introduction to energy management. week/lesson 10 air moving equipment: fans and ducts

Introduction to Energy Management

Week/Lesson 10

Air Moving Equipment: Fans and Ducts

After completing this chapter, you will be able to: State how fans add kinetic and potential energy to

air Identify features and drive arrangements of

centrifugal fans Identify the features of axial fans


Explain fan performance characteristics including pressure, flow rate, horsepower, efficiency and speed

Use a fan’s performance tables to check a fan’s capability

Evaluate pressure changes within a duct system


Fan basics Centrifugal fans• Force air in a direction perpendicular to the

motor shaft Axial fans• Force air in a direction parallel to motor shaft


Types of centrifugal fans Forward-curved-very noisy Backward-curved Backward-inclined Air foil


Centrifugal drive arrangements Air movement and control association Single or double width Single or double inlet Direct connection Belt and pulley connection


Tubular and roof ventilator centrifugal fans Tubular• Straight line air flow• Fan mounted transversely in the housing

Roof ventilators• Packaged units


Types of axial fans Propeller• Move large volumes of air• Create very small pressure increases

Tube axial• Propeller mounted in a tube

Vaneaxial – low air turbulence


Performance characteristics of fans Volume Pressure Horsepower Speed Efficiency


Volume (capacity) Amount of airflow through the fan Expressed in cubic feet per minute (cfm)

Pressure Potential and kinetic energy Pt = Ps + Pv (total, static, velocity)


Pressure sensing Static, Ps perpendicular to airflow Velocity, Pv parallel to airflow Total, Pt = Static, Ps + Velocity, Pv

Pitot tube


Horsepower Brake horsepower Energy required to operate fans

Speed Determines air volume Expressed in rotations per minute (rpm)


Efficiency Ratio of output to input Input is greater than output Friction losses effect efficiency Low friction losses high efficiency


Performance tables and curves for fans Compilation of fan characteristics Often called multi-rating tables Helps designers evaluate fan choices Horsepower, efficiency and pressure


Describing air flow in ducts Continuity principle• Q = V x A• Conservation of air mass

Air velocity changes with duct size Larger ducts lower air velocity


Duct characteristics – ideal Increase in duct cross section• Velocity pressure drops• Static pressure increases• Total pressure remains constant


Duct characteristics – ideal Decrease in duct cross section• Velocity pressure increases• Static pressure drops• Total pressure remains constant


Duct characteristics – actual Friction causes pressure drops Larger ducts lower pressure drops Lower velocity lower pressure drops Turbulence causes pressure drops Static pressure drops as air flows


Duct design and evaluation CFM = Qs / (1.08 X Δ T) Constant pressure drop method Velocity reduction method Static regain method


System performance and fan selection Airflow varies, duct system is fixed Friction increases at the square of flow rate System characteristic curve Operation point fan selection


Types of Fan Demo

Two basic types of fan are:

Centrifugal - rotating impeller to increase velocity of an airstream.

Axial - air is pressurized by the aerodynamic lift of the fan blade.

Centrifugal Fan Components

Axial Fan Components

Axial Fan Components

Vaneaxial fan shown

Centrifugal Fans - Blade Designs

Centrifugal - Airfoil• Blade curves away from direction of

rotation• Highest efficiency• Relatively low noise• High structural strength• Power reaches max near peak

efficiency, then reduces twoards free delivery (self-limiting).

• For given duty, has highest speed of centrifugal design

Centrifugal - Backward Inclined

• Efficiency slightly less than airfoil design

• Pronounced region of instability where low efficiency and pulsations (stall, surge, bi-stable flow)

• Power reaches max near peak efficiency, then reduces t0wards free delivery (self-limiting).

• Consideration for use in applications where airfoil blade erosion concern

Centrifugal - Backward Curved

• Power reduces towards free delivery (self-limiting).

• Promote smoother flow than BI fan

• Operate at higher speeds and therefore require sturdier construction than FC

• Less susceptible to flow/fan instabilities due to less pronounced instability region

Centrifugal - RadialRugged blade design, self-cleaning• Lowest efficiency centrifugal• Used for material handling due to

blade strength• Moderate to high pressure industrial

applications• Power rises continually to free

delivery (overloading).

Centrifugal - Radial Tip

• Blade is radial at outer edge of wheel, curved in direction of flow as it moves inward

• More efficient than Radial blade• Used for mildly erosive environments• Higher pressure than Airfoil

Centrifugal - Forward Curved

Also called “Squirrel Cage”• Blade curved forward in direction of

rotation • Low pressure HVAC applications

(residential, etc.)• Lower speed than other centrifugals • Single thickness blade, lightweight

construction

Axial Fans - TypesPropeller - generally operate

at < 1/2 inch water gauge, inefficient, steeply rising power curve (overloading).

Tubeaxial - propeller mounted in cylindrical tube, ranges up to ~ 4 inches water gauge, overloading.

Vaneaxial - utilizes guide vanes to recover rotational energy, up to 10 inches (single stage), overloading.

Axial Fan - Guide VanesFunction - reduce the helical flow pattern of the fan.

Inlet - vanes guide airflow into a rotational pattern opposite to fan rotation to provide neutralize deflection by blades and result in axial flow

Outlet - vanes take rotational pattern from blades and redirects it to an axial flow, converting some rotational velocity into static pressure.

Centrifugal Fan Volume Control

Generally 3 methods:• Variable speed – changing fan speed results in a

change in fan flow.• Outlet damper – functions by changing system

resistance, forcing the fan to operate against higher backpressure and reducing the flow.

• Inlet damper – combination of resistance change and potential flow characterisitic change (spin, reduced turbulence) at fan inlet.

introduction to energy management. week/lesson 10 air moving equipment: fans and ducts

Documents

features of axial fans

fans performance tables

air flowsair

ductsdescribing air

motor shaftaxial fansforce

ps velocity

duct systemair

ps perpendicular