UNIT ISTEAM GENERATORS

Types and classification Fire tube – Water tube

Low Pressure – High pressure Stationary – Mobile

Power generation – Processing Coal fired – Oil and gas fired Vertical – Inclined – Horizontal

Fire tube boilersCochranCornishLancashireMarineLocomotive

Water Tube BoilersSimple vertical boilerBabcock WilcoxStirling

1. Safety valve2. Pressure gauge3. Water level indicator4. Steam stop valve5. Fusible plug6. Manholes, handholes7. Blow off cock8. Feed pump

Superheater Economiser Steam water separator Air preheater

Performance testing To find

Equivalent evaporationBoiler efficiencyLossesHeat balance sheet

Factor of evaporation h-hf /2257 E = total heat required to evaporate

feed water from and at 100oC E= me(h-hf)/2257, where me is mass of

steam actualy produced in kg/kg of fuel or like units

Efficiency of boiler = ms (h-hf)/mf.C

Capacity required, pressure and temperature of steam

Base load or peak load Place of erection of boiler Fuel and water available (Quality and

quantity) Probable permanency of the station

Losses due to unburnt coal Losses due to moisture present in coal Losses due to sulphur like elements Heat lost in flue gases Radiation heat loss

Fire Tube boiler Water Tube Boiler

Low pressure boiler p<80 bar High pressure boiler p>80 bar

Shell must be present Shell need not be there

Forced circulation very difficult Forced circulation makes the heat

transfer more effective

Explosion risk less Explosion risk more

Transportation and Erection

difficult

Transportation and Erection easy

Fixed capacity Capacity can be increased by

increasing the pressure

Scale formation and thus less

heat transfer

Forced circulation and less or no

scale formation

Lancashire bolier, Cochran

boiler

Babcock and Wilcox boiler

STEAM NOZZLES

In steam turbines to increase velocity of steam

In steam injectors to pump water into the boiler

In processing plants for drying the chemicals etc

Isentropic expansion C2 = [2(h1-h2)]

1/2 m/s where C2 is the exit velocity, h1 and h2 are the enthalpy of steam at inlet

of the nozzle and at the exit of the nozzle respectively (in J)

Effect of friction •To increase dryness fraction of the steam•To reduce the total heat drop and thus reduce the exit velocity of the steam coming out of the nozzle

STEAM TURBINES

Rotary machine to convert heat energy of steam in to shaft work

Impulse turbine and reaction turbine Used in power plants First reaction turbine is hero engine Single stage – multistage Governing is needed to control the

speed vis-à-vis load

Steam TURBINE STEAM ENGINE

ROTARY Balancing and

lubrication easy Less vibration Less linkages Does not Need

flywheel Used in power plant Less losses Costly

RECIPROCATORY Balancing and

lubrication difficult More vibration More linkages Need flywheel Used in only small

engines More losses cheap

Impulse TURBINE Reaction turbine

Works on impulse principle

Small in size More losses More power per stage Nozzle present Symmetric blades Does not need pressure

tight casing Flow only through

nozzle Cheap DeLaval turbine

Works on reaction principle

Big in size Less power per stage No nozzles only guide

blades Aerofoil blades Air tight casing

needed Flow through the

entire annular space Costly Parson turbine

I C ENGINES

A reciprocating device that converts heat energy into shaft work

As per thermodynamic cycleOtto cycleDiesel cycleDual Cycle

As per StrokeTwo strokeFour stroke

Vertical engines Horizontal ingines Incline engines Inline engines Radial engines V-engines Opposed cylinder engines Single cylinder Multi cylinder engines

Automobiles Agricultural equipments Power generation Earth movers Marine applications Rail locomotives

To Cool the IC engine

To lubricate the moving parts of an IC Engine

To inject diesel into the combustion chamber at very high pressure for atomisation

Pushing out the burnt gases out of the cylinder before taking the fresh charge is called as scavenging.

