Radiator Fan Stacking

Two fans better than one? – Joe

SUMMARY: Two fans are better than one, but with a noise penalty.

I received a number of emails asking to see if I could show what happens when another fan is added to a radiator – this gives two fans in a push-pull configuration (one in the front, one at the rear – a “fan sandwich”). For a radiator with a 120 mm fan profile, this can yield some benefits in increased airflow – question is how much?

Rads

To shed some light on this, I used the flow chamber to determine cfms using a 1 ½” thick Black Ice Xtreme Radiator, powered by Delta 120 mm fans (WFB1212HH, 0.68 amp, 2800 rpm @ 95 cfm). As a side note, I also determined cfms for the same design radiator in the ¾” thick version to show the size impact.

Test Results

Stack

Radiator
1 ½” Radiator, one 120 mm fan

1 ½” Radiator, two 120 mm fans

¾” Radiator, one 120 mm fan

RPM @ CFM
2071 @ 30

2055 @ 46

2114 @ 45

RPM @ CFM
2696 @ 42

2558 @ 58

2597 @ 60

Looks like fan stacking pays off – the second fan increases cfms through the radiator on average by about 50% – a significant change. It’s also interesting to note that doubling the depth of the radiator (¾” to 1 ½”) exacts a significant toll – about a 50% reduction in cfms.

Note that fan rpms are relatively unchanged – fan stacking does not increase rpms. Two fans in series reduce the system’s resistance – hence, less resistance to air flow. This then yields an increase in air flow, as it’s now easier to get air through the fins.

Also note that a 50% increase in air flow does not necessarily mean a 50% increase in a radiator’s ability to dissipate heat. Based on the Black Ice Xtreme Radiator testing done earlier, about a 20% performance increase seems reasonable.

CONCLUSIONS

The deeper the radiator, the more it takes to push air through it. An effective measure to increase flow would be to add a second fan, although noise will increase somewhat.

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