TYPICAL V AND U BUTT PREPARATIONS FOR WELDING
FUEL TECHNOLOGY 72 REED'S GENERAL ENGINEERING KNOWLEDGE
ANALYSIS OF FLUE GASES
Various types of apparatus are available. The apparatus is suitable for a detailed analysis, and three types of recorder in fairly common use, are now described. It is suggested that for examination purposes the student should be familiar with the apparatus and any one type of recorder.
The Apparatus
The enclosed volume of the apparatus is 100 ml. Pipette A contains a solution of potassium hydroxide in distilled water which absorbs carbon dioxide. Pipette B contains a solution of
potassium hydroxide and acid in distilled water which
absorbs oxygen. Pipette contains a solution of cuprous chloride in hydrochloric acid (or ammonia). Pipettes must be used in sequence A , B then C. Aspirator bottle water is mixed with sulphuric acid or made salty to stop absorption and methyl orange or other acids or alkalis are commonly used as colouring fluids (see Fig. 2.1 1).
atmosphere cock Gas
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graduated and woter APPARATUS Fig. 2.11By using the aspirator, raising and lowering to act as a pump the sample is first drawn in, passed through each pipette in turn, measured, and then discharged. The reduction in
volume at each stage, always taking readings with water levels in eudiometer and aspirator coincident to ensure equal pressure, is the percentage volume of that gas. Gaseous will be registered in any hot gas analysis as unless previously gas washed out by, for example, a solution of manganese sulphate.
Typical results:
Always a dry flue gas analysis water vapour removed). This is a volumetric analysis, which is the usual method. The hot gases of exhaust containing water vapour and sulphur dioxide are quickly cooled in transit to the apparatus. Water vapour condenses and sulphur dioxide is very readily absorbed by the
water so that the apparatus measures CO, and
by difference quite correctly as no water vapour or (to affect the reading) are present in the dry sample entering the apparatus. If the gas sample were led directly to say a recorder by a fan so that the sample was hot then any gaseous sulphur dioxide would register as carbon dioxide unless the manganese sulphate chemical had been used to absorb the gas.
Chemical Absorption Type Recorder
Referring to Fig. 2.12, the in the sample diffuses through a porous pot and is absorbed. This causes a reduction in volume which creates a vacuum. The pressure difference can be read off on the gauge calibrated directly as
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CHEMICAL ABSORPTION TYPE RECORDER
Fig. 2.12
Thermal Conductivity Type Recorder
a c supply
THERMAL CONDUCTIVITY TYPE RECORDER
Fig. 2.13
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Referring t o Fig. 2.13:
Approximate thermal conductivities are in proportion: 2,
The sample enters via a filter and drier, water vapour must be removed as same conductivity as The wire cell resistance is proportional to heat dissipation, proportional to thermal conductivity of gas in cell, proportional therefore to content. Air is used in reference cell. Thus the only difference between gas sample and air, from the thermal conductivity viewpoint is (as removed and 0 2 and same value).
This assumes no CO or if these are present (normally only very small proportions) they will be registered as unless the sample is first passed over a burner and these two gases burned off before the reading.
Bridge electrical unbalance is dependent on content and the unbalance current is measured by the potentiometer.
Mechanical Type Recorder
n a r r o w
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t r u n k
gas trunk
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MECHANICAL TYPE RECORDER
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Referring to Fig. 2.14:
Density of flue gas is proportional to content. Couple (torque) of impeller to vane is proportional to density of the medium in the narrow gap. The torques if both in air would balance. Therefore the C02 content increases the density, hence torque, the torque unbalance which is directly proportional to the content is conveyed through the link mechanisms t o the recorder scale or pen. Gas and air are arranged t o be at equal total pressure, temperature and saturation. Stainless steel components are used and bearings are of the ball race type with spring loaded adjustments.
Clean Air Act
It is an offence to discharge smoke into the atmosphere, however in marine practice allowance is made for soot blowing, lighting up and breakdown. Emissions from a forced draught oil fired boiler furnace or an oil engine must not be longer than 10 minutes with dark smoke in the aggregate in any period of 2 hours and not more than 4 minutes continuous dark smoke except when soot blowing a water tube boiler.
The practical applications of smoke colour indications and combustion equipment effects are discussed later in this chapter. Smoke detectors are included in Chapter 8. There is an increasing use of R numbers and gas analysis indicators.
Dissociation
Most combustion reactions are reversible. At high temperatures the molecule bonds tend to disrupt and form molecules of the original form absorbing heat in the process.
There is an increase in volume which is resisted by high pressures so as pressure rises dissociation reduces.
1 bar 0.1% dissociation at 1760 K, 6% at 2260 K , 55% at 3250 K. 102 bar 0.01 dissociation at 1760 K, 0.1 at 2260 K, 17% at 3250 K. 1 bar 0.04% dissociation at 1760 K, 2% at 2260 K, 28% at 3250 K. 102 bar 0.004% dissociation at 1760 K, 0.3% at 2260 K, 3 % at 3250 K FUEL TECHNOLOGY
These figures only relate to the gas o r vapour by itself. Gas mixtures and rich oxygen contents tend to reduce dissociation considerably.
