Fan Considerations when Adding a Heat Exchanger

When designing a liquid cooling loop, there are several considerations relating to mating the fan and heat exchanger and installing the assembly into your system. This application note examines how these considerations, namely the use of a plenum, flow direction, and volumetric and mass flow rate, affect fan selection and integration.

The plenum distances the fan from the heat exchanger fins to ensure that the air is distributed across the entire face of the heat exchanger.

If the fan is placed too close to the heat exchanger, it reduces the effective size of the heat exchanger to approximately that of the fan (Figure 1). Since the air is now passing through a smaller area, the result is a higher air-side pressure drop and a reduced air flow. The combination of the smaller effective heat exchanger area and reduced air flow results in less heat transfer.

When placed the correct distance from the heat exchanger (see Figure 2), the fan moves the air across the entire fin area of the heat exchanger. Since the air flow is spread out over a larger area it results in a lower pressure drop, therefore greater air flow and better performance.

To obtain maximum performance from your heat exchanger, it is also important that the junctions between the fan, plenum and heat exchanger are airtight to avoid air leakage and ensure that all the air flows through the heat exchanger.

Most of Boyd’s standard heat exchangers feature an integral fan plate and plenum at the optimum distance for good air flow. This ensures the best performance when integrating the heat exchanger into your system.

Several conditions, including performance, fan life, and noise, impact the fan placement.

Performance

Provided that there are no external restrictions on air flow, a fan moves the same amount of air across a given resistance, regardless if it is pushing or pulling. This means that if you are simply attaching a fan to a heat exchanger in an open space, there is little performance difference whether you push or pull the air across the heat exchanger. If the fan is pushing the air across the heat exchanger, there may be a slight temperature rise in the air entering the heat exchanger and therefore decrease in performance due to heat generated by the fan. In most cases this is marginal.

However, where the air path is constrained like in a cabinet cooling application, one direction may be less restrictive than the other, resulting in a performance difference. Such situations need to be evaluated on a case-by-case basis.

Fan Life

Like all electrical devices, the motor of the fan will last longer when exposed to cooler temperatures. There can be as much as a 55% reduction in life when fans are operated in 60°C air as opposed to 20°C. If you are cooling the liquid, it is best to push the cool air across the heat exchanger so that the cooler air passes over the motor of the fan. Conversely, if you are cooling the air, fan life and performance will be improved if the fan draws the air across the heat exchanger.

Noise

Orienting the fan on the side of the heat exchanger furthest from the operator, exhausting the air away from the operator, provides the quietest operation. Other factors that can affect the noise level of the fan include overall airflow, blade size and design, and the speed at which the fan operates. Larger, slower moving fans are quieter than small, high-speed fans.

Volumetric Flow and Mass Flow

Cooling capacity depends on the mass flow rate. A fan provides a constant volume flow, not a constant mass flow. Mass flow and volume flow are related by the density of the air. Denser air affords a higher mass flow rate and therefore offers improved heat exchanger performance.

The density of air is determined by pressure and temperature. At a temperature of 59°F and a pressure of 14.7 psia, the density of air is 0.076 lb/ft3. Increasing the temperature or decreasing the pressure results in a lower density. When operating at elevated temperatures and altitudes, more volumetric flow is required to compensate for this lower density.

For example, our 6210 heat exchanger equipped with a Marin Fan has an air flow rate of 225 cfm. At 59°F and a pressure of 14.7 psa, this is equivalent to a mass flow rate of 17.1 lb/min. However, at an altitude of 20,000 ft, the mass flow rate is less than half of this value. Figure 3 shows how this mass flow rate varies with altitude and temperature.

Figure 3: Volumetric flow rate vs mass flow rate of our 6210 heat exchanger with a Marin Fan at various temperatures and altitudes