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The following are sources of additional information about fans and ventilation.
The list is not exhaustive, nor does the mention of any resource constitute
a recommendation or endorsement.
Energy experts at EREC provide free general and technical information to the
public on many topics and technologies pertaining to energy efficiency and renewable
energy.
A comprehensive online resource for DOE's energy efficiency and renewable energy information.
Good source of unbiased information about fans or other appliances.
In partnership with industry and government, develops, promotes, and integrates energy technologies and practices to make buildings more efficient and affordable. Here are some other program web sites:
How do fans make you feel cooler?
how do fans make you cooler, fans actually add heat to a room. One way to think
about it is like this: If you have a perfectly insulated room and you put an
electric fan in it, then the room will get warmer. All the electricity that
is driving the fan turns directly into heat.
So a fan does not cool the room at all. What a fan does is create a wind chill effect.
When weatherpeople talk about wind chill on a cold winter day, what they are referring to is how the wind increases convective heat loss for details on convection). By blowing air around, the fan makes it easier for the air to evaporate sweat from your skin, which is how you eliminate body heat. The more evaporation, the cooler you feel.
How does a water cooler air conditioner work? There are several different ways
that water is used in air conditioning. The most common use is in cooling towers.
A cooling tower can be any size, from really gigantic down to the size of an
automobile.
In a cooling tower, air blows over water to create evaporative cooling, and this cooler water is used in the heat exchanger of, say, an air conditioner. A more direct form of evaporative cooling is often found in greenhouses, farmers' markets and chicken houses. In these kinds of systems, water flows over a mesh, and a fan blows air through the wet mesh into the building. Humid, cooler air is the result.
Fans are by far the most economical cooling option in terms of both purchase and running costs. Fans are cheap to buy and cost an average of two cents per hour to run.
Fans provide a cooling effect by moving air either around a room or a person. Fans do not reduce actual room temperatures or humidity levels.
The table below will help you choose the most appropriate fan to suit the size of the room you wish to cool:
Selecting the best fan size
Room Area (metres2) Ideal Fan Width (sweep)
Up to 10 900 mm (36 inches)
10-20 1200 mm (48 inches)
15-30 1400 mm (58 inches)
+30 Two or more fans
Types of fans
Portable Fans
Available in desk, box or pedestal models.
Look for a fan that oscillates (changes direction) and has a variable speed
control.
Most suitable for personal cooling
Average purchase cost £20-£170.
Ceiling fans
Can be suspended in rooms with high ceilings.
Should be installed with a clearance of at least 2100mm above floor level.
Should be located higher than the light fittings to avoid flickering shadows,
or you can install a unit with a light fitting attached..
If more than one fan is required, the spacing between fans should be as least
three times the fan width.
A reversing function enables some fans to be used in winter to bring down the
heat that builds up at ceiling level.
Look for variable speed control and curved blades to produce more air movement.
Average purchase cost £50-£200, plus installation.
Air has mass and thermal capacity. It transfers energy. It can be compressed. When it's compressed, air transmits hydrostatic pressure. Although it has a rather low specific heat, compared to other fluids, air is the most commonly used medium for heat transfer. It is available everywhere on the surface of the planet.
Air is usually taken directly from the surrounding atmosphere and returned
to it with a revised thermal content. Materials or objects that are cooled or
warmed are almost always immersed in air. Even if another medium is used to
cool a heat source, the ultimate heat sink is still the atmosphere, and, even
in that case, the secondary heat transfer path to the atmosphere is usually
a moving air stream.
A standard density figure of 0.075 lb/ft3 is used to rate fan performance. The
thermal capacity of air, based on that value, is 0.569 W•min/°C/ft3.
For every cubic foot of air per minute in an air stream, 0.569 watts of power
dissipated as heat are transferred for an air temperature change of one degree
Celsius. The thermal capacity of air, when held at a constant pressure is often
expressed as 7.59W•min/°C/lb.
Air Flow Rates and Package Layout
In electronic equipment and circuits, power dissipation is generally a stated condition; an air flow rate requirement is dictated by the need to keep the temperature of one or more of the components within specified design limits.
Types of Fans and Blowers
Air moving devices are generally described as being either a type of fan (Fig.
1a) or a centrifugal blower (Fig. 1b). The main difference between fans and
blowers is in their flow and pressure characteristics. Fans deliver air in an
overall direction that is parallel to the fan blade axis and can be designed
to deliver a high flow rate, but tend to work against low pressure. Blowers
tend to deliver air in a direction that is perpendicular to the blower axis
at a relatively low flow rate, but against high pressure.
There are several types of fans, some of the most common being propeller, tube axial and vane axial styles. Propeller fans are the simplest type of fan, consisting of only a motor and propeller. One problem with propeller fans is that tip vortices are produced by the pressure differential across the airfoil section.
A tube axial fan (the most common type in electronic cooling systems) is similar to a propeller fan, but also has a venturi around the propeller to reduce the vortices. The vane axial fan has vanes that trail behind the propeller in the airflow to straighten the swirling flow created as the air is accelerated.
