Energy Savings on Fan Applications

Compiled by Arnold S. De Leon

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Types of Fans Axial or Propeller Fans

In an axial fan the air flows in parallel to the shaft.

Axial fans are suited for relatively large volumes compared to pressure.

PropellerVane Axial

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Roof Ventilator

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Types of Fans

Centrifugal Fan In a centrifugal fan the air

flows is in a radial direction relative to the shaft.

Suitable for higher airflow and pressure

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Classification of Centrifugal Fans

Straight radial blades. Self cleaning. Suitable for material transportCurved backward blades. High efficiency, low

energy consumption, changing in pressure have little influence on air volume. Low noise emission, stable in parallel running.

Curved forward blades. High efficiency, small dimensions, changing in pressure have little influence on pressure head

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Basic Terms in Fan Application

Static Head is defined as “the pressure exerted by a still liquid or gas,

especially water or air.

Figure 1, Showing Equal Air Pressure inside and outside the building. Here no exhaust fan running

Figure 2, with Exhaust Fan running Inside Air is lesser than Outside Air.

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Basic Terms in Fan Application

Flow (Q) Amount of liquid or gas necessary to maintain Static Pressure

express is Cubic Feet per Minute or Cubic Meters per Minute.

Point to remember 2.44 ft2 of vent box opening

can accommodate approximately 1500 CFM of air or 42 cu. Meters per minute

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Basic Terms in Fan Application

Brake Horse Power (BHP) The brake horsepower is the amount of real horsepower going to

the pump, not the horsepower used by the motor. In the metric system kilowatts (kW) is used.

Due to hydraulic, mechanical and volumetric losses in a pump or turbine the actual horsepower available for work on or from the fluid is less than the total horsepower supplied.

P BHP = * Q * h / 3300 /

P BHP = Brake Horse Power

= specific weight of air Q = is for flow in CFM h= is for head = stands for efficiency

P EL = P BHP / EL

P EL = Electric Motor Power

EL = stands for motor efficiency

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How to read a Fan Curve?

Points to Remember:

•The most valuable pieces of information supplied by fan manufacturers is the fan performance curve.

•Each Fan, has its own performance curve on a particular application.

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Fan Curve Basics

Fan Curve Figure on the right shows a typical

centrifugal fan produce at its outlet at a given RPM.

The curve is a plot of outlet pressure in inches of water versus the flow of air in CFM of a Fan. System Curve

The System Curve is added to the Fan Curve

This shows the requirement of the vent system from the fan to overcome losses and produce an air flow..

Fan Curve

System Curve

OperatingPoint

Operating Point The point where pressure and flow

requirement of the system can meet by the fan.

Without external influences, the fan will operate only at this point.ADL 06/07/2010

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Fan Curve Basics

Fan Curve

System Curve

OperatingPoint

BHP

BHP Curve Now the BHP curve is added. At Static Pressure of 4 units Head,

and Flow of 6.2 units CFM, the motor shaft will exert 6.2 BHP of power.

Computing for the Motor HP Say motor at full load efficiency of

90% P EL = P BHP / EL

P EL = 6.2 / 0.90

P EL = 6.8 HP Commercially available motors

are 5.5 and 7.5 HP, choose 7.5 HP ADL 06/07/2010

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Fan Curve Basics

Fan Curve

BHP

Effect of Resistance to the fan System Curve can shift due to

following factors,▪ Dirt in the filters▪ Damper throttling

Increase resistance

decreaseresistance

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Methods of Regulation in Fans

• Outlet Dampers

• Variable Inlet Vanes

• Pitch Control

• AC Drives or Variable Frequency Drive

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Outlet Damper

The outlet dampers affect the system curve.

As the outlet damper is closed, the fan’s point of operation moves left from the initial design point on the static pressure curve.

Depending on where the design point is, damper horsepower may be less, the same, or more than the full flow of horsepower.

Outlet dampers are typically the least expensive first cost option but also offer the least potential for energy savings.

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Variable Inlet Vane

Inlet-vane dampers affect the Fan Curve.

It spins the air in the direction of wheel rotation as it enters the fan. With this pre-spin, the wheel cannot develop its full output, yielding a reduced CFM at reduced horsepower.

Each damper setting creates new pressure and horsepower curves. With inlet-vane dampers, reduced airflow always results in reduced horsepower.

More expensive initially than outlet dampers, but inlet-vane dampers offer greater potential for energy savings.

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Variable Pitch Axial Fan

It has a sophisticated hub which carries the blades. Each blade is connected to a spindle, which is rotated by a lever. A servo-controlled hydraulic cylinder moves all the levers simultaneously while the fan impeller is rotating. This varies the output of the fan.

The blade/hub assembly is mounted on a shaft which rotates in a casing, and is then referred to as the rotor.

The casing may have an open inlet, but more commonly it will have a right angled bend to allow the motor to sit outside the ductwork. The discharge casing gently expands to slow down the air or gas flow and convert kinetic energy into useful static pressure.

