Thermal Engineering

Md. Mohiuddin

Lecturer

Department of Mechanical Engineering

ME 1105

Pure Substance

Pure Substance

Formation of Steam at Constant Pressure from Water

• Consider 1kg of water at 0° C contained in the piston-cylinder arrangement as shown in the figure.
• The piston and weights maintain a constant pressure in the cylinder. If we heat the water contained in the cylinder, it will be converted into steam

Formation of Steam at Constant Pressure from Water

• The volume of water will increase slightly with the increase in temperature as shown in the figure.
• It will cause the piston to move slightly upwards and hence work is obtained.
• This increase in the volume of water (or work) is generally, neglected for all types of calculations.

(i)

Formation of Steam at Constant Pressure from Water

• On further heating, temperature reaches boiling point.
• When the boiling point is reached, the temperature remains constant and the water evaporates, thus pushing the piston up against the constant pressure.
• Consequently, the specific volume of steam increases as shown in the figure.
• At this stage, the steam will have some water particles in suspension, termed as wet steam.
• This process will continue till the whole water is converted into wet steam.

(ii)

Formation of Steam at Constant Pressure from Water

• On further heating, the water particles in the suspension will be converted into steam.
• The entire steam, in such a state, is termed as dry or saturated steam

(iii)

Formation of Steam at Constant Pressure from Water

• On further heating, the temperature of the steam starts rising.
• The steam, in such a state, is termed as superheated steam

(iv)

Formation of Steam at Constant Pressure from Water

During the formation of the superheated steam, from water at the freezing point, the heat is absorbed in the following three stages:

• Heating up water from freezing point to boiling point.
• Change water to steam at a constant boiling point temperature.
• Heated up steam to the superheated region.

Temperature vs. Total Heat Graph during Steam Formation

• The heating of water up to boiling temperature or saturation temperature is shown by AB. The heat absorbed by the water is AF, known as sensible heat.
• The change of state from liquid to steam is shown by BC. The heat absorbed during this stage is FG, known as latent heat of vaporization.
• The superheating process is shown by CD. The heat absorbed during this stage is GH, known as heat of superheat.
• Line AH represents the total heat of the superheated steam.

Temperature vs. Total Heat Graph during Steam Formation

When P is increased to P₁ and P₂ than boiling point temperature is increased to B₁ and B₂, respectively, however, latent heat of vaporization is decreased.

Temperature vs. Total Heat Graph during Steam Formation

When P is increased to P₁ and P₂ than boiling point temperature is increased to B₁ and B₂, respectively, however, latent heat of vaporization is decreased.

Line AB₁ B₂E – Saturated liquid line or liquid line.

Line EC₁C₂A₁ – Dry saturated vapour line or Dry steam line.

Critical point: The latent heat of the vaporization of water decreases when the pressure and saturation temperature increase and becomes zero at a specific point, which is known as the critical point.

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Another way, the critical point is the point where the liquid line and vapor line merge.

Temperature vs. Total Heat Graph during Steam Formation

The temperature corresponding to the critical point is known as critical temperature and the pressure is known as critical pressure.

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• For steam the critical temperature is 374.15⁰C and the critical pressure is 221.2 bar.

Important Terms

• Wet steam: When the steam contains moisture or particles of water in suspension, it is said to be wet steam. It means that the evaporation of water is not complete. The region between the saturated liquid line and the dry saturated vapor line is wet steam.

Wet Steam

• Dry saturated steam: When the wet steam is further heated, and it does not contain any suspended particles of water, it is known as dry saturated steam. Steam on the dry saturated vapor line is dry saturated steam.

Important Terms

• Superheated steam: When the dry saturated steam is further heated at a constant pressure, thus raising its temperature, it is said to be superheated steam.
• Since pressure is constant, therefore the volume of superheated steam increases with increasing temperature.
• The region right side to the dry saturated line (A₁E) and above the critical point is known as the superheated region.

Important Terms

Dryness fraction or quality of wet steam: It is the ration of the mass of actual dry steam to the mass of the same quantity of wet steam, and is generally denoted with ‘x’.

Where,

m_g = Mass of actual dry steam.

m_f = Mass of water in suspension

m = Mass of wet steam = m_g + m_g

x = 0 on the saturated liquid line and left side of this line.

0 <x <1 in wet region.

x = 1 on the dry saturated vapor line and right side of this line and also above the critical point.

Wet Steam

Important Terms

Saturation Temperature: The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase.

Sensible heat of water: It is the amount of heat absorbed by 1 kg of water, when heated at a constant pressure, from the freezing point to the temperature of formation of steam, i.e. saturation temperature (t). The sensible heat is also known as liquid heat. It denotes with h_f.

Important Terms

Latent Heat of Vaporization: It is the amount of heat absorbed to evaporate 1 kg of water, at its boiling point or saturation temperature without change of temperature. It denotes with h_fg

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• It has been experimentally found that the value of h_fg decreases as the pressure increases and it is zero at critical pressure.
• If the steam is wet with a dryness fraction x, then the heat absorbed by it during evaporation is xh_fg

Important Terms

Important Terms

Important Terms

Specific volume of Steam: It is the volume occupied by the steam per unit mass at a given temperature and pressure and is expressed in m³/kg.

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• It is the reciprocal of the density of steam in kg/m³.
• The value of specific volume decreases with the increase in pressure.

Important Terms

Important Terms

Superheated Steam

Whenever dry steam or saturated steam is further heated, then the steam is termed as superheated steam.

