Wednesday 27 November 2013

Types of hydro-power plant

Types of Hydropower Plants

There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not. The images below show both types of hydropower plants.
Many dams were built for other purposes and hydropower was added later. In the United States, there are about 80,000 dams of which only 2,400 produce power. The other dams are for recreation, stock/farm ponds, flood control, water supply, and irrigation.
Hydropower plants range in size from small systems for a home or village to large projects producing electricity for utilities. The sizes of hydropower plants are described below.

Impoundment

The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.

An impoundment hydropower plant dams water in a reservoir.

Diversion

A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Photo of an aerial view of a river with a waterfall and no dam. The hydropower intake and outlet are labeled. The intake is above the waterfall; the outlet is below it.

The Tazimina project in Alaska is an example of a diversion hydropower plant. No dam was required.

Pumped Storage

When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

Sizes of Hydroelectric Power Plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.

Large Hydropower

Although definitions vary, DOE defines large hydropower as facilities that have a capacity of more than 30 megawatts.

Small Hydropower

Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 100 kilowatts to 30 megawatts.

Micro Hydropower

A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village.

Drawing shows a micro hydropower plant. Intake gates allow water to flow through the Penstock Powerhouse to the turbine.

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Hydroelectric Power Plants

Hydroelectric Power Plants

Hydroelectric power plants convert the hydraulic potential energy from water into electrical energy. Such plants are suitable were water with suitable head are available. The layout covered in this article is just a simple one and only cover the important parts of hydroelectric plant.The different parts of a hydroelectric power plant are

(1) Dam
Dams are structures built over rivers to stop the water flow and form a reservoir.The reservoir stores the water flowing down the river. This water is diverted to turbines in power stations. The dams collect water during the rainy season and stores it, thus allowing for a steady flow through the turbines throughout the year. Dams are also used for controlling floods and irrigation. The dams should be water-tight and should be able to withstand the pressure exerted by the water on it. There are different types of dams such as arch dams, gravity dams and buttress dams. The height of water in the dam is called head race.

(2) Spillway
A spillway as the name suggests could be called as a way for spilling of water from dams. It is used to provide for the release of flood water from a dam. It is used to prevent over toping of the dams which could result in damage or failure of dams. Spillways could be controlled type or uncontrolled type. The uncontrolled types start releasing water upon water rising above a particular level. But in case of the controlled type, regulation of flow is possible. Cross section of a
power house

(3) Penstock and Tunnel
Penstocks are pipes which carry water from the reservoir to the turbines inside power station. They are usually made of steel and are equipped with gate systems.Water under high pressure flows through the penstock. A tunnel serves the same purpose as a penstock. It is used when an obstruction is present between the dam and power station such as a mountain.

(4) Surge Tank
Surge tanks are tanks connected to the water conductor system. It serves the purpose of reducing water hammering in pipes which can cause damage to pipes. The sudden surges of water in penstock is taken by the surge tank, and when the water requirements increase, it supplies the collected water thereby regulating water flow and pressure inside the penstock.

(5) Power Station
Power station contains a turbine coupled to a generator (see the cross section of a power house on the left). The water brought to the power station rotates the vanes of the turbine producing torque and rotation of turbine shaft. This rotational torque is transferred to the generator and is converted into electricity. The used water is released through the tail race. The difference between head race and tail race is called gross head and by subtracting the frictional losses we get the net head available to the turbine for generation of electricity.

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Cooling Tower:TYPED BY METHOD OF HEAT TRANSFER

TYPED BY METHOD OF HEAT TRANSFER

All of the cooling towers described here are evaporative type towers, in that they derive their primary cooling effect from the evaporation that takes place when air and water are brought into the direct contact. At the other end os the spectrum is the Dry tower, where by full utilization of dry surface coil sections, no direct contact (and no evaporation) occurs between air and water. Hence sensible heat transfer cools the water totally.
IN between these extremes are the plume abatement and water conservation towers, wherein progressively greater portions of dry surface coil sections are introduced into the overall heat transfer system to alleviate specific problems or to accomplish specific requirements

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Cooling Tower:TYPES BY SHAPE

TYPES BY SHAPE

There are two different types:
RECTILINEAR:
These towers are constructed in cellular fashion, increasing linearly to the length and numbers of cells necessary to accomplish a special thermal performance.

