Dirty Designing

It’s one thing to design a lighting layout from the comfort of your office. It’s another to design a lighting system that works properly in the intended environment.

If you use standard designs (and design standards) without adjusting for the specific application, you risk having a dysfunctional design. Three common mistakes:

  • Treating Computer Aided Design (CAD) as Computer-Dictated Design.
  • Treating NEC minimums as design maximums.
  • Not compensating for the “dirt load” of the application.

In power design, an example of that third mistake is not compensating for harmonics. In a clean application, the “normal” design provides the desired output without extra cost, whether we’re talking about power quality or air quality. But not all applications are clean. Just as dirty power can cause problems in a power distribution system, so can a dirty environment cause problems with a lighting system.

For harmonics, the solution often involves derating a transformer. But for lighting, the solution isn’t just using larger ballasts or more lights. In many cases, derating for dirt isn’t part of the solution.

Your first step in designing for dirtiness is to assess how dirty the environment is. The IES (Illuminating Engineering Society) Handbook provides a table that classifies the environment into five categories of dirtiness. It’s called the Dirt Depreciation Table. If this is an existing installation, you can easily determine where it fits in this table. If it’s a new application, you’ll need to look at installations that have similar processes.

Your next step is to determine the average uniform horizontal illumination for the luminaire you’re planning to install. If the number is insufficient you’ll have to use a different luminaire. That’s one reason you do the calculation at this stage.

To get that value, divide the lumens by the area in square feet. Keep in mind:

  • Lumens: the rated output of the lamp.
  • Area: The target area you’re trying to light.

Even if the number is sufficient, you might need to use a different luminaire. Why? Because you’re dealing with the initial lumens produced by that particular combination of lamp, fixture, lens, and shade. The output of the lamp will slowly deteriorate over time, and then undergo rapid deterioration near its end of life. The lens may discolor over time, or accumulate some dirt, oil, or other environmental contaminant that blocks some light.

This does not mean you did that math for nothing. It means you have a working number to start with. You can solve lens issues by purchasing a lens suitable for your particular environment and setting up the recommended cleaning schedule.

What about lamp deterioration? For each lamp, the manufacturer should publish charts that show lumen output versus hours of operation. Simply identify the point on the curve that coincides with your planned relamping interval and adjust the design as needed.

Dirty Designing, Part 2

Source: Mark Lamendola | Mindconnection

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