It should be noted that the airflow requirement was calculated to be 10 CFM and the fan will provide 11.5 CFM of airflow. The operating point highlighted by the red circle in figure 4 indicates the pressure and airflow for the system with the selected fan. Figure 4: System requirements and fan performance Overlaying the system requirements from figure 2 on the dc fan characteristics from figure 3 produces the graph in figure 4. Figure 3: CUI Devices' CFM-6025V-131-167 performance graph The datasheet for the dc fan specifies 16 CFM of airflow with no back pressure, static pressure of 0.1 inH 2O with no airflow, and also provides the graph in figure 3. Figure 2: System requirements, airflow versus static pressureīased upon the curves in figure 2, CUI Devices' CFM-6025V-131-167 dc axial fan has been selected for the project. The dashed line denotes the minimum airflow required for the product (greater airflow is also acceptable), while the orange curve represents the relationship between pressure and airflow for the mechanical design of the product. The mechanical design of the product has been characterized to produce the airflow versus pressure graph shown in figure 2. Take for example a product whose airflow requirements have been calculated to be 10 CFM or greater based upon the heat to be removed and the air temperature limits. Datasheets from fan manufactures will provide a value of airflow rate with no back pressure, a value of maximum pressure with no airflow rate, and a curve of the airflow versus the pressure available from the fan. Figure 1: Characterizing and plotting airflow versus pressure Achieving Required Airflow and Pressureīased upon concepts from the previous two sections, an airflow rate and air pressure must be created by the fan (or fans) to provide the required cooling. Many CAD products are available to calculate the air pressure and airflow characteristics of a design, while anemometers and manometers can be used to measure the air speed and pressure characteristics once a design is completed. Calculating the required pressure will be a separate task for each unique product and cannot be simplified in a manner similar to the flow rate calculations. Fans should be selected to produce sufficient pressure to force the required volume of air through the product to enable the desired cooling. The path of the airflow through the product to be cooled will create a resistance to the flow of the air. It is also necessary to know the pressure at which the airflow is to be delivered by the fan. The above equations state the airflow rate required to cool a product. ΔT C = the temperature the air will rise when absorbing the heat to be dissipated in ☌ Air Pressure ΔT F = the temperature the air will rise when absorbing the heat to be dissipated in ☏ Q m = airflow in Cubic Meters per Minute (CMM) Q f = airflow in Cubic Feet per Minute (CFM) Using these values for density and specific heat the above equation simplifies to: The density of dry air at sea level at 68☏ (20☌) is 0.075 lbs/ft 3 (1.20 kg/m 3) and the specific heat of dry air is 0.24 Btu/lb ☏ (1 kJ/kg ☌). K = a constant value, dependent upon the units used in the other parameters ΔT = the temperature the air will rise when absorbing the heat to be dissipated Thus, the equation can be arranged as shown below.Īirflow = Power/(Density * Specific Heat * Temperature Rise)Īs discussed in our previous blog post, this equation is commonly written as: In most applications the excess power (inefficiency of the system) is known and the airflow (volume/time) is unknown. Power = (Volume/Time) * Density * Specific Heat * Temperature Rise Substituting the second equation into the first relates the energy dissipated to the volume of air involved.Įnergy = (Volume * Density) * Specific Heat * Temperature Riseĭividing both sides of the equation by time produces the following form of the equation. The mass of the moving air can be calculated from the volume of air being moved and the density of the moving air. The amount of energy transferred is dependent upon the mass of the moving air, the specific heat of the moving air, and the temperature change imparted to the moving air.Įnergy = Mass * Specific Heat * Temperature Rise Moving air is effective in cooling objects by absorbing heat from the object and then transferring that heat elsewhere to be dissipated. Download a Free Comprehensive Thermal Management eBook Important Airflow Parametersīefore a fan can be specified for a particular system, there are a few parameters that are important to understand regarding airflow and heat transfer.
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