Compilation of questions on thermal engineering for college students.
Q. 1. State the first and second law of thermodynamics.
Ans. First Law of Thermodynamics:
(a) For a closed system which has passed through a cycle, the first law of thermodynamics states:
When a closed system has passed through a cycle, the sum of the heat energy taken in across the boundary from the surroundings is proportional to the work delivered from the system to the surroundings.
ADVERTISEMENTS:
As heat and work are measured in the same unit, joule, the proportionality factor is unity. Therefore, for a closed cycle.
Heat flow into the system from the surrounding (Q) = work flow from the system to the surrounding (W).
(b) For a closed system which has been conducted through a non-flow process, the first law of thermodynamics states:
The difference between the sum of the heat flowing into the system and the work delivered from the system is equal to the increase in the internal energy of the system.
ADVERTISEMENTS:
Therefore Q – W = U2 – U1.
Heat flow into system from the surroundings = increase in the internal energy of the system + work delivered to the surroundings from the system.
The relation Q – W = U2 – U1 is known as non-flow energy equation.
The system energy can be changed by a transfer of heat alone or by work alone or by heat and work together. If the system is thermally insulated from the surroundings, no heat transfer is possible and processes taking place under these conditions are called Isentropic processes.
ADVERTISEMENTS:
Second Law of Thermodynamics:
The Second Law of Thermodynamics like the First law is a very important law and is profound in its application. It is based upon observable phenomena and states that the heat will not transfer up the gradient of temperature of its own accord. This does not mean that heat cannot be made to transfer up the gradient of temperature. External energy is required to transfer heat from low temperature to high temperature.
The second law of thermodynamics establishes the direction in which spontaneous thermal processes occur in nature, and determines the conditions under which heat is converted into work. It states that heat is spontaneously transferred only from hot bodies to cold bodies.
As regards the conversion of heat into mechanical work, the second law of thermodynamics establishes the following condition:
ADVERTISEMENTS:
In order to convert heat into work, we must have two bodies at different temperatures. A hot body is a source of heat (required for doing work), while the cold body is a heat absorber (sink).
In accordance with the second law of thermodynamics, the heat of the source can be converted into work only partially, since in this process some fraction of heat is always transferred to the sink, the efficiency of a heat engine being always less than unity.
Kelvin-Plank Statement:
It is impossible for any cycling device to exchange heat with only a single reservoir and produce positive work. The Kelvin-Plank statement says that heat cannot be continuously and completely converted into work- a fraction of the heat must be rejected to another reservoir at a low temperature.
ADVERTISEMENTS:
The second law thus places a restriction on the first law in relation to the way energy is transferred. Work can be continuously and completely converted into heat but not vice versa.
Clausius Statement:
There is no process possible whose sole result is the removal of heat from a reservoir at one temperature and the absorption of an equal quantity of heat by a reservoir at a higher temperature. This statement does not say that it is impossible to transfer heat from a lower temperature to a higher temperature body. We require external energy, with a refrigerator does when it receives an energy input, usually in the form of work.
Q. 2. What is a thermodynamic cycle? Explain its importance.
Ans. From the view point of thermodynamics any heat engine, should be designed to follow the Carnot cycle, because it has the highest possible thermal efficiency within a given temperature range. However, due to difficulties in design, it has been proved to be impossible to construct an internal combustion engine in which the processes of addition and rejection of heat would proceed along isothermal lines.
For modern internal combustion engines, the higher temperature of 1800°C and lower temperature of 15°C are common. If we were to design Carnot engine within this temperature range, the pressure of 3000 bar would be required, and engine would be of fantastic size. In existing internal combustion engines the pressure reaches 40 to 50 bar.
It was found to be most convenient to impart heat to the working substance in:
(i) Constant volume process.
(ii) Constant pressure process.
(iii) Combined constant volume and constant pressure process.
Accordingly, three theoretical cycles which are of practical importance have been developed for thermodynamic engines:
(i) Otto cycle in which heat is added at constant volume
(ii) Diesel cycle in which heat is added at constant pressure
(iii) Dual cycle in which heat is first added at constant volume and then at constant pressure.
