The affinity laws are a set of rules to describe the interconnected relationships between speed, flow, power, and pressure seen with fans, turbines, and similar devices. These rules are critical for heating and cooling installers, who must select products appropriate to their needs and calibrate them correctly. They are also useful in the design of other systems where turbines, fans, and pumps may be used. These laws are published in texts for the industry and are typically taught and discussed in training.
One of the affinity laws is the simple stipulation that motor speed and flow are proportionate. As the motor slows, the flow of air or other materials through a turbine will also slow. To increase the flow, it is necessary to increase the speed. The amount of pressure is also subject to affinity laws; pressure will equal the speed of the motor, squared.
Power can be an important factor in design. The faster a motor goes, the more power it requires. The affinity laws discuss the fact that the power needed to run a fan or similar device is equal to the cube of the motor speed. Other topics covered by affinity laws are the diameter of the impeller and the effect it can have on flow, power, and other performance issues. These rules are consistent across many types of systems, reflecting the basic physics behind fan design.
Installers of systems subject to affinity laws need to think about the uses of the system and install the appropriate equipment and support infrastructure. A high power draw, for instance, must be accommodated with an adequate power source. The installer considers the maximum stress on the system and accounts for it in the design, as for example in the case of air conditioning in a very hot climate where users may turn the system to the maximum setting in a heat wave. Failure to consider the maximum stress could result in problems, like a breakdown of the system under high demand.
Affinity laws can also be important in research and development. Researchers work with air flow and pressure in research and experimentation to explore topics like aerodynamics. They need to design repeatable, quantifiable experiments so they can report results accurately. Part of this process includes fully describing the conditions of the experiment, such as a vortex formed in an experimental chamber with fans to look at how aircraft perform in windy conditions.
By Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.