An important parameter to consider when specifying perforated tubing is “Open Area” or OA. OA has a big impact on appearance and governs how the tubing functions in some applications. Here we’ll explain what the term means and it’s influence on in-service performance.
All About the Ratio
Perforated tubes are formed from metal sheet or coil that’s been through a punch. The punch creates a pattern of holes. These can be of various shapes, sizes and spacing.
Open Area is the ratio of the total area of all the holes to the original area of sheet. For a simple example, picture a sheet 10” square. If 20 holes each of 1” in area are punched the total hole area is 20 sq in. This makes the OA 20 divided by 100 (10 x 10) or 20%.
In practice a 20% OA is towards the bottom end of what is produced. (Any lower and the material would look like solid sheet with a few random holes, rather than functional or decorative perforated tube.) Most perforated tubing has a higher OA, with 70% being about the upper limit. Above this the sheet tends to deform as metal is punched out.
You should note that OA says nothing about the shape, spacing or pattern of perforations. However, higher OA percentages need square, rectangular or hexagonal holes as these pack together closer than circles.
Options for Perforation Pattern
Most perforated tube is produced with circular holes. These are easier to punch and don’t leave potential weak points in the material. Typically each row of holes is offset from the one above to produce a “staggered” pattern. Any angle of “stagger” can be selected although 60° is common. It’s also perfectly possible to have no stagger at all, which results in a straight pattern.
For any given pattern there is a specific equation giving the OA percentage. (Our Product Specialists will be happy to help with these if needed.) In general terms though, a staggered pattern allows closer hole spacing and therefore a higher OA.
In some applications, particularly those where appearance takes center stage, square, rectangular or hexagonal holes may be preferred. Hexagons naturally nest in a staggered formation while squares tend to be used in a straight pattern.
One issue with square, rectangular and hexagonal holes is the sharp internal corners. These are potential points of cracking, particularly when the sheet is formed into tube or other shapes. A way around this is to form round-end slots.
Strength is another consideration in some applications. Rather obviously, removing material leaves the sheet lighter but also reduces stiffness and yield strength.
IPA Pattern Numbers
In an effort to simplify the specification and production of perforation patterns the Industrial Perforators Association (IPA) came up with a numbering system. Referred to as “IPA #,” each number defines a unique combination of hole size and spacing. Thus IPA #110 has round holes 3/32” diameter in a staggered pattern on 3/16” centers with an OA of 23%. Tables in the IPA handbook also give information on how the perforation pattern impacts strength.
The Influence of OA on Perforated Tube Applications
At the risk of oversimplifying, larger hole area, (higher OA,) means “things” can pass through more easily. While we often think of those “things” as fluids, they could also be solid, granular materials, sound or even light.
In fluid applications OA has a direct link to pressure loss. There are equations and graphs for calculating pressure drop as a function of OA. More generally though, just remember that lower OA equals more resistance to flow which means higher pressure drop. (If you need to get into details, our Product Specialists can help.)
Perforated tube is also used in dispensing granular or powdered material. Grain handling is another example. In these applications round-end slotted shapes often perform well, with OA determining distribution density. (A side benefit of using perforated tube is that hole size and shape acts as a filter, blocking the passage of larger material clumps.)
Another application for perforated tube is in sound attenuation. Here the tube is used to hold sound-absorbing material in place. For maximum attenuation the tube should be acoustically “transparent,” allowing sound through to the material rather than reflecting it back. This suggests a high OA is desirable, but the reality is a little more complex. In practice sound frequency also has an impact, so OA should be selected with a view to the frequencies that need attenuating. (Again, our specialists can help.)
Another good use for perforated material is as a shade against light. Light transmission is related directly to OA: higher OA means more light comes through. If the goal is to reduce light transmission a lower OA is desirable but if better visibility is needed go with a higher OA. If the shade is being used to create a specific pattern OA lets a designer calculate the initial intensity needed.
Perforated material is also used for shielding against microwaves. Similar to acoustics, perforations can either block or pass microwaves, depending on microwave wavelength and hole size and shape.
Calculate It Yourself, or Ask an Expert
When developing an application using perforated tube, give some thought to both perforation pattern and Open Area. Together these have a big effect on what passes through the perforated material, the speed of transmission, and what gets left behind. In general, a higher OA means greater transmission, but different applications have different needs. While everything can be calculated – from first principles if you want – an easier approach is to consult with a product specialist here at Perforated Tubes Incorporated.