Rotary compressor

1. Rotary Compressors

3. Root Blower

7. Vane Blower

8. Vane Blower

9. Centrifugal Compressor
• Centrifugal compressors accelerates the velocity of the
gases (increases kinetic energy) which is then converted
into pressure as the air flow leaves the volute and enters
the discharge pipe.
• Deliver much higher flow rates than positive displacement
compressors
• For low pressure ratios (< 4:1), if higher pressure ratio with
larger unit
– prefer axial flow compressor
• Usually operate at speeds > 3,000 rpm.
• Smaller length, contaminated atmosphere doesn't affect
the performance
• Disadvantages- larger frontal area and lower maximum
efficiency
Applications
Most well-known centrifugal compressor applications are gas turbines
and turbochargers.
Either or both axial and centrifugal compressors are used to provide
compressed air to Modern gas turbines which operate on the
Brayton cycle. The types of gas turbines that most often include
centrifugal compressors include turboshaft, turboprop, auxiliary
power units, and micro-turbines.

10. Centrifugal Compressors
A centrifugal compressor is a radial flow rotodynamic fluid
machine that uses mostly air as the working fluid and utilizes the
mechanical energy imparted to the machine from outside to
increase the total internal energy of the fluid mainly in the form of
increased static pressure head.
A centrifugal compressor essentially consists of three components.
1.stationary casing
2.rotating impeller
3.diffuser

13. Centrifugal Compressor
Inlet
The inlet to a centrifugal compressor is typically a
simple pipe. It may include features such as a valve,
stationary vanes/airfoils (used to help swirl the flow)
and both pressure and temperature instrumentation.
Figure 1.
Turbocharger
Construction and
Centrifugal impeller
The key component that makes a compressor centrifugal is the centrifugal impeller. It is
the impeller’s rotating set of vanes (or blades) that gradually raises the energy of the
working gas.
Diffuser
The next key component to the simple centrifugal compressor is the diffuser.
Downstream of the impeller in the flow path, it is the diffuser’s responsibility to convert
the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing)
the gas velocity. Diffusers can be vane less, vaned or an alternating combination.
.
Collector / Casing
The collector of a centrifugal compressor can take many shapes and forms. When the
diffuser discharges into a large empty chamber, the collector may be termed a Plenum.
When the diffuser discharges into a device that looks somewhat like a snail shell, bull’s
horn or a French horn, the collector is likely to be termed a volute or scroll. As the name
implies, a collector’s purpose is to gather the flow from the diffuser discharge annulus
and deliver this flow to a downstream pipe. Either the collector or the pipe may also
contain valves and instrumentation to control the compressor.

15. STATIC AND STAGNATION PROPERTIES
The velocities of air encountered in centrifugal
compressors are very high as compared to that in reciprocating
compressors. Therefore, total head quantities should be taken
into account in the analysis of centrifugal compressors. The total
head quantities account for the kinetic energy of the air passing
through the compressor.
Consider a horizontal passage of varying area of cross-
section, as shown in Fig. , through which air flows from left to
right. Assuming no external heat transfer and work transfer to
the system, the steady flow energy equation for one kg mass of air
flow can be written as:

16. Temperature T is
called the ‘static
temperature’; it is the
temperature of the air
measured by a thermometer
moving with the air velocity.
If the moving air is brought to
rest under reversible adiabatic
conditions, the total kinetic
energy of the air is converted
into thermal energy, thereby
increasing its temperature and
pressure.
This temperature and pressure
of the air are known as
‘stagnation’ or ‘total head’
temperature and pressure. The
stagnation quantities are
denoted by a suffix notation o.

