Master of Science (Eng.) in Environmental and Energy Engineering

You are required to carry out six laboratory sessions in the first semester to back up the combustion fundamentals course, CPE 630. The labs cover a wide range of topics and are designed to be of relevance to all parts of the course (not just Combustion Fundamentals).

Lab reports are marked and count towards the mark in Laboratory Practicals CPE 631.

The labs are to be held on weeks 2, 4, 6, 8, 10, 12 on Friday afternoons. The lab reports should be handed in to Dr Ristic two weeks after the lab was done.

The location of the experiments is as follows:

Calorific Value of coal F65 - Analytical lab

Flow Measurement B9 (same block as LT17) - 1st year labs

Performance of a Heat Exchanger B9 - 1st year labs

Analysis and Classification of coal F65

Boiler Efficiency B55

Fluidised Bed Heat Transfer B55

The students will be broken into groups of two.

The timetable of labs is as below. Please report to the lab at 2.00pm and wait for the lab supervisor. DO NOT start the lab without a supervisor present under any circumstances.

Week that the lab is held

Report-writing is an essential part of the engineer's craft. It is the formal means by which ideas and information are transferred to others. You must therefore be able to write and present your information clearly so that it can be easily understood by others.

A typical full report structure is:

Each report will be marked out of 10. The following marking scheme will be used by markers:

See also: Topping, J.: Errors of Observation and their Treatment

Lyons, L.: Data Analysis for Physical Science Students

Most scientific experiments require you to measure physical properties such as length and time. Such measurements are never perfect, that is to say, there is always some error or uncertainity associated with every measurement that you make. Maybe the instrument that you are using is not very accurate; maybe you are not very good at using it; maybe what you are trying to measure is fluctuating. Only if we take these errors and uncertainties into account can we judge how accurate our final result might be.

There are generally two types of error. Systematic errors are constant and reproducible for every reading that you take. For example, there may be a zero error or incorrect calibration for a particular instrument. Random errors, on the other hand, vary from one reading to the next, but in such a way that on average as many readings lie one side of the true value as lie on the other. In fact, the mean of an infinite number of readings will give the true value of the parameter being measured. Unfortunately, life is too short to take that many readings, so we must do our best to estimate the true value from the limited number of readings that are possible. In order to do this, we must first be able to assess quantitatively the errors associated with any reading, and then know how to determine the overall uncertainty by combining the individual experimental errors.

There are a number of ways of estimating random errors in the measurement of a given parameter:

(i) The preferred method is to take the reading several times and then calculate the mean and standard deviation. The more readings that are taken, the closer the mean is to the true value, and the smaller the standard deviation. This method necessarily assumes that the true value that you are trying to measure does not change while you are taking all the readings.

(ii) If you use a ruler to measure a length, or a stopwatch to measure a time, you will have an instinctive feeling for the accuracy (i.e. uncertainty) of any measurement that you make. For example, with a millimetre ruler you will probably measure to the nearest 0.5 mm, so you would give a reading as, say, 43.5 ± 0.25 mm. A similar principle applies to digital displays: if a voltmeter displaying two decimal places shows 2.38 V, this should be recorded as 2.38 ± 0.005 V.

(iii) If a display fluctuates as you attempt to take a reading, you have to judge visually the average value and its associated error, i.e. the range over which the value fluctuates.

(iv) Instrument manufacturers provide a figure for an instrument's accuracy in its specification. Modern laboratory instruments usually have a high inherent accuracy, i.e. there is very little instrument error. This error can therefore be safely ignored in your experimental work.

When we manipulate our data, we must know what to do with the associated errors. Some errors may augment each other, while others may partially cancel.

Consider the case of a ten-lap race, with person A measuring each lap time, and person B measuring the total race time. Suppose that the error in each time reading is ±  1 s. Person A can calculate the total race time by adding the ten lap times. What is the uncertainty in that figure? If errors are purely additive, this produces an error of ±  10 s. Is this reasonable? Person B has just one reading with an uncertainty of just ±  1 s. Would you expect A's calculation to be as much as 10 seconds different from B's measurement? Probably not – we are being unduly pessimistic by considering the worst possible case, where all the errors build up. As these are random errors, it is likely that some of A's lap-times are low, and some are high. This means that there will be some cancellation of errors when A's measurements are added. The probability of all the errors working against us at one time is very remote, even though the uncertainty of each original reading is not in doubt.

