One of the most important parameters in air-to-liquid cooling applications is airflow. To calculate the airflow required to cool a process, one must know the amount of heat to be dissipated and the change in air temperature. Airflow alone, however, is not sufficient in selecting a fan. The system impedance along the air path must also be calculated. Airflow and system impedance define the fan operating point necessary to cool a process. When selecting a fan for a heat exchanger, some other considerations include fans versus blowers, constant or variable flow, and AC or DC power.
As mentioned above, the first step in selecting a fan for a heat exchanger is approximating the airflow required to dissipate the heat generated in the process. The basic equation to estimate the required airflow is:
This is known as the heat capacity equation. By incorporating conversion factors and the specific heat and density of air at STP (Standard Temperature and Pressure) conditions, equation (1) is simplified to:
For equation (2), the units for volumetric flow rate are Cubic Feet per Minute (CFM), units for amount of heat transferred are Watts, and units for the change in temperature are degrees Fahrenheit. For example, to remove 145 W of heat from a small electronic cabinet to cool the air from 90°F (32°C) to 73°F (23°C), we need:
This is the airflow needed to dissipate the heat generated inside the cabinet at STP conditions. It should be noted that the mass of air, not its volume, determines cooling capacity.
Once the airflow is estimated, the system impedance or "airflow resistance" must be calculated or measured. System impedance is expressed in static pressure as a function of airflow. A typical system impedance curve is governed by equation (4):
This equation describes the relationship between the static pressure and the airflow required in a particular system.
Going back to the previous example, using equation 4 we calculate the static pressure through the cabinet to be 0.11 inches of water. In this application, there is up to 1 gpm of available facility water at 52°F (11°C). We need to select a fan that can provide at least 27 CFM of airflow at 0.11 inches of water and a heat exchanger that has the following performance when using water at 1 gpm or less:
ITD is the Initial Temperature Difference between the incoming hot air and cold water.
As Figure 1 below indicates, Lytron's 6105 copper tube-fin heat exchanger will provide 6.9 W/°C when paired with a fan that can provide at least 27 CFM and water flow greater than 0.25 gpm. If we select an Oriental Motor fan model MU1225S as shown in Figure 2, the pink vertical and horizontal lines show that at the required 0.11 inches of water this fan will provide 39 CFM, this is well over our requirement of 27 CFM. It's important to note, however, that fan accessories such as finger guards and filters can have an impact on fan performance, as shown by the difference between airflows at points A, B, and C in Figure 2. In a high impedance system such as our example, the effect on fan performance is minimal. With a low impedance system though, the impact to fan performance can be greater. If our cabinet had a clear airflow path, the system impedance would be relatively low and accessories would have a significant impact on fan performance, as shown by the airflow differences between points D, E, and F on Figure 1. Any significant drop in airflow from the required amount will impact the performance of the heat exchanger.
In addition to airflow and system impedance, other important factors must be considered when selecting a fan such as fan type, constant or variable flows, AC or DC power, air density, noise, life expectancy, EMI/RFI interference, and more. (Part 2 will cover the considerations of air density, audible noise, life expectancy, and EMI/RFI interference.)
After identifying the system impedance and the overall required airflow, the next consideration is generally what type of fan to use. The most prevalent types of fans are axial fans and blowers. An axial fan moves air in a direction parallel to the direction of the fan blade axis. They work well under low static pressure conditions and are preferred when low noise is a requirement. Blowers are centrifugal in design, with the air moving perpendicular to the axis of rotation. They are suitable for high-pressure applications, such as telecommunications and high-end servers, and operate at maximum efficiency near their peak static pressure.
Fans are often oversized because the sizing calculations were based on worst-case scenarios. For example, a fan may be sized based on the maximum heat dissipation required or based on an extremely high ambient temperature. In this case the extra performance provided by an oversized fan may only be needed in extreme situations. For many operating phases, a considerably lower airflow rate would be sufficient (e.g. for lower ambient temperatures or when devices are only operated with a partial load). "Intelligent" fans are an effective solution for such applications where adaptation to changing conditions is necessary. With this type of temperature-dependent fan control, the speed drops when the thermal load is low. Consequently, the noise emission and power requirement decrease.
Of course your systems available power may dictate the type of fan. If your application is power flexible, you should weigh the merits of a DC versus AC fan. A DC fan provides variable flow while an AC fan provides constant flow. In the past, DC fans were significantly more expensive than AC fans. Today, the price difference is almost non-existent and one can make decisions based more on performance and functionality. Even though AC fans are still widely used today, DC fans boast longer life, approximately 60% less power consumption, and lower levels of EMI (Electro Magnetic Interference) and RFI (Radio Frequency Interference).
When selecting a fan for a heat exchanger, it is important to look not only at cooling requirements and system impedance, but also at fan type, constant or variable flow, and AC or DC power operation. Part 2 of this article will discuss additional fan considerations such as life expectancy, air density, noise, and EMI/RFI interference.
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