Fans are mechanical devices designed to move air, both as stand-alone units and as installed in heating, ventilating and air conditioning systems. They move the air in terms of both circulation (which is concerned with the velocity that air is moved in a confined space) and ventilation (which is concerned with the volume of air moved).
Fan performance and capability is dependant on basic fluid dynamics principles, so the selection of a fan requires the understanding of some of the terms and concepts involved, as well as the types of fans available. Proper fan selection will also depend on the design of the duct system it is integrated with and the specific application the overall system is intended to address.
The basic principles in fan and ventilation engineering depend on the fact that a fan’s mechanical efficiency stays constant through it’s range of operating speed RPM. Detailed formulas of basic fan engineering can be found in the current edition of the ASHRAE Guide, available from the American Society of Heating, Refrigeration and Air Conditioning Engineers.
Certain standards and codes pertain to fans in HVAC systems. Always consult the local building codes and standards. In addition, the following associations offer relevant information on requirements for fan ventilation and circulation systems.
Air Horsepower (AHP): the amount of work done in the movement of a particular volume or weight of air at a particular speed.
Actual Cubic Foot per Minute (ACFM): a unit of volumetric capacity commonly used by manufacturers of blowers and compressors, this is the actual gas delivery with reference to inlet conditions. Whereas the cubic foot per minute (CFM) is an unqualified term and should only be used in general, and never be accepted as a specific definition without explanation.
Since the volumetric capacity refers to the volume of air or other gas at the inlet to the unit, it is also often referred to as Inlet CFM (ICFM) – essentially the same as ACFM. All three terms (CFM, ACFM or ICFM) are used interchangeably to mean basically the same thing.
Area: measures the square footage of any plane surface
Area of Duct: equals the width times the height of a duct multiplied by the air velocity, expressed in cubic feet of air per minute flowing through a duct
Brake Horsepower: measurement of the work done by a fan’s electric motor, expressed in horsepower delivered to the fan shaft, plus the loss in power caused by pulleys and drive belts. Brake horsepower is always a higher number than air horsepower.
Cubic feet per Minute: volume of air moved by a fan per minute.
Density: weight of air in pounds per cubic foot. At 70 degrees F and 29.92 inches barometric pressure, density is 0.075 lb per cubic ft.
Mechanical Efficiency (ME): the ratio of work input to work output, in this case the ratio of air horsepower to brake horsepower, expressed in a decimal number or percentage.
Outlet Velocity: outlet velocity of a fan, expressed in feet per minute
Revolutions per Minute (RPM): the speed a fan motor turns at
Static Efficiency: a fan’s static efficiency is the mechanical efficiency multiplied by the ratio of static pressure to the total pressure
Static Pressure (SP): a fan’s static pressure is the total pressure minus the fan velocity pressure, measured in inches of water.
Tip Speed (TS): the circumference of the wheel multiplied by the RPM. Also known as the peripheral velocity of wheel. For quiet fan operation, tip speed should not be greater than 3300 rpm.
Total Pressure (TP): the sum total of a fan’s static pressure and it’s velocity pressure. Represents the pressure rise from fan inlet to fan outlet.
Velocity: speed in feet per minute at which air is moving, measured at any location.
Velocity Pressure (VP): computed air velocity divided by 4005, with the result cubed. Velocity pressure applies only to air in motion; measured in inches of water. One inch water gauge is equal to 4005 fpm.
Types of Fans
Centrifugal fans have housings shaped like a nautilus shell mounted with an impeller through which air flows outward radially from the center. The blade can be inclined forward or backward which impacts the design of the fan. A centrifugal fan can only move air when the fan moves in one direction, in other words, it will not blow air backwards when the fan is motor is run backwards, as in a propeller type fan.
Axial flow fans are mounted in a ring or cylinder. Air flows parallel to the main axis through the impeller. Axial fans can be of several types, such as vane axial, tube axial, or propeller fans.
A vane axial fan is made up of an axial flow wheel and a series of vanes that increase efficiency and guide the air flow. Tube axial fans have an axial flow wheel mounted within a cylinder. Propeller fans have a disc wheel or propeller mounted in a plate or casing ring; they are the simplest type of fan construction, Propeller fans operate best against low resistance pressure.
Most electric motors used to power fans used in HVAC systems are small enough to be of the direct connected design, although larger centrifugal fans used in forced air furnaces can incorporate belt drives. One advantage of the belt drive arrangement is that speed adjustments can be made by changing the size of the pulley.
In terms of efficiency and maintenance, however, directly connecting the fan wheel to the motor shaft is the best option. If the fan is a centrifugal or propeller fan with a diameter of over 60 inches, direct connect fans are unfeasible, so a belt drive is required.
The typical direct connect fan motor is a single phase alternating current type; it may be split phase, shaded pole or capacitor type. When there are current limitations, is recommended to use a capacitor motor, because of its greater electrical efficiency.
A fan motor will normally run at s high temperature, too hot to hold a hand against. They do overheat and burn out, though and it is usually due to one or more of the following factors. The line voltage may be too high or low for the motor’s rated voltage.
On a belt driven fan, the belt could be too tight or loose, causing reduction of the cooling effect of the fan on the motor. Insufficient air intake or outlet area can starve a propeller fan, causing static pressure to rise and brake horsepower load to increase, overheating the motor.
Specifying a Fan
To select the proper fan for an HVAC system, typically there is a range of information that needs to be known. The following explains the various requirements.
The static pressure is the pressure of the system, including ducting, air intakes, and screens presented against the flow of air. It is calculated after duct sizing has been established. Fan selection should match static pressure of the system to the CFM capacity of the fan for that level of static pressure.
The volume of air that must be moved in order to meet the building’s requirements, expressed in CFM (cubic feet per minute) must be known. It is established by dividing the total cubic feet of the structure’s air space by the number of air changes required.
Application type will often dictate the kind of fan required. For a ducted system, for example, centrifugal fans are appropriate, where for a system without ducts, a propeller type fan is recommended. Other considerations include cost limitations and ambient air temperature.
The maximum allowable noise level associated with the fan needs to be known so that a suitable size and capacity can be selected.
The temperature of the airstream and the ambient temperature required must be known. Some applications will necessitate a fan capable of operating at a higher temperature, for example.
The line voltage available can vary from country to country and system to system, so must be known. Fans come in voltages of 115, 230 and 460 volts, as well as special order voltages such as 177 or 575 volts.
The catalogs and brochures from fan manufacturers usually have guidelines and installation information for assistance in selection of fans, so use these as rules of thumb to help you.