ROUND MECHANICAL DRAFT:

Are towers as the name implies, are essentially round in plan configuration, with fans clustered as close practicable around the center point of the tower. Multi-faceted towers, such as the octagonal mechanical draft (OMD) also fall in the general classification of “round” towers.

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Cooling Tower:SPRAY – FILLED

SPRAY – FILLED

This kind of towers has not a heat transfer surface, depending only upon the water break-up af-forded by the distribution system to promote maximum water-to-air

characterization by construction

we can see two different kinds of cooling towers by construction:

Field-erected
Factory-assembled

Field-erected:
The field-erected cooling towers are those on which the primary construction activity takes place at the site of ultimate use. All large towers, and many of the smaller towers, are prefabricated, piece-market and shipped to the site for the cooling towers manufacturer usually provides final assembly.
FACTORY-ASSEMBLED:
The factory-assembled cooling towers undergo virtually complete assembly at their point of manufacture, whereupon there are shipped to the site in as a few sections as mode of transportation will permit.

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Cooling Tower:CHARACTERIZATION BY AIR FLOW

CHARACTERIZATION BY AIR FLOW
The cooling towers by the relative flow are divided in several groups :
COUNTERFLOW:
IN the counterflow towers, the air moves vertically upward through the fill, counter to the downward fall of water. Because of the need for extended intake and discharge plenums; the use of high pressure spray systems; and the typically higher air pressure losses, some of the smaller counter flow towers are physically higher; require more pump head; and utilize more fan power than their cross flow counterparts. In a larger counter flow towers, however, the us of low pressure grativity-related distribution systems, plus the availability of generous intake areas and plenum spaces for the air management, is tending to equalize, or even reverse, this situation. The enclosed nature of a counterflow tower also restricts exposure of the water to direct sunlight, thereby retarding the growth of the algae.

CROSSFLOW:
The crossflow towers have a fill configuration throught, which the air flows horizontally, across the downward fall of water. Water to be cooled is delivered to hot water inlet basins located atop the fill areas, and is distributed to the fill by gravity throught metering orifices in the floor of those basins.

The crossflow towers can be divided in:

DOUBLE-FLOW:

In this kind of towers the fan is inducting air through two inlets and across two banks of fill.

SINGLE-FLOW:
This kind of towers only has one air inlet and one fill bank, the remaining three sides of the towers being cased. Single-flow towers are customarily used in locations where are unrestricted air path to the tower is available from only one direction.

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Cooling Tower:HYBRID DRAFT

HYBRID DRAFT
Tgey are equiped with mechanical draft fans to augment airflow. Consequenly, they are also referred to us fan-assisted natural draft towers. The intent of their desing is to minimize the horsepower required for the air movement, but to do so with the least possible stack cost impact. Properly desogned the fans may need to be operated only during pereiods ao high ambientsand peak loads.

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Cooling Tower:MECHANICAL DRAFT

MECHANICAL DRAFT
Mechanical draft towers uses fans (one or more) to move large quantities of air through the tower. They are two different classes:

Forced draft cooling towers
Induced draft cooling towers

The air flow in either class may be crossflow or counterflow with respect to the falling water. Crossflow indicates that the airflow is horizontal in the filled portion of the tower while counterflow means the air flow is in the opposite direction of the falling water.
The counterflow tower occupies less floor space than a crossflow tower but is taller for a given capacity. The principle advantages of the crossflow tower are the low pressure drop in relation to its capacity and lower fan power requirement leading to lower energy costs.
All mechanical towers must be located so that the discharge air diffuses freely without recirculation through the tower, and so that air intakes are not restricted. Cooling towers should be located as near as possible to the refrigeration systems they serve, but should never be located below them so as to allow the condenser water to drain out of the system through the tower basin when the system is shut down.
FORCED DRAFT
The forced draft tower, shown in the picture, has the fan, basin, and piping located within the tower structure. In this model, the fan is located at the base. There are no louvered exterior walls. Instead, the structural steel or wood framing is covered with paneling made of aluminum, galvanized steel, or asbestos cement boards.

