How do you read a fan curve?
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The performance of a fan can best be presented graphically. The accepted chart uses volumes as X axis and pressures, horsepower inputs, and efficiencies as the Y axis. The forms of the pressure and horsepower curves depend of the type of wheel used in the fan. They are characteristic of the particular type. Curves (see Fig. 1) are plotted for a given size fan at a given speed.
Static Pressure Curve
SP: Static pressure
This is the term used to identify the “push” needed to overcome the system’s resistance to airflow. The faster the air moves the greater the resistance to flow and the more energy is required to push or pull the air through the system. The unit of measure is “inches of water gauge” such as 30” SP which is equal to about 1 PSI.
CFM: Cubic Feet per Minute
The unit of measure for specifying the volume of flow. The static pressure curve provides the basis for all flow and pressure calculations. This curve is constructed by plotting a series of static pressure points versus specific flow rates at a given fan RPM. To find a fan’s point of operation, locate the required static pressure on the SP scale at the left of the curve. Then draw a horizontal line to the right, to the point of intersection with the SP curve. Next, draw a vertical line from that point to the CFM scale on the bottom to determine the fan’s flow capacity for that SP at the given speed. For example in (Fig. 1), the performance of this fan is 560 CFM and 35” SP at 3550 RPM. To arrive at a proper fan size may involve some trial and error, looking at several curves of various sizes and speeds. It should be noted that different fans can provide the same performance and that design conditions play a large part in the proper selection of a reliable solution.
Brake Horsepower Curve
BHP: Brake Horsepower or horsepower input required to operate a fan.
Once the CFM and SP have been determined, a BHP rating can be established. An accurate BHP rating is necessary to properly size the motor or to determine the operating efficiency of one fan as compared to another. Performance curves contain a BHP curve from which the BHP rating can be determined for specific capacities. To determine BHP at a specific point of operation, a horizontal line is drawn to the right from the point of intersection of the vertical CFM line and the BHP curve. So with the example above, 560 CFM in (Fig. 1) shows about a 6 BHP rating. You would select a 10 HP motor for this fan.
A good working knowledge of performance curves is necessary to understand the performance characteristics and capabilities of different fan equipment. Use of performance curves in the selection of fan types and sizing will assure stable and efficient operation as well as future flexibility. Static Efficiency Curve
SE: Static Efficiency
Static Efficiency is the ratio of the theoretical air power to the shaft power WITHOUT accounting for kinetic energy (velocity pressure). Static efficiency gives information on overall performance, that is, for the entire fan including rotor, casing, etc. This curve is usually presented along with the flow/pressure curve and power curves. A good working Knowledge of performance curves is necessary to understand the performance characteristics and capabilities of different fan equipment. Use of performance curves in the selection of fan types and sizing will assure stable and efficient operation as well as future flexibility.