Thermal Engineering Notes for Diploma and Engineering College Students!
Note # 1. Thermodynamics System:
Sometime we have to visit a general physician for certain health related problem. At that time, the physician examines our body for the accurate diagnosis of the illness. But, when we consult an ophthalmologist, he concentrates only a part of our body i.e., Eye. Thus, every doctor decides the portion of body upon which concentration is made, according to his scope of diagnosis and treatment.
Above mentioned fact is elaborated in view of thermodynamic studies and is specified with the terms like System, Surrounding and Boundary.
A Thermodynamic System is defined as quantity of matter or region of space upon which attention is concentrated in the analysis of the problem.
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Thermodynamic system may be defined as any part of the Universe which we want to study.
In order to fix the limits of a system, there must be a partition around the system. This partition is known as the boundary of the system.
Everything external to the boundary is called the Surrounding. The system along with surrounding forms a Universe.
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The working of a thermodynamic system depends on the energy interactions between the systems and surrounding, which takes place across the boundary. Hence we must start our studies with the type of boundaries and its properties.
The types of boundaries on the basis of its existence are:
(i) Real Boundary:
Consider an oven used for baking the food products. The wall of the oven physically encloses the heating coils and the food products. Thus it forms a real boundary. The cylinder-piston in IC engine provides a real boundary during expansion or compression of inside gases.
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(ii) Imaginary Boundary:
When we wish to study the working of a tap of water, we separate the valve system from rest of the thing using an imaginary boundary. In thermodynamics the systems like nozzles, turbines, carburettor of vehicle can be defined with a concept of such an imaginary boundary.
The types of boundaries on the basis of its position are:
(i) Fixed Boundary:
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This type of boundary has a definite shape and encloses a fixed region of universe. A wall of a furnace is example of fixed real boundary while the system of turbine has fixed imaginary boundary.
(ii) Moving Boundary:
This type of boundary may change its shape and the region of universe enclosed by it. A wall of balloon which changes its size and shape with the pressure of inside air is an example of moving real boundary.
Types of System:
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The types of systems may be viewed in a tree form as shown in Fig. 1.3
(a) Closed or Non-Flow System:
It is fixed mass i.e., there is no mass transfer across the boundary. But there may be energy transfer into and out of the system .
As an example- consider a system consisting of a gas, in a cylinder fitted with a piston . If heat is supplied to the gas, the gas expands i.e., the volume of the gas increases so work will be done by the gas on the piston. Thus heat and work will cross the system boundary. But since no mass crosses the boundary, this system is called a closed system.
(b) Open System or Flow System:
It is one in which both mass and energy can enter and leave the system . Most of the engineering devices are open systems.
It is one in which both mass and energy can enter and leave the system. Most of the engineering devices are open systems.
(c) Isolated System:
It is of fixed mass and energy and there is no mass or energy transfer across the system boundary. An isolated system is independent of all the changes that are taking place in the surroundings.
(d) Steady Flow and Unsteady Flow Systems:
If the mass flow rate and energy flow rate at inlet and outlet are same, then the system is known as steady flow system.
If the rates are varying, then it is called an unsteady or transient flow system.
(e) Adiabatic and Diabetic Systems:
When the system is perfectly insulated, so that neither heat enters nor it leaves the system, then it is known as adiabatic system.
When heat energy, either enters or leaves the system, it is called a diabetic system. It is not insulated.
Note # 2. Limitations of the First Law of Thermodynamics:
First-law states that, when a closed system undergoes any cyclic process, then the cyclic integral of work is equal to the cyclic integral of heat
Thus the law merely states that work transfer during a cycle is equal to the heat transfer and does not place any restriction on the direction of heat and the work transfer. It does not specify whether the process is possible in a particular direction or not at all. According to this law it can be assumed that the energy transfer can take place in either direction, since it does not specify the direction of energy transfer.
But by practical experience it is observed that, even if a proposed cycle satisfies the first law, it doesn’t ensure that the cycle will occur actually. This resulted in the formulation of Second Law of Thermodynamics. Thus a cycle will proceed only if both I and II laws of thermodynamics are satisfied.
