Short Notes: Air and Gas Compressors | Short Notes for Mechanical Engineering PDF Download

 Page 1


Air and Gas Compressors  
Compressor: A compressor compresses air or a gas by harnessing external work 
fed from a prime mover.
The increase in the gas pressure is accompanied by the temperature rise. If the 
compressor is perfectly insulated and the compression is adiabatic then it requires 
the minor work input to increase the gas pressure. All the generated heat of 
compression is expanded to compresses the gas as no heat is allowed to escape. 
So Q = 0
Mass flow route of the gas = M and Vi = V2
Z1 = Z2,
By applying SFEE hi = h2 + (-Wc) (-) sign before Wc refers to the work done on the 
gas (system) Wc = M(h2-hi) = MCP (T2-Ti ) Nozzle 
There is no work output: W = 0
Page 2


Air and Gas Compressors  
Compressor: A compressor compresses air or a gas by harnessing external work 
fed from a prime mover.
The increase in the gas pressure is accompanied by the temperature rise. If the 
compressor is perfectly insulated and the compression is adiabatic then it requires 
the minor work input to increase the gas pressure. All the generated heat of 
compression is expanded to compresses the gas as no heat is allowed to escape. 
So Q = 0
Mass flow route of the gas = M and Vi = V2
Z1 = Z2,
By applying SFEE hi = h2 + (-Wc) (-) sign before Wc refers to the work done on the 
gas (system) Wc = M(h2-hi) = MCP (T2-Ti ) Nozzle 
There is no work output: W = 0
No heat influx or escape Q = 0 
For a horizontal disposition 
Zi =Z2
And so the SFEE applied to the nozzle boils down to
t 1 ~
IfV2» l \ , then
K - - yjZCfiT, — T: »
Pump:
• Pump is a mechanical device generally used for raising liquids from lower 
level to a higher level.
• Pump is also used to add energy to the fluid to move it from one point to 
another. This is achieved by creating a low pressure at the inlet and the high 
pressure at the outlet of the pump. Due to the low inlet pressure the liquid 
rises from where it is available and the high outlet pressure forces it up where 
it is to be stored or supplied. However, work has to be done by prime mover to 
enable it to impart mechanical energy to the liquid which ultimately converts 
into pressure energy.
• Pump is also used to increase the flow rate of the liquid •
Classification of pumps:
• In Pump, the saturated liquid is compressed and reversible adiabatically 
ending at initial pressure.
• Steady Flow Energy Equation (SFEE) for the boiler (as control volume)
h4 + Qt = h! = > Qt = h! - h4
• SFEE for turbine (as control volume)
W r = h -i - h2
where, Wt = work done by turbine.
Page 3


Air and Gas Compressors  
Compressor: A compressor compresses air or a gas by harnessing external work 
fed from a prime mover.
The increase in the gas pressure is accompanied by the temperature rise. If the 
compressor is perfectly insulated and the compression is adiabatic then it requires 
the minor work input to increase the gas pressure. All the generated heat of 
compression is expanded to compresses the gas as no heat is allowed to escape. 
So Q = 0
Mass flow route of the gas = M and Vi = V2
Z1 = Z2,
By applying SFEE hi = h2 + (-Wc) (-) sign before Wc refers to the work done on the 
gas (system) Wc = M(h2-hi) = MCP (T2-Ti ) Nozzle 
There is no work output: W = 0
No heat influx or escape Q = 0 
For a horizontal disposition 
Zi =Z2
And so the SFEE applied to the nozzle boils down to
t 1 ~
IfV2» l \ , then
K - - yjZCfiT, — T: »
Pump:
• Pump is a mechanical device generally used for raising liquids from lower 
level to a higher level.
• Pump is also used to add energy to the fluid to move it from one point to 
another. This is achieved by creating a low pressure at the inlet and the high 
pressure at the outlet of the pump. Due to the low inlet pressure the liquid 
rises from where it is available and the high outlet pressure forces it up where 
it is to be stored or supplied. However, work has to be done by prime mover to 
enable it to impart mechanical energy to the liquid which ultimately converts 
into pressure energy.
• Pump is also used to increase the flow rate of the liquid •
Classification of pumps:
• In Pump, the saturated liquid is compressed and reversible adiabatically 
ending at initial pressure.
• Steady Flow Energy Equation (SFEE) for the boiler (as control volume)
h4 + Qt = h! = > Qt = h! - h4
• SFEE for turbine (as control volume)
W r = h -i - h2
where, Wt = work done by turbine.
SFEE for the condenser
Qi = hi - f> 3
©
A sim p le steam p o w er plant rep rese n tin g c y c le
• SFEE for the pump (ds = 0)
n p = \| J ~ ^2 = J. ^ ' d p \
v * '= k t - wp
• Efficiency of the Rankine cycle,
w: ~ w? (h i-h : )- (h 4-h^'
a a (1 > j — h4)
• Steam rate
1 ke
• Heat rate
g, 1 kJ
rrr - rrp t j k\vs
Rankine cy cle on p*V, T*s and h-a 
c o o r d in a te s
Q eco = h5 - h4 (Economiser) 
Q eco = h6 - h5 = hfg (Evaporator)
Page 4