During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. The drift eliminators located at the top of the tower remove water entrained in the air. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems.
INDUCED DRAFT
The induced draft tower show in the following picture has one or more fans, located at the top of the tower, that draw air upwards against the downward flow of water passing around the wooden decking or packing. Since the airflow is counter to the water flow, the coolest water at the bottom is in contact with the driest air while the warmest water at the top is in contact with the moist air, resulting in increased heat transfer efficiency.

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Cooling Tower: ATMOSPHERIC

ATMOSPHERIC
The atmospheric cooling towers utilize no mechanical fan to create air flow through the tower, its air is derived from a natural induction flow provided by a pressure spray.
We can see it in the following picture:

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TYPES OF COOLING TOWERS

TYPES OF COOLING TOWERS
Cooling towers are designed and manufactured in several types:

ATMOSPHERIC
MECHANICAL DRAFT

a. FORCED DRAFT
b. INDUCED DRAFT

HYBRID DRAFT
TYPED BY AIR FLOW

a. COUNTERFLOW
b. CROSSFLOW
a.1 DOUBLE-FLOW
a.2 SINGLE-FLOW
c. SPRAY-FILLED

TYPED BY CONSTRUCTION

a. FIELD-ERECTED
b. FACTORY-ASSEMBLED

TYPED BY SHAPE

a. RECTILINEAR
b. ROUND MECHANICAL DRAFT (RMD)

TYPED BY METHOD OF HEAT TRANSFER

a. EVAPORATIVE
b. DRY TOWER
c. PLUME ABATEMENT
d. WATER CONSERVATION

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COOLING TOWERS

COOLING TOWERS

The machines and processes of industry, as well as those devoted to human comfort and well being generated tremendous amounts of heat, which must be continuously, dissipated if those machines and processes are to continue to operate efficiency. Although this heat is usually transferred to a cool, flowing volume of water, final rejection is always to the atmosphere and, invariably, is accomplished by some form of heat exchanger.
The natural process of evaporation makes them very effective heat transfer mediums, although somewhat inefficient due to their limited surface area and their total dependence upon random winds.

Friday 22 November 2013

Evaporative Condensers

The vapor to be condensed is circulated through a condensing coil, which is continually wetted on the outside by a recirculating water system. Air is pulled over the coil, causing a small portion of the recirculating water to evaporate. The evaporation removes heat from the vapor in the coil, causing it to condense.

The evaporative condenser has a cabinet with a water-sprayed condenser, and it usually has one or more fans. The excess heat is removed by evaporating water. In an evaporative condenser the primary coolant of the cooling system is cooled, which is the opposite of a cooling tower. Evaporator condensers are more expensive than dry coolers and are primarily used in large cooling systems or systems where the outdoor temperature is high. In many locations around the world, regulations limit the physical size of a cooling system and this in turn limits the use of evaporative condensers.

Spraying a condenser with water exploits the fact that the dew point temperature is lower than the air temperature and that a wet surface transfers heat more efficiently.

1 Hot primary coolant

2 Cold primary coolant

3 Cold water

4 Water sprinklers

5 Centrifugal fan

Key benefits of Evaporative Condensers

Low system operating costs: lower condensing temperatures for a more compact compressor using less power
Low refrigerant charge, costs and environmental impact minimised
Space-saving: up to 50% area savings compared to comparable air-cooled installations

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Water-Cooled Condensers

Three main types of water-cooled condensers are pipe in a Pipe", " shell and Coil and plate. Each type performs the same task, approximately in the same conditions using different designs for ships, which contain water and refrigerant in the condenser. Consider every type, from the smallest to the largest capacity.

Pipe-in-pipe", sometimes called the double pipe or coaxial capacitor. Its name reflects its design. One tube is placed in a big pipe, and the ends of large tubes are sealed. Water circulates through one of the tubes, while the refrigerant passes through the other. In this figure, the refrigerant passes through the space between the inner and outer tube. This is the normal arrangement.

