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Cathar · 07-11-2004, 07:32 PM

Air-flow series hypothetical scenario:

Using BillA's ThermoChill radiator review here, and Panaflo fan specs here.

Using AA's figures above: The heat capacity of air is 1005 J/kgC and the density is 1.29 kg/m³. So 1m³/min (~35CFM) of air-flow has a thermal resistance of 21.6W/C (or 0.0463C/W)

Let's use the ThermoChill 120.1 performance to crunch some math, and let's use a 5C water-air inlet differential into the first radiator. Just rough eye-balling of data curves.

Let's consider the Panaflo L1A 12cm fan (~69CFM rated, 30dBA, 38mm thick fan)

For a single 120.1, the L1A would push around an estimated 0.75m³/min through the core. The radiator would have an estimated C/W of around 0.07.

If we now stuck two of the rads in air-flow series with the same fan, we'd see around about 0.5m³/min of air-flow through the rads.

At a 5C water-air differential, the first radiator would have a C/W of around 0.10. Now to work out the efficiency of the second radiator, we'll assume that our system is in equilibrium with a 5C air-water delta into the first rad. This means that the first radiator is dissipating 50W of heat into the air flowing through it. At 0.5m³/min, the air is therefore warming up by 50 / (21.6 * 0.5) =~ 4.6C as it flows through the first radiator. We'll ignore that the coolant will have cooled off somewhat after this as it flows into the second radiator.

So the second radiator now has a water-air differential of just 0.4C, or at a C/W of 0.10 with a 0.5m³/min air-flow rate, is providing just 4W of additional cooling effect after the air got warmed by the first radiator.

So overall with a 5C initial differential, the system is providing ~54W of cooling potential, or a total system C/W of around 0.09, which is definitely worse than just having a single radiator by itself.

Alternately, if we had the radiators in air-flow parallel, the same fan could push around 1.25m³/min through the pair of radiators, and offer a C/W of around 0.05 or so.

So with the L1A:

Single 12cm radiator: C/W =~ 0.07
Dual 12cm radiators in air-flow series: C/W =~ 0.09
Dual 12cm radiators in air-flow parallel: C/W =~ 0.05

The issue here being that at very low air-flow rates, these radiators are above 90% efficient in terms of water-air thermal transfer, and so the addition of a second radiator just chokes the air-flow, while being fed with air that has closely approached the coolant temperature. If you can push the air-flow rates up high enough where the radiator's thermal transfer efficiency drops away, then the second radiator starts to become more effective, but by that stage you do need fairly powerful and high pressure fans to push enough air-flow through the radiators for that to happen.

07-11-2004, 07:32 PM	#17
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Air-flow series hypothetical scenario: Using BillA's ThermoChill radiator review here, and Panaflo fan specs here. Using AA's figures above: The heat capacity of air is 1005 J/kgC and the density is 1.29 kg/m³. So 1m³/min (~35CFM) of air-flow has a thermal resistance of 21.6W/C (or 0.0463C/W) Let's use the ThermoChill 120.1 performance to crunch some math, and let's use a 5C water-air inlet differential into the first radiator. Just rough eye-balling of data curves. Let's consider the Panaflo L1A 12cm fan (~69CFM rated, 30dBA, 38mm thick fan) For a single 120.1, the L1A would push around an estimated 0.75m³/min through the core. The radiator would have an estimated C/W of around 0.07. If we now stuck two of the rads in air-flow series with the same fan, we'd see around about 0.5m³/min of air-flow through the rads. At a 5C water-air differential, the first radiator would have a C/W of around 0.10. Now to work out the efficiency of the second radiator, we'll assume that our system is in equilibrium with a 5C air-water delta into the first rad. This means that the first radiator is dissipating 50W of heat into the air flowing through it. At 0.5m³/min, the air is therefore warming up by 50 / (21.6 * 0.5) =~ 4.6C as it flows through the first radiator. We'll ignore that the coolant will have cooled off somewhat after this as it flows into the second radiator. So the second radiator now has a water-air differential of just 0.4C, or at a C/W of 0.10 with a 0.5m³/min air-flow rate, is providing just 4W of additional cooling effect after the air got warmed by the first radiator. So overall with a 5C initial differential, the system is providing ~54W of cooling potential, or a total system C/W of around 0.09, which is definitely worse than just having a single radiator by itself. Alternately, if we had the radiators in air-flow parallel, the same fan could push around 1.25m³/min through the pair of radiators, and offer a C/W of around 0.05 or so. So with the L1A: Single 12cm radiator: C/W =~ 0.07 Dual 12cm radiators in air-flow series: C/W =~ 0.09 Dual 12cm radiators in air-flow parallel: C/W =~ 0.05 The issue here being that at very low air-flow rates, these radiators are above 90% efficient in terms of water-air thermal transfer, and so the addition of a second radiator just chokes the air-flow, while being fed with air that has closely approached the coolant temperature. If you can push the air-flow rates up high enough where the radiator's thermal transfer efficiency drops away, then the second radiator starts to become more effective, but by that stage you do need fairly powerful and high pressure fans to push enough air-flow through the radiators for that to happen.