CHAPTER THREE

CENTRIFUGAL COMPRESSORS, FANS

AND BLOWERS

Contents

• Introduction

• Components and operation

• Effects of impeller shape on performance

• Velocity triangle and work/energy transfer

• Slip factor

• Diffuser

• Chocking, surging and rotating stall

• Pressure rise and loading coefficient

• Mach number

• Characteristic curve

Introduction • Power absorbing turbo machines

• Compressor, pump and fans are the devices used to increase the pressure of the fluid. But they differ in the task they perform

• centrifugal fan and compressor employ centrifugal effect to rise pr.

• All compressor, fan and blower are used to pump gas, but their difference is on the achieved pr. rise.

• A fan cause s only a small rise (up to pr. of 0.07 bar) of the flowing fluid. It consists of a rotating wheel (called the impeller), which is surrounded by a stationary member known as the housing.

• Energy is transmitted to the air by the power-driven wheel and a pressure difference is created, providing airflow .

• In blowers(0.07 to 3 bar) , air is com pressed in a series of successive stages and is often led through a diffuser located near the exit.

• Compressor for absolute pressure of 3 bar or above

• Single stage centrifugal compressor have pr ratio of 4:1 to 6:1

comparing centrifugal and axial flow compressor

Centrifugal compressor are smaller length

In centrifugal compressor, wide range of mass flow rates of gas

The best efficiencies are 3 to 4 percent bellow those obtained from axial flow compressors designed for the same duty.

In centrifugal compressor working fluid can be contaminating gas like exhaust gas.

At higher mass flow rate values, axial compressors have better efficiency but at lower flow rates centrifugal compressors have better performance.

The centrifugal compressor is not suitable when the pressure ratio requires the use of more than one stage in series because of aerodynamic problems

Disadvantage is larger frontal area and lower maximum efficiency

Centrifugal compressors found mainly in turbochargers, turbofan and turboprop aircraft engines

Components and Principles of Operation The principal components are impeller and diffuser

The rotor in centrifugal turbomachines is also named as impeller.

Energy is transferred from the impeller to the fluid by almost purely centrifugal

effect. A rotating particle about some axis of rotation usually tends to escape inthe outward radial direction.

Flow enters into the compressor through the impeller eye. The rotating impellerdrives the inlet flow to the larger radius exit by centrifugal effect. Subsequently,low pressure region is created around the impeller eye which allows fluid to besucked into the compressor.

The part of the impeller that creates low pressure region is called the inducer

The flow is induced through the impeller eye and is forced at impeller exit.

Inlet guide vanes are usually provided before the impeller eye.

The flow is accelerated through a nozzle inlet casing in some centrifugalcompressors before it enters the IGVs.

The fluid which gets out of the impeller is again diffuses through diverging area ofstationary blades (diffusers) to convert its kinetic energy into static pressure.

Energy is imparted to the air by the rotating blades, thereby increasing the static

pressure as it moves from eye radius r1 to tip radius r2.

The remainder of the static pressure rise is achieved in the diffuser. The normal

practice is to design the compressor so that about half the pressure rise occurs in

the impeller and half in the diffuser.

The air leaving the diffuser is collected and delivered to the outlet.

Volute casing surrounding the compressor stage collects the diffused flow

Cont.�

• Shrouded impeller has the advantage

of no tip clearance loss but it has

increased friction loss.

Fig: - Types of impeller construction

Fig: - Vane – less centrifugal compressor

Centrifugal compressors can be built with a double entry or a single entry

impeller.

�Three possibilities for impeller blade orientation : Radial blades, Backwardcurved blades, Forward-curved blades

�Backward-curved blades (β₂< 90°),

As shown in Fig. the value of Cw₂ (whirl component

at outlet) is much reduced, and thus, such rotors

have a low energy transfer for a given impeller tip speed

�Radial vanes (β₂= 90°)

have some particular advantages for very high

speed compressors where the highest possible pressure

is required.

�Better strength and design and manufacturing

simplicity are important justifications for choosing

radial blading.

Effect of blade shape on performance

Cont�• Forward-curved vanes (β₂ over 90°)have a high

value of energy transfer .

• Therefore, it is desirable to design for highvalues of β₂(over 90°), but the velocitydiagrams show that this also leads to a veryhigh value of C₂ which will cause a tremendousfriction loss in delivery pipes.

• As a result, backward curved blades areusually used in centrifugal pump design.

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Fig: Effect of blade

shape on performance

of pumps when mass

flow rate increases

�For the same rotor tip speed, the highest pressure ratio is attained in forward curved

impellers and the least pressure ratio is obtained in backward curved blades.

�The total pressure or head developed by the centrifugal impeller depends on the mass flow

rate in backward flow and forward flow blades, whereas it doesn’t pose any effect on radial

blade impellers.

�The curved blades tend to straighten due to centrifugal effect. Strength is least concern in

radial blade impeller.