In 4-stroke engine scavenging takes place in exhaust stroke.

If scavenging is poor, then power produced will be reduced

Supplying more air during the inlet or suction stroke by pressure is called supercharging.

This is done to improve volumetric efficiency

This increases the net power produced by the engine.

Supercharging is carried out by turbocharger, which is driven by the exhaust gas from the engine

In SI engine ignition takes place before the TDC of the piston due to certain circumstances (like preignition). This is called as detonation.

Isooctane has zero detonation characteristics and any fuel is measured in octane rating.

Due to the combustion, different wave fronts are formed inside the cylinder and the wavefronts compress the already compressed fuel. This increases the temperature and the compressed but yet to be ignited fuel burns and opposes the wave front thus producing knocking

Knocking is measured in Cetane rating

To find the power and performance characteristics, the performance tests such as brake power test, Morse test are conducted

Indicated power (IP) is the power produced inside the cylinder – measured by indicator

IP = pLANk/60 (Watt) Brake power (BP) is the power obtained in a

dynamometer outside the flywheel shaft BP = 2πNT/60 (Watt) Friction power = indicated power – Brake

power

Air standard efficiency Indicated thermal efficiency Brake thermal efficiency Mechanical efficiency Volumetric efficiency

Heat carried out by exhaust gases Heat carried out by cooling fluid Heat lost due to friction power Unaccountable losses

SI ENGINE Compression ratio 1:8 Petrol fuel Spark ignition Carburetor Need current for ignition More air std efficiency Lighter cylinder Less heat and vibration Lighter flywheel Cooling, balancing and

lubrication easy

CI ENGINE Compression ratio 1:22 Diesel fuel Compression ignition Fuel injector Does not need current Less air std efficiency Heavier cylinder Vibration and heat more Heavier flywheel Cooling, balancing and

lubrication difficult

One power stroke in one revolution

Lighter flywheel Suitable for small engines Lubrication difficult High specific power High speed More pollution, scavenging

difficult Starting easy Special design for piston No valves only ports High specific fuel

consumption Low volumetric efficiency

One power stroke in TWO revolutions

Heavier flywheel Suitable for heavy engines Lubrication easy Low specific power Low speed Less pollution, separate

exhaust stroke Starting difficult Simple design for piston valves present Low specific fuel

consumption High volumetric efficiency

GAS TURBINES

A rotary device, (a prime mover) transforms heat energy of gases into mechanical work or shaft work

An external combustion engine Works on Brayton thermodynamic cycle

(or reverese Joule’s cycle) Used in airplanes, turbochargers and

power generation Two types of gas turbines are Open cycle Closed cycle

Processes1-2 Isentropic compression 2-3 Constant pressure heat addition 3-4 Isentropic expansion (power process) 4-1 constant pressure heat rejection

Starting motor

Atmospheric air

Exhaust gases

Generator

Fuel

Air Compressor

Gas Turbine

Open cycle Mixing type

combustion chamber

Air and gas as medium

Aviation fuel as fuel Relatively cheap High specific power Used in airplanes Power cannot be

increased

Closed cycle Non-mixing type Helium or liquid

sodium medium Any low quality fuel Costly Low specific power Power plants Power can be

increased by increasing the pressure ratio

Gas turbine Rotary device High speed prime

mover Aviation fuel as fuel Less balancing Difficult to start Used in airplanes Lubrication easy No flywheel Governing difficult

IC Engine Reciprocating

device Low speed Petrol, diesel as fuel Complicated

balancing Easy to start Automobiles, Power

plants Lubrication difficult Flywheel must Governing easy

Net Power Produced = Work done by Turbine – Work done on compressor

W = Wt – Wc

Work ratio = W /Wt

Efficiency of the Turbine system= (Qs – Qr) /Qs

= [(T3-T2) – (T4-T1)] / (T3 – T2)

= 1 – (1 / rp (γ-1)/ γ)

Intercooling Reheating Regeneration Combination of the above