01 Properties of Steam

transcript

Chapter 1:

PROPERTIES OF STEAM

Steam:

Vapour form of water is called STEAM.

Water in solid phase: ICE

Water in liquid phase: WATER

Water in gaseous phase: STEAM

Dept. of Mechanical & Mfg. Engg.

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.

Properties

Definition:

The characteristics of a system that can be used to define

its condition/state is called as a property.

Example:

Suppose if you were to specify the condition of ice cubes

in a freezer. You may find out that the ice cube is at -4oC

and occupies 1m3 volume upon mesurement.

Here, the temperature and volume are the properties of ice.

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

Pressure

“P”

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

point “A” on the Temperature – Enthalpy graph

Temperature

Enthalpy

Temperature

Enthalpy

Temperature

TSat B

Sensible heat

Latent heat

Fig. 1

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 00C to the saturation temperature

Tsat0C at a given constant pressure “P”.

hf = m x Cp x ΔT kJ

Where, m = mass of water in kg.

Cp = specific heat of water

= 4.1868 kJ/kg0K

ΔT = Temperature Difference.

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

Sensible

Latent Heat

Temperature

Amount of

Superheat

Degree of Superheat

Temperature

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 wet steam.

Steam Wet of Mass Total

Steamin Wet present SteamDry of Mass x fraction, Dryness

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.

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 temperature of superheated steam poses

problems in lubrication.

Higher generation & initial cost.

Energy content of steam:

Superheated

Dry saturated

steam > >

Enthalpy equations for different states of

a) Enthalpy of Dry saturated Steam (hg):

hg = hf + hfg kJ/kg

Enthalpy A

Sensible

Latent Heat

Temperature

Amount of

Superheat

Degree of Superheat

Temperature

b) Enthalpy of Wet Steam (h):

Sensible

Latent Heat

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

Temperature

h = hf + x .hfg kJ/kg

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

Sensible

Latent Heat

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

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):

Sensible

Latent

Temperature

Enthalpy

Amount of

Superheat

Degree of Superheat

Temperature

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

e) Degree of superheat (DOS):

Sensible

Latent

Temperatur

Enthalpy

Amount of

Superheat

Degree of Superheat

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 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

1. Superheated temperature

2. Degree of superheat

3. 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 heat equal

to 600 kJ/kg. And 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:

Steam is generated in a boiler at a pressure of 9 bar and

with a dryness fraction of 0.98. Find the quality and

temperature of steam under 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

hf hfg hf

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.

01 Properties of Steam

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