1.1 PQ Curve and Working Point
-PQ curve refers to the curve of the fan's air pressure and air volume, and the design of the system's cooling capacity is directly related to the curve.
- With the different resistance of the system, the fan working point along the PQ curve, low resistance system fan air volume will be larger, the system cooling ability; high resistance system air volume is small, the cooling effect is not ideal. Therefore, the design try to reduce the system resistance, so that the air volume is maximized.
- Usually, the system design of the fan working point designed in the air volume of 1/2 ~ 2/3. For high resistance systems can be designed at 1/3 to 1/2 of the airflow.
1.2 Noise indicators
- Noise is divided into sound pressure level and sound power level, the fan is generally given as a noise parameter indicators of the full-speed state of the sound pressure level indicators,
test method for the distance from the fan inlet 1m at the measurement.
- Noise main standards: ETS 300 753, GR487, GR63,
ETS 300 753 indoor products at 23 ℃ conditions, outdoor products:
? Day 4.1: 40°C, 4.1E: 45°C;
? 3.4: 15°C, 3.5: 25°C at night;
NEBS? GR63 indoor product tested at 27C ambient;
GR487 outdoor product tested at 27C ambient;
-Relative to the human senses:
0 ~ 20 dBA very weak
20 ~ 40 dBA weak
40 ~ 60 dBA moderate60 ~ 80 dBA loud
80 ~ 100 dBA very loud
100 ~ 140 dBA deafening
1.3 Multiple Fan Noise Superposition Calculation
The noise value given in the fan specification is the parameter index at full speed. However, in practice, the fan is generally in the speed state, the noise and speed of the relationship between the formula:
spec1-spec2 = 50lg (rpm1/rpm2)
According to the noise of a single fan at room temperature, to assess the total noise of the system, the formula is:
Ln = L1 + 10lg n
L1 is 1 L1 is the noise of 1 fan
Ln is the noise of n fans
n is the number of fans
2, air volume calculation
2.1 The total required air volume Q:
Q = C * m * △T?
V = Q/(C * ρ * △T) unit: m3 / s
where: Q: heat dissipation; △T: the permissible temperature rise; C: specific heat of air , V: volume flow rate
2.2 Simplified calculation of the empirical formula:
? V=1.76*Q/△T Unit: CFM
? General: △T take 8 - 15 ℃.
3. Heat Dissipation Methods
3.1 There are two ways to dissipate heat in the system:
Blowing method:
Advantages: high efficiency of heat dissipation, the fan is located in the air inlet, high reliability;
Disadvantages: the flow field is not uniformly distributed, and sometimes there will be a dead zone of heat dissipation;
Extracting air method:
Advantages: Even flow field;
Disadvantages: low heat dissipation efficiency, the fan is located at the air outlet, low reliability
3.2 Applicable Scenarios:
Blowing method is suitable for the system with high localized power consumption;
Extracting air method is suitable for the system with more uniform internal power consumption.
4, fan selection
4.1 Determination of the working point:
According to the total air volume of the system's cooling requirements and the number of fans planned to determine the air volume of a single fan working point Q1;
4.2 Ensure that the fan's air volume working point is located in the 1/2-2/3 of the fan PQ curve:
That is, to determine the selected fan air volume Specifications Q = Q1 / (1/2) or Q1 / (2/3);
4.3 Noise:
At this stage of the consideration of fan selection is to take into account the noise requirements of the system design specifications;
4.4 Recommendations:
fan selection, it is recommended that the selection of PWM speed control function fan. Its purpose: to control noise, increase fan life, and extend the time of blocking the dust net.
5. Example Analysis
5.1 Cooling Requirements:?
Standard 19-inch 1U chassis.
Chassis width * height * depth 442mm * 43.6mm * 400mm.
The motherboard size is 410mm (width) × 220mm (depth), thermal power consumption 100W.
The number of power supply module 2, power 150W, power module conversion rate of 95%.
The thermal power consumption of the whole system has no centralized point and is relatively decentralized and uniform.
The fan supply voltage is 12V.
5.2 Calculation of air volume
The thermal power consumption of the whole machine: Q=100W+150W×(1-0.95)×2=115W
According to the calculation of the empirical formula: V=1.76×Q/△T, △T take 10 ℃
The whole machine air volume is: V=1.76×115/ 10=20.24CFM
5.3 Determine the heat dissipation
According to the whole system heat power consumption is not centralized, relatively decentralized and uniform, the whole machine heat dissipation using the way of extracting air.
5.4 Fan Selection
*Size: According to the layout size of the chassis motherboard and power supply, determine the fan to use five 40 * 40 * 10cm fan.
*Airflow: Based on the overall airflow of 20.24CFM, it is concluded that the airflow of each fan is 4.048CFM.
The system is designed to design the working point of the fan at 1/2 to 2/3 of the airflow
*Voltage: 12V
Based on the Fan Selection Handbook, the specific fan specifications are selected: AD0412UB-G70( T) or other fan models with similar air volume and pressure.
5.5 Noise Calculation
The noise of the selected fan is 29dB, according to the formula Ln=L1+10lg n
The total noise of the 5 fans is L5=29+10lg5=29+7=36dB
The noise of the 5 fans is L5=29+10lg5=29+7=36dB.