Performance of Evaporator

This post provides some information about what is mean by performance of evaporator, how performance of evaporator can be increased.

A rising film evaporator

 

Basically the performance of evaporator depends upon its capacity and economy. Let us discuss these terms one by one.

1) Capacity of evaporator:

Capacity of evaporator is defined as the number of kilograms of water vaporized/evaporated per hour.

The rate of heat transfer Q through the heating surface of evaporator is the product of heat transfer coefficient, heat transfer surface area and the overall temperature drop.

Therefore  Q = U×A×ΔT

Where

  • Q = Rate of heat transfer
  • A = area of the heat transfer surface
  • ΔT = overall temperature drop

The capacity of an evaporator depends upon the temperature of the feed solution. If the feed solution is at the boiling temperature corresponding to the pressure in vapor space of an evaporator, all the heat supplied will be utilized for evaporation, thus increasing the capacity of evaporator.

Similarly if the cold feed solution is fed in the evaporator, initially some energy will be required to increase the temperature of the feed solution to the boiling point of the solution corresponding to the vapor pressure inside an evaporator. In some cases this amount energy may be very high. Thus the capacity of evaporator will be reduced.

2) Evaporator Economy:

Economy of the evaporator is another important parameter which decides the performance of evaporator. It may be defined as the amount of steam used and is expressed in terms of pounds of vapor produced per pound of steam supplied to the evaporator train.

Following are some of the methods to increase the economy of evaporator;

a)      Use of multiple effect evaporation system ( Vapor recompression )

In multiple effect evaporation system, the vapor produced in the first stage is used as energy source (heat) for the second stage and so on. Thus increasing the economy of evaporator.

multiple effect evaporator

For example, in three effect evaporator if 1Kg. of steam is supplied to the first stage 2.5 Kg. steam is produced.

In vapor recompression method, vapor from the evaporator is compressed to increase its temperature so that it will condensate at temperature higher enough to permit its use as a heating media in the same evaporator.

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Working of hydroelectric power plant

In this article we are going to learn how Hydroelectric Power Plant works, what are the requirements of hydroelectric power plant for the selection of site, etc.

ThreeGorgesDam hydroelectric power plant

Hydroelectric Power Plant

In hydropower plant potential and kinetic energy of the water is used to rotate the turbine and hence generator to generate electricity.

Classification of Hydroelectric Plants:

  1. Conventional and pumped storage plants: in the conventional type, water is collected in the dam and its head is used for the rotation of turbine. In lean demand period, water is pumped back using water pump in the storage tank and used in peak demand period for the generation of electricity.
  2. Base and peak load plants: Hydel power plant is used as base load plant in lean demand period where as it can be used as peak load plant when there is shortage of water. Example of base load plant is conventional one and example of peak load plant is pumped storage plants.
  3. Base and head discharge: in hydel power plant potential energy of the water is converted into kinetic energy which is then converted in to mechanical energy and finally to electrical energy. All these energy conversions depend upon two main factors i.e. level of water in tank (head of water) and second is flow of water through the turbine.

Layout of Hydroelectric Power Plant (Hydro Station):

Following figure shows the basic layout diagram of the hydroelectric power plant.

Hydroelectric Power Plant Layout
Layout of Hydro Electric Power Plant

Construction & Working of Hydroelectric Power Plant:

Following are some of the main components of the hydroelectric power plant.

  1. Reservoir: water harvested from the catchment area is stored in the reservoir which is then used to generate the electricity.
  2. Dam: it is made in the path of the river to make the reservoir to hold the rain water.
  3. Spillways: Spillways are made to make the dam safe. When level of water is exceeds some defined point, it will discharge through these spillways.
  4. Forebay: when there is sudden change in the turbine load, in such cases there is need of temporary storage of water. This temporary storage of water near turbine is called as forebay.
  5. Surge tank: surge tank is build in between dam and the valve house. It is used to take care of the system load fluctuations.
  6. Penstock: it is water pipeline carrying water from dam to turbine.
  7. Prime mover or turbine: it is the main part of the power station. It is coupled with the generator. Turbine is rotated by the flow of water. As it is coupled with the generator, generator also rotates which produces electricity.
  8. Powerhouse: it consists of turbine, alternator and electrical equipment.
  9. Tail races: outlet water of the turbine is discharged to the river trough tail races.
  10. Selection of site:

Following are some of the factors that should be considered for Hydropower Power Plant while selecting a site.

  • Availability of water
  • Location of the dam
  • Head of water
  • Storage of water
  • Transport facilities
  • Distance from the load centre

Youtube video to understand working of hydroelectric power plant:

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Working of Gas Power Plant

In this article we are going to learn about how Gas Power Plant works? Gas power plant is also called as gas turbine power plant.

gas power plant
Gas Power Plant

Gas Power Plant:

In the previous article we have seen the overall idea of working of Thermal Power Plant which uses steam energy for the generation of electricity. Today we are going to discuss one more power plant which is known as gas turbine power plant. Gas power plant uses compressed air to rotate the generator and to generate electricity.

