Introduction and Summary

General Fan Performance Guide

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What this guide is:
A complementary article to the General Heat Transfer Guide
An introduction to the science behind fans
A means to estimate fan requirements
A guide to determine performance changes based on fan characteristics
An explanation of sound, sound generation, sound measurement, etc.
A summary of DC drives including the relationship between voltage, speed, and load.

What this guide is not:
A guide to specific fans
A complete in-depth analysis of all things associated with fans
An exposé that will convert you into an all-knowing guru of fans

The specific purpose of this guide is to take the science associated with fans and translate it into a meaningful document that will allow the reader to understand how fans work and how they apply to computers. It provides a brief summary of DC power and drives. It finishes with an introduction to the concepts of sound generation and measurement.

This guide will not discuss any equipment performance by manufacturer or model. The primary reason for this is that the available equipment changes too frequently. A secondary reason is a desire to avoid a subjective bias. The equipment available in the computer field changes daily. The equations that govern fans and sound do not.

I can’t possibly answer every single question concerning fans and how they relate to computers. I can only hope to answer most of them while providing the information you need to answer the rest.

A brief word about the author’s qualifications; I’m a mechanical engineer with a degree from UW-Madison. My last article about heat transfer condensed a lot of concepts from miscellaneous three credit engineering classes into a single document. This article condenses a lot of material not found in typical engineering classes. While some of the information contained herein is the domain of standard engineering classes, most falls under the category of on-the-job learning.

Quick Summary

  • No fan will push its rated volumetric flow rate when mounted in a real-world setting.
  • Cooling via air is not affected significantly by air density changes caused by elevation.
  • Cooling via air is significantly affected by local air temperature.
  • Cooling a heat sink requires sufficient air volume and air velocity
  • Fan flow is proportional to fan RPM provided external conditions remain constant (mounting, ambient airflow, etc.)
  • Fan peak static pressure is proportional to fan RPM^2
  • Fan power is proportional to fan RPM^3
  • Fan “stacking” is beneficial only when fan outlet pressure is significantly higher than ambient pressure or when significant flow resistance exists.
  • Neither of these conditions occurs commonly in computer cooling.
  • DC drive no-load speed is proportional to applied voltage.
  • dBA is a scale for measuring sound perceived by the human ear. For every 10 additional decibels, the sound seems twice as loud.
  • Total perceived sound from multiple fans may not be directly calculated, though it may be roughly estimated. In general, adding a fan of equal dBA rating will raise the total perceived sound by 3 about dBA.
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