Thus the second law involves the fact that processes will proceed in a certain direction but not in the opposite direction, for example-
(i) Consider the Joule’s experiment in which the fall of weight W rotates the paddle wheel and increases the temperature of water. Here work is converted into heat. However reverse of this process is not possible i.e., by heating water, paddle wheel will not rotate and lift the weight.
This shows that the system can operate in a cyclic process only when work is done on the system and heat is rejected out of the system. But it cannot work in a cyclic process when both work and heat transfer are positive even though such a process will not violate the first law of thermodynamics.
(ii) Consider hot tea in a cup, here heat flows from tea to the cooler surroundings. Once it is cooled, it can never be heated by the addition of heat from the cooler surroundings, without doing any work on it.
(iii) A cup of ice cream when kept in atmosphere absorbs heat and melts. But it will not solidify giving back heat without doing any work on it.
(iv) When a vehicle is stopped by applying mechanical brakes, the brakes get hot. The I.E. of the brake increases by an amount equal to the decrease in K.E. of the vehicle. According to the first law, the hot brake on cooling, should give back its increase in I.E. to the wheels of the vehicle, causing it to regain its speed.
This never happens in practice. From the above examples it is obvious that, even if the first law is satisfied the processes will proceed in a particular direction while they are impossible in the opposite direction. Thus the first law of thermodynamics is necessary but not sufficient condition for the processes to take place.
It has also been practically found that all forms of energies are not equally convertible into work and the first law is silent about the extent of conversion of energy. It is necessary to study Second Law of Thermodynamics in this regard.
Note # 3. The Regenerator:
The regenerator is a contrivance which alternately stores and rejects heat in a manner which, theoretically, is thermo- dynamically reversible. By the incorporation of a regenerator the cycles used by both of these engines were made to conform to thermodynamic reversibility; their ideal efficiencies were, therefore, the same as the Carnot cycle efficiency when working between the same temperatures limits.
A view of one form of a regenerator is shown in Fig. 23.6. The air in the space at the lower end is kept at a higher temperature T1 by the furnace underneath it. The low temperature air the upper space is kept cool at a temperature of T2 by a supply of cold water passing through tube in contact with it. The space between the two ends is filled with wire gauge for storing the heat. The temperature of the gauge varies uniformly from T1 at the base to T2 at the top, as shown by the temperature diagram to the right of the figure.
The hot air in the base in contact with the engine cylinder during a portion of the cycle. The air is forced upwards through the gauze, by means of a pump, and is thus cooled at constant volume to a temperature T2. In passing through the gauze, it is cooled at a uniform rate, the temperature gradient being approximately a straight line.
This process can be regarded as thermodynamically reversible as the heat is assumed to flow from the air to the gauze when at the same temperature. The heat is thus stored in the layers of wire gauze; the increase in temperature of the latter due to this storage is extremely small on account of its large bulk.
During another portion of the cycle the cool air in the upper space is in contact with engine cylinder. The cool air is then forced downwards through the wire gauze; it thus receives heat from the gauze at constant volume by a graduated method of heating which makes the process thermodynamically reversible. The air is now in the lower space at a temperature T1 and has absorbed from the gauze the whole of the heat stored during its previous upward flow.
It will be seen from this that the regenerator is able to store and reject heat alternately, in a reversible manner, owing to its uniformly varying temperature gradient. In order to approximate to the assumed processes the regenerator must be very large compared with the volume of air used in the engine cylinder. The pump forcing the air through the regenerator is driven off the engine crank-shaft.
Note # 4. Comparison of Otto, Diesel and Dual Combustion Cycle:
The important parameters in cycle analysis are compression ratio, maximum pressure, maximum temperature, heat input, output etc. The cycle can be analysed for the different parameters.
(a) For Same Compression Ratio and Same Heat Input:
Figure 23.13 shows the three cycles on P-V and T-S diagrams. All the cycles start from the same initial state 1 and air is compressed isentropically to the same point 2 as compression ratio is same.
It is seen that adding the heat at constant volume resulting the highest maximum temperature and pressures (Otto-cycle). Adding the heat at constant pressure results in the lowest maximum temperatures (Diesel cycle) while the values for the limited pressure cycle (Dual cycle) lie between Otto and Diesel Cycles.
