MEASUREMENT OF VELOCITY

Generally it is the volume flow rate which is the most important

quantity to be measured and from this it is possible to calculate a mean

flow velocity across the full flow area, but in some cases it is also

important to know the velocity at a point. A good example of this is in

a river where it is essential for the captain of a boat to know whatstrength of 

current to expect at any given distance from the bank;

calculating a mean velocity from the volume flow rate would not be

much help even if it were possible to measure the exact flow area over

an uneven river bed.

It was exactly this problem which led to one of the most common

velocity measurement devices. A French engineer called Pitot was given

the task of measuring the flow of the River Seine around Paris and

found that a quick and reliable method could be developed from some

of the principles we have already met in the treatment of Bernoulli’s

equation. Figure 3.2.17 shows the early form of Pitot’s device.

The horizontal part of the glass tube is pointed upstream to face the

oncoming liquid. The liquid is therefore forced into the tube by the

current so that the level rises above the river level (if the glass tube was

simply a straight, vertical tube then the water would enter and rise until

it reached the same level as the surrounding river). Once the water has

reached this higher level it comes to rest.

What is happening here is that the velocity head (kinetic energy) of

the flowing water is being converted to height (potential energy) inside

the tube as the water comes to rest. The excess height of the column of

water above the river level is therefore equal to the velocity head of the

flowing water.

MARINE DIESEL ENGINES

Diesel engines are produced by many manufacturers  in a range of power outputs, 
for very many applications.The largest diesel engines are to be found in ships
and these operate on the 2-stroke cycle, which makes them quite unusual.
The piston is bolted to a piston rod which at its lower end attaches
to a cross head running in vertical guides, i.e. a cross head bearing. A connecting 
rod then transmits the thrust to the crank to turn the crankshaft. The arrangement 
is the same as on old triple expansion steam engine, from which they were earn. 
They have the further peculiarity of being able to run in both directions
by movement of the camshaft. This provides astern movement without the expense of what 
would be a very large gearbox.These very large engines are the first choice for most 
merchant ships because of their economy and ability to operate on low quality fuel.
A typical installation on a container ship, for a instance,would be a six cylinder 
turbocharged engine producing 20 000k Wat a speed of about 100 rpm. The engine is connected directly to a fixed-pitch propeller.

GEAR SHAPES

References 2 and 16 include many recommendations for the geometric design of gears considering
strength, inertia, and molding condifions. Many smaller gears are simply made
with uniform thickness equal to the face width of the gear teeth. Larger gears often have a
rim to support the teeth, a thinned web for lightening and material savings, and a hub to facilitate
mounting on a shaft. Figure 9^1 shows recommended proportions. Symmetrical
cross sections are preferred, along with balanced section thicknesses to promote good flow
of material and to minimize distortion during molding.
Fastening gears to shafts requires careful design. Keys placed in shaft key seats and
keyways in the hub of the gear provide reliable transmission of torque. For light torques,
setscrews can be used, but slippage and damage ofthe shaft surface are possible. The bore
of the gear hub can be lightly press fit onto the shaft with care to ensure that a sufficient
torque can be transmitted while not overstressing the plastic hub. Knurling the shaft before
pressing the gear on increases the torque capability. Some designers prefer to use
metal hubs to facilitate the use of keys. Plastic is then molded onto the hub to form the
rim and gear teeth.

FRANCIS TURBINES

Francis turbines are very versatile. These are reaction turbines, i.e. during energy transfer from water to the runner there is a drop in static pressure as well as a drop in velocity head. Initially, the design of the slow runner (N; '" 60) was of the radial flow type, but dow they are of the mixed flow variety with radial entry and axial exit. Water from the penstock enters a spiral -or scroll casing which surrounds the runner (Fig. 10.28). The cross-section of the spiral diminishes uniformly along the circumference to ke~ the water velocity contant along its path. The water then enters the -guide vanes or wicket gates which are pivoted and can be turned suitably to regulate the flow and ostput. The guide vanes impart a tangential velocity or angular momentum to the water before entering the runner. The runner has a number of curved blades (12 to 22), welded. to the shrouds. The velocity of water gradually changes from radial to axial. 'JUter flowing past the runner, the water leaves through the draft tube, a closed flaring conduit, either straight or elbow type, increasing the pressure and reducing the velocity before falling into the tailrace. 

In Fran!is turbine the pressure of water at the inlet is more than that at the outlet. Thus, the water in the turbine must flow in a closed conduit. Unlike the Pelton wheel where the water strikes only a few of the runner buckets at a time, in the Francis turbine the runner is always full of water. After doing its work the water is discharged to the tailrace through the closed tube of gradually enlarging section, the draft tube, which does not allow water to fall freely to tailrace as in the Pelton turbine. The free end of the draft tube is submerged deep into the tail water to make the entire water passage from the head race to the tailrace totally enclosed. 