17. Velocity triangle

18. Velocity triangle Nomenclature:
Suffix ‘1’ denotes parameters at inlet and ‘2’ at
outlet.
β1 = Angle of the rotor blade at inlet
β2 = Angle of the rotor blade at outlet
α1 = Angle made by entering air or exit angle of
guide blade
α2 = Angle made by the outgoing air from rotor
blade
V1 and V2 = Absolute velocity of air at inlet and
outlet of rotor, m/s
Vr1 and Vr2 = Relative velocity of air at inlet and
outlet of rotor, m/s
Vf1 and Vf2 = Velocity of flow at inlet and outlet
of rotor, m/s
Vw1 and Vw2 = Velocity of whirl at inlet and outlet of rotor, m/s
Vb1 and Vb2 = Mean peripheral velocity of blade tip at inlet and outlet, m/s
R1 and R2 = Inner and outer radii of rotor, m
m = Mass flow rate of air, kg/s
At the inlet to rotor, air enters with absolute velocity V1 making an angle α1
to the direction of motion of blade (usually α1 = 90°), without any shock and
its whirl component Vw1 = 0.

19. Work Requirement (Euler’s Work) for a Centrifugal
Compressor:
The work required/kg of air in a stage of a centrifugal compressor
can be found by applying the moment of momentum theorem.
As per the Newtonian equation, force is given by rate of change of
momentum.
Similarly, rate of change of moment of momentum about the centre
of rotation gives torque.

20. Work Requirement (Euler’s Work) for a Centrifugal Compressor:

21. Different vane shape
• The impellers may be classified depending on the exit angle β2 into
(i) Backward curved vanes, (ii) Radial vanes and (iii) Forward curved
blades.
21

22. Slip Factor of Centrifugal Compressors:
Under ideal conditions, fluid particles follow exactly the same path
of blade profile such that relative velocity at impeller outlet tip is
inclined with the tangential direction at blade tip angle β2 only,
irrespective of mass flow rate, speed etc.
Such an ideal flow is possible when impeller has infinite number of
blades of no thickness.
In actual practice, when impeller has finite number of blades, fluid
is trapped between the impeller vanes due to its inertia and the fluid
is reluctant to flow over the impeller.
This causes a pressure difference across the blades. There is a high
pressure at the leading face and low pressure at the trailing face.
This pressure difference generates a relative velocity gradient and
formation of eddies.
The fluid is thus discharged at a certain average angle β’2 which is
less than β2.
Therefore fluid is said to have slipped with respect to impeller during
its flow across it.

23. Slip and Slip Co-efficient
• It is assumed that the velocities are constant over the cross sectional
area.
• But in actual practice this assumption is not correct as shown if fig,

24. Slip factor depends upon number of blades and is usually 0.9. It does not
reduces the efficiency but only reduces the head developed.

25. Pre-rotation or Pre-whirl
• We know that the velocity at
inlet have more effect on Mach
number at inlet.
• The relative velocity at the inlet
should be minimum, which
reduces the Mach number for a
given eye tip diameter.
• For a fixed eye tip diameter the
Mach number can be reduced by
providing pre-whirl at the inlet
using guide vanes.

26. Pressure coefficient

27. A centrifugal compressor runs at a speed of 1500 r.p.m. and delivers 30 kg of
air per second. Exit radius is 0.35 m, relative at exit velocity is 100 m/s at an
exit angle of 750 . Assume axial inlet and T1 = 300k and P1 =1 bar. Calculate (a)
The Torque (b) The Power required (c) The Ideal head developed
Speed N =15000 rpm, T1 = 300k ,
Mass flow rate m = 30 kg/s , Exit diameter D2 = 0.7 m,
Exit relative velocity Vr2 =100 m/s,
Outlet blade angle (β2) =750 Axial Inlet α1 = 900 ,
Vw1 = 0, (V1=Vf1)
Vb2 = (πD2N)/60 = (π x 0.7 x 15000)/60
= 549.8 m/s
(Vb2 – Vw2 )/ Vr2 = cos β2
Vw2 = 549.8 – 100 cos(750 ) = 523.89 m/s
(a) The Torque T = m(R2Vw2/g = (30 x 0.35 x 523.89 )/9.81= = 5500.91 N-m
(b) The Power required = T ω = (5500.91 x 2π x15000)/(1000x60) = 8640.8 kw
(c) The Ideal head developed (H) = Vb2 Vw2/g = 550 x 523.89/ = 2937.23 m.

28. A centrifugal compressor
develops a pressure ratio of 5
and air consumption of 30 kg/s.
The inlet temperature and
pressure are 15°C and 1 bar
respectively. For an isentropic
efficiency of 0.85, the power
required by the compressor will
be nearly.