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.

FANS

FD and ID fans operate continuously for long periods. up to I or I t years. So. these must be well designed, ruggedly constructed, well balanced, and highly efficient over a wide range-of outputs. Typical fans have capacities 0(700 m3/s of volume flow producing 152 mm water static pressures (about OJ 5 bar). 
There are two types of fans. viz., centrifugal and axial. In the centrifugal fan, the gases arc accelerated, radially through curved or flat impeller blades' from rotor to a spiral 'or volute casing. In the axial fan. gases arc accelerated parallel to the rotor axis. This is similar to a table fan, but here the fan is housed in a casing to develop static pressure. Axial fans have higher capital costs . 

Centrifugal fans can have forward-curved. flat or backward curved impeller blades . The velocity triangles at exit from the tip of the blades are shown, where the absolute velocity of gas V is the same in all the three cases. It is seen that for the same V the blade tip velocity Vb is the highest for the backward-curved blades (c) and the lowest for the forward-curved blades (a). Since Vb = (;rDN)J60, for the same tip diameter D, the rpm N is the highest for the backward-curved and the lowest for the forward-curved blades. The FD fans should have high Vb so as to rotate at high speeds and handle large volume flow of air. Therefore, centrifugal fans with backward-curved blading arc normally used for FD fans. The IDfans handle dust-laden flue gases and so the blades are subject to erosion by the fly ash. The erosion rate of blades is lower if the blade tip speed Vb is less and the fan rotates at lower speeds. Therefore, centrifugal fans having forward-curved or flat blading arc used for ID fans. Low-speed fans with flat blades are used for particularly dirty or corrosive gases. 

FLICKERMETER

Flicker is fundamentally a term used to describe the behaviour of lighting, and
its perceived effect on the human observer. Historically various experiments
have been undertakin to a determine the relationship between perception or
irritation on the one hand and both frequency and magnitude of lighting level
disturbance on the other. The concern of the power system engineer is that the
most usual cause of flicker is a distortion of the voltage waveform of the electric
supply to a lamp, and flicker is the most likely cause of complaint arising
from voltage fluctuations. Curves exist, derived from lighting perception
results, relating percentage voltage dip, frequency of disturbance and acceptability.
An example is given in Fig. 25.2, but such curves must be treated with
caution because they make assumptions as to lamp characteristics.

The IEC approach is more sophisticated, and takes account of both shortterm
sensitivity (a value calculated every 10 minutes) and long-term sensitivity
(a combination of 12 short-term values). Intensity of flicker annoyance can be
measured with a flickermeter (IEC 60868). IEC 61000-3-7 provides indicative
levels for acceptable annoyance on a network, but recognises that the absolute
limits will vary between utilities depending on specifics of the loads served
and the supply network. In the UK, for example, utilities apply Engineering
Recommendation P28 to determine acceptability. In Europe EN 50160 gives
power system flicker levels at consumers’ terminals

PLC SELECTION

The development of a control system may be divided into various stages as

shown in the project development life cycle . A management

decision, based on timing and resource availability, is made as to the

best stage to obtain competitive tenders for remaining design, supply

 or installation work from specialist contractors. The first step is to 

carefullydetail the system to be controlled together with possible future

 expansion requirements. This initial description must carefully detail

 the hardware and software interfaces and addresses such questions

 as the physical location of devices, supervisory control connections,

 motor or actuator loads and physicalenclosure protection.

The second step is to define the operational control requirements in a concise

and accurate descriptive form. At this second stage, it is essential that full

consultation is made with operatives and maintenance crews as well as the

engineers in order to ensure the correctness of the descriptions and definitions

of the user’s wishes. These descriptive control requirements are then converted

in a particular format as a sequence of logical events. A specification (sometimes

 termed Functional Design Specification orFDS) is next prepared for both the 

 hardware and software. The hardware specificationshould cover the following points:

HRC FUSES

The high rupturing capacity (HRC) fuse has excellent current and energy limiting characteristics and is capable of reliable operation at high prospective rms symmetrical current fault levels (typically 80 kA at 400 V and 40 kA at 11 kV).

Fuses are available in ratings up to 1250 A at low voltages and, say, 100 A at 11kV, and normally packaged in cartridge format. The fuse operates very rapidly under short circuit fault conditions to disconnect the fault within the first

half cycle and therefore limit the prospective peak current.

The fuse element traditionally consists of a silver element. Recent research and development by some manufacturers has allowed copper to be used when problems of increased pre-arcing I2t, less pronounced eutectic alloying (‘M’)

effect and surface oxidation are overcome. In some cases the performance of the copper element fuses actually surpasses that of the silver types.

The silver or copper strip element is perforated or waisted at intervals to reduce power consumption and improve the tolerance to overloads .

 The fuse operation consists of a melting and an arcing process. Under high fault currents the narrow sections heat up and melt. Arcing occurs across

the gaps until the arc voltage is so high that the current is forced to zero and the fuse link ruptures. The operation of a typical 100 A HRC-rated fuse under short circuit conditions .

SUBSTATION LAYOUTS

The major practical  service continuity  risk for the transformer-feeder substation is when the substation supply cables are both laid in the same trench and suffer from simultaneous damage. Much of the substation cost savings would be lost if the supply cables were laid in separate trenches since the civil trench work, laying and reinstatement costs are typically between 33% and 40% of the total
supply and erection contract costs for 132 kV oil filled and 33 kV XLPE, respectively.
In congested inner city areas planning permission for separate trenches in road ways or along verges is, in any case, seldom granted. The civil works trenching and backfill costs for two separate trenches (one cable installation contract
without special remobilization) are typically 1.6 times the cost of a single trench for double circuit laying. The choice depends upon the degree of risk involved and the level of mechanical protection, route markers and warnings utilized. 

BINARY VAPOUR CYCLES

No single fluid can meet allthe requirements as mentioned above. Although in the overall evaluation, water is better• than any other working fluid. at high temperatures, however, there arc a few better fluids. and notable among them are: (41) diphenyl ether. (CC.H5hO. (b) aluminium bromide AlBrJ and (e) liquid metals like mercury. sodium, potassium and so on. Among these. only mercury has actually been used in practice. Diphcnyl ether could be considered but it has not yet been used because like most organic substances, it decomposes gradually at high temperatures. A luminium bromide is a possibility and yet to be considered. 

As at pressure of 12 bar. the saturation temperatures for water, aluminium bromide, and mercury arc I ~p 0(" 4X2.5°(' and 560°(', I c-pcctivcly. The highest cyclic temperature consistent with the best available material for use in powerplant is about 560°C. Therefore, mercury is a better working fluid in the high temperature range because at 560°C, its vaporization pressure is relatively low. Its critical pressure and temperature are IOXO bar and 1460 cc, respectively, 

But in the low temperature range. mercury is unsuitable because its saturation pressure becomes exceedingly low. and it would bc impractical to maintain such a high vacuum in the condenser. At 30°C. the saturation pressure of mercury is only 2.7 x 10 4 em Hg. Its specific volume at sutJh a low pressure is very large, and it would bc difficult to accommodate such a large volume flow. 

For this reason. to take advantage of the beneficial features of mercury in the high temperature range and to get rid of its deleterious effects in the low temperature range, mercury vapour leaving the mercury turbine is condensed at a higher-temperature and pressure, and the heat released during the condensation of mercury is utilized in evaporating v. atcr to form steam to operate on a conventional turbine. 

Thus. in the binary (or two fluid) cycle. two cycles with different working fluids arc coupled in series, the heat rejected by one being utilized in the other .