Thermodynamic 2

8/12/2019 Thermodynamic 2

1/33

Thermo 2

Lecture set 1

ByEngr. Rowie Carpio


2/33

Heat engine

A device that converts (part of) heat energy towork.

Operates on a cycle

Example:

Internal combustion engines

Gasoline and diesel engines Steam powerplant

Rankine cycle


3/33

Heat engines

Essential to all heat engines are:

1. absorption of heat into the system at

high temperature;

2. rejection of heat to the surroundings

at a lower temperature;

3. and the production of work


4/33

Heat engine efficiency

The efficiency of all heat is fundamentally limited by Carnot's theorem!


5/33

Carnot engine

2ndLaw of Thermodynamics

How efficient can a heat engine be?

What determines the upper limit?


6/33

Carnot theorem

States that: All irreversibleheat engines between two heat

reservoirs are less efficient than a Carnot

engineoperating between the samereservoirs.

All reversibleheat engines between two heat

reservoirs are equally efficient with a Carnotengine operating between the same

reservoirs.


7/33

Carnot engine

is a heat engine operating at completelyreversible manner.

Ideal engine

Carnot cycle:


8/33

Carnot engine

The efficiency of all heat engines is fundamentally limited by Carnot's

theorem!

No engine can operate with efficiency greater than that of the Carnot

engine.

Carnot engine is an ideal engine! It does not exist in reality!

The temperature is in the Kelvin or absolute scale

This efficiency is called the Carnot efficiency


9/33

Carnot cycle

1-2: Reversible isothermal expansion of the gas at the "hottemperature, T

H (isothermal heat addition or absorption).

2-3: Adiabatic expansion of the gas (isentropic work output).

3-4: Reversible isothermal compression of the gas at the "cold" temperature, TC.

(isothermal heat rejection).

4-1: Isentropic compression of the gas (isentropic work input).

Recall:Adiabaticisentropic


10/33

Carnot cycle


11/33

Example 1

Carnot theorem/engine


12/33

Example 2



13/33

Example 3



14/33

Example 5


15/33

Real heat engines

Take note:

All real engines are irreversible!!

No engine can operate with efficiency greater than that of the Carnot engine.

Rankine Cycle Steam engine Steam power plant

Stirling cycle

Otto cycle Gasoline/petrol

enginesDiesel cycle Diesel engine

Ericsson cycle

Brayton cycle Jet engine


16/33


17/33

Mollier (H-S) diagram


18/33

Rankine cycle

Is the basis of operation of a the steam powerplant.

The working fluid most commonly used is water,though other liquids can also be used.

Rankine cycle design is used by most commercialelectric power plants.

The traditional steam locomotive is also acommon form of the Rankine cycle engine.

The Rankine engine itself can be either a pistonengine or a turbine.


19/33

Rankine cycle

Consists of four steps:


20/33

The steam power plant


21/33

Practical power cycle

Accounts for the irreversibility of the turbine in practical (real situation)


22/33

The steam power plant


23/33

Effect of Pressure and Temperature in Rankine Cycle (1)

Effect of exhaust pressure andtemperature on the rankine cycle

Exhaust pressure drop from P4 to P4 with the

corresponding decrease in temperature at

which heat is rejected. The net work is increased

by area 1441221 (shown by the

shading). The heat transferred to the steam is

increased by area a22aa. Since these two

areas are approximately equal, the net result is

an increase in cycle efficiency. This is also evident from the fact that the

average temperature at which heat is rejected is

decreased.

Note, however, that lowering the back pressure

causes the moisture content of the steam

leaving the turbine to increase.

This is a significant factor because if themoisture in the low-pressure stages of the

turbine exceeds about 10%, not only is there a

decrease in turbine efficiency, but erosion of the

turbine blades may also be a very serious

problem.


24/33

Example 2Consider the ammonia Rankine-cycle power plant shown in Fig. 1, a plant that was designed to

operate in a location where the ocean water temperature is 25C near the surface and 5Cat some greater depth. The mass flow rate of the working fluid is 1000 kg/s.

a. Determine the turbine power output and the pump power input for the cycle.

b. Determine the mass flow rate of water through each heat exchanger.

c. What is the thermal efficiency of this power plant?


25/33


Effect of boiler pressure onRankine-cycle efficiency.

Increasing the superheating of the steam

from 3-3 increased work (area) 334

43, and the heat transferred in the boiler

is increased by area 33bb3. Since

the ratio of these two areas is greater

than the ratio of net work to heat

supplied for the rest of the cycle, it isevident that for given pressures,

superheating the steam increases the

Rankine-cycle efficiency.

This increase in efficiency would also

follow from the fact that the average

temperature at which heat is transferred

to the steam is increased. Note also that

when the steam is superheated, the

quality of the steam leaving the turbine

increases.


26/33


Effect of effect of superheatingthe steam in the boiler

Increasing the superheating of the steam

from 3-3 increased work (area) 334

43, and the heat transferred in the boiler

is increased by area 33bb3. Since

the ratio of these two areas is greater

than the ratio of net work to heat

supplied for the rest of the cycle, it isevident that for given pressures,

superheating the steam increases the

Rankine-cycle efficiency.

This increase in efficiency would also

follow from the fact that the average

temperature at which heat is transferred

to the steam is increased. Note also that

when the steam is superheated, the


increases.


27/33


Effect of boiler pressure onRankine-cycle efficiency.

Finally the influence of the maximum pressureof the steam must be considered, and

this is shown in Fig. 11.6. In this analysis the

maximum temperature of the steam, as well as

the exhaust pressure, is held constant. The

heat rejected decreases by area b44bb.

The net work increases by the amount of thesingle cross-hatching and decreases by the

amount of the double cross-hatching.

Therefore, the net work tends to remain the

same,but the heat rejected decreases, and

hence the Rankine-cycle efficiency increases

with an increase in maximum pressure.

Note that in this instance too the average

temperature at which heat is supplied

increases with an increase in pressure. The


decreases as the maximum pressure increases.


28/33


In summary, the efficiency ofthe Rankine cycle can be

increased by:

lowering the condenser

pressure;

raising the boiler pressure

during heat addition; and

superheating the steam.


29/33

The thermal efficiency of a steam power cycle

In real situation, power plants

Universally operate with condenser pressures

as low as practical,

Seldom operates at pressure much above

10,000 kPa or temperatures much above

600oC, mainly due to required capital

investment.


30/33

Seatwork 1


31/33

is the production of more than one useful form of energy

(such as process heat and electric power) from the same

energy source.

Cogeneration


32/33

The Regenerative Cycle

Water from the condenser,

rather than being pumped

directly back to the boiler, is

first heated by steam

extracted from the turbine.

This is normally done inseveral stages, with steam

as drawn from the turbine

at several intermediate

states of expansion

The purpose of heating the feedwater in this manner is to raise the average temperature

at which heat is added to the boiler. This increases the thermal efficiency of the plant,

which is said to operate on a regenerative cycle

Typical operating conditions


33/33

Example 2

Regenerative cycle

Thermodynamic 2

Documents