We therefore need a method of combining errors which is not over-pessimistic, but which gives a good indication of the likely error. We need a measure of the probable error. Statistical techniques can be employed to estimate the overall probable error from a sequence of random errors. Suppose that each measurement we make has an error e associated with it. For the simple calculations that you will be carrying out, the following relations can be used.

i.e. the probable overall error is the square root of the sum of the squares of the individual errors. So if we take our example of the race time, the probable error in A's summation of lap times is given by:

Clearly this is a larger error than with B's single measurement, but is much less than the over-pessimistic 10 seconds obtained by simple addition. Remember that 3.16 s represents the probable error; it is still possible (but unlikely) that the true value lies outside the limits of the error.

So, the probable overall relative error is the square root of the sum of the squares of the individual relative errors.

Consider an example of calculating the volume of a cylinder whose diameter and length have been measured as 18.5 ± 0.25 mm and 53.0 ± 0.25 mm respectively.

One final point. Note that because the errors are squared, it only takes one error to be much greater than the others for that error to dominate over all the others. You should therefore always be aware during experiments of which errors and uncertainties are having the greatest effect on your answer.

Measurements are often made on how one quantity changes as another is varied. The results are then plotted on a graph, which, in many cases is designed to give a straight line in the form y = mx + c. In practice your points will not lie exactly on a straight line, i.e. there may be a degree of scatter, or a degree of curvature. There are two main reasons for this:

(i) Slight curvature in the line suggests that the theory which predicts a straight line is, in fact, inaccurate. Maybe a poor assumption has been made in deriving the equation. Check for this, and mention it in your discussion of the experiment.

(ii) Scatter either side of a line is usually due to experimental errors of the type described above.

Whatever the reasons for the scatter, we usually need to determine a slope and an intercept from the plot. We need to determine the best values and the error in each.

There are statistical techniques available which calculate the best-fit line and values for the slope and intercept. Calculations are extremely tedious to do by hand, but are available in most graph-plotting computer packages. If these are not available, or you are not confident enough to use them, you will need to draw the graph by hand, draw the best-fit straight line by eye and determine the slope and gradient graphically.

You then need to calculate the uncertainity in the slope and intercept. Software packages rarely do this, so it usually has to be done by hand. The following method will suffice:

To determine the calorific value (enthalpy of combustion) of a sample of coal.

The physical and chemical properties of coals vary enormously according to their geographical source and geological history. Their differing properties form the basis of various classification systems, all of which attempt to group coals according to their general characteristics and main uses. A number of classification schemes have been introduced based on chemical composition and calorific value.

The calorific value of a fuel is defined as the heat released by the fuel when it is completely burned at standard pressure (1 bar) and reference temperature (298 K). Calorific values for solid and liquid fuels are normally given per unit mass, whereas gaseous fuels are normally given per unit volume. Obviously, the higher the calorific value, the greater the heat release. High quality coals, such as anthracites and bituminous coals, can be expected to have calorific values in the range 25 to 33 MJ/kg; low quality coals, such as lignites, and peat-based fuels have calorific values of under 20 MJ/kg.

The determination uses a bomb calorimeter (Figure 1), in which a sample of coal is burned in oxygen under standardised conditions. The fuel is placed in the central container (the bomb), which is surrounded by a water jacket. The fuel is ignited, and the energy liberated is transferred to the water. A thermometer measures the rise in temperature, from which the amount of heat released may be calculated. The calorific value can then be determined after corrections are made for the heat liberated by the ignition wire. It is customary for calorific value to be quoted per kilogram of dry fuel. Thus the moisture content must also be measured and the appropriate correction made.

Weigh about 1 gram of coal (to the nearest 0.1 mg) into a crucible.

Stretch a piece of nichrome wire between the electrodes of the bomb cap and tie around it a 90 mm length of cotton. Place the bomb cap on the special stand, and place the crucible in its holder, making sure that the cotton is in contact with the sample.

Assemble the bomb, making sure that sealing ring is correctly positioned in its groove in the bomb cap, and that both metal surfaces are clean. The bomb cap retaining ring must be tightened by hand only.