For the thermodynamic analysis of internal combustion engine cycles the real processes taking place in the engine cylinder and accompanied by friction and heat transfer are arbitrarily replaced with reversible processes. It is assumed that the working fluid is an ideal gas and the process developing in the engine follows a closed cycle.
In actual engines, however, the working fluid does not return each time to its initial state, but is exhausted from the cylinder and replaced with a new working fluid (intake). Nevertheless, the study of the theoretical cycles of processes taking place in the internal combustion engines is of great importance for establishing the most efficient conditions under which these engines should be operated to convert heat into work and for determining the factors effecting engine efficiency.
Efficiency of Cycle:
The thermal efficiency of a thermodynamic cycle which can be defined as the ratio of available heat energy converted to work and the heat received by the body.
Air Standard Efficiency of Cycle:
This is the theoretical thermal efficiency of an ideal cycle. This efficiency is based on the thermodynamic cycle working with air as a standard medium. This efficiency is a yard stick to compare the thermal efficiencies of actual engines working on thermodynamic cycle. It may be considered that the thermal efficiencies of the machines working on these cycles cannot be more than the efficiencies of these cycles.
Q. 3. Explain the working of heat machine.
Ans. This is a machine operating on thermodynamic cycle that converts heat energy into mechanical energy.
Therefore this heat machine has three main elements as shown below:
(1) Hot Body – This is a source of heat for the cycle.
(2) Cold Body – This receives the rejected heat during the cycle.
(3) Working Fluid – This receives heat from hot body and rejects the heat to cold body after performing the useful work.
The function of heat machine is to receive heat from a hot body and convert a part of it into useful work and the balance heat is rejected by the cold body.
If W is the useful work. Then
W = Q1 – Q2 = Q
where Q1 is the heat received from hot body.
Q2 is the heat rejected to a cold body.
Q. 4. Name the alternate fuels used as substitute fuel for conventional petroleum fuels.
Ans. There are varieties of alternative fuels which can be used as substitute fuel for conventional petroleum fuels with some modifications in the system. The fuels can be used as a dual fuel or partial substitution with conventional fuels.
The various alternative fuels are:
(1) LPG (Liquefied Petroleum Gas):
The LPG is a mixture of propane and butane with a small percentage of unsaturated. The LPG contains those hydrocarbons which are gaseous at normal atmospheric pressure, but are condensed to the liquid state at normal temperature by the applications of moderate pressures.
The LPG requires less storage space because it has liquid state storage and its combustion is smooth because it is in the gaseous state. It mixes easily with air and results into complete combustion of fuel. It has high calorific value and low air-fuel ratio. It has very less emissions and pollution because of complete combustion of fuel. The cost of fuel is low as composed to conventional fuel.
(2) CNG (Compressed Natural Gas):
The natural gas contains methane as main constituent of about 95% of total volume. The natural gas has high calorific value requiring no storage facilities. It mixes with air easily and does not produce smoke or soot. It has no sulphur content. The gas can be stored under high pressure in a tank.
(3) Alcohols:
The alcohols like ethanol (C2H5OH) and methanol (CH3 OH) can be used as a fuel for I. C. Engines and other purposes. The methanol and ethanol can be used as substitute fuels. The methanol can be substituted as fuel partially for petrol engines because it replaces petrol fuel and improves octane ratings of petrol.
The latent heat of vaporization of methanol is high which reduces the inlet air temperature. The ethanol can be used in diesel engines and can be blended with diesel fuel, known as E-diesel. The use of alcohols increases the engine efficiency and performance. The alcohols can be produced from corn or waste from sugar industries.
(4) Hydrogen:
The hydrogen is the simplest, lightest and high calorific value fuel. It is made up of one proton and electron. In a normal gaseous state, hydrogen is colourless, odourless tasteless, non-toxic and burns invisibly.
