STEAM ENGINES & TURBINES CLASS NOTES FOR MECHANICAL ENGINEERING

 

https://mesubjects.net/wp-admin/post.php?post=14522&action=edit   MCQ St Turnbines 

https://www.mesubjects.net/wp-admin/post.php?post=3681&action=edit          Steam Nozzles

https://www.mesubjects.net/wp-admin/post.php?post=3613&action=edit    Q. ANS. S Turbine

STEAM ENGINE & TURBINE CLASS NOTES

FOR MECHANICAL ENGINEERING

  

Steam engine works with steam made in

the boiler. It converts heat energy of steam

into mechanical motion.  A steam engine

uses Rankine cycle ( modified Carnot Cycle)

and is thus most efficient. Thomas Savory,

Thomas Newcomen and James Watt made

the best contribution in the invention and

improvement of the steam engine. It made

an industrial revolution. Even today steam

engine is very common for electricity generation.

Mean effective pressure is a constant pressure

which acts on the piston throughout the stroke.

It produces the same work as is produced under

actual operating conditions. In steam engines,

high pressure steam moves the piston to produce

motion and power. On the other hand, steam turbines are

used to produce large power in thermal plants. Steam

turbines have better efficiency than steam engines. Steam

turbine use convergent divergent nozzles to produce a jet.

This jet strikes the turbine blades and produces power.

STEAM ENGINE

Mean effective pressure with zero clearance volume is
pm = (p1/re)(1+lnre -pb)
Mean effective pressure with clearance volume, Vc
pm = [(p1/re)(1+lnre) -pbV2-(p1-p2)Vc]/Vs
  BHP =pm LAN/60
  1. Brake Thermal Efficiency
ηB = HP/ m­ (h 1—h L)
where m ­s is the steam used per sec
1 is the enthalpy of steam entering the steam engine
L is enthalpy of feed water at back pressure
  1. Mechanical efficiency
η m e c h = BHP/Indicated HP
Friction HP = IHP –BHP = energy lost in friction
  1. Overall efficiency
ηo = BHP/heat energy obtained by burning of fuel per second = B HP/m f x C
where m f is the mass of fuel consumed per second=kg/s
C is the calorific value of the fuel per kg
  1. Indicated Thermal Efficiency
ηI  = I HP/m ­s(h 1—hL)
where m­ s is the steam used per sec
1 is the enthalpy of steam entering the steam engine
L is enthalpy of feed water at back pressure
  1. Relative Efficiency
ηRe = Thermal efficiency/Rankine efficiency

 STEAM ENGINE TERMS

Cylinder feed, m cf
Mass of steam drawn from the boiler per working stroke is cylinder feed.
Missing quantity
= (mass of steam when dry saturated — mass of steam when wet) per stroke
Specific Steam Consumption
It is expressed in either of the two ways i.e. either based on IHP or based on BHP
= Mass of steam per hour / IHP    kg/kWh
OR
= Mass of steam per hour / BHP     kg/kWh

STEAM TURBINES

Steam turbine is a prime mover which gets its rotary motion from the gradual change in momentum of steam jet. It simply changes the direction of the jet. The jet is coming at high velocity from nozzles. The jet is striking the blades of the turbine. This jet strikes the vanes and produces centrifugal force. This force runs the rotor of the turbine. Rotor coupled to a generator to produce electric power. Steam turbines produce a large percentage of power in the entire world.  Steam turbines have the highest efficiency.  Efforts are being made to improve the efficiency up to 50 % by 2024. It may be possible by reducing the losses and using improved material of construction. Steam turbines (350 bar and 3000C) use iron and nickel based alloys. Improved coatings  reduce oxidation, corrosion and erosion in the materials

 Types of steam turbines

 Impulse turbine, Reaction turbine, Combination of impulse and reaction turbine, Velocity compounded, Pressure compounding, Velocity & pressure compounding, Single stage, Multi-stage, Axial flow, Radial flow, Tangential flow, Mixed flow, Super critical pressure turbines ( p≥225 bars), Very high pressure turbines (p≥170 and p ≤ 225  bars), high pressure (p≥40 and p ≤ 170  bars), medium pressure ( 2 to <40 bars), low pressure steam turbines(p>1.2 and <4 bars), single & multi-shaft turbine, throttle governing, nozzle governing, bypass governing, Stationary turbines with constant & variable speed, Moving turbines with varying speed, Condensing turbines with regeneration, Condensing turbines with one or more extractions, Back pressure turbines and topping turbines

STEAM TURBINES-LOSSES

Losses in the steam turbine is loss of power. Losses must be minimum. There are two main types of losses in a steam turbine.
(a) Internal losses (Due to flow of steam inside the turbine): Losses due to steam Leakage, Nozzle friction losses, Blade friction losses, Disc friction losses, Gland leakage losses , Residual velocity loss, Carry-over loss, Losses due to erosion and corrosion, Throttling losses in steam turbines, Loss due to boundary layer formation on the blade surfaces
(b) External losses in steam turbines: These are the losses which do not concern the loss associated with the steam. External losses include  Bearings friction, Fluid friction at the outside rotating surfaces of the turbine, Energy loss in the bearings and governing mechanisms &Pumping power losses  

COMPARISON STEAM TURBINES & RECIPROCATING STEAM ENGINES

Sr.

No.

STEAM TURBINE

STEAM ENGINE

1.
In this there is a double conversion of energy. A jet coming from nozzles rotates the steam turbine.
Steam is directly used to overcome
the external resistance and there is no dynamic action of steam.
2.
It is versatile in application. Produces large power. It runs big ships. This runs pumps, compressors and fans.
Has limited application.
3.
Used with widely fluctuating loads.
Not suitable with fluctuating loads
4.
Thermal efficiency is high.
Thermal efficiency is low.
5.
Perfect balancing possible.
Perfect balancing is not possible
6.
Do not require robust foundations because of perfect balancing.
Require robust foundations because of imperfect balancing.
7.
There are no rubbing parts. Hence lubrication is much simpler.
Rubbing parts are present. Hence lubrication posses problems.
8.
Much higher speeds and hence more power.
Much higher speeds are not practical and hence less power.
9.
There is no additional device to achieve rotary motion.
Eccentric converts reciprocating motion into rotary motion. Thus there will be more friction and more losses.
10.
Steam turbines overloaded with little drop in efficiency.
Sudden big drop in efficiency under over-load
conditions.
11.
Rate of steam consumption does not increase with years of use.
Rate of steam consumption increases with years of use due to development of leakages.
12.
Its life is long.
Its life is less due to the rubbing action.
13.
These produce power from a few kW to around 200 MW or even higher.
Power producing capacity of steam engines is small and limited.
14.
This type of turbine develops power at a uniform rate and thus does not need a flywheel.
Development of power is non-uniform and thus  needs a flywheel.
15.
There is a less Repair maintenance
Repair maintenance is more.