Connect the filling tube to the bomb, tighten the union connexion by hand, and fill the bomb with oxygen to a pressure of 25 atm. Remove the filling tube and fit the sealing cap to the inlet tube. The tommy-bar facilitates lifting the bomb into the calorimeter.

After filling the bomb with oxygen, stand it on top of the apparatus, and test the firing circuit to check that the wire has not been disturbed while assembling and filling the bomb.

Place the calorimeter vessel into the calorimeter so that the short locating peg on the side of the calorimeter vessel engages the slot in the vertical portion of the front left-hand foot of the spider support.

Place the bomb into the calorimeter vessel. Carefully lower the lid of the water jacket complete with thermometer and thermistors, making sure that nothing is misaligned. Press the bomb firing plug (NOT the FIRE switch) and check that it has engaged the socket on the bomb by confirming that the READY TO FIRE lamp lights up.

Switch on the heater and allow about 5 minutes for the temperature to stabilise. Read the temperature of the water in calorimeter vessel every minute. When three consecutive readings agree to within 0.001° C, depress the FIRE switch to ignite the sample, and zero the counter. [WARNING: Do not extend any part of your body over the calorimeter either during firing, or for at least 20 seconds after firing.] A satisfactory ignition is indicated by the READY TO FIRE lamp staying off when the FIRE switch is released.

Leave the apparatus for 5 minutes, then start recording the temperature of the calorimeter to the nearest 0.001° C every minute until the temperature has stabilised. Remove the bomb from the calorimeter. Examine the contents of the bomb to check that complete combustion has occurred. Incomplete combustion is indicated by the presence of sooty deposits on the inner surface of the bomb, or residual carbon in the crucible. Any such evidence invalidates the test.

Assuming that the bomb is perfectly insulated, then the heat absorbed by the water in the calorimeter is equal to the heat released by the fuel. Now, the heat gained by the water (Q) is given by:

Q = C D T

where C is the heat capacity of the bomb (also known as the bomb factor) in Joules/K, and D T is the rise in temperature of the water. We must subtract from this the heat generated by combustion of the cotton and the ignition wire. The demonstrator will provide you with these values. Thus the calorific value of the fuel is given by:

To determine the proximate analysis for two different coals and hence to classify the coals according to the ASTM classification.

The physical and chemical properties of coals vary enormously according to their geographical source and geological history. Their differing properties form the basis of various classification systems, all of which attempt to group coals according to their general characteristics and main uses. A number of classification schemes have been introduced based on chemical composition and calorific value.

The chemical composition is central to the classification of coals. Two types of analysis are in general use: proximate analysis and ultimate analysis. Here you will be using the proximate analysis, in which coal composition (by mass) is given in terms of four constituents: fixed carbon, moisture content, volatile matter (gases emitted during the thermal decomposition of coal in an inert atmosphere) and ash (inorganic matter left after combustion). In a proximate analysis, the moisture content, volatiles content and ash content are each measured separately; the fixed carbon is then determined by simple difference.

Coals can be divided into four generic classes (in descending carbon content):

More formal classifications of coal split each of these four groups into further sub-groups. The American Society for Testing and Materials (ASTM) system, widely used in North America, classifies each coal according to its fixed carbon content and calorific value.

Notes:

Weigh an empty crucible. Gradually add about 1 gram of coal and record the weight of the crucible and contents. Tap the crucible gently to spread the sample evenly over the bottom of the crucible. Place the crucible in an oven at a temperature of 105°C to 110°C for one hour. (During this period you can start analyses B and C.) Cool the crucible in a dessicator and reweigh. The percentage moisture can be calculated from:

Weigh an empty crucible. Gradually add about 1 gram of coal and record the weight of the crucible and contents. Tap the crucible gently to spread the sample evenly over the bottom of the crucible. Place the crucible in a high temperature furnace. Heat the sample to 750°C and leave it at that temperature for one hour so that all the combustible material is completely burned. Remove the crucible from the furnace. Let it cool for about one minute in the laboratory, then place it in the dessicator until it has cooled to room temperature. Reweigh the sample. The percentage ash can be calculated from:

