Views: 0 Author: Site Editor Publish Time: 2023-04-23 Origin: Site
Centrifugal fans use the centrifugal force provided by the rotation of the impeller to increase the kinetic energy of the air/gas.When the impeller rotates, the gas particles near the impeller are thrown off by the impeller and enter the fan casing.Therefore, the kinetic energy of the gas is measured as pressure due to the system resistance provided by the casing and tubing.The gas is then directed to an outlet through an outlet conduit.After the gas is thrown out,the gas pressure in the middle area of the impeller decreases.Gas rushes in from the eye of the impeller to normalize it.This cycle is repeated so that the gas can be delivered continuously.
The characteristic that distinguishes the centrifugal fan from the blower is the pressure ratio it can achieve.In general, blowers can produce higher pressure ratios.According to the American Society of Mechanical Engineers (ASME), the ratio the ratio of discharge pressure to suction pressure is used to define fans, blowers, and compressors.The fan ratio is as high as 1.11, the blower ratio is from 1.11 to 1.20, and the compressor ratio exceeds 1.20.Blowers and compressors are generally much more robust in construction than fans due to the higher pressures involved.
Performance
Ratings in centrifugal fan performance tables and curves are based on standard air SCFM.Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds per cubic foot (1.2 kg/m3), a sea-level air pressure of 29.92 inches ).Centrifugal fans selected to operate in conditions other than standard air require both static pressure and power adjustments.
At standard altitude (sea level) and above standard temperature, air is less dense than standard.Air density corrections must account for centrifugal fans specified for continuous operation at higher temperatures.Centrifugal fans remove a constant volume of air in a given system regardless of air density.When specifying a centrifugal fan for a given CFM and static pressure under non-standard conditions, an air density correction factor must be applied to select the proper size fan to meet the new conditions.Since 200 °F (93 °C) air is only 80% as heavy as 70 °F (21 °C) air, centrifugal fans create less pressure and require less power.To obtain the actual pressure required at 200 °F (93 °C), the designer must multiply the pressure at standard conditions by an air density correction factor of 1.25 (ie 1.0/0.8) for proper system operation.To obtain actual power at 200 °F (93 °C), the designer must divide the power at standard conditions by the air density correction factor.
Air Movement and Control Association (AMCA)
The centrifugal fan performance tables provide fan speed and power requirements and static pressure at standard air densities.When centrifugal fan performance is not at standard conditions,the performance must be converted to standard conditions.Centrifugal fans rated by the Air Movement and Control Association (AMCA) are tested in a laboratory in a test setup that simulates a typical installation for that type of fan.Typically, they are tested and rated as one of four standard installation types specified in AMCA Standard 210.AMCA Standard 210 defines a uniform method for laboratory testing of enclosed fans to determine airflow rate, pressure, power, and efficiency at a given rotational speed.The purpose of AMCA Standard 210 is to define the exact procedures and conditions for fan testing so that ratings provided by different manufacturers are based on the same basis and can be compared.For this reason, fans must be rated in a standardized.