Advantages of superheated steam

• The superheated steam contains more heat, and hence its capacity to do work is also increased without increasing its pressure.
• The superheating is done in a superheater, which obtains heat from waste furnace gases.
• The high temperature of the superheated steam results in an increase in thermal efficiency.
• Since the superheated steam is at a higher temperature than that corresponding to its pressure, therefore it can be considerably cooled during expansion in an engine cylinder and, thereby, becomes wet. It is thus obvious, that heat losses due to condensation steam on cylinder walls, etc., are avoided to a great extent.

External Work Done during Evaporation

Whenever water at boiling temperature is heated at a constant pressure, it gets converted into steam after absorbing the latent heat. This latent heat is unused in the following two ways

• In overcoming the internal molecular resistance of water in changing its state from saturated water to dry saturated steam.
• In overcoming the external resistance to the movement of the piston due to an increase in volume during evaporation.
2. The first effect is called internal work or internal latent heat, as the change takes place within the body of the steam, and represents the energy stored in the steam (Internal Energy).
3. The second effect is called external work of evaporation and represents the energy that has been taken out of the steam.

External Work Done during Evaporation

p = Pressure on the piston in bar = p × 10⁵ N/m².

v_f = Volume of water in m³ at pressure p, and

v_g = Volume of steam in m³ at pressure p

External Work Done during Evaporation

Internal Energy of Steam

It is the actual heat energy stored in steam, above the freezing point of water. The internal energy may be calculated by subtracting the external work done during evaporation from the enthalpy or total heat of steam.

Internal energy of steam (u)

= Enthalpy or total heat – External work done during evaporation.

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Steam Table

Steam Table: The properties of dry saturated steam like saturation temperature, sensible heat, latent heat of vaporization, enthalpy or total heat, specific volume, entropy, etc. vary with pressure and can be found by experiments only. These properties have been carefully determined and made available in a tabular form known as steam tables.

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There are two important Steam tables

• in terms of absolute pressure
• in terms of temperature

Steam Table

In terms of temperature

Steam Table

In terms of absolute pressure

Problem

Calculate the enthalpy of I kg of steam at a pressure of 8 bar and dryness fraction of 0.8. How much heat would be required to raise 2 kg of this steam from water at 200 C?

Problem

Determine the quantity of heat required to produce 1kg of steam at pressure of 6 bar from a temperature of 25°C under the following conditions.

• when the steam is wet having a dryness fraction 0.9;
• when the Steam is dry saturated; and
• when it is superheated at a constant pressure at 250° C assuming the mean specific heat of superheated steam to be 2.3 kJ/kg K.

Problem- Assignment

Steam enters an engine at a pressure of /2 bar with a 67° C of superheat. It is exhausted at a pressure of 0.15 bar and 0.95 dry. Find the drop in enthalpy of the steam.

Problem

Determine the volume of! kg of superheated steam at a pressure of 20 bar and temperature of 300° C.

Problem

A boiler is supplied with feed water at a temperature of 45⁰C. The water is converted into steam at pressure of 5.5 bar and a temperature of 188⁰C. Determine the quantity of heat supplied per kg of steam. Assume suitable data.

Problem

Find the external work done during evaporation per kg of steam at a pressure of 15 bar when the steam is (a) 90% dry and (b) dry saturated.

Problem

Steam at 18 bar and dryness 0.9 is heated at constant pressure until dry and saturated. Find the increase in volume, heat supplied and work done per kg of steam. If the volume is now kept constant, find how much heat must be extracted to reduce the pressure to 14 bar.

Phase

• A phase is identified as having a distinct molecular arrangement that is homogeneous throughout and separated from the others by easily identifiable boundary surfaces.
• There are three principal phases solid, liquid, and gas
• A substance may have several phases within a principal phase, each with a different molecular structure.
• Carbon, for example, may exist as graphite or diamond in the solid phase.

Property Diagram

T-V Diagram

Property Diagram

P-V Diagram

At 150⁰ C the saturation pressure is 0.4762 MPa

Property Diagram

Triple Point/ Triple Line

• Under some conditions all three phases of a pure substance coexist in thermodynamic equilibrium. On P-V or T-V diagrams, these triple-phase states form a line called the triple line.
• The states on the triple line of a substance have the same pressure and temperature but different specific volumes.
• For water, the triple-point temperature and pressure are 0.01⁰C and 0.6117 kPa, respectively.
• That is, all three phases of water coexist in equilibrium only if the temperature and pressure have precisely these values.
• No substance can exist in the liquid phase in stable equilibrium at pressures below the triple-point pressure.

Property Diagram

T-V Diagram- Extending the Diagrams (For Water)

Property Diagram

P-V Diagram- Extending the Diagrams

Property Diagram

• There are two ways a substance can pass from the solid to the vapor phase:
1. it melts first into a liquid and subsequently evaporates
2. it evaporates directly without melting first.
• The latter occurs at pressures below the triple-point value since a pure substance cannot exist in the liquid phase at those pressures.
• Sublimation: Passing from the solid phase directly into the vapor phase is called sublimation.

Property Diagram

P-T Diagram

For water

For normal substance

• Sublimation curve: Separate solid and gaseous phase.
• Vaporization curve: Separate liquid and gaseous phase.
• Melting or fusing curve: Separate solid and liquid.
• These three lines meet at the triple point, where all three phases coexist in equilibrium.

Melting curve shows that:

• For normal substances, It has a positive trend. Indicate melting temperature increases slightly with increasing pressure.
• For water, the curve has a negative trend. Indicate melting temperature decreasing with increasing pressure.

Property Diagram

P-V-T Diagram

P-v-T surface of a substance that contracts on freezing.

Property Diagram

P-V-T Diagram

P-v-T surface of a substance that expands on freezing (Water).

Property Diagram

P-V-T Diagram

Property Diagram

Mollier Chart (h-s diagram)

Thank You