The hydrogen is made from natural gas through a process of reforming. The reforming process separates hydrogen from hydro carbons by adding heat. It can be produced from a variety of sources including water and bio-mass. The hydrogen can be produced by electrolysis of water.
The storage of hydrogen is hazardous because of problem explosion under high temperature conditions. It causes brittleness in some materials including metals which can generate electrostatic charges and sparks through flow or agitation.
The gas can be used in I. C. engines optimized to burn hydrogen instead of gasoline. The engine can reach upto efficiency of 38% and has zero emissions.
(5) Vegetable Oils:
The vegetable oil can be used as substitute fuel for I.C. engine. The oil such as edible oil, sun flower oil, coconut oils etc. can be used as a fuel for I.C. Engines. They have low calorific value and operation is not efficient.
(6) Bio-Diesel:
The bio-diesel is an alternative fuel produced from renewable resources such as soyabean or used restaurant grease. The bio-diesel contains no petroleum, but it can be blended with petroleum diesel to create bio-diesel blend. It can be used in diesel engines with no major modifications.
Bio-diesel is simple to use and it is bio-degradable, non-toxic, free of sulphur and aromatics. The use bio-diesel reduces unburned hydrocarbon, CO and particulate matter compared to emissions from diesel fuel. It can be blended up to 20% in diesel fuels- and requires no major modifications.
(7) Gas to Liquid Fuels:
The gas to liquid (GTL) fuels can be produced from natural gas, coal and bio-mass using Fischer-Tropsch chemical reaction processes. The liquid produced includes naphtha, diesel and chemical feed stocks. The GTL can be purely used or blended with diesel fuel in the existing engines.
These fuels provide an opportunity to reduce dependence on petroleum based fuels and reduce tail pipe emissions. The GTL fuel has no sulphur, aromatics and toxics. The use of fuel shows loss of fuel economy upto 3-4%. The GTL fuels are desirable as compared to natural gas because they are less expensive to transport than oil. Therefore converting natural gas into liquid reduces its cost.
(8) Gobar Gas:
The gobar gas is the most suitable fuel for diesel engines and cooking purposes because it is gas produced from waste material such as cow dung etc. The gobar gas can be produced by simple tank embedded in the waste material.
The fuel contains large amount of methane and other hydrocarbon which can burn efficiently. This fuel has very low cost and can be produced in villages in large quantity. The gas can be used for stationary diesel engines and cooking purposes.
Q. 5. Derive equations for heat losses during combustion.
Ans. The heat of combustion can be usefully absorbed or converted into work. However, it often happens that an appreciable amount of potential heat is lost due to incomplete combustion. The fuel is used in boiler furnaces and in internal combustion engines.
The principal ways in which heat may be lost from furnace are the following:
(i) As sensible heat in dry flue gases
(ii) As sensible heat and latent heat in the water vapour formed by burning the hydrogen of the fuel
(iii) Heat in vapour evaporated from the fuel
(iv) Due to incomplete combustion of carbon, hydrogen, etc.
(v) Loss due to combustible matter in ashes and clinker
(vi) Unaccounted losses such as due to radiation, due to moisture in combustion air, sensible heat in ashes, etc.
The heat losses from an internal combustion engine may be in the following ways:
(i) Sensible heat in dry flue gases
(ii) Sensible heat and latent heat in the water vapour formed by burning the hydrogen of the fuel
(iii) Losses due to incomplete combustion
(iv) Heat carried away by circulating water
(v) Unaccounted losses.
We shall consider briefly how the above losses can be estimated with sufficient accuracy:
(i) Heat carried away by dry flue gas per kg of fuel = m Cp (T-t) where m is the mass of dry flue gas per kg of fuel, Cp is the average value of the specific heat of flue gases and T and t are the exit and inlet gas temperatures respectively. The average value of specific heat may be taken as 1.005 kJ/kg-K.
