Radiator System Resistance

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Determing how much air gets through a radiator – Joe

SUMMARY: Expect about half the fan’s rated free flow through a radiator due to its resistance.

After I wrote about Measuring Airflow Resistance, I received requests to take a look at air flow resistance through radiators. Not a bad idea, so I did some testing on a few radiators to find out how much airflow does get through.

For this test, I tested three radiators:

Rads
  • BE Cooling – ¾” core, two loops
  • Black Ice – ¾” core, 12 channels
  • Swiftech – 1 ¾” core, four loops – two deep

These may not be the most current models, but I picked them as representative of three different types.

The Test

The radiators were mounted on the flow chamber and system resistance was measured. As an example of what a 120 mm fan would do when attached to these radiators, I used a 120 mm Nidec Beta V TA450DC, Model A34346-58, rated at 92 cfm @ 2550 rpm, and measured it and its flow through the radiators:

Rad Flow

As the radiators’ system resistance curves show, it takes a LOT of pressure to get a LOT of airflow through these radiators. Mounting a 92 cfm fan on a radiator does not mean you’ll get anywhere near 92 cfm through it – more like 50-60%.

To determine what to expect from a particular fan with these radiators, you plot the fan’s PQ curve onto the system resistance curves – where it intersects tells you how many cfms will get through. In this case, the Nidec was tested on the airflow chamber and its PQ curve was plotted (note: This data is also available from the manufacturer) – this is the black dashed line in the chart above.

As a final test, the Nidec was mounted on each radiator and its flow determined through each, with the following results:

  • BE Cooling – 53 cfm
  • Black Ice – 63 cfm
  • Swiftech – 45 cfm

The Swiftech and Black Ice are about as expected, but wait a sec: Based on the graph, the BE Cooling radiator should show MORE airflow than the others (less system resistance) – so how come it tested LESS than the Black Ice?

When I first saw this, I thought maybe there was a slip up in transcribing results. Back the next day to re-test and make sure what was what. No problems there – so what gives?

Then it hit me – the Swiftech and Black Ice radiators use a plenum and the BE Cooling does not – the fan was mounted right on top of its fins. This caused interference with the fan blades, thereby lessening flow compared to the other two radiators.

CONCLUSIONS

One aspect of picking a radiator for a watercooling system is the amount of airflow through it. The “denser” the radiator, the less air gets through. In addition, a radiator with a plenum for the fan is more likely to yield better flow than directly mounting a fan on its fins.

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