In the past, our understanding of the fan, most of them stayed on the air volume, wind pressure, speed, noise and other parameters, but after the recent period of study, we found that in fact, our previous cognition is still slightly superficial, relying solely on the maximum air volume and maximum wind pressure given by the manufacturer is actually unable to judge the heat dissipation ability of the fan, must be combined with the P-Q characteristic curve, in addition to the formula in the fan law to calculate the various parameters of the fan at different speeds, so as to judge how a fan will behave in actual use.
However, even if we understand the performance of the fan in actual use, this does not mean that the fan is necessarily suitable for our application scenario. We often face some very real problems, such as what kind of fans do we need for a 200W power CPU to meet the heat dissipation needs? Obviously, at this time, it is meaningless to look at the working parameters of the fan.
The amount of air required for heat dissipation can be calculated
First of all, we need to make it clear that among the so many parameters of the fan, only the air volume can determine the heat dissipation capacity, so only when the air volume actually provided by the fan is greater than the heat dissipation demand, we can say that the fan meets the use demand. So how do we determine how high an airflow fan the system needs? There are actually formulas that can be calculated here, but here we will first explain some parameters and conditions related to formulas.
- 1cal is equal to the heat required when water with a weight of 1 g and a temperature of 0 °C lifts 1 °C;
- 1cal is approximately equal to 4.2J;
- 1W of power works for 1 second with an energy of 1J;
- The constant pressure specific heat of air is 0.24 (cal/g°C)
- Air at a temperature of 25 °C at 1 standard atmospheric pressure, with a density equivalent to 1.185 Kg per cubic meter;
- CMM and CFM are what we often call air volume Q, the former is cubic meters per minute, the latter is cubic feet per minute;
- 1CMM is equivalent to 35.3CFM
After confirming that the above conditions are determined, let's look at the formula like this:
Heat (H) = specific heat (Cp) * Weight (W) * Temperature rise (△Tc)
The above formula is applied to our actual use scenario, heat actually refers to the heat that the air can take away, the specific heat and weight are naturally the specific heat and weight of the air, and the allowable temperature rise is the temperature difference between the air before entering the heat source and leaving the heat source, that is, the temperature difference between the air inlet and the outlet. If the air volume of the inlet air is large enough, it means that the more heat can be taken away by the air, and when the heat taken away by the air matches the heating rate of the heat source, it is equivalent to meeting the heat dissipation needs.
Next, we apply this formula to the above conditions, we can know how the heat dissipation efficiency of 25 ° C air can theoretically provide. The weight of air is the product of density and volume, and the volume of air is calculated by CMM or CFM, which is what we often call air volume, so the weight of air W = Q/60 * 1185 (Q takes CMM), which is the weight of air every second. After applying the above formulas and conditions and further simplifying them, we can get the following simplified formula (the specific process can be deduced by yourself).
Q = 1.8 * P / △Tc
In the formula, P is our heat source power, △Tc is the temperature difference, and Q is the calculated air volume demand. Based on this formula, we can calculate the amount of air that theoretically needs to be provided for heat dissipation in order to determine the heat source of power over a period of time.
However, there is a very easy error here, some students will apply △Tc directly to the temperature difference between CPU temperature and room temperature, for which you will calculate a very low air volume demand, but in fact, the △ Tc here is not the temperature difference between the two, but the temperature difference between the inlet and the outlet of the heat dissipation system, which can also be said to be the allowable heating range of the heat source.
Taking the common 240mm cold drain cold radiator as an example, when it is used in a heat source with a 200W power, if we require that the temperature difference between the cold exhaust air inlet and the air outlet does not exceed 10 °C, then the △Tc at this time should be calculated by taking a value of 10 °C, and the result is about 36CFM.
Is the reality and the calculation very different? Because only the effective air volume is calculated
However, the maximum air volume that the current 12cm fan can provide is basically far more than the number we calculated, and products above 60CFM abound, so why use a combination of 2 fans or even 3 fans? The actual calculated number is a theoretical value, which belongs to the effective air volume. In fact, when the fan is working, the effective air volume provided is lower than its maximum air volume, for example, there is a key parameter called the air rate in the fresh air system, that is, the ratio between the effective air volume and the maximum air volume, the greater the ratio, the higher the efficiency of the fresh air system, and the same is true for the heat dissipation system.
Here we have to talk about the role of wind pressure, we all know that the maximum wind pressure and maximum air volume of the fan can not appear at the same time, when the fan is applied to different occasions, it will show different wind pressure and different air volume, and the wind pressure and air volume here is the effective value.
In fact, there is such a saying in the industry, that is, "air volume is the result, wind pressure is the means", which can be understood in combination with the P-Q characteristic curve. As an example, when the Yajun M12 fan is applied to the cold exhaust, its actual wind pressure is at the level of 1.6mm H2O, and the actual air volume is around 35CFM, whether it is the actual wind pressure or the actual air volume will be lower than the maximum value marked, but this is the performance of this fan in practical applications.
Similarly, the maximum air volume of the owl's NF-F12 fan is only about 65CFM, and the maximum wind pressure is close to the level of 4mm H2O, which is applied to the cold drain, and the actual wind pressure that can be provided is actually very close to the Yajun M12, so the heat dissipation efficiency provided by the two fans at this time is very close, and the measured results are the same.
Therefore, even if we calculate the air volume required for the entire cooling system, it is not that we only need to look at the air volume to choose the fan is enough, we also have to consider whether the fan's wind pressure is enough to resist the overall impedance of the system, so that the fan can provide sufficient effective air volume, only when the effective air volume can meet the use needs, the fan can be regarded as able to meet the use needs.
So how much fan does a 200W CPU need for heat dissipation?
In fact, the answer to this question is not so easy to answer, the key is how much °C the temperature difference your cooling system can bring to the CPU, that is, what is the value of △Tc. In our actual experience, the value range of △Tc is generally about 8 °C to 10 °C, here we may wish to take 8 °C directly, the value of P depends on the power consumption of the CPU, 200W is calculated according to 200W, so according to the above formula, 200W CPU for heat dissipation, △Tc at about 8 °C, then the effective air volume that the fan needs to provide is not less than 45CFM.
And there is no fixed conversion formula between the effective air volume and the maximum air volume, the best practice is to combine the P-Q characteristic curve selection, the fast method is to choose the larger, generally speaking, 1.5 times will be more realistic, according to this calculation, the maximum air volume is not less than 68CFM Fan will be more appropriate.