AC Units and Control ME 536

8/14/2019 AC Units and Control ME 536

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ME 536ir Movers and Fan

Tec no ogy. .

Mechanical & Marine Engineering Dept.Arab Academy for science, technology and

Maritime Transport


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References

Burmeister L.C. Elements of Thermal-FluidSystem Design, Prentice Hall, 1998.

Equipment, 1992.


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Fan principles,

,

Fan selection,

Fan installation design,

Effect of variable resistance devices.


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Prime movers for Air Conditioning

applications:

Fans: move air.

.

Compressors: move gases or vapour

re r gerants.


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,axial, special designs (including radial)

-clockwise) is important because the blades and

direction

ressure rop roug e sys em mus e nown

to choose a fan. Fans are quietest when they operate near peak

efficiency; efficiencies are often provided on fan

curves.


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Ventilation: Natural: [free] by free convection.

.Using Fans.

energy that is transformed by a means of.

Uses of Fans: Ducts to supply air.

Coils for forced convection.


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A FAN is an air pump that creates a pressure differencean causes a r ow.

The impeller does work on the air, imparting to it boths a c an ne c energy, w c vary n propor on,

depending on the fan type.

the flow.

blades of the impeller impart kinetic energy to the air by

changing its velocity. Velocity change is in the tangential and radial velocity

components for centrifugal fans, and in the axial and

tangential velocity components for axial flow fans.


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rect r ve:

[Small fan- Coil units- Most Axial fans]

More compact assembly.

Belt drive: [most applications]

Flexibility in fan speed (can be changed by altering.

Built guard are required for safety.

Normal torque motors are generally used for fan duty.


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Fan Types:Centrifugal: Axial:

-

Special Design:

-Straight fins.-Forward

.

-Tube axial- Propeller.

.

Power roof ventilation.-Centrifugal

- .


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Centrifugal fan

Increase of air static pressure is created by the conversion.

from its inlet to its outlet.


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Direction of rotation and discharge position

for centrifugal fans.


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Direction of rotation and discharge position

for centrifugal fans.


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Axial Fan

Increase of air static pressure is created by the

conversion of velocit ressure to static ressure

Direction of airflow is parallel to the axle of the fan.


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, ,

Tubeaxial: impeller is inside a tube to guide

Vaneaxial: like a tubeaxial except vanes either up

or owns ream o e mpe er are use o re uceswirl and improve performance

Used to deliver large flow rates but small

increase in pressure Examples include fans used for ventilation

without ductwork, mobile room fans, and fans

used to cool computers


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Performance curves:


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Fans cause a pressure increase through two methods

Centrifugal force is created by the rotation of the column of air.

Kinetic energy is supplied to the air through the impeller

Total pressure = velocity head + static pressure

Blades are airfoil-type, backward-curved, forward-curved, or radial (straight)

Airfoil-types are complex and expensive but very efficient;theyre used for large systems where the cost is justified.


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Method of Obtainin Fan Performance Curves


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Highest efficiencies occur at 50 to 60% of wide open volume.

. Power reaches maximum near peak efficiency and becomes

lower or self-limitin toward free deliver .


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- - ,

correctly the motor wont overheat or burn out even if

High efficiency and stable operation make this bladet e o ular.

Choose the operating point to be just to the right of thepeak pressure flow rate to achieve both high efficiencyand a stable flow rate.

This type of fan operates stably because the pressuredifference provided by the fan drops if the flow rate goesup. If the opposite were true, increased an increased

,

unstable.


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Fan Curve, Backward-Curved Centrifugal Blade

Efficiency only slightly less than airfoil fan.

Ten to 16 single-thickness blades curved orinclined away from direction of rotation.

Efficient for same reasons as airfoil fan.


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-

lower efficiency than the

-backward-inclined.

Do not rate fan in the

pressure curve dip to the leftof peak pressure.

Power rises continuallytoward free delivery. Motor

selection must take this intoaccount.


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Comparison of Centrifugal Fan

Types:em orwar -curve a a ac war -curve

Efficiency Medium

-

Medium

-

High

-

Speed required Small Medium Medium

S eed for iven Low Medium Hi hpressure rise

Noise Fair Poor (very highnoise

Good

Application

DPAverage Average High

Q m3/s


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Comparison between Various Types of Fans


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Before proceeding with any fan selection the following Air volume requirement [CFM, l/s, m3/hr..]

Air density. [Altitude and Temperature] Types of service: [Environment, Materials, vapours to be

ex aus e , opera ng empera ure .

Noise criteria. .

Number of fans.

Fan type. Expected fan life in years. Rotation. sc arge.

Motor position.


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ow a e. Pressure. ype Noise


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Air conditioning capacity: [ACC]

ACC = mo

* (hi ho) Volume flow rate (fan capacity) Qo

by the fan in a unit of time.mo = ACC / h h

Qo = mo (kg/s) * v (m3/kg)

N.B:ACC from calculations

(h h ) & v from chart


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Static Power: is the part of the total power

that is used to roduce the chan e instatic pressure.


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Total pressure: [Pt]Pt = Pdynamic + Pstatic

tat c ressure: sIs the pressure exerted on the walls of the air duct and is parley dispelled on

overcoming the various resistances met on its way.

