steam engine

By:Naphis AhmadAssistant professorJIT, Barabanki

05/01/2023 Jahangirabad institute of technology 1

Unit III

WORKING OF STEAM ENGINE


Simple steam engine shown is a horizontal double acting steam engine having cylinder fitted with

cylinder cover on left side of cylinder. Cylinder cover has stuffing box and gland through which the

piston rod reciprocates. One end of piston rod which is inside cylinder has piston attached to it.

Piston has piston rings upon it for preventing leakage across the piston. Other end of piston rod

which is outside cylinder has cross head attached to it. Cross head slides in guide ways so as to

have linear motion in line with engine axis. Cross head is connected to the small end of connecting

rod by the gudgeon pin. Big end of connecting rod is mounted over crank pin of the crank.

Reciprocating motion of piston rod is transformed into rotary motion of crankshaft by cross head,

connecting rod and crank.Cross head transmits the motion of piston rod to connecting rod. Cross

head guide ways bear the reaction force.Crank is integral part of crank shaft mounted on bearings.


D slide valve opens and closes the exhaust and inlet passages from steamchest to engine cylinder. Steam chest has two openings

one for inlet of live steam and other for exit of dead or expanded steam. Live steam refers to the steam having sufficient

enthalpy with it for doing work in steam engine. Dead steam refers to the steam having insufficient enthalpy with it and does

not have capability to produce work. High pressure and high temperature steam (live steam) enters from main inlet passage into

steam chest. D-slide valve occupies such a position that passage (port 1) from the steam chest to engine cylinder gets opened.

High pressure steam enters cylinder and forces piston towards other dead centre. Linear motion of piston is transformed into

rotation of crankshaft through crosshead, connecting rod, gudgeon pin and crank.When piston reaches other dead centre then

the corresponding displacement of valve rod causes shifting of D-slide valve such that other passage (port 2) from steam chest

to cylinder gets opened and passage 1 comes in communication with the exhaust passage. power.


Let us take steam engine without clearance in which live steam enters the cylinder at state 1 and

steam injection continues up to state 2. Point 2 is showing the state at cut-off. Subsequently steam is

expanded up to state 3 theoretically and stroke gets completed. The expansion process is of hyperbolic

type. Hyperbolic expansion process is one having the P-V = constant. In actual practice this expansion

is not continued upto 3 due to the fact that positive work available from engine in later part of stroke is

much less than negative work required for maintaining piston movement. Hence expansion process is

terminated even before this piston reaches the extreme position (dead centre).

Modified rankine cycle


Here expansion process is shown to be terminated at state 5 and exhaust port is opened at this point,

thereby causing sudden drop of pressure as shown in 5-6. Exhaust of dead steam occurs from 6 to 4. It is

obvious from P-V diagram that the terminating expansion process before piston reaching dead centre

shortens the expansion stroke length although actual work output from engine does not get affected. This

modified form of cycle is called as ‘modified Rankine cycle’. In case of steam engine with clearance

volume the modified Rankine cycle is as shown by 125641 while for engine without clearance volume it is

given by 125641. Practically steam engine always has clearance volume but for the ease of mathematical

analysis it may be considered to be without clearance volume. Therefore such cycle without clearance is

also termed as hypothetical cycle. T-S representation of the modified Rankine cycle is shown in Fig.


INDICATOR DIAGRAM

Indicator diagrams for steam engine are shown in Fig. Hypothetical indicator diagram shown by cycle abcde and the actual indicator diagram for an engine are given here. Hypothetical indicator diagram is obtained considering all processes in cycle to be ideal and assuming no heat loss and pressure drop etc.


Hypothetical indicator diagram shows the indicator diagram having steam admission beginning

at a and continuing up to b. Expansion occurs between b and c in hyperbolic manner. At c there

occurs sudden pressure drop due to opening of exhaust valve up to d. Piston travels from d to e

and steam injection begins at a and thus cycle gets completed. In this hypothetical diagram all

processes are considered to occur as per their theoretical assumptions. While actual diagram is

based upon the actual occurrence of all processes. This is a reason why actual diagram shown

by abcde differs from hypothetical indicator diagram. In hypothetical diagram it is assumed that

there occurs no pressure drop, valves open and close instantaneously, expansion occurs

following hyperbolic process and admission of steam and its exhaust occur at end of strokes

etc.The actual indicator diagram differs from hypothetical indicator diagram because of the

following factors.


