Subject: Cooling vs filtration
By: Ray (IP: 86.128.78.*)
Written on: 23-03-2009 00:08

Long post, sorry!

The original system included an 80mm fan running at 2/3rds speed, with a medium filter.
Operating temperatures were good, heat-down* was acceptable but dust incursion through the filter was poor.

The first try at improving filtration was to increase the PPI** through a fine filter of the same 80mm size as previouly used. This caused operating temperatures to rise far too much.
Dust incursion through the finer material could not be checked because this unacceptable increase in operating temperatures (see table) required a quick fix. Heat-down was still acceptable.

Removing the filter completely confirmed that there was no fundamental system issue:- the problem stemmed from the new, finer air filter.
The decision was made to stay with the new filter material. So the problem was, how to reinstate the previous operating temperatures.
Some quick, basic, math plus practical considerations and the ready availability of parts seemed to indicate that a new, larger, 120mm filter to replace the 80mm filter might work.
Explanation:- for a given size only a given volume of air can flow through the filter. This reduces as a finer filter material is used. Increasing fan speed would help but noise increased unacceptably and \"confused***\" the CPU fan resulting in slightly worse operating temperatures.
Therefore, by reverting to the original 2/3rds fan speed and increasing the area of the filter, sufficient airflow might be possible whilst all other parameters would remain unchanged.
Note that a 120mm filter offers approx 2.25 times the filter area of an 80mm filter. There are other benefits but they are relatively minor, so not included.

After fitting the 120mm filter, temperatures have reverted to acceptable levels.
Long-term dust incursion will be checked in a month, as will tell-tale dust pattern on the filter itself (should be dispersed over the entire surface of the filter if it\'s all effective airflow).

Cost note: zero materials cost (parts iused came from the \"junk\" box) and zero extra operational power consumption.

* heat-down = the time taken for a system to return to normal temperature after a period of peak activity (say to 39C from 50C). This is a reliable measure since the impact on the entire system by a single change is shown when nothing else is happening that might influence the result.
** PPI = Pores per inch:- the more pores, the finer the filter. So a 10 PPI filter will have large airspaces and poor filtration whereas a 60 PPI filter will have tiny airspaces and relatively excellent filtration.
*** Explanation = if air is blown AT a fan it can either speed-up or slow down that fan:- it depends on the angle of the air as it blown onto that fan, amongst other things.

Table. Temperature is in Celcius for motherboard/CPU/power

a) Baseline (80mm medium filter, 2/3rds fan speed)
21 / 37 / 28

b) No filter (2/3rds fan sped)
21 / 36 / 28

c) 80mm Fine filter fitted (2/3rds fan speed)
22 / 44 / 35.5

d) 80mm Fine filter fitted (100% fan speed)
21 / 37.5 / 28.5

e) 120mm fine filter (2/3rds fan speed)
21 / 37.5 / 29

All comments welcome.
Cheers, Ray

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