- Joined
- May 31, 2004
The following points summarize how air flows through and around fans. It's a summary from an article I read by NMB engineering:
0. One rear fan produces the best overall case cooling effect. Combine that with a fresh air duct to the CPU for added effect
1. Fan intake is smooth and laminar in flow
2. Fan exhaust is turbulant
3. Turbulant air has up to twice the heat dissipation ability as laminar flow
4. The turbulant air only extends a few centimeters from the exhaust face.
5. Up to 90% of air flow can be lost due to recirculation (churning) and care must be taken eliminate recycling hot air.
6. Baffles or ducts can be used to block recirculation of air.
7. Air flow will always take the path of least resistance.
8. An exhaust vent should always be at least 50% larger than the fan.
9. Dust accumulation can significantly block cooling so intake fans should have filters.
10. The case should have a slight positive pressure to prevent dust from entering components.
11. Components with critical cooling requirements should be placed close to air inlets. High temperature components should be placed near outlets.
12. Hot air flowing through a fan can decrease its life time by 2/3. It is better to have intake fans and avoid exhaust fans.
13. Avoid placing obstructions in high air velocity areas (like fan filters too close to the intake) this will reduce the noise generated and increase intake velocity. Filters should be mounted at least 2cm from the fan surface.
14. Use vibration isolators on all fans to eliminate mechanical transfer of noise to the case which then acts as a nice amplifier.
15. Use structural reniforcements to control enclosure resonant frequencies. Use sound absorbing material inside the case to reduce noise.
16. Mount a fan on an interior surface rather than an exterior one.
17. Obstructions near fan intakes create more noise than the same obstructions placed near fan exhaust (eliminate intake vents holes and replace with fan guards).
18. Two or more fans in parallel produce less noise than one fan producing the same CFM.
19. Two fans in series increase pressure but only increase CFM marginally in a low pressure environment (like a PC case).
20. Improving air flow within the enclosure can significantly improve cooling. (making sure there are no dead spots and use baffles to redirect air to places that need cooling). Tie up wires, place hot objects above cool ones.
21. Each fan added to a system also increases heat. Six, 5 watt fans produce 30 watts of heat that needs to be exhausted. Minimize fan use. In some cases the heat from an additional fan might be greater than its cooling effect.
22. Each additional fan increases noise, heat, cost and may disrupt air flow.
23. Experience shows that an empty enclose reduces air flow by 5 to 20%. A densely packed enclosure reduces air flow by 60% or more.
24. Finger guards produce small losses and small increases in noise so use them.
25. Fan adapters cause CFM loss. The more sloped the sides the greater the air flow loss. In theory a 10% slope produces minimal loss for an exhaust. But a 20% slope is fine for air intake. (ie: to go from 120mm to an 80mm HSF the duct must be 8" long for no air flow loss...at 5" you see a 50% loss).
0. One rear fan produces the best overall case cooling effect. Combine that with a fresh air duct to the CPU for added effect
1. Fan intake is smooth and laminar in flow
2. Fan exhaust is turbulant
3. Turbulant air has up to twice the heat dissipation ability as laminar flow
4. The turbulant air only extends a few centimeters from the exhaust face.
5. Up to 90% of air flow can be lost due to recirculation (churning) and care must be taken eliminate recycling hot air.
6. Baffles or ducts can be used to block recirculation of air.
7. Air flow will always take the path of least resistance.
8. An exhaust vent should always be at least 50% larger than the fan.
9. Dust accumulation can significantly block cooling so intake fans should have filters.
10. The case should have a slight positive pressure to prevent dust from entering components.
11. Components with critical cooling requirements should be placed close to air inlets. High temperature components should be placed near outlets.
12. Hot air flowing through a fan can decrease its life time by 2/3. It is better to have intake fans and avoid exhaust fans.
13. Avoid placing obstructions in high air velocity areas (like fan filters too close to the intake) this will reduce the noise generated and increase intake velocity. Filters should be mounted at least 2cm from the fan surface.
14. Use vibration isolators on all fans to eliminate mechanical transfer of noise to the case which then acts as a nice amplifier.
15. Use structural reniforcements to control enclosure resonant frequencies. Use sound absorbing material inside the case to reduce noise.
16. Mount a fan on an interior surface rather than an exterior one.
17. Obstructions near fan intakes create more noise than the same obstructions placed near fan exhaust (eliminate intake vents holes and replace with fan guards).
18. Two or more fans in parallel produce less noise than one fan producing the same CFM.
19. Two fans in series increase pressure but only increase CFM marginally in a low pressure environment (like a PC case).
20. Improving air flow within the enclosure can significantly improve cooling. (making sure there are no dead spots and use baffles to redirect air to places that need cooling). Tie up wires, place hot objects above cool ones.
21. Each fan added to a system also increases heat. Six, 5 watt fans produce 30 watts of heat that needs to be exhausted. Minimize fan use. In some cases the heat from an additional fan might be greater than its cooling effect.
22. Each additional fan increases noise, heat, cost and may disrupt air flow.
23. Experience shows that an empty enclose reduces air flow by 5 to 20%. A densely packed enclosure reduces air flow by 60% or more.
24. Finger guards produce small losses and small increases in noise so use them.
25. Fan adapters cause CFM loss. The more sloped the sides the greater the air flow loss. In theory a 10% slope produces minimal loss for an exhaust. But a 20% slope is fine for air intake. (ie: to go from 120mm to an 80mm HSF the duct must be 8" long for no air flow loss...at 5" you see a 50% loss).
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