Theory of Producing Vapour

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

Creation of the Working Fluid using A Pure Substance …..…..

Triple Point

• Water:

• Triple point temperature : 273.17 K

• Triple point pressure : 0.6113 kPa

• Ammonia

• Triple point temperature  : 195.2 K

• Triple point pressure  : 6.111 kPa

Production of Vapour : Ancient Method

Production of Vapour : Modern Method

Rudolf Christian Karl Diesel • Diesel was born in Paris, France in 1858 the

second of three children of Elise and Theodor Diesel.

• At age 14, Rudolf wrote a letter to his parents stating that he wanted to become an engineer.

• Diesel was graduated in January 1880 with highest academic honours. • Started working as director of company working on design and construction of a modern refrigeration and ice plant from 1981.• In early 1890, Diesel moved to Berlin.• Diesel understood thermodynamics and the theoretical and practical constraints on fuel efficiency.• He first worked with steam, his research into thermal efficiency and fuel efficiency leading him to build a steam engine using ammonia vapour.

• He spent many months in a hospital, followed by health and eyesight problems.

• He then began designing an engine based on the Carnot cycle, and in 1893, Diesel published a treatise entitled

• Theorie und Konstruktion eines rationellen Wärmemotors zum Ersatz der Dampfmaschine und der heute bekannten Verbrennungsmotoren.

• Theory and Construction of a Rational Heat-engine to Replace the Steam Engine and Combustion Engines Known Today.

Starting from Liquid State

Let's consider the results of heating liquid from 20°C

20C

For Ammonia Pressure must be greater than 857.5kPa

For Ammonia Pressure must be greater than 2.339 kPa

State 1

20C

Liquid Ammonia @ 1 MPa

Liquid Water @ 100 kPa

• In the compressed liquid region, the properties of the liquid are approximately equal to the properties of the saturated liquid state at the temperature.

State 2 : Saturated Liquid

• Process 1-2:

• The temperature and specific volume will increase from the compressed liquid, or subcooled liquid, state 1, to the saturated liquid state 2.

state 2 Saturated Liquid Ammonia @ 1 MPa &24.9C

Saturated Liquid Water @ 100 kPa & 99.62C

• Process 2-3:

• At state 2 the liquid has reached the temperature at which it begins to boil, called the saturation temperature, and is said to exist as a saturated liquid.

• Properties at the saturated liquid state are noted by the subscript f and v2 = vf.

• During the phase change both the temperature and pressure remain constant.

• Water boils at 99.62°C when the pressure is 100kPa .

• Ammonia boils at 24.99°C when the pressure is 1000 kPa ).

• At state 3 the liquid and vapor phase are in equilibrium and any point on the line between states 2 and 3 has the same temperature and pressure.

State 3 : Equilibrium Mixture of Saturated Liquid Vapour

State 4 : Saturated Vapour

• Process 3-4:

• At state 4 a saturated vapor exists and vaporization is complete.

• The subscript g will always denote a saturated vapor state.

• Note : v4 = vg.

Temperature PressureSpecific Volume, m3/kg

0C MPaSaturated Liquid Saturated Vapour

100 0.1013 0.001044 1.673120 0.1985 0.00106 0.8919150 0.4759 0.00109 0.3928200 1.554 0.001156 0.1274250 3.973 0.001251 0.05013300 8.581 0.001404 0.02167

Saturated Water Vs Saturated Steam

Temperature PressureSpecific Volume, m3/kg

0C MPaSaturated Liquid Saturated Vapour

100 6.254 0.002188 0.01784120 9.107 0.002589 0.01003

132.3 11.33 0.004255 0.004255

Saturated Liquid Ammonia Vs Saturated Vapour Ammnia

State 5 : Superheated Vapour• Process 4-5:

• If the constant pressure heating is continued, the temperature will begin to increase above the saturation temperature.

• State 5 is called a superheated state because T5 is greater than the saturation temperature for the pressure.

Superheated Ammonia @ 1 MPa &300C

Superheated Water @ 100 kPa & 300C

Constant Pressure Process

The Theory of Producing Steam

• Water and steam can be easily used as heat carriers in heating systems.

• Water boils and evaporates at 100°C under atmospheric pressure.

• By higher pressure, water evaporates at higher temperature - e.g. a pressure of 10 bar equals an evaporation temperature of ~179.90C.

• At a constant pressure of 10 MPa the saturation temperature is 311.10C.

Wet Vapour

• Wet vapour is a mixture of vapour and liquid water at same temperature and pressure.

• Saturation pressure is the pressure at which the liquid and vapor phases are in equilibrium at a given temperature.

• Saturation temperature is the temperature at which the liquid and vapor phases are in equilibrium at a given pressure.

• Saturation Pressure is function of temperature or vice versa.T = F(p)

1.5 2.5 5* * * *. . . .

ln( )satr sat

r

a b c dP

T

The Wagner-Ambrose equation

* 1 satrT

satsat

rc

TT

T

Equations for Saturation Conditions of Water

Saturation Properties of Water :

cT

T1

Many Constant Pressure Processes

• If all of the saturated liquid states are connected, the saturated liquid line is established.

• If all of the saturated vapor states are connected, the saturated vapor line is established. • These two lines intersect at the

critical point and form what is often called the “steam dome.”

The critical point of water is 374.14oC, 22.09 MPa

The critical point of ammonia is 132.3oC, 11.33 MPa

Density of Saturated Liquid

Density of Saturated Vapour

• The region between the saturated liquid line and the saturated vapor line is called by these terms: – Saturated liquid-vapor mixture

region, – Wet region,– Two-phase region, and just– The saturation region.

• The trend of the temperature following a constant pressure line is to increase with increasing volume.

• The trend of the pressure following a constant temperature line is to decrease with increasing volume.

The Vapour Dome

Peculiar Nature of Wet Vapour

• Pressure and temperature are not independent properties.• Either p & V or T& V are independent pair.• P & v or T & v can also be considered.• A new property is to be defined for steam for ease of

design.• This is called Quality or dryness fraction of wet steam.

Quality and Saturated Liquid-Vapor (Wet) Mixture

• Now, let’s review the constant pressure heat addition process for water shown in Figure.

• The state 3 is a mixture of saturated liquid and saturated vapor.

• How do we locate it on the T-v diagram?

• To establish the location of state 3 a new parameter called the quality x is defined as

gf

g

mm

mmassx

total

vaporsaturated

mass

• The quality is zero for the saturated liquid and one for the saturated vapor (0x 1).

• The average specific volume at any state 3 is given in terms of the quality as follows.

• Consider a mixture of saturated liquid and saturated vapor.

• The liquid has a mass mf and occupies a volume Vf.

• The vapor has a mass mg and occupies a volume Vg.

Volume of Wet Mixture