How to select the right fan?

1. Fan Selection
There is not a single method for fan selection that is suitable for all applications. It is the careful balance between different and often contradicting considerations that leads to an optimum solution. Below are a few common design considerations when selecting a fan.

► Minimize the power consumption.

► Minimize the acoustic noise.

► Maximize the service life and reliability.

► Minimize the total life cycle costs.

1-1. Step 1 – Estimation of overall cooling air flow requirement
The prime function of a cooling fan is moving sufficient air to transport unwanted heat energy to the ambience. The amount of energy that air can carry away, i.e. the air cooling capacity, is a function of the air temperature rise between the inlet and exhaust of a system.

Large air temperature rise gives high cooling capacity. However, this also means the system and the individual heat dissipating components would be operating at a higher temperature, especially near the system exhaust.

High operating temperature could affect life and performance of a system. Design engineers have to first decide the desired operating conditions for the system concerned and the resultant permissible air temperature rise. Once these two factors are decided, the cooling air flow requirement can then be determined.

Using the concept of conversion of energy, the transport of heat to moving air is given by

Where

H

= overall heat dissipation, W

= mass flow of the moving air, kg/s

Cp

= air specific heat capacity, J/kg.K (for dry air at 20°C, Cp≈1000J/kg.K)

△T

= cooling air temperature rise between inlet and exhaust, K

Rearranging the equation for fan selection in CFM

Where Q = cooling air flow requirement, CFM

1-2. Step 2 – System impedance estimation
After the air flow requirement is determined, the design engineers have to estimate the resistance cooling air flow needs to overcome inside the system. This is normally achieved by computational simulation, experiment or even simple analytical calculation. The result (system characteristic) is typically presented as system impedance curves as shown in Figure 1 below.

Figure 1 – System characteristic in the form of system impedance curves.

1-3. Step 3 – Selecting a suitable fan
The fan air performance curves given in this article show the boundary of the possible flow delivery at different static pressure. By overlaying the system impedance curve with the air performance data of a selected fan, the fan operating point can be deduced from the intersect of the two lines as shown in Figure 2. In general, a fan is suitable if the intersecting point lies on the rising portion of the fan air performance curve.

If the system impedance characteristics discussed in Step 2 is not available, it is recommended to select a fan with the maximum flow delivery 20 to 30% higher than the cooling air flow requirement discussed in Step 1.

Figure 2 – Matching the fan air performance to system impedance in order to determine the operating point (i.e. intersect of the two curves)

2. Fan Installation

Fans can be installed either at the inlet side or the outlet side of a passive thermal component or a thermal system. Under ideal conditions, the overall air delivery follows the system characteristic curve and there is no difference wherever the fan is installed.

However, in practice, there are advantages and disadvantages in each of the approaches and user can decide what is best for their application.

2-1. Advantages of installing fans at the inlet side

► Improve overall cooling capacity due to unsteady flow field downstream of fan.

► Longer fan life because of the lower inlet air temperature.

► The whole system is pressurized and this helps to stop inhaling dust and dirt from unwanted cracks and openings.

2-2. Advantages of installing fan at the exhaust side

► Reduce the influence of low velocity, re-circulating zone immediately downstream of fan.

► Better air flow control by careful selection of inlet opening locations.

► Lower fan acoustics as exhaust openings are typically positioned away from users.

► In general, distance of at least one fan depth should be allowed between fan and its adjacent components to minimize interference and undesirable losses.

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