Weigh an empty crucible plus lid. Gradually add about 1 gram of coal and record the weight of the crucible (plus lid) and contents. Tap the crucible gently to spread the sample evenly over the bottom of the crucible. Place the covered crucible into a high temperature furnace which has already been preheated to 925°C. Heat the sample for exactly seven minutes. Remove the crucible from the furnace. Let it cool for about one minute in the laboratory, then place it in the dessicator until it has cooled to room temperature. Reweigh the sample. The percentage volatiles quoted in a proximate analysis excludes water. However, both water and volatiles are driven off during heating. Thus the volatiles content is given by:

M = % moisture content

Fixed Carbon is not determined directly, but is taken to be everything other than what has already been measured. Thus it can be calculated from:

The demonstrator will provide you with a calorific value for each of the coals used.

From your results, calculate the fixed carbon and volatile content on a dry, ash-free basis, i.e. ignoring the moisture and ash content.

From these figures and the calorific values provided, determine the ASTM Group to which each coal belongs.

You have determined the dry, ash-free content of the coal, but the ASTM table refers to a dry, mineral matter-free basis. The mineral matter in a coal is usually about 10-15% higher than the ash content, because some minerals react or decompose during the analyses carried out above. Discuss whether using an ash-free basis instead of a mineral-free basis makes any difference to your classification.

Cooper, B.R. & Ellingson, W.A. (eds.): The Science and Technology of Coal and Coal Utilisation, Plenum, 1984 662.62

Francis, W. & Peters, M.C.: Fuels and Fuel Technology (2nd ed), Pergamon, 1980 662.6

Smoot, L.D. & Pratt, D.T. (eds.): Pulverised-coal Combustion and Gasification, Plenum, 1979 662.62

Speight, J.G.: The Chemistry and Technology of Coal, Dekker, 1983 662.622

To determine the carbon and hydrogen content of a coal sample.

The physical and chemical properties of coals vary enormously according to their geographical source and geological history. It is important to be able to determine the chemical composition of various coals, as this often provides the key to their different combustion characteristics. Two types of chemical analysis are in general use: proximate analysis and ultimate analysis. You will be performing one part of an ultimate analysis, in which coal composition (by mass) is given in terms of the chemical elements that make up the coal (such as carbon, hydrogen, nitrogen, sulphur, chlorine and oxygen).

Check that the furnace is at 1350°C and that the silver gauze is in its correct position. Connect the absorption train to the combustion tube and pass oxygen through the system at 18 litres/hour for 10 minutes. Disconnect the absorption train from the combustion tube and connect to the air purification train. Draw purified air through the train at a rate of 12 litres/hour for 10 minutes. Disconnect the absorption train, wipe each absorber with a clean, dry cloth. Allow the absorbers to cool to room temperature (usually about 15 to 20 minutes), and then weigh.

Weigh about 0.5 g of coal (to the nearest 0.1 mg) into a clean, dry sample boat. Spread the sample evenly over the bottom of the boat and cover it with about 0.5 g of aluminium oxide. Reconnect the weighted absorption train. Remove the rubber stopper carrying the silica pusher and insert the sample boat into the combustion tube to such a position that its centre is 240 mm from the centre of the hottest zone (the first mark on the push-rod). [BEWARE: The end of the push-rod may be very hot.] With the silica pusher fully withdrawn, replace the rubber stopper and continue to pass oxygen at 18 litres/hour. At the end of each of the next six one-minute periods, push the boat forward 40 mm, withdrawing the silica pusher each time in order to prevent its distortion. Allow the boat to remain in the hottest part of the tube for a further 4 minutes. Disconnect the absorption train, purge with purified air, wipe and cool as before (leaving for one hour if possible) and reweigh.

The carbon content of the coal is given by the equation:

Calculation of the hydrogen content of the coal is complicated by two factors. First, some of the water absorbed will have come from moisture in the aluminium oxide used to oxidise the coal; second, some of the water absorbed will have come from moisture content of the coal. Neither of these should be included in the hydrogen content of the coal. The hydrogen content is therefore given by:

M = % moisture in the coal sample.