(i) Practically there shall be pressure drop due to friction, throttling and wire drawing etc.Friction, throttling and wire drawing occur in valves and ports connecting steam chest and cylinder thereby causing pressure drop.(ii) Inlet and exit valves (ports 1, 2 and 3) can never be opened or closed instantaneously, which means that there shall always be some time required for completely opening or closing the valves. Therefore, in order to ensure entry of maximum amount of high pressure and temperature (live steam) steam it is desired to advance the opening of inlet valve and suitably modify the operation of valves for remaining processes. Thus it may be said that actuallythe inlet, cut off and release occur gradually.(iii) Expansion in steam engine does not occur hyperbolically in actual process due to varying heat interactions. This causes shift in expansion line from bc in hypothetical engine to the actual expansion line as shown in actual indicator diagram (bc). It may be understood that due to condensation of steam the expansion follows some other law.(iv) Exhaust of steam begins at c in order to overcome limitation of gradual opening of exhaust valve. Also the sudden pressure drop is not possible. Actual pressure drop occurs during cd in non-instantaneous manner upto back pressure value.


SATURATION CURVE AND MISSING QUANTITY

In a steam engine it could be seen that the same passage acts as the passage for inlet of live steam (high

pressure and high temperature steam) and exit of dead steam (low pressure and low temperature steam).During

the steam admission stroke passage walls, valve face/port and cylinder walls become hotter and in exhaust

stroke these surfaces become cooler due to low temperature steam passing through passages. Subsequently

when high temperature steam again enters the engine cylinder then the hot steam comes in contact with cool

surfaces which lead to condensation of a portion of fresh steam entering. Condensation is visible till the

temperature of contact surfaces equals the hot steam temperature.Later on during exhaust stroke when cooler

steam exits through valve then cold steam comes in contact with hot surfaces (surfaces are hot due to hot steam

admission). Due to cold steam contact with hot surfaces reevaporation of condensed steam occurs. This

reevaporation of steam slightly increases work and reduces wetness of steam. During admission of steam the

condensation causes loss of steam without doing work.


Condensation during intake generally causes increase in steam consumption by up to 40%.Condensation of

steam can be prevented to some extent by one or more of the following ways.

(a) Superheated steam may be supplied to engine thereby offering great margin upto the condensation state. It

will allow for only reduction in degree of superheat due to contact with low temperature surfaces and thus

condensation is prevented.

(b) Condensation can also be controlled by providing steam jacket around cylinder wall so as to maintain

engine’s contact surfaces at high temperatures thereby, preventing condensation.

(c) Valve timings can be modified so as to result in greater degree of compression prior to admission of steam.

This increased compression yields increased temperature of residualsteam therefore, causing increased

temperature of engine surfaces.


Thus it is seen that condensation and reevaporation are

processes which shall always be there whether in large

quantity or small quantity in an actual engine. Because of this

condensation the actual volume occupied by steam will be

less than hypothetical (theoretical) volume. This difference

between the actual volume of steam and theoretical volume of

steam at any point is known as ‘missing quantity’. If we

assume all steam states during expansion to be dry and

saturated and obtain the expansion curve throughout the

stroke, then such curve is called saturation curve. Figure

shows the saturation curve and missing quantity of steam.


Here bc shows the actual expansion curve and bc is the expansion curve considering all states ofsteam

during expansion to be dry and saturated. At any point during expansion the amount of steam condensed

can be accounted by missing quantity. At some pressure, line ghi shows the volume occupied by steam in

actual engine as Vgh and theoretical volume occupied being Vgi. Length ‘gh’ and ‘gi’indicate these

volumes. Using these volume values dryness fraction neglecting leakage and other losses can be given by,

xh =gh/gi

Similarly dryness fraction at ‘k’, xk =jk/j l


CHOKED FLOW

Let us consider a converging nozzle as shown in Fig. with arrangement for varying back pressure.

A valve is provided at exit of nozzle for regulating the back pressure at section 2-2. Let us denote

back pressure by pb. Expansion occurs in nozzle from pressure p1 to pb.

Initially when back pressure pb is equal to p1 there shall be no flow through the nozzle but as back

pressure pb is reduced the mass flow through nozzle increases. With the reduction in back pressure a

situation comes when pressure ratio equals to critical pressure ratio (back pressure attains critical

pressure value) then mass flow through nozzle is found maximum. Further reduction in back pressure

beyond critical pressure value does not affect the mass flow i.e. mass flow rate does not increase

beyond its’ limiting value at critical pressure ratio. Thus under these situations flow is said to be choked

flow or critical flow.