(ii) Heat carried away by water vapour formed due to combustion of hydrogen in fuel per kg of fuel = 9H x 4.1868 [(100 – t) + Lo + 0.48(T – 100)] where H is the amount of hydrogen per kg of fuel, t is the atmospheric temperature, Lo is the latent heat of steam at atmosphere pressure, 0.48 is the specific heat of superheated vapour for MKS system and 2.01 kJ/kg-K for SI system and T is the exit gas temperature.
(iii) Heat carried away by moisture in the fuel per kg of fuel
= Mm [4.1868 (100 – t) + Lo + 0.48 (T – 100)]
where Mm is the mass of water evaporated from 1 kg of fuel.
It should be noted that the moisture in the fuel has to be heated to its boiling point, evaporated and then superheated to the exit gas temperature.
(iv) Heat lost due to incomplete combustion consists of two parts:
(a) Incomplete combustion of carbon
(b) Incomplete combustion of hydrogen and hydrocarbon.
Heat lost due to incomplete combustion of carbon –
where CO and CO2 represent the per cent by volume of carbon monoxide and dioxide in the flue gases and C is the amount of carbon in one kg of fuel.
The heat lost due to incomplete combustion of hydrogen and hydrocarbon is not usually assessed, but in coal fired furnaces this loss may be equal to the loss due to incomplete combustion of carbon. In I.C. engines, a very considerable loss may result from the partial or non-combustion of a portion of the fuel.
Loss due to methane in exhaust gas per kg of fuel –
CH4 represents the percentage by volume of methane in exhaust gases.
(v) Heat lost due to combustible in ashes:
This loss is usually calculated on the assumption that the combustible in ashes is carbon.
Heat lost due to carbon in ashes = C’m/100 × 35000 kJ/kg
where C’ is the percentage of carbon in ashes, m is the mass of ash and 35000 is the calorific value of carbon in kJ.
(vi) Heat carried away by circulating water in case of i.c. engine is calculated by the following relation:
Heat carried away by circulating water = m1 × Cw × (t1 – t2)
where m1 is the weight of water used per minute and t1 and t2 are outlet and inlet temperatures respectively and Cw is the specific heat of water.
(vii) The unaccounted losses are obtained as the difference between the total heat supplied and the total heat accounted for. The unaccounted losses include those due to radiation, heating moisture in the air used for combustion and experimental errors.
The most serious losses which can be controlled are those due to incomplete combustion and the heat carried away in the dry flue gases.
Q. 6. Explain combustion of various fuels with the help of equations.
Ans. The main constituents of any fuel are carbon and hydrogen. Some sulphur may also be present. The combustion of a fuel is a chemical combination of these constituents of a fuel with oxygen as a result heat is evolved. The combustion can be represented by a chemical equation quantitatively and qualitatively. The smallest quantity of a gas which takes part in a chemical reaction is a molecule.
The following table gives the approximate atomic and molecular weights, of various elements and compounds, which will be necessary in the calculation work for combustion of a fuel.
1. Combustion of Carbon:
When carbon burns in plentiful supply of oxygen, the process is represented by the chemical equation:
The carbon monoxide produced during the combustion is a fuel and therefore it should not be allowed to escape otherwise it would be a loss. It is highly poisonous and has no smell therefore it is dangerous if it escapes.
2. Combustion of Carbon Monoxide:
When hydrogen burns with oxygen it produces water vapour.
The combustion can be represented by a chemical equation:
Sulphur is of very minor importance in contributing to the calorific value of a fuel because only small quantities are present and its calorific value is very low being 9160 kJ/kg. It has got injurious effects because it forms, on burning, sulphur dioxide which gets converted into sulphurous acid in presence of moisture which corrodes the metal and therefore fuel having higher percentage of sulphur is undesirable particularly for steam raising purposes.
When sulphur burns with oxygen it produces sulphur dioxide.
The combustion can be represented by a chemical equation:
i.e. sulphur + oxygen = sulphur dioxide
32 + 32 = 64
∴ sulphur + oxygen = sulphur dioxide
1 kg + 1 kg = 2 kg.