The accurate measurement of fluid flowrate is essential if a process is to be properly controlled. This experiment investigates two widely-used pieces of hardware for measuring flowrate in pipes - the orifice plate and the venturi meter. These devices are placed between sections of a pipeline and each takes advantage of the fact that introducing a flow restriction results in a pressure decrease, the magnitude of which is related to the flowrate. In the case of the orifice plate, the restriction is a plate with a hole, which is bolted between two pipe sections. The pressure difference between a position upstream of the plate and one downstream of the plate is measured. Although the orifice plate is inexpensive, the kinetic energy of the jet of fluid formed at the plate hole is almost entirely converted to heat rather than being recovered in fluid pressure. The pressure drop and power needed to pump the fluid are large as a result. The venturi meter employs a gradual reduction in flow area followed by a gradual increase in flow area back to the pipe cross-sectional area to produce the pressure decrease. This smoother flow shape results in a smaller pressure drop, but means that the venturi meter is relatively expensive and physically much larger than the orifice plate.

The principle of both the orifice plate and the venturi meter is the same and is usefully represented by Bernoulli’s equation:

(1)

where: U = fluid velocity (m/s)

z = the elevation (height) of the fluid (m)

Bernoulli's Equation expresses the relation between kinetic energy, gravitational potential energy, and the potential for the pressure forces to perform work in the steady flow along a streamline when the fluid density is constant (as with a liquid). A further relation constraining the flow through these flow meters is statement that the mass flowrate must be the same at each section of the flow, i.e.:

r AU = constant (2)

where A is the cross-section area of the pipe.

This is a theoretical estimate of velocity in the pipe (at section 1), assuming that Bernoulli's Equation holds. The theoretical volume flowrate is then

The value of C for orifice plates is significantly less than 1.0 largely because flow through the plate hole has an effective area that is much smaller that the hole area, an effect called the ‘vena contracta’. The venturi meter with its smooth-flow transition has a discharge coefficient much nearer 1.0. The discharge coefficient is constant at large flowrates but at low flowrates is a function of Reynolds number, where the Reynolds number is defined as:

A duct is composed of a series of rectangular and circular sections, and is fed with air from a centrifugal blower. At three different flowrates Pitot-static velocity measurements will be made at uniformly-distributed positions over the cross-section of the transparent, rectangular section in order to find the mean velocity and hence the true flowrate. For the three flowrates the flow meter pressure differences will be recorded so that the theoretical flowrate and hence the discharge coefficient of each flow meter at each flowrates can be determined.

1. Familiarise yourself with the position of each flow section, the pressure measurement positions for each flow meter and the Pitot-static probe and the route of each tube conveying pressure to the meters.

2. Switch on the blower and check that a pressure difference develops for each flow meter and for the Pitot-static probe. Vary the flowrate using the outlet blockage plate and check that the pressure differences increase as the flowrate increases. See the notes below on how to measure the pressure difference. Record the ambient air temperature in the laboratory.

The electric manometers convert a strain produced in a wall across which the pressure difference is allowed to act into an electric signal which is then converted through calibration of the instrument to a pressure reading. The meter connected to the Pitot-static tube further converts the pressure readings into a velocity in m/s, the assumption being made that the pressure difference results from a Pitot-static probe exposed to an ambient air flow.

From Step 3 above, plot the square root of pressure difference for the venturi meter versus the square root of pressure difference for the orifice plate. According to the relation presented above, this plot should be a straight line through the origin if the discharge coefficient remains constant with flowrate (Reynolds Number).

Dimensions inside duct: Pipe diameter 5.5 inches

Rectangular section 4.5 ´ 5.0 inches

Orifice plate hole diameter 4.25 inches

Venturi throat diameter 3.75 inches

Properties of air:

Describe in a little detail the reasons for the actual flowrate being less than that predicted by Bernoulli's Equation.

Coulson, J.M. & Richardson, J.F.: Chemical Engineering Vol.1 (4th ed), Pergamon, 1990

660.2

Massey, B.S.: Mechanics of Fluids (6th ed), van Nostrand & Reinhold, 1989 532

White, F.M.: Fluid Mechanics (3rd ed), McGraw-Hill, 1994 532

plus most books on general fluid mechanics.

To compare the performance of a heat exchanger in co-current and counter-current operation and to determine the overall heat transfer coefficient and the effectiveness of heat exchange for a range of operating conditions.

