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$: PROPERTIES OF STEAM - EngineeringDuniya.com Year/Mech Engg... · PROPERTIES OF STEAM ... FORMATION OF STEAM EXPERIMENT (Constant Pressure) ... The dryness fraction of a steam is defined$
MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 1

Chapter 1:

PROPERTIES OF STEAM

Steam:

Vapour form of water is called STEAM.

Water in solid phase: We call it as ICE

Water in liquid phase: We call it as WATER

Water in gaseous phase: We call it as STEAM


Application of steam

Food processing industry.

Cooking: hotels, restaurants etc.

Used as a working fluid in steam engines and steam

turbines.

Used in industries for process heating.

Petrochemical industry.

Washing / drying / sterilizing in hospitals.

Health clinic / gym.


FORMATION OF STEAM EXPERIMENT (Constant Pressure)

Consider 1 kg of water at

0oC taken in a cylinder

fitted with a freely moving

frictionless piston as

shown in figure.

Cylinder

I kg of

water at

OoC

Pressure

“p”


The initial condition of water at 0oC is represented by the

point “A” on the Temperature – Enthalpy graph

A

Temperature

Enthalpy

(h)

Temperature

(ToC)


A

Temperature

Enthalpy

(h)

Temperature

(T)oC

TSat B

hf

Sensible heat

hfg

Latent heat

TSup

AOS

C

D

DOS

Fig. 1

W W

W W

W

Fig. 2 Fig. 3 Fig. 4 Fig. 5

A B C D


Definitions

Sensible heat (hf): (Enthalpy of water)

It is the amount of heat required to raise the temperature

of 1 kg of water from 0 0C to the saturation temperature

Tsat0C at a given constant pressure “p”.

hf = m x Cp x Tsat kJ

Where, m = mass of water in kg.

Cp = specific heat of water

= 4.1868 kJ/kg0K


Saturation temperature (Tsat):

It is the temperature at which the water begins to boil at

the stated pressure.

Latent heat (hfg): (Enthalpy of evaporation)

It is the amount of heat required to evaporate 1 kg of

water at saturation temperature to 1 kg of dry steam at the

same saturation temperature and at the given constant

pressure “p”.

Superheated temperature (Tsup):

It is the temperature of the steam above the saturation

temperature at a given constant pressure.


Amount of superheat (AOS): (Enthalpy of superheat)

It is the amount of heat required to raise the temperature

of dry steam from its saturation temperature to any

desired higher temperature at the given constant

pressure “p”.

AOS = m x Csup (Tsup - Tsat) kJ

Degree of superheat (DOS):

It is the difference between the superheated temperature

and the saturation temperature.

DOS = (Tsup – Tsat)


Different states of steam

The steam as it is being generated can exist in three

different states,

1. Wet steam

2. Dry saturated steam (dry steam)

3. Superheated steam


Wet Steam:

A wet steam is defined as a two-phase mixture of

finely divided water particles and steam at the

saturation temperature corresponding to a given

stated pressure.


Enthalpy A

B C

Sensible

Heat

Latent Heat

hfg

Tsup

Temperature

Amount of

Superheat

Degree of Superheat

Ts

hf

D

Temperature

hg


The quality of wet steam is specified by the dryness

fraction which indicates the amount of dry steam present in

the given quantity of wet steam and is denoted as “x”.

The dryness fraction of a steam is defined as the ratio of

mass of the actual dry steam present in a known quantity of

wet steam to the total mass of the mixture.

Mass Total

Steamin Wet present SteamDry of Mass x fraction, Dryness


Let,

mg = Mass of dry steam present in the sample quantity of

wet steam

mf = Mass of suspended water molecules in the sample

quantity of wet steam

The dryness fraction of wet steam is always less than 1.

The dryness fraction of dry steam is equal to 1.

g

f g

mx

m m


Dry Saturated Steam:

(dry steam)

Dry saturated steam is the steam at saturation

temperature and at a given pressure having no water

molecules entrained in it.


Superheated Steam:

A superheated steam is defined as the steam at a

temperature higher than the saturation temperature at the

given stated pressure.


Advantages of Superheated Steam:

Superheated steam possess very high energy compared

to dry saturated steam or wet steam at the same pressure,

hence its capacity to do the work will be higher.

It doesn’t create any problems like rusting or corrosion of

blades of turbine / engine cylinder.