For the same heat input and compression ratio, minimum heat is rejected by the Otto-cycle, maximum heat is rejected by Diesel cycle and the heat rejected is in between Otto and Diesel in case of dual cycle. Consequently, work output and efficiency is maximum in Otto-cycle and the least work done and efficiency in case of Diesel cycle. Dual cycle work and efficiency lie in between the two-Otto and Diesel cycles.
(b) For Constant Maximum Pressure and Same Heat Input:
Otto-cycle 1-2-3-4-1
Diesel cycle 1-2′-3′-4′-1
Dual cycle 1-2″-3″-5″-1
Figure 23.14 compares the three cycles on the basis of same maximum pressure and same heat input.
For the same maximum pressure the points 2, 3’ and 3″, must be on the same pressure line and the heat input, area 2356 (Otto) = area 2’3’5’6 = area 2″.3″ and 5″6. From the diagram, heat rejected its maximum in Otto-cycle, minimum in Diesel cycle and intermediate in dual cycle.
Therefore, for the same heat input, Diesel cycle produces more work, Otto-cycle less work and dual cycle intermediate work. Therefore, for maximum pressure and same heat input. Diesel cycle is the most efficient, dual cycle come the next and Otto-cycle has the least efficiency.
(c) For the Same Maximum Pressure and Temperature:
Figure 23.15 compares Otto-Diesel cycles for the same maximum pressure and maximum temperature. It is clear from the Figures diagram that the heat rejected by both Otto and Diesel cycles is same. But the heat supplied to Diesel cycle is more than that for Otto-cycle. Hence in this case, Diesel cycle is more efficient than Otto cycle. The dual cycle efficiency again lies/falls between the two to others.
(d) For the Same Maximum Pressure and Output:
For same work output, point 3′ will be on left side of 3. Therefore heat rejected in Diesel is less.
Note # 5. Systems of Refrigeration:
A refrigerator may be defined as a machine for producing cold. Refrigerators are used for the manufacture of ice and for the cooling of storage chambers in which perishable food is stored. Theoretically, any reversible heat engine would act as a refrigerator when run in a reversed direction by means of external power; the engine then becomes a heat pump, which pumps heat from a cold body and delivers it to a hot body.
The cold body usually consists of a tank of brine which is kept at a low temperature by the refrigerator; the cold brine is circulated through pipes, around the storage chamber which is to be cooled. The hot body is a supply of water which abstracts the heat from the working substance of the refrigerator.
The net action of the refrigerator is to pump heat from the brine and to deliver this heat to the cooling water. By doing this it is transferring heat from a cold body to a hot body; this operation, according to the Second Law of Thermodynamics, can only be performed by the aid of external work. Hence, it is necessary to supply power from an external source in order to drive the refrigerator.
There must be a closed loop for the refrigerant (condensing) so that no fresh supply of the refrigerant is required and thus avoiding the hazard to surroundings because of the refrigerant coming out of the system. In this arrangement, heat addition to the refrigerant i.e. evaporation and heat rejection by condensation of the refrigerant take place.
Depending upon the equipment employed the systems of refrigeration may be broadly classified into three types:
(a) Vapour compression system.
(b) Vapour absorption system.
(c) Steam-jet refrigeration system.
Note # 6. NC and CNC Machine:
In machine tools say a lathe machine-metal is removed from a job to create required shape-using cutting tools. For this purpose travel of the tool both along the bed of the lathe and the depth of the cut, speed etc. are to be controlled. This is done by skilled worker who controls these variables manually.
Manual operations have its limitations like accuracy, low production and high rejections. In numerically controlled machines most of the operations can be done automatically. A machine tool which operates automatically or semi-automatically as per coded instructions given to it is called numerically controlled machine (NC machine). In NC machines servos motors replace human operators in positioning of the work piece and in positioning/operation of the cutting tools.
For the NC machines first programme to do the required operation has to be written and punched on the tapes. The reader reads the tapes and sends required electrical signals to the controller to execute the job. Block diagram of an NC machine is given below.
Illustrative Example of NC Programming:
To operate the NC machine tool as programmed apart from the coded instructions—electronic/electrical and mechanical interfacing is required to move and control the cross slide, tool post, control speeds etc.
