[Test-Equipment] Choosing a quiet replacement fan

[Eric Lemmon]

I know that it should be obvious, but it is a fact that a fan cannot cool
anything.  A fan simply allows air to transfer heat from one object or area
to another, provided that there is a difference in temperature.  If a heat
sink was at 100 degrees F, and the ambient air was also at 100 degrees F,
adding one or more fans will make no difference!  Indeed, the fans
themselves add heat of their own, so- ironically- adding a fan will increase
the heat sink temperature.  I found that statement hard to swallow, until I
saw it proved in a thermodynamics class.

I have had several pieces of Hewlett-Packard test equipment that had fans
that were real screamers, and the noise really irritated me.  I ran some
tests on these fans, and was able to select replacements that ran much
slower but produced nearly the same air flow.  Keep in mind that certain
modes of equipment operation produce more heat than others, so if your
planned use does not include those modes, you can use less air flow.  Also
keep in mind that the OEM fan might be overkill, being selected to provide
sufficient airflow despite a nearly-clogged filter.

73, Eric Lemmon WB6FLY
 

-----Original Message-----
From: test-equipment-bounces at mailman.qth.net
[mailto:test-equipment-bounces at mailman.qth.net] On Behalf Of Dave C
Sent: Thursday, January 03, 2013 9:03 AM
To: Test equipment list; hp_agilent_equipment at yahoogroups.com
Subject: [Test-Equipment] Choosing a quiet replacement fan

I read occasionally that owners of T&M equipment tire of the noise a cooling
fan makes and asks for help to determine if/how a quieter replacement can be
found. There are many factors at play in such a decision, none as simple as
"it's quieter". 

This web site addresses choosing a fan for PC enclosures, and many of the
criteria are the same (heat being the most important one for us):

<http://www.silentpcreview.com/Fan_Test_System_2010>

A quote:

"Over the years, we have observed one clear phenomenon about fans and
cooling: The relationship between airflow and temperature invariably becomes
exponential at some point. Increase airflow from nothing to something, and
the drop in temperature can be dramatic. Keep increasing airflow, and the
cooling improvement becomes less and less significant, until at some point,
the temperature hardly drops at all. The trick, for the PC builder who seeks
both good cooling and low noise, is to find the point where any decrease in
airflow (or fan speed) effects a significant increase in temperature, while
only a very large airflow increase effects a significant temperature drop.
In other words, once you have enough airflow, additional airflow has very
little cooling effect, so all you're doing is increasing noise. "Enough
airflow" is not a constant, of course, it varies for each system of
components."

FYI,
Dave
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