A nozzle operating with maximum mass flow rate condition is called choked flow nozzle. At the critical pressure ratio the velocity at exit is equal to the velocity of sound. If the back pressure is reduced below critical pressure then too the mass flow remains at maximum value and exit pressure remains as critical pressure and the fluid leaving nozzle at critical pressure expands violently down to the reduced back pressure value. Graphical representation of mass flow rate with pressure ratio and variation of pressure along length of nozzle explain the abovephenomenon. State a refers to the state having back pressure more than critical pressure, state c refers to the state having back pressure equal to critical pressure and state d refers to state having back pressure less than critical pressure.


OFF DESIGN OPERATION OF NOZZLE

Design operation of nozzle refers to the nozzle

operating with pressure ratio equal to critical pressure

ratio and maximum discharge rate per unit area then

nozzle is said to be operating under design conditions.

If the nozzle does not operate under design conditions

then it is called off design operation of nozzle.

Depending upon the back pressure value in reference to

design value of pressure at exit of nozzle, the nozzle

can be classified as under-expanding, over-expanding

nozzles.


Inspite of the inside surface of nozzle being smooth the frictional losses always prevail due to friction

between fluid and nozzle surface and friction within fluid itself. Due to friction prevailing during

fluid flow through nozzle the expansion process through nozzle becomes irreversible. Expansion

process since occurs at quite fast rate and time available is very less for heat transfer to take place so

it can be approximated as adiabatic. Friction prevailing during flow through nozzle causes heat drop

by about 10–15% and reduces the exit velocity. For the flowing fluid to be gas the T-S diagram

representation is as follows:

EFFECT OF FRICTION ON NOZZLE


Non ideal operation of nozzle causes reduction in enthalpy drop. This inefficiency in nozzle can be accounted for by nozzle efficiency. Nozzle efficiency is defined as ratio of actual heat drop to ideal heat drop. Nozzle efficiency, Nozzle =Actual heat drop/Ideal heat drop


Nozzle is called under-expanding if the back pressure of nozzle is below he designed value of pressure at exit of nozzle. Nozzle is called over expanding if the back pressure of a nozzle is above designed value of pressure at exit of nozzle. Detail discussion about the off design operation of nozzle is given ahead for convergent and convergent-divergent nozzle.

Convergent nozzle: Let us look at convergent nozzle having arrangement for varying back pressure. Fluid enters the nozzle at state 1, say pressure p1. Variation of back pressure using valve at exit of nozzle shows the pressure and velocity variation as shown in Fig. Following significant operating states are shown here.


The one dimensional steady flow through a duct is shown above. For control volume shown the principle of conservation of mass, energy and Newton’s law of motion may be applied.By Newton’s law of motion, F = m a where F is the resultant force acting on system of mass ‘m’ and acceleration ‘a’.Pressure waves and Sound waves: Let us consider a cylindrical duct having piston on one end for generating the pressure wave. Figure 13.3 shows the arrangement for producing a pressure wave moving to right with velocity ‘a’. Sound wave is also a small pressure disturbance that propagates through a gas, liquid or solid with velocity ‘a’ that depends on the properties of medium.


Supersaturation

The phenomenon of supersaturation in steam nozzles is also called

as supersaturated flow or metastable flow in steam nozzle. When

superheated steam flows through a nozzle and expands upto the

back pressure such that exit state of steam lies in wet region, then

during expansion steam vapours expand isentropically and slowly

get condensed up to exit state. During such expansion steam also

passes across saturated steam line or saturation line having unity

dryness fraction. Thus it is obvious that expansion of steam is

accompanied by simultaneous state change from superheated state

to wet state.At every point along expansion line there exists a

mixture of vapour and liquid in equilibrium. An expansionprocess

starting at 1 goes up to state 2 in thermal equilibrium as shown on

T-S and h-s diagram.


Superheated steam undergoes continuous change in state and becomes dry saturated steam at state 3 and subsequently wet steam leaving steam turbine at state 2. Some times expansion of steam occurs in metastable equilibrium or in equilibrium in which change of steam state could not maintain its pace with expanding steam. This phenomenon in which change of steam state could not occur simultaneously with expanding steam in nozzle is called phenomenon of supersaturation and flow is called supersaturated flow or metastable flow.


Phenomenon of supersaturation causes increase in discharge by 2–5% because of increase in density at throat and

also the heat drop gets slightly reduced thereby causing reduced velocity at exit. Supersaturation causes slight

increase in dryness fraction and entropy.


steam engine

Engineering