![]() ![]() For these altitudes, we need to recalculate our heat transfer equation using the appropriate density for the altitude the fan will operate in. For instance, at 5,000 feet, the density is. As the elevation increases, there are fewer molecules weighing down and the density of the air decreases. This value is created by all of the other molecules in the atmosphere weighing down on the molecules at sea level. Density Change at High AltitudeĪt sea level, the density of air is. See Appendix A1 for derivation of these equations. From the heat transfer equation, we calculate that 40 CFM is required. Suppose we want to cool a box at sea level that emits 340 watts of energy and we want to maintain a 15☌ temperature differential. sea level), the equation then becomes simplified: If the specific heat of air and the density are held constant (eg. Consequently, the reverse applies also.Īt sea level, the density remains fairly constant and we calculate the CFM using a heat transfer equation: If we pack more molecules of air into a given volume, increasing the density, we have more mass per volume and the ability to transfer heat increases. If we count the number molecules for a given volume we obtain the density of the air (mass/volume). Every molecule of air has a mass, and this mass has the ability to absorb or emit energy also known as transferring heat. But within that volume of gas, and in our case – air, we can evaluate the quality of the air and its ability to transfer heat. CFM is strictly a rate of volume measurement. It has no considerations as to what gas it pertains to nor the density of the gas. What is CFM?ĬFM is Cubic Feet per Minute a measurement of volume over time. When choosing the right fan or blower for an application, the choices are always specified as being able to supply CFM. ![]() The scope of this paper will be to explain how elevation effects the cooling performance of a fan or blower. The density is different at different elevations. But why do tests performed at elevations greater than 3,000 feet not match the manufacture's data? When comparing test results to the manufacturer's data, the test results show similar performance. For instance, most cities are no more than 2,500 feet in elevation. When testing cooling fans and blowers, the tests are typically performed in locations that are at or near sea level. ![]()
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