system layouts.
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.
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.
|
Layflat (inches) |
Diameter (inches) |
Minimum (cfm) |
Recommended (cfm) |
Maximum (cfm) |
| 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.
| Perforation Diameter (inches) |
Air throw (feet) |
| 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.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.
Ducts longer than 120ft. (36.5m) - The holes will be spaced equidistantly.
When ordering the customer may specify the following alternative positions:
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.
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.
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