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Heat Exchanger Fan Selection – Part 2 of 2

 heat exchanger fanFigure 1 – Heat Exchanger Fan

In air-to-liquid or liquid-to-air cooling applications, airflow is one of the most important parameters. Therefore, selecting the right fan is just as important as selecting the right heat exchanger. In Heat Exchanger Fan Selection Part 1, we calculated airflow and system impedance requirements and discussed selection factors such as AC or DC power, constant or variable flow, and the choice of a fan or blower. Part 2 of this article will discuss other important heat exchanger fan selection factors such as air density effects, noise, life expectancy, and EMI/EMC interference.

Air density effects

As mentioned in part 1 of this article, it is the mass of air that determines cooling capacity, not its volume. This is because every air molecule has mass and this mass has the ability to absorb or transfer heat. The more air molecules you have in a given volume, the more heat this mass will absorb or transfer. However, the mass of air in a given volume varies with altitude and temperature. When we speak of airflow, the definition of density is better expressed as a function of time with the equation:

equation(1)(1)

Where:

equation defined(a)

Fans provide a constant volume of airflow regardless of air density. In other words, a fan will supply 300 CFM whether the air temperature is 70°F at sea level or 250°F at 10,000 feet above sea level.

To illustrate this, let’s review an example. If we consider a volumetric flow rate of 300 CFM of dry air supplied by a fan, what would be the air mass flow rate at 70°F at sea level, at 250°F at sea level, and at 250°F at 10,000 feet above sea level?

Table 1 – Comparison of Air Densities & Mass Flow Rates at Various Temperatures & Elevations

Condition Density (lbs/ft3) Mass Flow Rate (lbs/hr)
Sea level, 70°F 0.075 1350
Sea level, 250°F 0.056 1008
10,000 ft, 250°F 0.038 684

From Table 1, we can see that by using equation (1) above, 70°F dry air at sea level weighs 34% more than 250°F dry air at sea level and 97% more than 250°F dry air at 10,000 feet of altitude. As a result, the system would need 402 CFM (1.34 x 300 CFM) of 250°F dry air at sea level in order to provide the same cooling capacity as 70°F dry air at sea level. The system would require 591 CFM (1.97 x 300 CFM) of 250°F dry air at 10,000 feet above sea level in order to provide the same cooling capacity as 70°F dry air at sea level. Note that the same temperature difference between the incoming hot liquid temperature and the cooling air temperature was assumed for this analysis.

Although humidity has a negligible effect on fan sizing, it can have an impact on the performance of a heat exchanger when a fan is used in a suction mode, downstream of the airflow. When warm humid air condenses, water droplets can accumulate on the heat exchanger fins, causing a decrease in performance and potentially corrosion. Condensation can also cause a short circuit in the fan.

Noise

Another important factor when selecting a fan is noise. Noise has no direct effect on fan performance, but it needs to be considered when selecting a fan for two important reasons. First, noise can effect work efficiency, or, in some extreme cases, can cause long-term hearing problems. Standards such as OSHA’s (U.S. Occupational Safety and Health Administration) Occupational Noise Exposure – 1910.95, limit exposure to various sound levels without hearing protection so that loss of hearing does not occur.

Secondly, noise can have a significant affect on the system’s operation and overall reliability. Noise can affect the function of some electronic devices, which may act as vibration absorbers and become fatigued by the vibration. Also, some operating environments such as laboratories contain noise sensitive instrumentation.

Fan design can minimize some broadband noises generated by air separation from the fan blade surface and trailing edge. Noise can be minimized by proper pitch angle and notched or serrated trailing blade edges.

Life expectancy

Fan life expectancy is defined as the period of time a fan can be operated continuously without losing significant rotational speed or emitting so much noise that it can no longer be used. Fans typically require a long life without failure to provide high system reliability. Bearing failure causes most fan failures. However, unlike motor or gear-head bearings that carry very large loads, bearings used on cooling fans typically have negligible loads. Therefore, fan life can be determined by the deterioration of the lubricant in the bearings. Since fans have low running and starting torque compared with motors used to drive heavy machinery, they will not rotate properly if the lubricant deteriorates. If this occurs, the starting voltage will increase and the fan may not start. Deterioration of lubricant also increases fan noise generated by the bearings.

The two most widely used methods for specifying fan life are the more commonly used L10 life method and MTBF (Mean Time Between Failure). The difference between the two is that L10 life specifically refers to the amount of time it takes for 10% of a group of fans to fail. MTBF for fans can be approximated as the time when 50% of fans have failed. Fan L10 lives are typically in the range of 60,000-70,000 hours under normal operating conditions of between -40°C and 50°C at 75% RH. MTBF life ranges are typically between 200,000–300,000 hours under the same conditions.

For long life expectancy, high quality ball bearing fans are considered the most reliable. When using an L10 method to compare life expectancy of sleeve bearing and ball bearing fans at temperatures ranging from 25°C-60°C, ball bearing fans outlast sleeve-bearing fans by 50% on average. In general, there is not much difference in life between sleeve and ball bearing fans when the temperatures are close to ambient. Traditionally, ball bearings provided a longer life than sleeve bearings in fan motors at high ambient conditions. Recent technical improvements in sintered sleeve bearings have led to reliability figures at least as good as ball bearings at significantly lower costs.

EMI & EMC interference

Another variable in fan selection is EMI (Electromagnetic Interference) and EMC (Electromagnetic Compatibility). EMI, by definition, is any electrical imposition that can interfere with the normal operation of equipment. There are two broad areas of EMI interference: conducted interference and radiated interference.

Conducted interference refers to any undesired signal conducted through power and signal lines. Radiated interference refers to any undesired signal that radiates from a source and may affect the normal operation of equipment. Conducted EMI is usually more of a problem than radiated EMI. In fact, when dealing with brushless DC fans, conducted EMI is normally the only concern.

Typically, AC induction motors running sine wave voltages do not present EMI concerns. There may, however, be small magnetic interference present close to the motor and its input leads. DC motors, either mechanically or electronically commutated, and AC motors powered by electronic controllers have EMI signatures. EMI is produced by the switching of the DC voltage, which is necessary to produce rotation of the magnetic fields in the motor.

EMC (Electromagnetic Compatibility) can best be described as the ability of equipment to operate without generating unwanted electromagnetic interference that can affect the operation of other electronic equipment, as well as its ability to not be negatively affected by unwanted interference generated elsewhere.

In summary, fan selection is a very important part of liquid-to-air and air-to-liquid cooling applications. It takes more than just airflow and static pressure calculations to size the proper fan for an application. As discussed in part 1 and 2 of this article, there are some other very important factors that must be considered by a designer when sizing a fan to provide a reliable system. These include air density effects, noise, life expectancy, and EMI/EMC interference.