Disadvantages of Superheated Steam:

The high superheated temperatures poses problems in

the lubrication.

Higher initial cost.

Energy content of steam:

Superheated

steam

Dry saturated

steam

Wet

steam > >


Enthalpy equations for different states of

steam

a) Enthalpy of Dry saturated Steam (hg):

hg = hf + hfg kJ/kg

Enthalpy A

B C

Sensible

Heat

Latent Heat

hfg

Tsup

Temperature

Amount of

Superheat

Degree of Superheat

Ts

hf

D

Temperature

hg


b) Enthalpy of Wet Steam (h):

A

B C

Sensible

Heat

Latent Heat

hfg

Tsup

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

Tsat

hf

D

Temperature

h = hf + x .hfg kJ/kg hg = hf + x hfg kJ/kg


c) Amount of superheat (AOS): (Enthalpy of superheat)

A

B C

Sensible

Heat

Latent Heat

hfg

Tsup

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

Ts

hf

D

Temperature

AOS = Csup (Tsup - Tsat) kJ/kg

where, Csup = Specific heat of the superheated steam

= 2.25 kJ/kg0K


d) Enthalpy of Superheated Steam (hsup):

A

B C

Sensible

Heat

Latent

Heat

hfg

Tsup

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

Tsat

hf

D

Temperature

hg

hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg


e) Degree of superheat (DOS):

A

B C

Sensible

Heat

Latent

Heat

hfg

Tsup

Temperatur

Enthalpy

Amount of

Superheat

Degree of Superheat

Tsat

hf

D

Temperature

DOS = (Tsup - Tsat) 0C


a) Enthalpy of Dry saturated Steam:

hg = hf + hfg kJ/kg

b) Enthalpy of Wet Steam:

h = hf + x .hfg kJ/kg

c) Enthalpy of Superheated Steam:

hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg

d) Degree of superheat (DOS):

DOS = (Tsup - Tsat) 0C

e) Amount of superheat (AOS):

AOS = Csup (Tsup - Tsat) kJ/kg


Properties of steam: Numerical

Problem 1: Find the enthalpy of 1 kg of steam at 12 bar

pressure when,

(a) steam is dry saturated,

(b) steam is 22% wet and

(c) Steam is superheated to 250°C.

Assume the specific heat of superheated steam as 2.25

kJ/kg°K


Problem 2: Determine the conditions of steam from the

following data:

a) Pressure is 10 bar and temperature 200°C,

b) Pressure is 12 bar and enthalpy of 2600 kJ/kg.


Problem 3:

Given enthalpy of 1 kg of steam at 30 bar is 3681 kJ. Is the

steam is wet or superheated?

If it is wet; find its dryness fraction.

If it is superheated; find its degree of superheat.


Problem 4:

By actual measurement, the enthalpy of steam at 6 bar is

found to be 2500 kJ/kg.

a) What is the quality of steam?

b) If 500kJ of heat is added to this steam, what is the

Superheated temperature

Degree of superheat

Enthalpy of superheat


Problem 5:

2 kg of water at 300C is heated continuously at constant

pressure of 5 bar. The total amount of heat added is 500 kJ.

Determine the dryness fraction or degree of superheat of

the resulting steam as the case may be.


Problem 6:

5 kg of water is heated from 400C to superheated steam

at 1500C with constant pressure of 3 bar. Find,

a) The total amount of heat added in the heating process

b) Amount of superheat


Problem 7:

2 boilers, one with super heater and another without super

heater are delivering equal quantities of steam into a

common main.

The pressure in the boiler and main is 20 bar. The

temperature of steam from a boiler with a super heater is

3500 C and the temperature of steam in the main is 2500 C.

Determine the quality of steam supplied by the other boiler.


Problem 8:

1000 kg of steam at a pressure of 16 bar and 0.9 dry is

generated by a boiler and it enters the super heater, where

its temperature is raised such that the degree of superheat

is 1800C. If the temperature of feed water is 300 C,

determine

a) Total heat added to feed water in the boiler.

b) Total heat absorbed in the super heater.


Problem 9:

A dry saturated steam at a pressure of 16 bar is

generated in a boiler. Dry saturated steam leaves the

boiler to enter a super heater, where it looses 600 kJ/kg

of heat. In the super heater, steam is super heated to

temperature of 380oC. If temperature of feed water is

30oC, determine:

Total heat supplied to feed water in the boiler

Dryness fraction of steam at the entry of super heater

Total heat supplied in the super heater.