CLOSED FEEDWATER HEATER

Closed feedwater heaters are shell-and-tube heat exchangers. They are basically small condensers which operate at higher pressures than the main condenser because bled steam is condensed on the shell side, whereas the fcedwater, acting like circulating cooling' water in the condenser, is heated on the tube side. 

It was shown in Chapter 2 that the temperature rise in each heater and cconomiser is equal for maximum cycle efficiency. Thus the heaters receive bled steam from the turbine at pressures determined roughly by equal temperature rise from the condenser to the boiler saturation temperature. They are classified as low pressure (LP) and .high rrcssttr~ (HP) heaters depending upon their locations in the cycle. The LP heaters arc usually located between the condensate pump and the deacrator, which is followed by the boiler ked pump. (BFP). The HP heaters are located between the BFP and the eeonomiscr, 

When bled steam entering H fccdwater heater is superheated, as in a HP beater, the heater includes a dcsupcrhcating zone where steam is cooled to its saturation temperature. It is followed by a condensing zone where the steam is condensed to a saturated liquid rejecting the latent heat of condensation. This liquid, called heater drain, is then cooled below its saturation temperature in a subcooling zone or a drain cooling zone before the drain is cascaded backward or pumped forward. 

Figure 8.13 shows the schematic diagram and the temperature profiles of a three-zone closed feedwater heater. There are, however, two-zone heaters that include a desuperheating and a condensing zone or a condensing and a subcooling zone. There are also single-zone heaters that include only a condensing zone. A drain-cooling zone, instead of being a part ofthe shell, may be located outside it. It is then called a drain cooler. 

MODERN WATER TUBE BOILER

It is now usual in public utilities to have only one boiler per turbine. This has made it possible to build even the largest power plant in unit design thus simplifying the piping systems and facilitating boiler and turbine control, especi~ in plants using steam reheating. 

The appearance of water-cooled furnace walls, called water walls, eventually led to the integration of furnace, economiser, boiler, superheater, reheater, and air preheater into the modem steam generator. Water cooling is also used for superheater and economiser compartment walls and various other components, such as screens, dividing walls, etc. 

Three design concepts of water tube boilers are illustrated in Fig. 6.16. Type A is a boiler with natural circulation as is type (a). Heat transfer to the water tubes around the walls is mostly by radiation from the fuel flame and less by convection from flue gases. Natural circulation is used up to steam pressures of approximately 180 bar, with separation of the steam from the water taking place in the boiler drum. Boilers with forced circulation by a special pump, originally known as La Mont boilers, are shown schematically as type B and also (b) in Fig. 6.16. They offer a certain amount of freedom in the arrangement of evaporator tubes and the boiler drum. Such boilers can be adapted to limitations in height and space. They are suitable for steam pressures up to 200 bar. Boilers operating at subcritical pressures «221.2 bar) which rely on a drum and recirculation, either natural or forced, are commonly known as drum boilers

FLUIDIZED BED COMBUSTION

When air is passed through a .fixed or packed bed of particles, air simply percolates through the interstitial gaps between the particles. As the air flow rate through the bed is steadily increased, a point is eventually reached at which the pressure drop across the bed becomes equal to the weight of the particles per unit cross-sectional area of the bed. This critical velocity is called 'the minimum fluidization velocity, UrnC' at which the bed is said to be incipiently fluidized. As the air velocity is increased further, the particles are buoyed up and imparted a violently turbulent fluidlike motion, with the drag forces exerted by the fluid on the particles exceeding their weight. There is a high degree of particle mixing and equilibrium between gas and particles is rapidly established. This is called a fluidized bed.  '  air supplied by a centrifugal blower is passed through a perforated or porous plate, called the distributor, and then a bed of particles of wide size distribution. A few distributors are shown in Fig. 5.36(b). The air flow rate is regulated by a bypass valve along with a control valve, and it is measured by a rotameter. Dividing the mass flow rate, so measured, by the product of the bed cross-sectional area and density of air, the superficial velocity of air, U, is estimated. For each mas's flow rate or superficial velocity, which is gradually increased, the pressure drop across the bed is measured. Figure 5.37(a) demonstrates the variation of bed pressure drop with superficial velocity. The pressure drop /!.p varies with the superficial velocity linearly along AB till it approaches WIA"where Wis the weight ofparticles in the bed and AI is the bed cross-sectional area. This is the fixed bed regime. With further increase in air.