251 RankineCycle MAR

7/28/2019 251 RankineCycle MAR

1/17

1

Vapour Powered Cycles

Prepared by

PM Muhammad Abd Razak

FKM, UiTMPP


2/17

2

VAPOUR POWERED CYCLE

From the previous chapter

The Carnot cycle is the most efficient cycle operating betweentwo constant temperature limits (TH & TL).

But it is not practical for power cycles.

Why??? Process 1-2 : the heat transfer

process in the saturation region isseverely limited. (eg 374C for waterand as P , hfg )Process 2-3: The turbine cannot

handle steam with a high moisturecontent which will cause erosion andwear on the blades.Process 4-1: not practical to design

a compressor that handles twophases(L+V)


3/17

3
http://prevpage/


4/17

4

The impracticalities of the Carnot cycle are removed by modifyingthe cycle to the Rankine Cycle:

State 4 is allowed to becomes saturated liquid fully.Compression is in the liquid state, therefore less powerneeded.

Heat addition is now at constant pressure (instead of constanttemperature & pressure)

Heat addition into the cycle can be increased by superheatingat constant pressure.

4

1

2

3qout

qin


5/17

5

The Ideal Rankine Cycle now consist of :1 2 Isentropic compression in liquid state (in a pump)2 3 Constant pressure heat addition (in a boiler)

3 4 Isentropic expansion (in a turbine)4 1 Constant pressure heat rejection (in a condenser)

Ideal Rankine Cycle

Rearranging the state points.


6/17

6

From SFEE


7/17

7

Then

Net work, wnet = w = w34 + w12

= (h3 h4) - (h2 h1)

Heat supplied q23 = (h3 h2)

The Ideal Rankine Cycle thermal efficiency

)(

)()(

23

1243

hh

hhhh

q

w

in

netRankine

==

And .this is a criteria of performance

Note:The term (h2 h1) is the pump work which is very small

compared to the turbine work and sometimes is ignored.


8/17

8

Other criteria of performances are

the work ratio

turbine

pumpturbine

w

ww

outputworkgross

outputworknetratiowork

==

)(

)()(

43

1243

hh

hhhhratiowork

=

the specific steam consumption (ssc) which is the amount ofsteam required to produce a unit power output

)(

1

)( 4334hhwm

mssc

==


9/17

9

DEVIATION OF ACTUAL VAPOUR POWER CYCLES FROMIDEALIZED ONES

The actual vapour power cycle differs from the ideal Rankinecycle due to irreversibilities in various components. 2 mainsources are :

Fluid friction (normally ignored) heat loss to the surroundings (deviation from adiabatic &isentropic process)


10/17

10

Isentropic efficiency of pump isentropic efficiency of turbine

HOW TO INCREASE THE EFFICIENCY OF THE RANKINECYCLE

The thermal efficiency can be improved by: increase of the average temperature of heat addition to the

working fluid in the boiler

decrease the average temperature of heat rejection from theworking fluid in the condenser


11/17

11

1. Superheating the Steam to Higher Temperatures

Through superheating the steam: Increase in heat input Increase of the net work

decreases the moisture content ofthe steam at the turbine exit Most importantly, increase in

thermal efficiency

The superheat temperature is limitedby metallurgical considerations.


12/17

12

2. Increasing the Boiler Pressure

For a fixed turbine inlet temperature: the cycle shifts to the left the moisture content of steam at the

turbine exit increases.

This side effect can be corrected byreheating the steam.

3. Lowering the Condenser Pressure

The condenser usually operatesbelow the atmospheric pressure.

Side effect: increases the moisture

content of the steam at the turbine.


13/17


14/17

14

A single reheat in a modern power plant improves the cycleefficiency by 4 to 5%.

The reheat temperatures are very close or equal to the turbineinlet temperature.

Heat transfer to the feedwaterfrom 2-2 is at a relatively low

temperature.

This lowers the average heat-addition temperature and thus

the cycle efficiency.

THE IDEAL REGENERATIVERANKINE CYCLE

Tavg


15/17

15

Some steam is extracted from the turbine at various points(eg at 6) and is used to heat the feedwater.

The device where the feedwater is heated by regeneration iscalled a regenerator, or a feedwater heater (FWH).

Heat is transferred from steam to the feedwater either by

mixing the two fluid streams directly (open feedwaterheaters) or without mixing them (closed feedwater heaters).


16/17

16

An open (or direct-contact) FWH is basically a mixingchamber

the steam extracted from the turbine mixes with theincoming feedwater, releasing its latent heat to thefeedwater.

the mixture leaves the heater as saturated liquid at theheater pressure.

For a unit mass of steam supply to the boiler, the amountextracted at 6 is y kg.

SFEE for the FWH


17/17

17

For the regenerative cycle 1 kg

1-y kg

y kg

1-y kg

1 kg

Documents

251 RankineCycle MAR