Cooling Fan: Air Flow - Static Pressure Characteristics
Pressure Loss
When you try to flow air into a certain channel, there will be blowing resistance in that channel in a direction that impedes the flow.
For example, comparing the case in the figure below, the upper device has mostly hollow spaces, so the blowing resistance is low, and there is not much decrease in air flow. However, as obstacles to the airflow increase, as seen in the device below, the blowing resistance increases, resulting in a decrease in air flow.
This is very similar to the flow of current, where in a circuit with low impedance, a large current flows, and in a circuit with high impedance, the current is small. This blowing resistance becomes the pressure energy that increases the static pressure inside the device, and is called pressure loss and is expressed by the following formula.
- V
- Flow speed [m/s]
- ρ
- Air density [kg/m3]
- ζ
- Duct-specific resistance coefficient
- A
- Cross-sectional area of duct [m2]
- Q
- Air flow [m3/s]
From the perspective of the cooling fan, this equation implies that in order to achieve a certain air flow Q, the cooling fan must be capable of generating a static pressure (denoted as P in equation 1) that can increase the pressure within the device.
Air Flow - Static Pressure Characteristics
The characteristics of a cooling fan are generally represented by the air flow - static pressure characteristics, which shows the relationship between the attempted air flow and the static pressure value. For example, let's say that at the required air flow Q1, the pressure loss of the device at that time is P1. In the case of the cooling fan characteristics shown in the next figure, the cooling fan has a static pressure value of P2, which is greater than the required static pressure P1. Therefore, it is possible to obtain a sufficient air flow at the required static pressure.
Because pressure loss is proportional to the square of air flow, doubling the air flow requires selecting a cooling fan that not only has twice the air flow but also has four times the static pressure simultaneously.
Measurement Method of Air Flow - Static Pressure Characteristics
There are two types of methods for measuring air flow - static pressure characteristics: the wind tunnel measurement method using a Pitot tube and the measurement method using a double chamber.
Among these, the double-chamber method has higher accuracy compared to the wind tunnel method and is widely used internationally. Therefore, our company adopts this method.
Furthermore, our measuring equipment is based on the AMCA Standard 210, a widely recognized standard for cooling fan testing methods worldwide. This method, as shown in the figure below, determines the air flow - static pressure characteristics of the tested cooling fan by measuring the differential pressure ΔP before and after the nozzle and the pressure Ps inside the chamber.
(AMCA: The Air Moving and Conditioning Association)
Our double-chamber system is the most versatile measuring equipment, suitable for use with or without intake and discharge pipes for the cooling fan.
In this method, the speed of the fluid flowing through the nozzle can be determined from the pressure difference between Chamber A and Chamber B. Therefore, the air flow Q can be expressed as the product of the speed V flowing through the nozzle, the nozzle area A, and the flow coefficient C. It can be represented as the following formula (2).
- A
- Cross sectional area of nozzle [m2]
- C
- Flow coefficient
-
\(\overline{V}\)
- Average flow speed of nozzle [m/sec]
- ρ
- Air density [kg/m3] (ρ=1.2 kg/m3 at 20 °C and 1 atmosphere)
- ΔP
- Differential pressure [Pa]
To measure the air flow - static pressure characteristics, the pressure in chamber A can be changed by controlling the pressure in chamber B using an auxiliary blower, and each point on the characteristic curve can be measured. Furthermore, by connecting it to a computer, we enable quick and highly accurate measurements in a short period.