Fans are turbomachines used to move vapours and gases. They are widely used in air-conditioning, refrigeration, dust extraction, drying and many other installations. Correct fan selection and assessment of proper operation require knowledge of the fan characteristic.
Fans are turbomachines used to move vapours and gases. They are widely used in air-conditioning, refrigeration, dust extraction, drying and many other installations. Correct fan selection for a given application, as well as assessment of whether the fan is working correctly, requires knowledge of its characteristic. Fans can be classified in many ways, taking into account mounting location, intended use, etc.; essentially, however, centrifugal and axial fans are distinguished. This classification follows from their design and is linked to the direction of fluid flow through the impeller. Centrifugal fans draw air in axially and discharge it radially, while axial fans draw in and discharge air in the axial direction. Axial fans are used when higher volume flow rates are required, whereas centrifugal fans are used when higher total pressure rise is needed.
Characteristic of the tested fanCharacteristic of the tested fan together with system load characteristics
Depending on the total pressure rise ΔPc obtained, fans can be divided into:
low-pressure – ΔPc < 720;
medium-pressure – 720 < ΔPc < 3600;
high-pressure – 3600 < ΔPc < 30000.
Other classification criteria for fans are also used, related to:
application: explosion-proof, roof, for pneumatic conveying;
number of impellers: single-stage, multi-stage;
mode of fluid intake: single-inlet, double-inlet
The basic parameters that characterise fan operation are: volume flow rate (capacity), total pressure rise, drive power demand, useful power and efficiency. Fan capacity is the volume flow rate of the fluid delivered by the unit, while total pressure rise is defined as the difference between total pressure at the outlet and total pressure at the inlet. The increase in total pressure results from the energy transferred to the fluid by the fan impeller. Useful power is calculated as the product of capacity and total pressure rise and a compressibility correction factor for the gas. Total fan efficiency is the ratio of useful power to shaft power.
Schematic of the test rig
Determining the fan characteristic
The fan characteristic describes the internal operating conditions of the fan and is the relationship between pressure and volume flow rate. For a given fan and each rotational speed n there is a curve that can be determined experimentally. Knowledge of such a characteristic is essential for correct fan selection for specific applications and for assessing correct operation. Example characteristics are shown in Figures 1 and 2. The test rig enables: the Central Refrigeration Centre performs fan testing in accordance with standards ANSI / AMCA 210 and ANSI / ASHRAE 51.
determination of the maximum air volume flow rate at “free delivery”, i.e. simulating air discharge from the unit into open space, as under real operating conditions;
determination of the maximum static pressure the test object can achieve when discharging air into a closed space;
determination of the operating characteristic of the test object (relationship between static pressure and air volume flow rate) – the number of characteristic points is set individually depending on the required accuracy.
Tested fan with a five-blade impellerTested five-blade impeller of a different shape
and blade pitch angleThe Central Refrigeration Centre offers its clients the following fan tests:
measurement of actual fan volume flow rate;
determination of fan pressure rise;
determination of fan operating characteristics;
full set of tests for product conformity assessment against the LVD directive;
electrical safety testing (B mark);
determination of useful power;
determination of total efficiency;
determination of air flow rate of fan–evaporator and fan–condenser assemblies;
determination of fan characteristics for different combinations of components and design parameters such as motors, impellers and blade pitch angles.
Additional tests:
electrical safety tests according to PN-EN 60335-2-80;
rotational speed measurement;
thermography (using a thermal imaging camera)
assessment of correct bearing operation, brush sparking and contactors, relays and fuses in electrical enclosures using an ultrasonic detector;