Velocity Diagram• The design velocity diagram involves a purely axial entry at the inlet through

the impeller eye and the blade is radial at exit

• a represents when the air enters the

impeller in the axial direction. In this case,

absolute velocity at the inlet, C1=Ca1

• b represents velocity triangle at the inlet

to the impeller eye and air enters through the

inlet guide vanes.

• Angle Ɵ is made by C1 and Ca1 and is known as

the angle of prewhirl. The absolute velocity C1

has a whirl component Cw1.

Cont.�• In the ideal case, air comes out from the impeller tip after making an angle of

90degree (i.e., in the radial direction), so Cw2=U2. (fig C)

But there is some slip between the impeller and the fluid, and actual values of

Cw1 are somewhat less than U2. This results in a higher static pressure on the

leading face of a vane than on the trailing face. Hence, the air is prevented from

acquiring a whirl velocity equal to the impeller tip speed.

• d represents the actual velocity triangle.

Slip factor

From above we can say that, there is no assurance that the

actual fluid will follow the blade shape and leave the

compressor in a radial direction. Thus, it is convenient

to define a slip factor σ (less than unity) as,

� =���

��

If radial exit velocities are to be achieved by the actual

fluid, the exit blade angle must be curved forward about 10–14

degrees.

Cont.. For radial vaned impellers, the formula for ��is given by Stanitz as :

Where n is number of vanes. A slip factor of about 0.9 is typical for a compressor with 19–21 vanes.Work done

Theoretical work done Wc=Cw2r2ω= Cw2U2= �� (considering �). In a real fluid,

some of the power supplied by the impeller is used in overcoming losses (windage, disk friction and casing friction losses)

To take account of these losses, a power input factor (values between 1.035 and 1.04)can be used. Thus the actual work will be� = ψ��

(assuming Cw1=0, although this is not always the case.)

Temperature equivalent of work done on the air and compressor isentropic efficie

T02 at the impeller exit, As no work is done on the air in the diffuser, T03= T02, whereT03’ is the isentropic stagnation temperature at the diffuser outlet

Mollier diagram for the complete centrifugal compressor stage

Pressure ratio

= � ��� − ��� = ψ��

��� − ��� =����

�

��

�� =�������

������ =

�� !���!�

��

������ =

!���!�

��

������ /�� =

�����

#$ #��

������ /��

% & = 1 +)* ������

��

#

#$

% & = 1 +)*����

�

����

#

#$

Diffuser • Diffuser in compressor used to convert the KE imparted by the impeller to

pressure rise. the maximum permissible included angle of the vane diffuser passage is about 11degree. Any increase in this angle leads to a loss of efficiency due to

boundary layer separation (eddy formation) on the on diverging passage walls. Diffusers can be

a. Volute or scroll collector: it is circular passage of increasing X-sectional area, as increment of discharge and results constant axial velocity around the volute, and equal pr around compressor casing and hence no radial trust. It is low cost

b. Veneless diffuser: diffusion takes place in parallel side passage and governed by the principle of conservation of angular momentum of the fluid. Vaneless diffusers has wide range of mass flow rate, but due to long flow path friction effect are important and efficiency is low.

c. Vaned diffuser: the vanes (surrounding impeller outlet) are used to diffuse the outlet KE at much higher rate, In shorter length, and higher efficiency than vaned diffuser.

Examples Example 1

Also calculate 1)the rise in total temperature during compression

If change in KE is negligible. 2)external diameter of the eye if the

Internal diameter is 15cm and mass flow rate is 10kg/s.

Compressibility effect

Mach number in the Diffuser

Pre whirl is used to reduce pressure loss by restricting Mach at the inlet by adding

inlet guide vanes. It also used to reduce the curvature of the impeller at the inlet.

But it ha disadvantages like, reduction in work capacity of compressor, introducing

additional part(adds weight)

Centrifugal compressor characteristics

Performance characteristics curve is plotted

for compressible flows, the one we

encounter in centrifugal compressor stage.

The pressure ratio ��

� is plotted against the

value of +,��

� relative to the design point,

for various speed-

�� and constant

efficiency lines.

The surge points for each operating

conditions can be indicated on the plotted

surge line, which is a critical design

consideration in compressors.

Choking When the velocity of fluid in a passage reaches the speed of soundat any cross – section, the flow becomes chocked (air ceases toflow). The choking behavior of rotating passages differs from thatof stationary passages, and therefore it is necessary to makeseparate analysis for impeller and diffuser, assuming onedimensional, adiabatic flow, and that the fluid is a perfect gas.

Inlet Choking

• Impeller choking

The above equation indicates that for rotating passages, mass flow is

dependent on the blade speed.

• Diffuser Choking For choking in the diffuser, we use the stagnation conditions for

the diffuser and not the inlet, b/c stagnation conditions at the

diffuser inlet are dependent on the impeller process.

ρ0 and a0 refer to inlet stagnation conditions, which remain unchanged

ExamplesExample 1:

Example 3

Example 4