Term Paper: Attic Fan

Pages: 10 (2857 words)  ·  Bibliography Sources: 5  ·  Level: College Senior  ·  Topic: Physics  ·  Buy This Paper

SAMPLE EXCERPT:

[. . .] A high quality thermostat is also necessary since it helps in energy conservation and is also time saving since it eliminates the need of the home owner climbing up to the attic in order to reset the fan. It is also necessary to include a firestat that helps in shutting off the fan incase of a fire outbreak. The fan's motor should be thoroughly lubricated at all times to ensure smooth operation.

There exist two types of attic fans. The roof top type and a gable-end type. The diagram below shows the effect of the attic fan on the overall ventilation of the house.

Determination of the attic ventilation requirement for the attic fan

1.Determination of the attic area's square footage to be ventilated

In order to determine the area of the attic to be ventilated by the attic fan, one need to multiply the length of the attic by the attic's width.

For example: Let's assume we have an attic whose length is 50 feet by 25 feet. Then it follows that the attic area to be ventilate d would be 50' x X 25'= 1250 square feet.

2.Determination of the required total net free area..

After determining the attic square footage, divide it by 150 in order to get the net free area needed to effectively ventilate the attic

Calculation:

1250 sq. ft + 150 = 8.333 square feet of net free area.

3.Determination of the required amount of intake and exhaust net free area.

In order to achieve an optimum performance in the attic ventilation, a balance must be achieved between the intake and the exhaust vents. This is achieved in calculation by dividing the result of step 2 by two.

Calculation

8.333 + 2= 4.16

4. Conversion to square inches

Ventilation product's specifications are usually listed in square inches. Therefore we must convert our calculations to square inches. To do this we multiply the results obtained by 1.

Calculation:

4.16 sq. ft x 144 = 599 sq. in. Of intake net free area and The attic fan design

The fan should be highly efficient in the exhaustion of air from the attic. The fan includes highly optimized airflow blades with a twisted configuration. The blades should be capable of achieving a rotational speed of up to 600 RPM.The choice of material for the blades should be plastic. That means that the blades can be remolded on to a hub in order to form one piece (unit).Metal too can be used in the design of the blades.

The assembly can then be mounted on an exhaust outlet possessing a conical shaped diffuser on the roof. The other alternative is to have the fan assembly in a portable form. This arrangement allows for the fan to be moved anywhere where need for ventilation arises. The blades are put in motion by the action of a solar powered motor. The blade and motor assembly should be capable of achieving approximately 1050 cfm while at the same time utilizing no more than 17 Watts.

The arrangement below portrays the basic configuration of the attic fan design.

The following factors are the most important in the design of the attic fan:

a) Uniform flow

In order for the fun wheel to impart a uniform stream of air of required velocity and pressure over the entire net attic area, the wheel must possess two very important characteristics.

i. The blades should be narrow at the tip as that is the area where the velocity is highest. The blade should then widen towards the hub. The widening of the blade towards the hub should is done in order to achieve more blade area that is required because of lower blade velocity.

ii. The blade's angle of rotation to the plane of rotation should be minimum at the tip and should increase towards the hub. It is necessary to calculate the width and angle for each point of the blade in this manner in order to achieve uniform velocity and pressure.

It is paramount to maintain uniform velocity and pressure over the blades because of the following reasons:

In case certain areas of the blade do not have same pressure as other portions of the blade, then there will be back flow of air in those points. Such points are located near the hub in blades such as the ones for an airplane propeller.

Inequality in the discharge velocity also results in power wastage in excess velocity pressure. Because velocity pressure increases as the square of velocity, inequality in discharge velocity, if translated converted into velocity pressure, would result to a higher velocity pressure. A good example being that fans which develop most of their velocity and pressure near the tip of the blade usually have very low efficiency.

b) Hub size.

The hub's aerodynamic purpose is to prevent back flow of air through the center of the fun and hence it increases the fan's efficiency. The hub should never be too large or too small. A large hub would result in an increase in velocity pressure. That would result in loss of efficiency due to the small net opening. A very small hub also results in the deterioration of air flow near the hub.

c) Section of the blade

The selection of the aerofoil shape of the blade should be done to ensure maximum efficiency. That is achieved by ensuring that the blade exhibits high lift and low drag. It is also important to consider the abruptness at which stalling occurs. It is important to consider the following factors while selecting a blade for the fan.Lift, the drag and the angle of attack.

d) Pitch and the blade angle

Pitch is the advance of air stream per revolution of the fan wheel. It may be expressed in terms of feet per revolution. For convenience, it is expressed as a dimensionless quantity. It is often expressed in terms of diameter of the hub.

It is worth noting that blade setting and blade angle is not the same thing as pitch. Blade angle shows the angle of the blade with the plane of rotation at a particular point on the blade

e) Number of blades

The number of blades that a fan has is not important. What is of paramount importance is the effective surface area of the blades. The total blade surface area for each radius is what really matters. The width of the blade is always limited by physical considerations in an effort to prevent the fan wheel from being too deep. Because the importance attached to the fan's efficiency, it is important to avoid excess or insufficient blade surface area. It is advantageous to select any particular number of blades up to the point when there is a limitation posed by the physical conditions of the design.

f) Speed of the blade.

It is important to correlate the design of the wheel with the speed of the blades. This is because the average pressure imparted by any wheel is directly proportional to the square of the speed of rotation in RPM (Rotation per minute)

Conclusion

A solar powered attic fan is an essential part of every household due to its applicability and affordability. The current global financial crisis coupled with the eminent environmental and energy crisis therefore call for more affordable, efficient and environmentally friendly alternatives.

The solar powered attic fan is therefore an appropriate use of renewable solar energy and its use should be advocated for and even be adopted by the governments and corporations in order to aid in the use of more efficient technologies.Further research and development in the use of solar power is necessary to match equal improvements in the design of the fan blades and hub assemblies.

A solar powered attic fan is therefore a work of true ingenuity.

'Bibliography

ANSI/AMCA Standard 210-99, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating"

Eckert, Michael, "The Dawn of Fluid Dynamics: A Discipline Between Science and Technology"

Frank Bleier, "Fan Handbook: Selection, Application, and Design"

Vermass, Wim. "An Introduction to Photosynthesis and Its Applications."… [END OF PREVIEW]

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