Diagram:

Following figure shows the schematic arrangement of a gas turbine power plant.

gas turbine power plant
Gas turbine power plant

Working of Gas Power Plant

In gas power plant air which is also known as the working fuel is compressed using air compressor at very high pressure. This compressed air is then fed into the combustion chamber where the fuel burns and produces hot flue gases (exhausts) at high pressure. It is then expanded in the turbine at high pressure to rotate the turbine. Turbine is coupled with the generator. Therefore when turbine rotates, generator also rotates and generates electricity.

In gas power plant, the compressed air is preheated before going to combustion chamber by the regenerator which uses hot air from the turbine to increase the efficiency.

Some part of the turbine energy is also used to run the air compressor so as to increase the overall efficiency.

Advantages and disadvantages of Gas Turbine Power Plant:

  • No special requirement for site selection. Gas turbine power plant can be built anywhere.
  • Initial cost of gas power plant is lowest as compare to hydel, nuclear or thermal power plant.
  • Running cost of gas power plant is higher.
  • But the maintenance and transmission & distribution cost is low.
  • Impact of gas power plant on environment is lower.
  • Overall efficiency of gas power plant is 40%. Therefore gas turbine power plant is quite efficient.
  • Space requirement is less.
  • Gas power plant is reliable.

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Construction and working of thermal power plant

Thermal power plant uses steam energy for the generation of the electricity. Fossil fuels like coal, oil, natural gases are burnt in the boiler, a hot flue gases thus generated are used to heat the feed water. A superheated steam is generated. This steam under high pressure then expanded in stem turbine to rotate the turbine. Turbine is coupled with the generator. When turbine rotates, generator also rotates and produces electricity. This arrangement of coupling turbine and generator is called turbo-alternator.

The capacity of the thermal power plant is generally few MW to 1000MW. But now a day it becomes a trend to build thermal power plants of capacity higher than 1000MW.

Following figure shows the general block diagram of the thermal power plant.

Thermal power plant Layout
Thermal power plant Layout

Site Selection for Thermal Power Plant:

  1. Land requirement: The land for the thermal power plant should be large enough so that the present installation and future expansion of the plant can be easily done. Cost and bearing capacity of the land also plays important role while selecting a site for thermal power plant.
  2. Water supply: Site should be near to the river so that water required for the ash disposal, boiler feed water, cooling and circulating water for condensers should be available easily.
  3. Fuel supply: Plant should be near to the coal mines, because cost of transmission of electricity is less than cost of transportation of fuel.
  4. Ash disposal: Making ash ponds is a part of installation of the plant. Enough space should be there for ash ponds and water supply should available for it.
  5. Transport facilities: For fuel supply – road and railway links. Located near coal mines because if quality of coal is poor transportation of coal is costly.
  6. Environment requirement: Thermal power plant produces lot of pollution. Ash ponds may produce water and air pollution, smoke from chimneys produces air pollution. To avoid affects of pollution site should be far from populated area.

Main power plant can be subdivided n to several units as follows:

  • Feed water unit
  • Boiler unit
  • Turbine unit
  • Generator unit
  • Cooling water unit
  • Fuel handling unit
  • Ash handling unit

Following are main elements of thermal power plant:

Boiler Turbo-alternator
  • Boiler feed pump
  • Economizer
  • Super heaters
  • Coal mills
  • Air preheater
  • Draught system
  • Steam turbine
  • Condenser
  • Cooling tower
  • Generator or alternator
  • Governors / Speed controllers
  • Alternator cooling system

A) Boiler

It is a steam generation unit in thermal power plant. There are two types of boilers: a) fire tube, b) water tube. In fire tube boilers, hot combustion gas flows through tubes which are surrounded by water. In case of water tube boilers, feed water tube pass through combustion chamber where hot flue gases flow over them.

  1. Boiler feed pump: function is to feed the water to be steamed in the boiler of thermal power plant. Condensate water to be coming out from condenser and make up water coming from water treatment plant ad together forming total feed water.
  2. Economizer: economizer is used as a heat exchanger. The remaining heat energy of the flue gases is passed through economizer so as to increase the efficiency of the plant.
  3. Superheater: In thermal power plant a saturated steam is first generated in the boiler which contains some amount of water droplets (or moisture). So to make the saturated steam from superheated stem a superheater is used.
  4. Coal mills: in modern power plant pulverized coal is used. Once pulverized, coal mills are employed for crushing the coal into a powder form. This increases the efficiency of boiler hence the plant.
  5. Air preheater: the remaining heat energy of the flue gases after passing through economizer is utilized for air preheating. The air required for completer combustion of the coal is first passed through air preheater so as to increase the efficiency of the boiler.
  6. Draught system: to overcome the resistance offered by the pipelines, air ducts, fuel beds, dampers, chimney, etc. for the flow of air, flue gases draught system is required in thermal power plant. Normally FD (forced draft) fan, ID (induced draft) fan and chimney facilitate this function. FD fan provides air from atmosphere to the boiler with the positive pressure, where as ID fan along with chimney sucks an air and waste gas from boiler to the atmosphere.