As way of illustration a simple turning job where 2 mm material is required to be removed from a shaft of 50 mm length, then:
Step (a) Movement of the tool post through 2 mm along-Z-axis
Step (b) Move of the tool by 50 mm in-X-direction
Step (c) Move the tool post by 2 mm in + Z-axis
Step (d) Move the tool post by 50 mm along + X-axis
Note # 7. Prop-Jet Turbine Plant:
A combined propeller and jet drive has been developed for aircraft propulsion. In this device a gas turbine drives a propeller and the exhaust gases are used as a propulsion jet, such a combination is termed a prop-jet engine.
It is being developed for heavy bombers and civil airliners for which it provides a satisfactory method of propulsion, because the propeller has a high efficiency at the lower altitudes whilst the efficiency of the jet propulsion increases at the higher altitudes.
During a flight test on a Theseus (Th-21) prop-jet engine installed in a plane, 1500 kW, was developed in the propeller shaft, together with a 227 kg jet thrust at sea level and a speed of 480 kmph.
The fuel consumption at this speed was found to be 0.352 kg/kW, the fuel used in the test being aviation kerosene. The equivalent brake power is defined as the propeller shaft power plus jet-power divided by the propulsion efficiency which is about 80%.
The Armstrong Siddeley prop-jet engine ‘Python’ had a shaft power of 3000 kW plus a thrust of 450 kg. It will be noticed that in both of these prop-jet engines the propulsion power obtained from the jet is 25 % of the propeller shaft power.
Advantages of Turbo-Prop-Jet Engine:
1. The turbo prop-jet, has good take off and climb characteristics in the lower subsonic speed range due to the large thrust and power available.
2. The turbo prop-jet engine combines the advantages of turbojet and turbo-prop engines i.e., low specific weight, small frontal area, simplicity, low vibrations and merits of propeller engine like high power for takeoff and high propulsive efficiency at low speeds.
3. The overall efficiency of the turbo-prop i.e., the fuel economy is superior to the turbo-jet engine at speeds below 800 kmph.
4. The turbo prop-jet engine has high thrust per m2 of frontal area compared with reciprocating engine.
5. This has less drag losses.
Disadvantages:
1. The addition of the reduction gear and propeller adds weight to the prop-jet engine.
2. At the altitude increases, the thrust of turbo-prop decreases.
Note # 8. Superheater:
The steam generated from a simple low pressure boiler is generally wet. So, for superheating the steam, superheaters are used. In the superheater, wet steam is first dried at the same temperature and pressure and then it is superheated at constant pressure.
Generally, same heat of the flue gases is used for superheating purpose and hence superheaters are placed in the path of flue gases. However in bigger installations superheaters are provided with independent furnaces. And such superheaters with independent furnaces are known as Independently Fired Superheaters.
Advantages of Superheating the Steam:
(1) Increase in amount of work output, for the same amount of steam and hence increase in cycle efficiency.
(2) Loss due to condensation of steam in the pipe line connecting the boiler to the steam engine or steam turbine is reduced.
(3) Loss due to condensation of steam engine/steam turbine is reduced.
(4) When the superheated steam is used, moisture will be absent; hence it will reduce corrosion and erosion of steam engine and steam turbine parts.
Figure 11.19 shows Sudgen’s Hair pin type of superheater arranged with Lancashire/Cornish boiler. The superheater is placed at the rear end of the boiler and the flue gases after their first pass, they pass through the superheater. Here the temperature of the flue gases is not less than 550°C. This superheater gives a superheat of 40 – 95°C.
This superheater consists of two Mild Steel boxes or headers and a number of tubes bent to U-shape. The ends of the tubes, in the headers will be either expanded or welded in order to have gas tight joint. Between front end rear headers some space is provided and this space is covered by means of a cover. This gives access for cleaning, inspection and repair purpose.
As long as temperature of the flue gases is less than 550°C then there is no danger of burning of tubes. But when the temperature increases beyond this and also when the delivery of steam from the boiler to the steam turbine through the superheater is suspended for a long time, then the following arrangements are to be made so as to protect the tubes from burning out.
(1) Flooding the Superheater Tubes – This water is to be drained before the supply of steam through the superheater again starts.
(2) Diverting the Flow of Flue Gases – This is done by Damper D which is operated by means of handle H.