Is the pressure required to set air motion and correspondents to the kinetic

energy.

Air intake louvers. Mixing box with air damper. Spray humidifiers. Preheating coil

After-coolin coil Ducts

Then we calculate the required external pressure.


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mp e uc ys em w es s anceto Flow Represented by Three 90

Elbows

Resistance Added to Duct

System

Resistance Removedfrom Duct System


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ect ons

MIXING BOX

DOUBLE MIXING BOX PRE FILTER

BAG FILTER

HEATER HUMIDIFIER (WASHER)

HEPA FILTER

MULTI-ZONE

SOUND ATTENUATOR

SUPPLY PLENUM


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Selection

equ re r an ng un w acapacity of 3200 c.f.m.

3200 c.f.m. = 5440 m3/Hr

-(2.78m/s)

CS-60: lower velocity but with higher.


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Fan selection

Flow rate

Static pressure

Find

Dynamic pressure

Using flow rate and total pressure:

RPM

Sound level


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Practical problems

. .

= 3060 m3/Hr @ 1000 pa

S eed: 800 RPM

So:

Outlet Velocity: 10 m/s


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Practical problems

Static problems:

Too little air

Too much air

Overloading of motors


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General Ventilation Locate intake and exhaust fans to make use of prevailing winds Locate fans and intake ventilators for maximum sweeping effect over theworking area

If filters are used on gravity intake, size intake ventilator to keep intake lossesbelow 1/8" SP Avoid fans blowing opposite each other. When necessary, separate by at least6 fan diameters se ass nsu ate motors w ere am ent temperatures are expecte to e

high for air-over motor conditions If air moving over motors contains hazardous chemicals or particles, use

- ,requirements For hazardous atmosphere applications use fans of non-sparking construction


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Collect fumes and heat as near the source of generation as possible Make all runs of ducts as short and direct as possible

or padicles being collected When turns are required in the duct system use long radius elbows

After calculating duct resistance, select the fan having reserve capacity beyond the static pressure determined Genera[ Ventilation guidelines above Install the exhaust fan at a location to eliminate any recirculation into

other arts of the lant When hoods are used, they should be sufficient to collect a!J contaminating fumes or particles created by the process


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Manufacturer will provide a fan curve for each

fan he or she produces. The fan curves predict the pressure-flow rate

performance of each fan.

Choose a fan that gives you the volumetric flowr n f r r m r r r .

Choose a fan that has its peak efficiency at or

. Sometimes will provide data in a table rather

.


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Conventional Fan Performance Curve Used byMost Manufacturers

Generalized Fan Curves


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Generalized Fan CurvesThese kinds of curves can be used to help choose a fan.

Fan Laws


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a a sFan data for geometrically similar fans can be collapsed onto a single curve using

dimensionless numbers


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the same point of rating on the performance curve.

will be constants. For example, if fan operation movesfrom point 1 to point 2, the values of the dimensionlessparameters will not change and thus can be used to

estimate system effects.

e care u a out us ng t e an aws to eterm ne t e

effect of fan speed change you may move to a very

,invalidate your results.

Fan Laws


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Fan Laws

It may be easier to see how these work in a different form:


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Law 1 relates to effect of changing size,

s eed or densit on volume flowpressure, and power level

,

pressure, or density on volume flow rate,speed, and power

volume flow, or density on speed,

,


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This exampleapplies the fanlaws to a casewhere the fanspeed N ischanged from 600to 650 RPM for afan of a given

size.


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For two identical fans in parallel, you can

make our own fan curve b takin theoriginal fan curve and doubling the.

For two identical fans in series, you canmake your own an curve by doubling the

pressure drop for a given volumetric flow

rate.

computer chasses?


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total ressure Pt = static ressure Ps +

velocity pressure (Pv)


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L t th t th f f th i


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Lets assume that the fan from the previous

[1.65 m3/s] at 2.0 in. H2O [491 Pa] of static

, . . . At these conditions, the fans static efficiency would

e:


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Fan total efficiency expresses the

ercenta e of in ut ower that isrealized

.

is calculated by substituting total pressure for static pressure in the static


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an e ec on s Forward curved (FC)

, ,cost u Higher airflow, higher static pressure, higher

efficiency s Vaneaxial u Limited space

s Variable-pitch vaneaxial (VPVA) u Large systems, higher airflow


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Low Capacity or Pressure

Incorrect direction of rotation. Make sure the fan rotates in same direction as the arrows on themotor or belt drive assembly


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motor or belt drive assembly. Poor fan inlet conditions. There should be a straight clear duct at the inlet.

improper wheel alignment.Excessive Vibration and Noise

Damaged or unbalanced wheel. Belts too loose; worn or oily belts.

Speed too high. Incorrect direction of rotation. Make sure the fan rotates in same direction as the arrows on the

mo or or e r ve assem y. Bearings need lubrication or replacement. Fan surge.

Overheated Motor . Incorrect direction of rotation. Make sure the fan rotates in same direction as the arrows on the

motor or belt drive assembly.

Cooling air diverted or blocked. .

Incorrect fan RPM. Incorrect voltage

Overheated Bearings . Excessive belt tension.


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AC Units and Control ME 536

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