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Unread 09-26-2004, 07:56 PM   #13
ferdb
Cooling Neophyte
 
Join Date: Sep 2004
Location: texas
Posts: 68
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Clearly a fan rated for 40 CFM in free air will flow less than that when put in a real system that has flow resistance through the radiator and the rest of the air path in the case. I refered to actual airflow through the system, not the free air CFM rating of the fans. I'm not saying that you can calculate exact results based on CFM ratings of fans. I was trying to clarify how the thermal system responds to increasing airflow, and to how to calculate numbers for an ideal system. Of course the systems we build are not ideal systems because of the fan CFM/backpressure curves and the thermal transfer characteristics of the radiator. However the equations can give you some reality checks on systems you are planning, especially in systems designed for quiet operation, where air velocities are lower and the system more closely approaches an ideal system.
1) Given a system, if you double the radiator area, and double the number of fans you will double the airflow through the system and reduce the coolant temperature above the ambient air temperature to roughly half what it was.
2) Since Fan CFM is directly related to RPM on a first order approximation, if you double the fan RPM you will double the airflow and half the temperature differential. This increases the fan noise greatly
3) If you reduce the fan RPM to 1/2, you'll halve the airflow and double the temperature differential. You will reduce the fan noise greatly.
4) If you double the radiator area, double the number of fans, but reduce their RPM to half you will end up with roughly the same system temperatures but much reduce fan noise.
5) If your coolant temperature is only 2C above ambient air temperature, no matter how fast you run your fans or how much radiator area you add you will not be able to reduce the system temps by more than 2C.
6) if your coolant temperature is 10C above ambient, then doubling your fan speed will roughly reduce your system temps by 5C.
If you are intending to hook up an overclocked/overvolted AMD64 CPU,6800GT video card, and Northbridge chip that is dissipating 200W into BI pro radiator, a Panaflo L1A running at full rpm couldn't possibly pull better than 50 CFM through that radiator and thus your coolant will be at least 7 C above ambient air temp, if not more.
The equations and all these examples are only to help you make prudent design decisions. If your coolant temperature is close to ambient air temperature already, adding fans and bigger radiators is not going to improve the system significantly. If your fans are too noisy, doubling the radiator size will let you halve their rpm and thus the noise. If your coolant temperature is significantly above ambient air temperature then the airflow through your system is poor because of fans, restrictions, or sucking hot exhaust air back in. Fix accordingly.
Make use of the information as you see fit.
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