B) Turbo-alternator:

In thermal power plant, turbo alternator is usually coupled unit of steam turbine and electrical generator. Steam turbine rotates with the help of superheated steam expanded from high pressure and high temperature. As the turbine rotates, alternator also rotates gives rise to generation of electricity.

  1. Steam turbine: there are two types of steam turbine: impulse turbine and reaction turbine. In impulse turbine the heat energy of the steam is first converted in to kinetic energy by first passing through fixed nozzles. The steam coming out of fixed nozzles at high velocity impinges on the blades of the rotor. In reaction turbine the steam expands partially while passing through fixed nozzles and partially over the moving blades.
  2. Condenser: exhaust gases from the turbine are passed through condensers fro two purposes. To condensate steam to water to recirculation as a feed water and to remove the incondensable gases through water circuit.
  3. Cooling tower: In thermal power plant, ordinary water pumped from river, canal or pond is used to cool the exhaust steam from the turbine in the condenser. This process requires large amount of water. So to reuse that water, cooling towers are used to cool the heated water coming out from condenser.
  4. Generator: this is the main unit of any power plant. In this unit the generation of electricity is done. It is mechanically coupled with the turbine, so that when turbine rotates, it also rotates. The speed of the generator is given by,

1

Where N = speed of generator,

f = frequency of the system (50Hz in India)

P = number of poles.

5. Governors (Speed controllers): In thermal power plant, the speed of the governor should be kept at the constant rate (50Hz in India) ± some tolerance. So to keep the speed constant the flow of stem in the turbine has to be controlled. So to overcome this problem governors or electrical speed controllers are used.

6. Alternator cooling system: In thermal power plant, large amount of heat is generated in the windings due to copper and Eddy current losses. This heat should be removed continuously so as to keep the insulating material from damaging. There are two methods of cooling systems, open-circuit and closed-circuit method.

For low capacity generators open-circuit cooling system is employed in which atmospheric air is passed through one side and sucks out from the another side.

For medium capacity generators closed circuit cooling system is employed in which same air passed again and again after cooling it. In case of high capacity generators same closed circuit cooling system is employed only with difference of air is replaced with oxygen.

Finally the overall efficiency of the thermal power plant is given by,

efficiency of thermal power plant
efficiency of thermal power plant

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What is evaporation and what are the types of evaporation process?

Evaporation process and Evaporators terminology.

In this article we are going to discuss what is evaporation process?, its definition, types of evaporation processes.

A rising film evaporator
A rising film evaporator

Evaporation:

Evaporation process may be defined as a process in which aqueous solution is concentrated in a vessel or group of vessel in which concentrated solution is the desired product and indirect heating (usually steam) is the energy source.

Evaporators can be arranged in forward-feed, reverse feed or parallel feed. vapors of the previous stage is the energy source for a next stage .

The efficiency of the evaporators can be increased by arranging number of evaporators in series.

Evaporator’s terminology:

Following are some of the basic terms used in the evaporation process.

1) Single effect evaporation:

In single effect evaporation process the dilute solution which is to be concentrated is contacted only once with the heat.

Following figure shows the single effect evaporation process.

single effect evaporator
single effect evaporator

2) Multiple effect evaporation:

Multiple effect evaporation is the process of evaporation in which the vapors coming out in the one stage is used as an energy source (as a heat) for the next stage.

In short, vapors of first stage are used as energy source for the next stage.

Examples: 1) In paper industries for Kraft liquor concentration six-effect evaporators are used. 2) 20 effect evaporators are used in desalinization plants.

Following figure shows the two stage evaporator.

multiple effect evaporator
multiple effect evaporator

3) Boiling point rise:

It may be defined as the difference in °F of boiling point of constant composition solution and boiling point of pure liquid at the same pressure.

4) Economy:

Economy of evaporator may be defined as the amount of steam used and is expressed in pounds of vapors produced per pound of steam supplied to the evaporator train.

5) Capacity:

Capacity of evaporator is measured in terms of its evaporating capability.

It is the pounds of vapor produced per unit time.

Steam requirement = Capacity ÷ Economy.

6) Co-current operation:

In evaporator if the feed and steam follow parallel path (i.e. in same direction) through the evaporator train.

7) Counter-current operation:

The feed and steam enter the evaporator train at opposite ends.

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Tags: Types of evaporators, Evaporators terminology, introduction to evaporators, single effect evaporators, multiple effect evaporators.