These capacitors are flexible in their location and because they are long, as a rule, they are in different forms to fit the space limitations of the application. Product shown here, a boxed product, designed for installation indoors. The compact nature of pipe-in-Pipe " capacitor makes it ideal for this product.

Tube-in-tube condensers most often are used for air conditioning products up to 5 tons of cooling capacity. They are also used, however, the commercial products through approximately 20 tons, as this vertical air conditioning (VAC). This design is ideal for adding air conditioning to old buildings being renovated...

Air-cooled condenser types

There are two types under this category, viz.

(a) natural convection and

(b) forced air type.

Natural Convection Condenser

Air movement over the surface of condenser tubes is by natural convection. As air comes in contact with the warm-condenser tubes, it absorbs heat from the refrigerant and thus the temperature of the air increases. Warm air being lighter, rises up and in its place cooler air from below rises to take away the heat from the condenser. This cycle goes on. Since air moves very slowly by natural convection, the rate of flow of heat from the refrigerant to air will be small. Thus a natural convection condenser is not capable of rejecting heat rapidly. Therefore a relati vely large surface area of the condenser is required. Hence the use of this type of condenser is limited to very small units such as domestic refrigerators. It, however, requires very 45 Refrigeration Equipment little maintenance.

In the small units, the condenser is fixed at the rear of the refrigerator cabinets. Generally, steel tubes are used, steel being cheaper than copper. To increase the heat-transfer area, wires are welded to the condenser tubes.These wires provide mechanical strength to the coil as well. In certain designs,widely-spaced fins are used. It is necessary to space the fins quite widely to avoid resistance to free (natural convection) air movement over the condenser. Still another design is the plate-type. The condenser coil is fastened to aplate. The plate being in contact with the condenser tubes, the surface area of the condenser is increased. The plate-type condenser is mounted on the back of the refrigerator cabinet with a small gap between the cabinet and the plate. This gap gives an air-flue effect and facilitates better natural convection air currents.It is obvious that while locating refrigerators or deep-freezes cabinets with a natural convection condenser fixed on the cabinet, sufficient care should be taken to allow free air movement. Also they should not be near an oven or

any warm location.

Forced-air Circulation Condenser

This type employs a fan or blower to move air over the condenser coil at a certain velocity. The condenser coil is of the finned type. Fins in such coils are closely spaced (ranging between 8 and 17 fins per inch). The space between the fins gets choked with dirt and lint. Therefore to obtain optimum capacity, the fins should be kept clean. For circulating air over the condenser, fans are mounted on the shaft/pulley of the compressor motor.

For bigger-capacity plants a separate motor is used to drive the fan or blower as also for hermetic-compressor units

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Types of Condenser

1.Air Cooled
2.Water Cooled
3.Evaporative

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Condenser

The functions of the condenser are to desuperheat the high pressure gas, condense
it and also sub-cool the liquid.
Heat from the hot refrigerant gas is rejected in the condenser to the condensing
medium-air or water. Air and water are chosen because they are naturally
available. Their normal temperature range is satisfactory for condensing
refrigerants.
Like the evaporator, the condenser is also heat exchange equipment

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Convective superheaters

Convective superheaters are most commonly found on locomotives. Much like a convection oven, this type of superheater utilizes the hot gases from the burner to reheat the steam. A convection superheater can be extremely efficient, because most of the thermal energy is given only to the boiler tubing, and what would normally be exhaust instead heats the superheater tubes. These are also found in power plants, but they were mostly implemented in steam locomotives.

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Superheater

A superheater is a device found in steam boilers that is used to convert wet, saturated steam into dry steam. Superheaters are a very beneficial part of the steam cycle, because dry steam contains more thermal energy and increases the overall efficiency of the cycle. Not only that, dry steam also is less likely to condense within the cylinders of a reciprocating engine or the casing of a steam turbine. Boiler superheaters can be found in three varieties:

1.Radiant superheaters
2.Convection superheaters
3.Seperately fired superheaters.

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Wednesday 27 November 2013