Air and Gas Compressors  
Compressor: A compressor compresses air or a gas by harnessing external work 
fed from a prime mover.
The increase in the gas pressure is accompanied by the temperature rise. If the 
compressor is perfectly insulated and the compression is adiabatic then it requires 
the minor work input to increase the gas pressure. All the generated heat of 
compression is expanded to compresses the gas as no heat is allowed to escape. 
So Q = 0
Mass flow route of the gas = M and Vi = V2
Z1 = Z2,
By applying SFEE hi = h2 + (-Wc) (-) sign before Wc refers to the work done on the 
gas (system) Wc = M(h2-hi) = MCP (T2-Ti ) Nozzle 
There is no work output: W = 0
No heat influx or escape Q = 0 
For a horizontal disposition 
Zi =Z2
And so the SFEE applied to the nozzle boils down to
t 1 ~
IfV2» l \ , then
K - - yjZCfiT, — T: »
Pump:
• Pump is a mechanical device generally used for raising liquids from lower 
level to a higher level.
• Pump is also used to add energy to the fluid to move it from one point to 
another. This is achieved by creating a low pressure at the inlet and the high 
pressure at the outlet of the pump. Due to the low inlet pressure the liquid 
rises from where it is available and the high outlet pressure forces it up where 
it is to be stored or supplied. However, work has to be done by prime mover to 
enable it to impart mechanical energy to the liquid which ultimately converts 
into pressure energy.
• Pump is also used to increase the flow rate of the liquid •
Classification of pumps:
• In Pump, the saturated liquid is compressed and reversible adiabatically 
ending at initial pressure.
• Steady Flow Energy Equation (SFEE) for the boiler (as control volume)
h4 + Qt = h! = > Qt = h! - h4
• SFEE for turbine (as control volume)
W r = h -i - h2
where, Wt = work done by turbine.
SFEE for the condenser
Qi = hi - f> 3
©
A sim p le steam p o w er plant rep rese n tin g c y c le
• SFEE for the pump (ds = 0)
n p = \| J ~ ^2 = J. ^ ' d p \
v * '= k t - wp
• Efficiency of the Rankine cycle,
w: ~ w? (h i-h : )- (h 4-h^'
a a (1 > j — h4)
• Steam rate
1 ke
• Heat rate
g, 1 kJ
rrr - rrp t j k\vs
Rankine cy cle on p*V, T*s and h-a 
c o o r d in a te s
Q eco = h5 - h4 (Economiser) 
Q eco = h6 - h5 = hfg (Evaporator)
Q sh = hf - h6 (Superheated)
_ Heat supplied - Heat rejected 
Heat supplied
= h iS - S .h T ^ jS .- S ,) 
T A S j- S , )
= 1 - -5- (S-- 5, = S ,- S ,)
T. ~ - 1
Turbine
• A turbine is a rotatory mechanical device that extracts energy from a fluid flow 
and converts it into useful work.
• A turbine is a turbo machine with at least one moving part called a rotor 
assembly, which is a shaft or drum with blades attached moving fluid acts on 
the blades so that they move and import rotational energy to the rotor.
The simplest turbines have one moving part, a rotor assembly, which is a shaft or 
drum, with blades attached. Moving fluid acts on the blades, or the blades react to 
the flow, so that they move and impart rotational energy to the rotor.
Gas, steam, and water turbines usually have a casing around the blades that 
contains and controls the working fluid.
Steam Turbine: A steam turbine is prime mover which converts high pressure 
energy and high temperature steam supplied by steam generator into shaft work 
with the low temperature steam exhausted to a condenser.
Impulse Turbines: All pressure drops of steam occur in the nozzles. No pressure 
drops as steam flow through the passage between two blades.
Before entering nozzle p0, V1
After p7 , Vi
After deflection p2, V2
• The steam flows through the nozzles and impinges on the moving blades
• The steam impinges on the buckets with kinetic energy.
• The steam may or may not be admitted over the whole circumference.
• The steam pressure remains constant during its flow through the moving 
blades.
• The blades are symmetrical.
• The number of stages required is less for the same power developed.
Momentum Conservation in Blades: Momentum of steam jet at inlet to the blades- 
momentum of steam jets at exit from the blades = momentum absorbed by wheel
Page 5