This chemical equations are known as stoichiometric or the equation for the correct mixture, since the amount of oxygen present is exactly sufficient to burn the combustible completely. They represent the re-arrangement of atoms during the combustion process and hence, the number of atoms of each element must be the same on each side of the equation.
We know that all chemical reactions are accompanied by heat exchanges. In some of them heat is evolved, others require a supply of heat to carry on the reaction. In cases in which heat is developed we call these reactions exothermic and in cases in which heat is absorbed we call them endothermic reactions. All the above chemical reactions are exothermic.
The energy released on combustion is accompanied by an equivalent reduction in mass. In our calculations the loss of mass is negligible and we can expect to measure equal masses of reactants and products of combustion.
Let us write the stoichiometric equation for any substance given by the chemical formula (CaHbOc).
The equation will be of the form –
Q. 7. How to calculate the flow through steam nozzles? Explain with derivation.
Ans. The flow of steam from the nozzle is considered to the isentropic process in which no loss of heat occurs from the nozzle and the heat is also not lost overcoming the friction. In other words the process is a frictionless adiabatic process.
Although the friction between the flowing fluid and the nozzle is always present which reduces the velocity of steam from the nozzle but also increases the enthalpy of steam. The steam expands from high pressure to low pressure converting the pressure energy into kinetic energy and hence the work is done during the process. This is an expansion of steam in which the enthalpy of steam is reduced.
Thus, the flow of steam through a nozzle may be regarded as either a reversible adiabatic (isentropic) flow or adiabatic flow modified by friction and super-saturation.
Fig. 10-9 shows a sketch of P-V diagram for steam nozzle.
The work done at the inlet of the nozzle is given by P1v1
The work done at the out let of the nozzle is given by P2v2
Then the net work done during the process is –
Alternatively, this work done is equal to the change of internal energy, (U1 – U2) as during isentropic expansion work is done at the cost of internal energy. This work amounts to P2v2 which is equal to the final flow work spent in forcing the steam out to make room for fresh charge of steam.
Example:
Steam at the pressure of 10 bar enters the steam nozzle in dry condition and leaves the nozzle at pressure of 5 bar with dryness fraction of 0.7. Calculate the work done during the process.
Q. 8. Derive an expression for specific volume of steam.
Ans. The specific volume of superheated steam may be found with reasonable accuracy by applying the general gas law PV/T = constant, because it is assumed that superheated steam behaves as a perfect gas from the beginning of superheating i.e., from dry saturated condition. The steam is superheated at constant pressure. By applying the gas law to steam at the beginning of superheating and at the end of superheating, we get-
Tsup and Tsat – Absolute temperature of superheated and saturated steam
Vsup and Vg – Volume of superheated and saturated steam
Psup and Psat – Pressure of superheated and saturated steam.
By knowing the pressure of steam generation and temperature of superheated steam, the specific volume of superheated steam can be directly obtained with the help of the steam tables for superheated steam.
The specific volume of wet steam can be obtained by the formula which can be derived as under:
If x be the dryness fraction of steam, then the mass of dry steam will be x and the mass of water particles will be (1 – x) per unit mass of wet steam. The volume of wet steam will be the sum of the volume of water particles and the volume of dry steam.
∴ Specific volume of wet steam Vw = (1 – x) vf + xvg.
But the specific volume of water is small compared with that of vapour and the product (1 – x) vf will be still smaller so the first term can be conveniently neglected and for all practical purposes the formula for the specific volume of wet steam becomes Vw = xvg.
Q. 9. What is a pyrometer? Name the pyrometers commonly employed in engineering practice.
Ans. Temperatures above 150°C are measured by instruments known as pyrometers.
In mercurial pyrometer, nitrogen under pressure is enclosed in a tube above mercury and this can be used for measuring temperatures upto 550°C.
Pyrometers commonly employed in engineering practice are: 1. Thermo-electric pyrometer 2. Radiation pyrometer 3. Optical pyrometer 4. Pyrometric cone or Seger cone pyrometer.
