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| 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
Cabinet Cooling Hints Whole-House Fans How Whole-House Fans Work You can regulate cooling by simply closing windows in the unoccupied areas and opening windows wide in occupied areas. Many people cool the bedrooms at night and the living areas during the daytime. Whole-house fans should provide houses with 30 to 60 air changes per hour (varies with climate, floor plan, etc.—check with a professional to determine what is appropriate for your home). The air-change rate you will choose depends on your climate and how much you will depend on the whole-house fan for cooling. Cooler, shadier areas don't require as much ventilation as warmer, sunnier ones. Houses entirely dependent on whole-house fans require a bigger fan because there is no air conditioning to fall back on. Sizing a Whole-House Fan Installing and Using a Whole-House Fan Attic ventilation must be increased to exhaust the fan's air outdoors. You'll need 2 to 4 times the normal area of attic vents, or about one square foot of net free area for every 750 cfm of fan capacity. The net free area of a vent takes into account the resistance offered by its louvers and insect screens. More vent area is better for optimal whole-house fan performance. Some fans come with a tight-sealing winter cover (or you can build one). If you switch between air conditioning and cooling with a whole-house fan as the summer weather changes, build a tightly sealed, hinged door for the fan opening that is easy to open and close when switching cooling methods. Drawbacks of Whole-House Fans Evaporative Coolers Hot outside air enters the swamp cooler. The air passes over water-saturated pads, and the water evaporates into the air. The energy used removes heat from the air. The 15- to 40-degree-cooler air is then directed into the home, and pushes warmer air out through windows. Because this process also humidifies the air, swamp coolers are best used in areas with low summertime relative humidity. Evaporative coolers use less than one-third the energy of air conditioners, and cost about half as much to install. Unlike central air conditioning systems that recirculate the same air, evaporative coolers provide a steady stream of fresh air into the house. Sizing and Selection Installation Most people install down-flow evaporative coolers on the roofs of their houses. However, many experts prefer to install ground-mounted horizontal units, which feature easier maintenance and less risk of roof leaks. Small horizontal-flow coolers are installed in windows to cool a room or section of a home. These portable evaporative coolers work well in moderate climates, but may not be able to cool a room adequately in hot climates. Room evaporative coolers are becoming more popular in areas of the western United States with milder summer weather. They can reduce the temperature in a single room by 5 to 15 degrees. Operation Control the cooler's air movement through the house by adjusting window openings. Open the windows or vents on the leeward side of the house to provide 1 to 2 square feet of opening for each 1,000 cfm of cooling capacity. Experiment to find the right windows to open and the correct amount to open them. If the windows are open too far, hot air will enter. If the windows are not open far enough, humidity will build up in the home. You can regulate both temperature and humidity by opening windows in the areas you want to cool, and closing windows in unoccupied areas. Where open windows create a security issue, install up-ducts in the ceiling. Up-ducts open to exhaust warm air as cooler air comes in from the evaporative cooler. Evaporative coolers installed with up-ducts will need additional attic ventilation. Filters remove most of the dust from incoming air—an attractive option for homeowners concerned about allergies. Filters can also reduce the tendency of some coolers to pull water droplets from the pads into the blades of the fan. Most evaporative coolers do not have air filters as original equipment, but they can be fitted to the cooler during or after installation. Evaporative Cooler Maintenance The more a cooler runs, the more maintenance it will need. In hot climates where the cooler operates much of the time, look at the pads, filters, reservoir, and pump at least once a month. Replace the pads at least twice during the cooling season, or as often as once a month during continuous operation. Some paper and synthetic cooler pads can be cleaned with soap and water or a weak acid according to manufacturer's instructions. Filters should be cleaned when the pads are changed or cleaned. Caution: Be sure to disconnect the electricity to the unit before servicing it. Two-Stage Evaporative Coolers Drawbacks of Evaporative Coolers They require maintenance (albeit easy) about once a month. If the cooler is installed on the roof, there is some roof deterioration caused by routine maintenance trips. A sunlit rooftop cooler will be about 1 degree Fahrenheit less effective than a shaded cooler. Rooftop maintenance also requires using a ladder, which may be an inconvenience. |
Cooling Fan UK is a website that can help you find a fan and cooling device . |
Managing Your Home's Ventilation
Remember the following if you plan to cool your home with ventilation:
- Learn how air flows naturally through your home.
- Take advantage of cool night air, and ventilate your home by natural or
mechanical methods.
- Keep a clear path for airflow both inside and outside your windows.
- Close windows, doors, and window coverings in the morning before your home
starts to heat up.
- Avoid producing heat in your home when it's hot outside.
- Wear cool clothing.
- Raise your air-conditioning thermostat to a more economical temperature..
Turn it off completely if you'll be gone for an extended period.
The most affordable office fan and cooling device can
be had if you look round. To get the best evaporative cooling fan ,
shop around and compare each fan cooling
from the different sites.
Fans can be thought of as low pressure air pumps that utilize power from a motor to output a volumetric flow of air at a given pressure. A propeller converts torque from the motor to increase static pressure across the fan rotor and to increase the kinetic energy of the air particles. The motors are typically permanent split capacitor AC induction motors or brushless DC motors. We shall now look at this system in more detail.
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.
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