Crossflow Fan Characteristics
When a fan is applied to an air moving requirement, its basic function is to supply some pre-determined quantity of air through the system. However, it must also overcome the inherent resistance of the system to flow, caused by fluid friction and turbulence. To achieve this, the fan increases the total energy content of the air by the mechanical action of the rotating impeller blades. This energy increase is of two types:
Kinetic energy: when any fluid is moving, it has kinetic energy. If the airstream is brought to rest by placing an obstruction in its path, such as a flat plate, a force is exerted on the plate. This force is a measure of the kinetic energy of the airstream, and when expressed as a pressure, it is termed Velocity pressure.
Potential energy: This is the energy in the airstream directly attributable to its pressure. The measure of this energy is the Static pressure.
The total energy increase in the airstream can be expressed as Total Pressure, equals the sum of Velocity and Static pressure. In the design of fan and air moving systems, the concept of Static pressure is most important. Pressures are expressed either as force per unit area – Pascals, N/m², or in linear measure of the water column supported by this pressure – mm H2O. The fan performance characteristics shown in this catalogue are for air at normal ambient temperature and for a 240 volt/50Hz electrical supply. During product development it is important to note that the speed of shaded pole motors can vary by ±10%.
Principles of Operation
A cylindrical vortex of spinning air is set up, with the axis of the cylinder parallel to the axis of the fan, and is stabilised by the vortex former in a position between the inlet and outlet a short distance from the inner edges of the impeller blades. This vortex couples with the incoming air and pulls it in at a speed many times faster that the actual speed of the blades. The moving blades meet this column of incoming air at a shock-free angle. With the help of the vortex, they swing it round to encounter the blades once more. Thus a fast, smooth laminar flow of air comes from the output duct at a very high velocity for a given motor speed.
Advantages of Crossflow Fans
Small size and, where necessary, a long shallow frontal area. High relative efficiency, especially for small pressure and placement. A long ‘throw’ of air for direct ventilation. Widely controllable characteristics. Very low load when throttled. Low speed. Extremely low noise.
The resistance imposed by a system against airflow is normally expressed as a pressure, and called system resistance. This is simply an energy loss, caused by friction and turbulence within the system, and is influenced by such things as inlet and outlet grille design, free space within the system etc. For any given piece of equipment there is a unique relationship between air flow and pressure drop, pressure being directly proportional to the square of the airflow. The relationship is termed the System Resistance (see fig. 1).
Whatever the choice of fan motor the operating point must lie somewhere on the system characteristic curve. As the operating point must also lie on the characteristic curve for the fan, the point of operation of the fan and system will lie at the intersection of these two curves (see fig. 2).
The effect of different fans on a given system can be easily evaluated and it should be particularly noted that it is impossible to vary the airflow through any system without there being a corresponding change in pressure drop.
Guide to Installation
Consort’s engineers will be pleased to assist you in your product design, as placement of the fan within your application is critical. If the fan is starved of air this may lead to a significant breakdown in fan performance (see fig. 3)