Problem 10:

The steam initially at a pressure of 9 bar and dryness

fraction 0.98. Find the final quality and temperature of

steam at each of the following operations.

a) When steam loses 50 kJ/kg at constant pressure

b) When steam receives 150 kJ/kg at constant pressure


Problem 11:

The steam initially at a pressure of 0.9 MPa and 2% wet

expands in a turbine such that it losses 80 kJ/kg at

constant pressure.

a) What is the quality of steam after expansion?

b) If it receives 160 kJ/kg of heat before expansion, what

would be the final state and temperature of the steam as

it comes out of the turbine?


Critical Temperature &

Pressure.

Pc = 221.2 bar

Tc = 374.150C

At a particular pressure

water is directly converted

into dry steam without

going through the phase of

evaporation. i.e., hfg = 0 .

This point is called critical

point.

Temperature

Enthalpy

Tc

Pc

Pi

P3

P2

P1

hf hfg


Critical pressure:

It is the pressure at which the water is directly converted

into dry steam without undergoing the state of

evaporation.

Critical temperature:

It is the corresponding temperature at the critical point.


Throttling:

Throttling is a type of expansion where steam passes

through a narrow passage and expands with a fall of

pressure without doing an external work.

In this case, there is no interchange of heat similar to an

adiabatic process.

The enthalpy remains constant during this operation.

So,

Enthalpy before throttling = Enthalpy after throttling

In this process, steam becomes drier and nearly

saturated steam becomes, superheated.


Measurement of Dryness Fraction

Dryness fraction of a sample of steam may be

determined by means of steam calorimeters.

Types of calorimeters:

1. Tank calorimeter

2. Throttling calorimeter

3. Separating calorimeter

4. Combined calorimeter


Throttling calorimeter:

It is known that at the time of

throttling the steam becomes

drier and nearly saturated

steam becomes superheated.

Limitations:

This type of calorimeter fails

when the steam is not

superheated after expansion.


Separating calorimeter:

If the steam is very wet then

the dryness fraction will be

measured by this type of

calorimeter.

Limitations:

Only an approximate dryness

fraction is measured, because

the steam discharged from the

calorimeter is not absolutely

dry.


Combined calorimeter


Separating calorimeter does not give an accurate result

and the throttling calorimeter fails if the steam is not

superheated after throttling.

A combination of separating and throttling calorimeter is

therefore found most suitable for accurate measurement of

dryness of steam.


The steam first passes through a separating calorimeter

and then through a throttling calorimeter.

Let m' = mass of water collected in the separating

calorimeter

Let m = mass of steam passed from the separating

calorimeter into the throttling calorimeter.

It is then finally condensed in a condenser.

Therefore, m + m' = total mass of steam entered into the

separating calorimeter,

So,

2

mx

m m


Let “x2” be the apparent dryness fraction determined by

the separating calorimeter.

So,

Let “x1” be the dryness fraction of wet steam which enters

into the throttling calorimeter. This dryness fraction can be

determined with the help of equation:

2

mx

m m

1 1 1 2g pf fg s fh c tx th h


Where,

x1 – dryness fraction of steam before throttling

hf1 – liquid enthalpy of steam before throttling

hfg1 – enthalpy of evaporation of steam before throttling

hg2 – enthalpy of dry steam after throttling

ts – temperature of steam after throttling

tf – saturation temperature of steam after throttling

cp – specific heat of superheated steam


So the actual amount of dry steam passes from the steam

main is “x1m”.

Now if “x” be the actual dryness fraction at the steam

main, then

11 1 2

x m mx x

m m m mx x


Problem 1:

A combined calorimeter is used to determine the dryness

fraction of steam at a pressure of 9.8 bar. The steam is

discharged from the calorimeter at atmospheric pressure

and at a temperature of 105°C. During the test 0.2 kg of

water is collected into the separating calorimeter and 2 kg

of steam is discharged to the throttling calorimeter. Find

the dryness fraction of steam. Assume Cp = 2.25 kJ/kg0K


Solution:

m = 2 kg, m’ = 0.2kg, P = 9.8 bar, ts = 105°C, tf = 100°C

With the help of separating calorimeter,

With the help of throttling calorimeter,

2

20.909

2 0.2

mx

m m

1 1 1 2g pf fg s fh c tx th h

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