New Fans Flashcards
What is the purpose of a fan?
Create a pressure difference that provides a change in air pressure that is usually intended to promote air movement through a duct system
How are fans selected?
For certain applications depending upon it’s adequacies and inadequacies
How to identify a centrifugal fan?
If airflow through the fan is perpendicular to the fan shaft
How to identify an axial fan?
Airflow is parallel to the shaft
3 types of axial fans?
Propeller, vane axial, tube axial
Application of a propeller fan?
Factories, welding shop, foundries, furnace rooms, labs, laundries, HVAC condenser and evaporator fans
6 operational characteristics of propeller fan?
Slower RPM, moderate temperatures, large volumes of air flow, low static, no ductwork, usually mounted onto an opening
Why is tube axial more efficient than a propeller fan?
Higher speed and close tolerance allow tube axial to operate at higher static
Greatest inefficiency of a tube axial fan?
Discharge air in spiral motion producing high duct system losses (turbulence)
Design improvement that makes vane axial more efficient than tube axial?
Guide vanes. reduces turbulence and promotes laminar flow
More efficient? Air foil blade or cross flow blade?
Air foil blade fan
Centrifugal fans are constant displacement. What does that mean?
Move constant volume rather than constant mass
List 4 kinds of backward inclined centrifugal fan blades
Airfoil, backward curved, backward inclined, radial tip
How fast do we run a backward inclined fan blade?
Twice the speed of forward curved fan
How much static can a backward inclined fan produce?
8-10 inches
How many blades does a backward inclined flat blade fan usually have?
10-20
What is the purpose of curved blades on a backward curved centrifugal fan?
Aerodynamic shape reduces noise levels and power requirements
Deficiencies of backward curved fan blade when compared to forward?
Larger and produces humming noise
How fast is backward inclined airfoil fan rotated?
3-4 times higher than forward curved fans
What static do radial tip fan blades operate at?
Medium to high discharge pressure. Greater than 2” (medium) and greater than 7” (high)
Wheel size for radial tip fan blades?
20.5 - 90.75 inches
Describe a radial fan blade
Straight, flat, paddle type, mounted perpendicular to shaft
How much static does radial fan blade produce?
It can generate high static pressure (over 10”)
What static does forward curved centrifugal produce?
Low (less than 2”)
Greatest diameter forward curved fan blade?
24”
2 applications for a cross flow fan?
Air curtains, ductless split AC evaporators
Another name for a mixed flow fan?
Tubular centrifugal
Application for a mixed flow fan?
Supply and return air when low sound levels are critical
6 things to consider when selecting the right fan?
- Static and velocity requirements
- Efficiency (HP requirements)
- Noise
- Air quality
- Physical fan size
- Initial cost
Backward curved fan blade tip velocity vs air velocity?
Tip velocity is greater than air velocity making it suited for high pressure low volume applications
Most important when selecting fan blade type?
Static pressure vs air volume
How does air density affect CFM?
It does not
If you want to double rotational speed of centrifugal fan what happens to HP requirements?
Factor of 8 times larger
Increased air temperature does what to motor current?
Decrease motor current (higher temp is less dense)