1. Thermo-Electric Pyrometer:
It is used for measuring temperatures from 150°C to 1600°C. It consists of a thermo-couple, an indicating or recording device and suitable connecting wires. When one end of a thermo-couple is heated, an electromotive force is generated which is applied either to a milli-voltmeter or potentiometer which is calibrated to read in degrees.
The magnitude of electromotive force developed depends upon the temperature difference of the hot junction and cold junctions.
It is used for measuring temperature above 550°C. This instrument utilizes the heating effect of energy radiated from a hot object as an index of its temperature.
It consists of four essential elements, namely:
(i) Cylindrical tube containing a concave mirror
(ii) Lens focused on the hot object
(iii) Small thermo-couple on which mirror concentrates its rays
(iv) Measuring device which is the same as that used with thermo-couple.
This is an instrument which is suitable only for high temperature work. The hot body is viewed through a telescope. A small electric filament lamp is arranged inside this telescope so that the image of the filament is superimposed on that of the radiation of hot body. A variable resistance is arranged inside this telescope so that the image of the filament is superimposed on that of the variation of hot body.
A variable resistance arranged in filament circuit enables to vary the temperature of the filament. This resistance is adjusted so that the image of the filament merges or disappears into the image of the hot body. Under these conditions these temperatures are similar. So resistance control can be calibrated in terms of temperatures.
4. Pyrometric Cone or Seger Cone Pyrometer:
It is used for measuring the furnace temperature. It can be used for temperatures from 600°C to 2000°C.
Q. 10. What is the function of draught gauge?
Ans. The function of a draught gauge is to measure the draught. It is sometimes known as a manometer. In its simplest form it consists of a glass tube having U-shape.
The U portion is partially filled with water. One end of the U-tube is extended into the place where the draught is to be measured and the other end is open to atmosphere. When there is no pressure difference between the atmosphere and the closed space where the draught is to be measured, the liquid stands at the same level in both the limbs of the U-tube.
If the atmospheric pressure is greater than the space where the draught is to be measured, the water in the leg which is connected to the space is forced up and the difference in water levels in two limbs gives the measure of draught intensity.
This type of draught gauge would not be sufficiently accurate because of the short length of their scales. In a manometer of a differential gauge type, more accuracy is obtained by inclining one leg of the tube and using light oil in it instead of water.
By this arrangement we get a long scale. The inclined tube is vertical. But the distance moved through is much greater. This arrangement renders the inclined tube instruments easy to read.
Q. 11. Explain the theory of inclined tube manometer.
Ans. The theory of the inclined tube manometer is as follows:
Let E in fig. 7-6, be the reservoir of cross-sectional area A and B be the inclined tube of cross sectional area a.
When P1 = P2,
let the level of the manometer liquid stand at CD. Assuming now that P1 becomes slightly greater than P2 the level in E sinks a distance equal to h2 while level in the tube rises say a distance h1, both distances being measured vertically.
Let the vertical distance between the two levels after the balance is established be h and let d be rise in the tube as measured along its length.
Q. 12. What are the instruments employed in a boiler plant? Explain their functions.
Ans. There are various instruments used in the boiler house to obtain maximum efficiency of boiler house plant.
The boiler house instruments may be broadly divided into two parts:
(i) Those which give information about performance.
(ii) Those which help to control the performance.
The main instruments employed in a boiler plant are as follows:
(a) Coal Meter:
With mechanical stokers coal can be measured volumetrically by means of instruments which are calibrated in terms of kg of coal passing through the stoker. This pre-supposes an average value for the density of coal. These meters differ in design according to that of the stoker with which they work.
The coal consumption is given on an integrating meter dial. Such instruments require frequent re-calibration to allow for variation in density ratio, and their accuracy may be greatly affected by the amount of wet stick used, as this has a tendency to bridge over in the stoker hopper.
(b) Water Meter:
These are of various types, the most commonly used being of the integrating pattern where the total flow is registered in litres but no record of the rate of flow is given.
