Two kinds of fans are used in HVAC applications: centrifugal and axial. Centrifugal fans can generate higher pressures, such as those needed to overcome the high resistance of duct systems, while axial-flow fans can deliver higher volumes of air most efficiently. It’s important to use the proper fan in every application so that energy is not wasted.
The centrifugal fan is a wheel with blades around the circumference. When the wheel rotates, air flowing into the center of the wheel is directed to the periphery by a shroud. Air is moved and accelerated by centrifugal force, as it is forced outward. There are two types of blade arrangements: forward-curved and backward-curved. Forward-curved blades deliver a higher volume, at lower speeds, than a backward-curved fan.
Axial-flow fans are the familiar arrangement of blades rotating on a shaft. There are three types: propeller, tube-axial, and vane-axial. The propeller fan is a set of blades rotating within a sheet-metal casing to restrict “blow-back” from the output side of the device. The tube-axial fan is of the same arrangement, but the fan aperture is a long tube. This improves flow consistency and generates a steady air pressure at the fan discharge. It is also more efficient because there is less air leakage at the blade tips. The helical airflow output from a tube-axial fan is transformed into a straighter, laminar pattern with vanes in the tube: a vane-axial fan. This increases the pressure of the air output, though at the cost of slightly more motor horsepower to overcome the flow resistance across the vanes.
The most critical selection criterion for fans is the static pressure they can overcome. The propeller fan is best for low-pressure applications. The tube-axial operates best at higher pressures and the vane-axial is ideal for the highest-pressure applications.
Following are the abbreviations used in the typical listing of fan laws:
D = fan size
N = fan speed
^p= air density (rho)
Q = flow rate
P = pressure
W = power input
The fan laws describe the relationship between a principal fan and its operation under modified conditions, as indicated by these variables. The most commonly cited fan law states that the horsepower input to a fan varies as the cube of the speed—all other conditions being unchanged.
Hp2 = Hpl * (N2/N1)3
This means that an increase in the fan speed by 10 percent will require 33 percent more horsepower. This is why variable-speed motor controllers are so economical, as even a slight reduction in air- flow will have great energy savings. This implies that it’s crucial to carefully match the operating point between the motor and fan. Both the motor and fan usually have adjustable sheaves (for belt-driven equipment), which are directly proportional to the shaft rpms:
Fan rpm/motor rpm = motor sheave diameter/fan sheave diameter
If the equipment is not properly adjusted to supply the design airflow, then the motor can be overloaded, and much energy will be wasted (and the motor can burn out).
Another implication of this fan law is that the power input is directly proportional to the air density: when the air is heavier, the fan must work harder to maintain a given flow rate. A corollary to this is that the warmer the air, the less the density, and the lower the power required to drive the fan.
It may seem to be paradoxical that the fan horsepower input actually decreases as the system pressure increases. However, consider that the fan must move a smaller volume of air as the resistance to airflow increases the apparent pressure. The decrease in air volume more than offsets the pressure effects, in most instances, so less power input is needed. (Hence the intrinsic advantage of high-pressure HVAC systems.)
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Excerpt from Energy Conservation Projects.