Mechanically unreliableHigh Electrical Efficiency

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Variable Pitch Axial Fan

Mechanically Linked Variable Pitch Axial Fan

Servo ControlledVariable Pitch Axial Fan

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AC Drive or VFD

VFD controllers provide energy savings by directly controlling fan speed.

From the fan laws, CFM varies directly with the change in fan speed and horsepower varies by the change in fan speed cubed.

For example, a 20% reduction in fan speed yields a 20% reduction in airflow and a 49% reduction in BHP.

VFD provides the greatest potential for energy savings.

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Which the best Regulation Method?

Outlet Damper Inlet VaneVFD

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Mathematical Concept

System Curve Formula;

P = K * Q2

Where

P = the pressure required to produce a given flow in the system

K = is a function of the system and represents the friction to air flow. The outlet vanes affect the K factor.

Q = is the airflow desired

Fan Affinity Law;

Volume CapacityQ1 / Q2 = (n1 / n2)         (1a)

Head or PressureP1 / P2 = (n1 / n2)2          (2a)

PowerHP1 / HP2 = (n1 / n2)3          (3a)

Where

P = the pressure required to produce a given flow in the system

n= is the motor speedQ = is the airflow desiredHP = is the Horse Power of Motor

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Mathematical Concept

Note that when System Curve Formula and Affinity Law are combine,

P = K * Q2 ; K = P / Q2

From Affinity Laws Volume Capacity:

Q1 / Q2 = (n1 / n2)    ;   {Q1 / Q2} 2 = (n1 / n2) 2 

From Affinity Laws Pressure Vs. Speed:

P1 / P2 = (n1 / n2)2   ;   P1 / P2 = {Q1 / Q2} 2   

P1 / Q12 = P2 / Q22 (This is same equation for System Curve “K”)  

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Mathematical Concept

The mathematical equation shows, that VFD regulation can achieves flow control in a way that closely matches the system or load curve

As the fan speed is reduced, a

significant reduction in power requirement is achieved.

For example, a 20% reduction in fan speed yields a 20% reduction in airflow and a 49% reduction in BHP.

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Sample Calculation

A Fan was selected to run at 300 RPM, 100 CFM and 4 inches of Static Pressure.

Determine the best regulation method.

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Sample Calculation- Outlet Damper

CFM Duty HPFrom Fan

Curve

Weighted HP

100 10% 35 3.5

80 40% 35 14.0

60 40% 31 12.4

40 10% 27 2.7

Total 32.6CFM

Dwell timeOutlet

DamperWithout

Regulation

Weighted HP 32.6 35

Equivalent in KW

24.3 26.1

Dwell time 496 496

Power rate PHP 8/ kW-hr

PHP 8/ kW-hr

Total Energy Cost/month

PHP 96,424 PHP 103,565

Savings using Outlet Damper PHP 7,141.00

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Sample Calculation- VFD Regulation

CFM Duty HPUsing Affinity

Law

Weighted HP

100 10% 35 3.5

80 40% 18 7.2

60 40% 7.56 3.02

40 10% 2.24 0.22

Total 13.94CFM

Dwell timeVFD

RegulationWithout

Regulation

Weighted HP 13.94 35

Equivalent in KW

10.4 26.1

Dwell time 496 496

Power rate PHP 8/ kW-hr

PHP 8/ kW-hr

Total Energy Cost/month

PHP 55,314 PHP 103,565

Savings Using VFD PHP48,251

Using Fan Affinity Law;

Power versus Speed or Cube LawHP = n 3

@ 100% CFM ; HP = 100%

@ 80% CFM ; HP = (0.8)*(0.8)*(0.8)= 0.512*

35HP= 18HP

@ 60% CFM ; HP = (0.6)*(0.6)*(0.6)= 0.216*

35HP= 7.56HP

@ 40% CFM ; HP = (0.4)*(0.4)*(0.4)= 0.064*

35HP= 2.24HP

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Savings Comparison

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Without Regulation

Outlet Damper

VFD Regulation

Weighted HP 35 32.6 13.94

Equivalent in KW

26.1 24.3 10.4

Dwell time 496 496 496

Power rate PHP 8/ kW-hr PHP 8/ kW-hr PHP 8/ kW-hr

Total Energy Cost/month

PHP 103,565 PHP 96,424 PHP 55,314

Savings using Outlet Damper

PHP 7,141.00

Savings Using VFD per Month PHP48,251

Difference in Savings per Month PHP 41,110.00

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Welcome to VACON

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Vacon HVAC Proven Technology

Heathrow Airport, Terminal 5, UK

Inox multiplex cinema, Pune, India

DR multimedia house, Copenhagen, Denmark

Raffl es private hospital, Singapore

Jumeirah Emirates Towers, Dubai, UAE

Sello shopping centre, Espoo, Finland

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Questions?