In case of hot water meters calibrated in litres a simultaneous record of temperature is required so that the mass of the water can be ascertained. The provision of a feed water thermometer is imperative in order to arrive at the true performance of a boiler.
(c) Steam Meter:
A dial type of indicator is provided at each boiler front. The steam meters and flow recorders are provided and by comparing the results with water meter we can know the losses in blowing down and from safety valves.
All steam meters operate under the impulse of a difference in pressure across a restriction or orifice in the path of the steam flow and are required to be calibrated for individual conditions of pressure and superheat in each particular case so as to read accurately in kg of steam.
(d) Temperature Measurement:
The following tables give the location and types of temperature measuring instruments usually employed in a boiler house.
(i) Feed Water Circuit (Water and Steam):
(ii) For Gas and Air Circuit:
(e) CO2 Recorder:
It shows the amount of excess air passing away in the gases and in conjunction with gas temperature provides the necessary data to calculate the heat carried away by flue gases.
The majority of CO2 recorders or indicators operate on one of the following two different principles:
(i) By automatic measurement of the difference in volume of the gases before and after the measured sample has been passed through a suitable solution of KOH.
(ii) Electrical Method:
It employs the difference in heat conductivity between air and gases, composed of air, and CO2. Two identical coils of resistance wire are connected to form two legs of a Wheatstone Bridge, which is supplied with current from a battery. These two coils are each situated within a separate cavity in a massive block of metal.
Air is passed through one of the cavities and flue gases through the other, both being at the same temperature. The heat conductivity of flue gases will vary with % content of CO2 and the out of balance set up between the resistances of the two coils, due to the difference in conductivity, is the measure of CO2 in the gases. This is indicated by a milli-voltmeter calibrated suitably.
(f) Draught Gauges:
They measure the draught in the boiler installation.
Draught indicators and recorders are of two kinds, viz.-
(i) Liquid displacement types.
(ii) Aneroid types.
The former are further classified in two types, viz., the ‘Plain U-tube draught gauge’ and the ‘Inclined tube gauge’.
Schedule of Instruments:
(i) Electrical CO2 Recorder:
It is connected to the main flue and close to the damper of each boiler.
(ii) Steam Flow Recorder:
This is placed on each steam main leaving the boiler house.
(iii) Steam Flow Indicator:
This is placed on each boiler.
(iv) Water Meter:
It registers total water fed to the whole battery.
(v) Recording Thermometers on Economizer Inlet and Outlet:
These are placed at inlet and outlet of economizer
(vi) Steam Pressure Gauge:
This is used for recording the steam pressure in a boiler.
(vii) Recording Thermometer:
These are placed on steam outlet from each boiler’s superheater.
(viii) Electrical Recording Pyrometer:
It is connected to main flue and close to dampers of each boiler.
(ix) Combined Electrical CO2 Recorder and Pyrometer:
It is connected to exit flue at chimney base.
(x) Draught Gauges:
It is connected to the furnace combustion space and at dampers of each boiler and to the ashpit of each furnace with forced draught and at economizer inlet and outlet flue.
Q. 13. Explain the operation of governor in steam engine.
Ans. The operation of the governor is as follows:
When the speed of the engine increases, the speed of rotation of the governor balls increases as well. The consequent increase in centrifugal force causes them to move outwards against the force of gravity and the control spring. Due to this the inner end of the arms of governor balls press down upon the valve spindle and partly close the valve.
This reduces the steam pressure, acting on the piston, due to the throttling at the valve. The power developed is also reduced and the engine speed drops to the normal value. Similarly, a decrease in speed will cause the weight to be lowered and the valve spindle to be raised. The pressure acting on the piston then increases resulting in an increase of the speed to the normal value.
For any given speed of the engine the balls occupy a position in which their centrifugal force just balances the force of gravity and that of the control spring. If the engine speed changes mostly due to change in load, the pressure will rise or fall until it gets adjusted to suit the load to be carried.
The speed at which the engine will run can be adjusted within limits by turning the adjusting screw, and thus changing the loading of the spring. Decrease in tension will decrease the speed of the engine and vice versa.
