A heat exchanger is a device designed to efficiently transfer or "exchange" heat from one matter to another. When a fluid is used to transfer heat, the fluid could be a liquid, such as water or oil, or could be moving air. The most well known type of heat exchanger is a car radiator. In a radiator, a solution of water and ethylene glycol, also known as antifreeze, transfers heat from the engine to the radiator and then from the radiator to the ambient air flowing through it. This process helps to keep a car's engine from overheating. Similarly, Aavid’s heat exchangers are designed to remove excess heat from aircraft engines, optics, x-ray tubes, lasers, power supplies, military equipment, and many other types of equipment that require cooling beyond what air-cooled heat sinks can provide.
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:
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):
After identifying the system impedance and 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. For example, using lower airflow 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, noise emission and power requirement decrease.
Your system’s 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.
Learn more about our Axial Fans and our High Performance Double Width, Double Inlet (DWDI) Blowers.