Once the temperature falls the molecules reform (re- combustion) and heat is again evolved. Thermal decomposition is non reversible split up under heat, thermal dissociation is reversible split up under heat.
In an I.C. engine dissociation causes reduction of maximum combustion temperatures and heat re-appearance during expansion occurs, which tends to raise the curve above the adiabatic.
In a fire there is a danger that the use of superheated steam as an extinguishing agent (say sootblowers on an air heater fire) could in fact feed the fire and accelerate the growth. For example the displacement which occurs about 707°C
Heat
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Hot 3 F e+
12 3+
goes t o completion giving liberation of volatile hydrogen and makes the combustion more rapid. Such fires are sometimes called 'Rusting' fires. Although the total displacement reaction is reversible the main factor is decomposition of steam vapour and dissociation of steam vapour plays a relatively minor part, however as dissociation is regarded chemically as reversible decomposition then the process is often regarded by engineers as dissociation.
The Boiler Combustion Heat Balance
A typical analysis of the heat utilisation in an oil fired boiler would be as in Table 2.4. ITEM HEAT IN FUEL (hcv) 100 IN STEAM 80 IN EXCESS AIR IN DRY GASES 3 8 IN WET VAPOURS 3 UNBURNED GASES RADIATION 5 100
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TABLE 2.48 REED'S GENERAL ENGINEERING KNOWLEDGE
It can be seen that dry combustion gases, excess air and wet vapours are losses. Minimum excess air and lowest practical flue gas temperature (bearing in mind complete combustion and corrosion in uptakes, etc.) reduce these losses together with close attention to content, to reduce unburned combustion gas loss.
527 F G temperature
Fig. 2.15
The flue gas (F.G.) loss can be seen to increase with flue gas temperature increase, increase with excess air increase and increase with fall in gas content (see Fig. 2.15).
The condition of the gases leaving the funnel is often the best indication of combustion conditions. Black smoke due to insufficient air (among other things), white smoke due to too much air, blue smoke due to burning of lubricating oils (in Z.C. engines), yellow smoke indicative of high sulphur bearing fuels, etc.
However content is often required to give the efficiency of combustion for a particular plant. Each plant however will have its own optimum figure and this may vary for boilers between 10 and 14% depending on many variables. ,
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COMBUSTION EQUIPMENT
Good combustion is essential for the efficient running of the boiler, it gives the best possible heat release and the minimum amount of deposits upon the heating surfaces. To ascertain if the combustion is good, measure the content (and in some installations the content) and observe the appearance of the gases.
If the content is correct (or the content low) and the gases are in a non smoky condition then the combustion of the fuel is correct. With correct content the excess air required for combustion will be low and this results in improved boiler efficiency since less heat is taken from the burning fuel by the small amount of excess air. If the excess air supply is increased then the content of the gases will fall.
Condition of burners, oil condition, pressure and temperature, condition of air registers, air supply pressure and temperature are all factors which can influence combustion.
Burners. If these are dirty or the sprayer plates are damaged then atomisation of the fuel will be affected. Types include pressure jet, in various forms, rotary cup, steam jet and ultrasonic.
Oil.
If the oil is dirty it can foul up the burners. (Filters areprovided in the oil supply lines to remove most of the dirt particles but filters can get damaged. Ideally the mesh in the last filter should be smaller than the holes in the burner sprayer plate).
Water in the oil can affect combustion, it could lead to the burners being extinguished and a dangerous situation arising. It could also produce panting which can result in structural defects.
If the oil temperature is too low oil does not readily atomise since its viscosity will be high, this could cause flame impingement, overheating, tube and refractory failure. If the oil temperature is too high the burner tip becomes too hot and excessive carbon deposits can then be formed on the tip causing spray defects, these could again lead to flame impingement on adjacent refractory and damage could also occur to the air swirlers.
Oil pressure is also important since it affects atomisation and lengths of spray jets.
80 REED'S GENERAL ENGINEERING KNOWLEDGE
registers. Good mixing of the fuel particles with the air is essential, hence the condition of the air registers and their swirling devices are important, if they are damaged mechanically by corrosion then the air flow will be affected. Pressure drops over the venturi of 25 mm water gauge give air speeds of about
20 Modern swirler type stabiliser designs give more efficient mixing with pressure drops up to 300 mm water gauge and air speeds up t o 70
Air: The excess air supply is governed mainly by the air pressure and if this is incorrect combustion will be incorrect.
bolt
body
handle
dowel
O.F. BURNER (WALLSEND-HOWDEN) Fig. 2.16
Fig. 2.16 shows a simple pressure jet burner arrangement for a boiler (Wallsend-Howden). Preheated pressurised fuel is supplied t o the burner tip which produces a cone of finely divided fuel particles that mix with the air supplied around the