Centrifugal blowers may have a forward curved wheel, a backward curved wheel, or be of the squirrel cage variety.
Basic Aerodynamics
Fans are of such common use that a basic understanding of the aerodynamics is
appropriate. This understanding begins with the recognition that the blades
of a fan propeller resemble the wing of an airplane, and as such follow the
same aerodynamic laws. For example, a fan blade produces lift when the chord
is elevated from the direction of the relative wind as shown in Figure 2.
The elevation angle is referred to as the angle of attack (AOA). The greatest airflow delivery from a fan occurs when the AOA is at a minimum, but the pressure differential across the fan is zero. As the AOA is increased, the airflow delivery decreases and the pressure differential increases. The airflow can decrease to nearly zero, but will also deliver the maximum pressure differential in this condition, which is called the shut-off point. When an AOA is reached where the air will no longer flow smoothly and begins to separate from the blades, an "aerodynamic stall" condition exists.
Since a fan is a constant volume machine, it will move the same volumetric flow rate of air irrespective of the air density. However, the mass flow rate does change as the density changes. This becomes important when equipment is expected to operate at altitudes significantly greater than sea level. Therefore the volumetric flow rate required at altitude (low density air) will be greater than that required to achieve the same cooling as at sea level.
The Fan Laws
Sometimes it may be necessary to determine the output of a given fan under other
conditions of speed or density, or to convert the known performance of an air
mover of one size to that of another geometrically similar unit of a different
size. The fan laws permit this.
Geometrically similar fans can be characterized by the following four equations:
Volumetric Flowrate: G = KqND3
Mass Flow Rate: m· =Km ND3
Pressure: P = KpN2D2
Power: HP = KHPN3D5
where:
K = constant for geometrically and dynamically similar operation
G = volumetric flow rate m· = mass flow rate
N = fan speed in RPM D = fan diameter
HP = power output = air density
From these relationships, it is possible to calculate a fan performance at a
second condition. Table 1 is a summary of the fan law equations in a form useful
for fan analysis.
Cabinet Cooling Hints
In addition to selecting a fan, there may be some choice in the location of
the fan or fans, and in this regard, the illustration in Figure 4 may prove
useful. The following comments should also be kept in mind with regard to fan
location:
1). Locate components with highest heat dissipation near the enclosure air exits.
2). Size the enclosure air inlet and exit vents at least as large as the venturi opening of the fan used.
3). Allow enough free area for air to pass with velocity less than 7 meters/sec.
4). Avoid hot spots by spot cooling with a small fan.
5). Locate components with the most critical temperature sensitivity nearest to inlet air to provide the coolest air flow.
6). Blow air into cabinet to keep dust out, i.e. pressurize the cabinet.
7). Use the largest filter possible, in order to:
a. Increase dust capacity
b. Reduce pressure drop.
The right ceiling fan will help cool down sizzling July afternoons and add
a nice touch to any room Ceiling fans are a happy meeting of the economical
and the esthetic. They're one of the few household appliances that can save
you money — as much as 15 percent off your energy bills — and look
good enough to enhance the decor of nearly any room.
Over the years, the choice of fan styles has broadened beyond the traditional
Tiffany model with its dark wood blades and elaborate glass shade. You can now
find designs that complement your clean-lined kitchen or contemporary living
room as well as traditional bedroom or dining room.
There's also a fan to meet any budget. The typical retail display includes models
that range from less than £100 for a bare-bones unit to well over £1,000
for an ornate design with cutting-edge controls and light fixtures. Here's a
primer on finding the fan that's right for your home.
Choosing a Fan
When shopping for a fan, you'll need to know what size and style are right for
your room and if any of the optional features, such as light fixtures or remote
controls, make sense for that environment. Fans are sized by the length of their
paddles, which should be matched to room size. The paddle span on residential
fans ranges from 29 to 54 inches. Select paddle size based on the room you want
to cool; see "Size It Right."
If you live in a three- or four-season locale, a reversible fan can provide year-round benefits. During the summer, the forward (counterclockwise) motion of the fan cools the room. With a fan, you'll conserve power without compromising on comfort. You can typically save between 4 and 8 percent of your cooling expenses for every degree you raise the thermostat in summer.
Ceiling fans can also help lower heating bills — up to 2 percent on heating costs for every degree the thermostat is lowered in winter. To get savings, switch the fan to run slowly in reverse: The clockwise movement breaks up the warm air that collects at the ceiling and pushes it down into the room. (Some fans have a special winter setting, in which intermittent bursts of speed blend warm and cool air.) This can be especially effective in rooms with a very high, angled ceiling or cathedral ceiling that collects a lot of heat. However, some authorities argue the benefits can't be felt in rooms with standard 8-foot ceilings. If you are buying a fan for its cooling ability, experiment during the heating season. But not all fans have reverse switches, so double-check before purchasing a unit.
Where can I buy the best cooling fan ?
Cooling Fan UK
gives you most of the sites you would probably want to visit for the best cooling fan .