An arrangement is also provided so as to pass the steam through superheater or directly to the main steam pipe as may be required M is the main steam pipe, P, Q and R are stop valves. When the superheater is in action valves P and Q are open R is closed. When steam is to be taken out directly from the boiler to the steam pipe, then valves P and Q are closed and R is opened.
Note # 9. Economiser:
One of the major heat losses in the boiler plant is the heat carried away by the exhaust gases. An economiser is a device or appliance, which recovers some of the heat of the exhaust gases. This recovered heat in utilised for preheating the water. This preheated water when it is supplied to the boiler drum, it requires less heat for its conversion into steam. This reduces the amount of fuel required and thus the efficiency of boiler plant increases. Figure 11.20 shows the Green’s economiser. It is used with stationary low pressure boilers.
It consists of a number of vertical pipes A. These pipes are arranged in groups of 4, 8, 12, 16 etc. Upper ends of the pipes are connected to the upper boxes B. These boxes B are in turn connected to the common header D. Lower ends of the pipes are connected to lower boxes. These lower boxes are connected to the horizontal pipe C through the cross pipes. All the tubes are enclosed in the brick work of the economiser except header D and pipe C.
Water will be pumped by means of a feed pump, water enters the economiser through the stop valve at the inlet F and passes into the pipe C, from where it flows through the cross pipe and then rises through the vertical tubes A. Since the economiser is provided in the path of flue gases the water rising through the tubes recovers heat and becomes hot. Hot water will be then collected into the top common header.
This hot water from the top header is taken out through the stop valve H, from where it goes to the boiler drum. K is the safety valve for the economiser.
Since the economiser is provided in the path of flue gases, soot (or ash) is likely to be collected over the tubes. It retards the rate of heat transfer from the flue gases to the water inside the tubes. In order to avoid this, soot is scrapped by means of scrapers. Scrapers of the two-adjacent tubes are coupled to form one pair and two such pairs of scrapers are connected by means of a chain, which passes over pulley P. Pulley P is connected to worm shaft, in such a way that when one pair of scrappers comes to top most position, another pair will be at the bottom most position, then it automatically reverses the direction of motion. The scrapers are kept in motion as long as economiser is in action.
By Pass Arrangement:
By Pass arrangements are provided for the flue gases and for feed water, so that the economiser may be put out of action when not required. Figure 11.21 shows such an arrangement for two Lancashire boilers fitted with an economiser. When the economiser is in service, then dampers L and M are open and the damper N is closed. For isolating the economiser damper N is opened and dampers L and M are closed.
Note # 10. Analysis of Coal:
Coal as found in nature is neither a pure substance nor of uniform composition. A definite chemical formula cannot be therefore written for a coal found in coal mines. Consequently two different methods of analysis are employed to know the composition of coal. They are known as Ultimate analysis and Proximate analysis.
In ultimate analysis a complete chemical breakdown of coal into its chemical constituents is carried out by chemical process. This analysis is required when important large scale-trials are being performed. The analysis serves the basis of calculations of the amount of air required for complete combustion of the kg of fuel. The analysis gives the percent content on mass basis of carbon, hydrogen, nitrogen, oxygen, sulphur and ash, and their sum is taken as equal to 100%. Moisture is expressed as a separate item. The analysis also enables us to determine calorific value or heating value of coal.
Proximate analysis is the separation of coal into its physical components and can be made without the knowledge of analytical chemistry. The analysis is made by means of a chemical balance and a temperature controlled furnace. The sample of fuel is heated in the furnace or oven. The component in the analysis we fixed carbon, volatile matter, moisture and ash.
These components are expressed in percent on mass basis and their sum if taken as 100%. Sulphur is determined separately. This analysis also enables us to determine the C.V. of coal. Compacted vegetation, in the absence of air and under the influence of pressure and temperature, is converted into coal.
The various stages of conversion are as follows:
Wood —> Peat —> brown —> coal —> lignite —> sub-bituminous coal —> bituminous coal —> anthracite.
This conversion takes 10 – 300 million years — the peat being the youngest and the anthracite the oldest one.
The ultimate analysis consists of determining the percentages of the ultimate constituents:
(a) Carbon
(b) Hydrogen
(c) Oxygen
(d) Sulphur
(e) Nitrogen
(f) Ash
Ultimate analysis is used for calculative and scientific work.