Most of the better type engines are governed by varying the points of cut-off and hence the total volume of steam supplied to the cylinder. This system is known as cut-off governing, as the period of steam admission is usually altered by changing the time of cut-off, leaving the beginning of the admission period unchanged.
In this case the intake steam pressure remains constant and hence full use is made of steam pressure. By making the cut-off smaller, however a smaller quantity of steam is admitted to the cylinder and a smaller diagram area is produced.
Hence a smaller amount of work is done in the cylinder. This method produces a much more valuable use of the steam than the other method known as throttle governing. Cut-off governing will, therefore, be generally more efficient.
There is no reduction in the ideal thermal efficiency at light loads with this type of governing. But it cannot be used with simple D-slide valve type engines. It requires Meyer’s expansion type of valve and allied attachments.
For an engine having a throttle governor, a straight line is obtained when the total mass of steam, m, used per hour is plotted against the indicated power. This particular relationship was observed first by Mr. Willan and is known as Willan’s line.
The equation of the straight line will be –
m = A x indicated power + B kg/hour.
Where A is the slope of the straight line and B is the intercept on the steam consumption axis. The indicated power is zero here and hence B represents loss due to condensation, leakage, and so on.
When a graph of steam consumption per hour for an engine governed by cut-off governing, is drawn against indicated power a curve is obtained as shown in fig. 9-14. For comparison purposes, the curves of steam consumption of two identical engines of the same indicated power are drawn, one governed by a throttle governor and the other by a cut-off governor.
It is evident from the figure that the steam consumption per hour at full load is the same for both the engines; but at fractional loads the steam consumption is higher for throttle governing than that for cut-off governing.
Q. 14. What is heat? Explain with example.
Ans. During an energy transfer process which results from the temperature difference between one body and another, the energy so transferred is called heat. The heat having been transferred will then disperse into other forms of energy such as internal energy or work, the dispersal being a function of the system employed.
Heat is a transient quantity, it being descriptive of the energy transfer process through a system boundary resulting from temperature difference. If there is no temperature difference, then there is no heat transfer.
Since the term heat is used to describe a transfer process, then heat energy ceases to exist when the process ceases. Thus heat is not a property.
Heat energy is given the symbol Q, and the unit of heat is joule.
By convention, heat flowing into a system is termed positive and heat flowing from a system is termed negative. On the other hand, work flowing from a system is called positive and work flowing into a system is called negative.
Heat and work are forms of energy in transition and are the only forms in which energy can cross the boundaries of a system. Hence neither heat nor work can exist as stored energy. Let us understand the problem with the help of an analogy suggested by Prof. Zemansky. Let us consider a lake or reservoir into which rain is falling.
After the rain has entered the lake, it becomes additional water in the lake and no longer exists as rain. The water in the reservoir is analogous to stored energy and the rain is analogous to either heat or work.
After heat or work has crossed the boundary and entered a system it ceases to exist as heat or work, and manifests itself as additional stored energy. It is, therefore, incorrect to speak of a body or system containing heat. It can contain energy and may gain or lose energy, which in transit may be in the form of heat or work.
The amount of heat which is taken in or given out by a working substance depends upon the following three quantities:
(i) Mass of the substance in a body
(ii) Specific heat capacity
(iii) Change of temperature.
The specific heat capacity of a substance is the quantity of heat necessary to change the temperature of unit mass of the substance through one degree. Therefore the quantity of heat taken in or removed from the substance is equal to (mass of the substance x specific heat x temperature change).
Q = m Cp Δ T
Specific heat capacity may vary with temperature. The practical unit for specific heat capacity is kJ/kg-K.
A particular case in the use of specific heat capacity arises in the use of water as a measuring device in calorimetry.
Water Equivalent of a body is defined as that mass of water requiring the same amount of heat to raise its temperature by one degree as will raise the temperature of the body by one degree. It is the product of the mass of the body and its specific heat divided by specific heat of water.