Air and Gas Compressors  
Compressor: A compressor compresses air or a gas by harnessing external work 
fed from a prime mover.
The increase in the gas pressure is accompanied by the temperature rise. If the 
compressor is perfectly insulated and the compression is adiabatic then it requires 
the minor work input to increase the gas pressure. All the generated heat of 
compression is expanded to compresses the gas as no heat is allowed to escape. 
So Q = 0
Mass flow route of the gas = M and Vi = V2
Z1 = Z2,
By applying SFEE hi = h2 + (-Wc) (-) sign before Wc refers to the work done on the 
gas (system) Wc = M(h2-hi) = MCP (T2-Ti ) Nozzle 
There is no work output: W = 0
No heat influx or escape Q = 0 
For a horizontal disposition 
Zi =Z2
And so the SFEE applied to the nozzle boils down to
t 1 ~
IfV2» l \ , then
K - - yjZCfiT, — T: »
Pump:
• Pump is a mechanical device generally used for raising liquids from lower 
level to a higher level.
• Pump is also used to add energy to the fluid to move it from one point to 
another. This is achieved by creating a low pressure at the inlet and the high 
pressure at the outlet of the pump. Due to the low inlet pressure the liquid 
rises from where it is available and the high outlet pressure forces it up where 
it is to be stored or supplied. However, work has to be done by prime mover to 
enable it to impart mechanical energy to the liquid which ultimately converts 
into pressure energy.
• Pump is also used to increase the flow rate of the liquid •
Classification of pumps:
• In Pump, the saturated liquid is compressed and reversible adiabatically 
ending at initial pressure.
• Steady Flow Energy Equation (SFEE) for the boiler (as control volume)
h4 + Qt = h! = > Qt = h! - h4
• SFEE for turbine (as control volume)
W r = h -i - h2
where, Wt = work done by turbine.
SFEE for the condenser
Qi = hi - f> 3
©
A sim p le steam p o w er plant rep rese n tin g c y c le
• SFEE for the pump (ds = 0)
n p = \| J ~ ^2 = J. ^ ' d p \
v * '= k t - wp
• Efficiency of the Rankine cycle,
w: ~ w? (h i-h : )- (h 4-h^'
a a (1 > j — h4)
• Steam rate
1 ke
• Heat rate
g, 1 kJ
rrr - rrp t j k\vs
Rankine cy cle on p*V, T*s and h-a 
c o o r d in a te s
Q eco = h5 - h4 (Economiser) 
Q eco = h6 - h5 = hfg (Evaporator)
Q sh = hf - h6 (Superheated)
_ Heat supplied - Heat rejected 
Heat supplied
= h iS - S .h T ^ jS .- S ,) 
T A S j- S , )
= 1 - -5- (S-- 5, = S ,- S ,)
T. ~ - 1
Turbine
• A turbine is a rotatory mechanical device that extracts energy from a fluid flow 
and converts it into useful work.
• A turbine is a turbo machine with at least one moving part called a rotor 
assembly, which is a shaft or drum with blades attached moving fluid acts on 
the blades so that they move and import rotational energy to the rotor.
The simplest turbines have one moving part, a rotor assembly, which is a shaft or 
drum, with blades attached. Moving fluid acts on the blades, or the blades react to 
the flow, so that they move and impart rotational energy to the rotor.
Gas, steam, and water turbines usually have a casing around the blades that 
contains and controls the working fluid.
Steam Turbine: A steam turbine is prime mover which converts high pressure 
energy and high temperature steam supplied by steam generator into shaft work 
with the low temperature steam exhausted to a condenser.
Impulse Turbines: All pressure drops of steam occur in the nozzles. No pressure 
drops as steam flow through the passage between two blades.
Before entering nozzle p0, V1
After p7 , Vi
After deflection p2, V2
• The steam flows through the nozzles and impinges on the moving blades
• The steam impinges on the buckets with kinetic energy.
• The steam may or may not be admitted over the whole circumference.
• The steam pressure remains constant during its flow through the moving 
blades.
• The blades are symmetrical.
• The number of stages required is less for the same power developed.
Momentum Conservation in Blades: Momentum of steam jet at inlet to the blades- 
momentum of steam jets at exit from the blades = momentum absorbed by wheel
in producing shaft work.
Avw = velocity of whirl at inlet - velocity of whirl at outlet, 
a = Nozzle angle subtended by nozzle axis with the direction of rotation of wheel 
Pi = Inlet blade angle 
P2 = Exit blade angle
- K = K c o s ^ U -A -J .
Blade friction factor
K= -1= (V 1 cos a - F j)(l+ k„)
For symmetrical blades: j87 = j 82
Vb = mean peripheral velocity of the blades (mean blade velocity)
d k n
= K ------
60
Dm = mean diameter of the blade
• Tangential thrust impressed by jet on the blades Pt = W s . AVwW s = steam flow 
rate
• Axial thrust
P« = W 'J - AVa > 
where
Av = v sin B, — v sin 5,
• Area of the flow or blade annulus.
4 = 2) . ( A - A 2)= xD wfi
D7 is the root diameter, D2 is the tip diameter, h*, is the height of the blades.
Blading or diagram efficiency