The process of heat exchange from a hot fluid to a cold fluid is important in many process industries. Heat exchangers can be classified according to the flow arrangement of the two fluids. The simplest heat exchanger is the annular exchanger (Figure 1) which consists of two concentric tubes. Fluid flow can then be arranged to be either co-current (Figure 1a), where the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end, or counter-current (Figure 1b), where the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.

In any exchanger the temperature of the hot fluid must always decrease, and that of the cold fluid must always increase. However, the two flow arrangements give different temperature profiles along the heat exchanger (Figure 2).

Note that the temperature difference between the two fluids is not constant, but varies along the heat exchanger.

The rate of heat transfer (Q) from the hot fluid to the cold fluid is given in general by the relationship:

Q = UA"D T" (1)

Let the diameter of the inner tube be D and the length of the heat exchanger be L.

Consider an element of thickness dx at a distance x from the left-hand end.

Let the temperature of the hot and cold fluids be T and t at point x, and T + dT and t + dt at a point x + dx. [Note the convention used for temperature, irrespective of the flow direction and irrespective of whether the temperature actually increases or decreases.]

Let the mass flowrates of the hot and cold fluids be W and w respectively.

If the heat exchanger is perfectly insulated, such that no heat escapes to the surroundings, then an energy balance on the element gives:

Heat lost by hot fluid = Heat gained by cold fluid = Heat transferred through the wall

where U is the overall heat transfer coefficient from the hot fluid to the cold fluid.

This leads to two differential equations.

(2)

(3)

Let us define D T = (T -  t), i.e. the temperature difference between the two fluids at any point in the heat exchanger. Then, subtracting equation (3) from equation (2), we obtain:

(4)

Note that in this integration we have assumed the mass flowrates, specific heat capacities and the heat transfer coefficient are all constants. Whether this is actually the case in practice depends on the operating conditions and the fluids involved.

Equation (4) is derived from an energy balance on an element of heat exchanger. Alternatively, we can carry out an overall energy balance, i.e. on the whole heat exchanger. The rate of heat transfer (Q) in the whole exchanger is given by:

which can be rearranged in the form:

Substitute into Eqn.(4):

(6)

Equation (6) is in the same general form as our original Equation (1), which means that the quantity represents an average temperature difference between the two fluids. This is generally called the logarithmic mean temperature difference (LMTD). Equation (1) can therefore be written:

Q = UA(LMTD) (7)

where A is the surface area available for heat transfer in the heat exchanger.

Note that although this has been derived for counter-current flow, the same equation applies for co-current flow (you may like to prove this yourself).

Arrange the valves in the cold water pipework for co-current flow, i.e. V2 and V3 open, V1 and V4 shut. Set the hot water flow rate to 10 l/min, and the cold water flowrate to 2 l/min. Allow the inlet and outlet water temperatures to stabilise (at least 5 minutes) and record the temperatures from the thermocouple display unit. [It is important for every change of flowrate that you leave sufficient time for the temperatures to stabilise, otherwise erroneous data will be obtained.] Repeat the procedure for cold water flowrates of 4, 6, 8 and 10 l/min. [When changing the cold water flowrate you should also adjust the hot water flowrate back to its proper value if it has drifted.] Reduce the hot water flowrate to 5 l/min and record water temperatures for cold water flowrates of 2, 4, 6, 8 and 10 l/min.

Arrange the valves in the cold water pipework for counter-current flow, i.e. V1 and V4 open, V2 and V3 shut. Repeat all combinations of hot and cold water flowrates.

For each combination of flowrate, calculate:

For each value of hot water flowrate, plot a graph of overall heat transfer coefficient (U) against cold water flowrate. Comment on your results.

Determine h for each operating condition. Comment on your results.

The two methods of calculating Q may give different values. Comment on the possible reasons.

Holman, J.P.: Heat Transfer (various editions), McGraw-Hill 536.2

Incropera, F.P. & De Witt, D.P.: Introduction to Heat Transfer (2nd ed.), Wiley, 1990 536.2

Kay, J.M. & Nedderman, R.M.: Fluid Mechanics and Transfer Processes, CUP, 1985 532