Part 4 of four-part series: Fan System Effects

A perforated plate can appear highly open and still create noticeable airflow resistance. The reason is that air does not move through the sheet as though it were passing through empty space; it must accelerate through many small openings, contract at the hole edges, and re-expand after it exits. Those repeated losses add up, so a plate with a high open-area percentage can still behave like a meaningful aerodynamic restriction.

Open Area Is Only Part of the Story

Open area tells you how much of the sheet has been removed, but it does not describe how the holes are shaped, how thick the plate is, or how the openings are arranged. Two plates with the same open area can have very different pressure drops if one uses many small holes and the other uses fewer larger ones. In practice, hole size, pattern, thickness, and edge geometry all influence resistance.

Small perforations are especially effective at creating distributed resistance because each hole acts like a tiny nozzle or orifice. That means the flow path is repeatedly forced to contract and expand, which increases turbulence and pressure loss. So even when the sheet looks “open,” the aerodynamic cost can still be significant.

Why Small Holes Create Pressure Drop

When air enters a small hole, it does not fill the full opening instantly. It contracts into a smaller effective jet, known as a vena contracta, and then expands after leaving the opening, which wastes energy as turbulence. The sharper the edge and the smaller the hole relative to the plate thickness, the stronger that effect tends to be.

Plate thickness matters because a thicker sheet makes each hole behave more like a short duct than a simple opening. That increases friction and can raise the effective resistance even if the open area remains unchanged. The result is that “high open area” does not guarantee “low pressure drop”.

What This Means for Fans

In fan systems, a perforated plate can change the operating point by adding resistance to the airflow path. If the fan must push through many small perforations, airflow may fall unless the fan has enough static pressure capability to overcome the added loss. That is why perforated guards, intake screens, and diffuser faces can have a real impact on measured performance.

Small perforations can also influence noise. The flow through many tiny openings creates numerous small jets, and those jets can generate broadband sound. In some applications that is desirable for airflow distribution or acoustics, but in others it becomes an unwanted penalty.

Uniformity and Resistance

Perforated plates are often used to improve airflow uniformity, not just to let air pass through. A plate with many small perforations can help spread flow more evenly across a surface, which is useful in diffusers, plenum faces, and equipment enclosures. The tradeoff is that the same geometry that improves uniformity can also increase pressure drop.

This is why design decisions should not be based on open area alone. A plate may have a high percentage of open space yet still be too restrictive if the holes are very small, the sheet is thick, or the pattern forces the air to repeatedly accelerate and merge. In other words, the airflow behavior depends on how the openings are made, not just how much material was removed.

Practical Design Points

When evaluating a perforated plate, ask:

• What is the required airflow?
• What pressure drop can the fan tolerate?
• How large are the holes relative to the plate thickness?
• Is the goal uniform distribution, low resistance, or acoustic control?
• Will the perforated plate be installed near a fan or in a location where added resistance matters?

For airflow focused applications, a larger hole diameter, a thinner sheet, and a well chosen pattern often reduce resistance. For distribution or sound control applications, smaller perforations may be appropriate, but the added pressure drop should be included in the fan selection. That way, the plate is treated as part of the system rather than as a decorative element.

Bottom Line

Small perforations can restrict airflow because each hole adds its own entrance, contraction, friction, and exit losses. Even when the open area is high, the plate may still create enough resistance to affect fan performance, increase noise, and shift the operating point. For engineers and contractors, the key is to evaluate hole size, plate thickness, hole pattern, and required pressure drop together and not open area by itself.

Four-Part Series: Fan System Effects

If you missed any part of our series, you can read them here:

Part 1: Fan System Effects: What and How
Part 2: Fan System Effect: Abrupt Change in Cross-Sectional Area
Part 3: Fan System Effect: Elbows & Other Obstructions