Write short notes on the following: a) Calandria b) Cyclone separator c) Benson boiler d) Circulating Fluidised Bed Combustion (CFBC) e) Air pre-heater f) Compounding of Impulse Turbine g) Components of Nuclear Reactor

Answer:

a) Calandria:

Calandria is a special type of evaporator. It has a heat exchanger integral with vapoour body. The level is maintained in the upper portion of the tubes or above the top tube sheet and the circulation pattern is up through the tubes and down through a central pipe called a 'down comer'. Circulation is created by the difference in specific gravity between the body liquor and

 

the heated liquor and vapour generated inside the tubes, plus a vapour lift effect. The calendria evaporator can be used for salting type applications, however, an agitator located inside or beneath the down comer is recommended to suspend the salt crystals in the lower portions of the body. Although the agitator creates some flow through the tubes, most of the flow is still created by Steam thermoxyphon. The calendria is also used for batch type concentration of liquors.

b) Cyclone separator:

Cyclone separator utilize a centrifugal force generated by a spinning gas stream to separate the particulate mater from the carrier gas. The centrifugal force on particulate matters in a spinning gas stream is much greater than gravity; therefore, cyclones are effective in the removal of smaller particles than the settling chamber. The efficiencies of cyclone separator can be above 90% for the particles larger than 5µm and drop rapidly for the small particle sizes.

 

 

 

Advantages:

 

1. Low initial cost
2. Simple construction & operation
3. Low pressure drop
4. Low maintenance
5. It has no moving parts
6. Continuous disposal of solid particles

Disadvantages:

Low collection efficiency for the particles below 5µm in diameter Equipment is subjected to severe abrasive deterioration.

 

c) Benson Boiler is a high pressure, drum less, supercritical, water tube steam boiler with forced circulation. This boiler was invented in the year 1922 by Mark Benson. This boiler is a super critical boiler in which the feed water is compressed to a supercritical pressure and this prevents the formation of bubbles in the water tube surface. The bubbles do not form because at supercritical pressure the density of water and steam becomes same. It was Mark Benson who first proposed the idea before heating into boiler and due to this the latent heat of water reduces to zero. As the latent heat of water reduces to zero the water directly changes into steam without the formation of bubbles.

 

The main parts of Benson boiler are:

1. Air Preheater: It preheats the air before entering into the furnace. The preheated air increases the burning efficiency of the fuel.

2. Economiser: It heats the water to a certain temperature.

3. Radiant Superheater: It is super heater which heats the water with radiation produced by the burnt fuel. It raises the temperature to supercritical temperature.

4. Convection Evaporator: It evaporates the superheated water and converts them into steam. It does so by the convection mode of heat transfer to the water from the hot flue gases.

5. Convection Superheater: It superheats the steam to the desired temperature (nearly 650 degree Celsius).

6. Furnace: It is the place where the fuel is burnt.

7. Feed Pump: It is used to supply the water inside the boiler at supercritical pressure of 225 bars.

d) Circulating Fluidised Bed Combustion (CFBC):

This is one way of burning pulverised coal in electricity generation plant. The coal is burnt in a bed of hot particles maintained in flowing air. At adequately high airspeed, the bed acts as a fluid resulting in rapid mixing of the particles. The fluidised action promotes complete coal combustion at relatively low temperatures and provides a means to transfer combustion heat efficiently from the bed to the steam tubes. The boiler in which coal is burnt in an environment of high concentration of bed material (mineral matter) derived from combustion of coal retained by using cyclone / other means. This bed material is fluidized by primary air (a part of combustion air).

The high concentration of bed material along with staged air supply ensures that bulk combustion temperatures do not exceed 950°C making it environment friendly (lesser production of NOx) means of utilizing coal.

It adopts a process different from Pulverized Fuel where coal is milled to a fine powder (similar to talcum powder) and fired. The concentration of ash is dependent on coal ash (mineral matter) content and ash (mineral matter) goes along with flue gases out of the boiler. In CFBC boiler, this ash (mineral matter) is separated using a cyclone or similar device and RECYCLED back and hence termed as CIRCULATING fluidized bed.

e) Air Pre-heater:

Air Pre-heater:

The function of the air pre-heater is to increase the temperature of air before it enters the furnace. It is generally placed after the economizer. So the flue gases pass through the economizer and then to the air pre-heater. An air-pre-heater consists of plates or tubes with hot gases on one side and air on the

other. It preheats the air to be supplied to the furnace. Preheated air accelerates the

combustion and facilitates the burning of coal.

Degree of pre heating depends on:

i) Type of fuel 

ii) Type of fuel buming equipment and ii) Rating at which the boiler and furnace are operated

There are three types of air pre heaters:

i) Tubular type

ii) Plate type

iii) Storage type

i) In tubular type air pre heater the gases reverse their direction near the bottom of the air heater, and a soot hopper is fitted to the bottom of air heater easing to collect soot. ii) In the plate type air pre heater the air absorbs heat from the hot gases being swept

through the heater at high velocity on the opposite side of a plate. iii) In the storage type air pre heater the temperature of the gases leaving the stack should be kept as low as possible so that there is minimum loss of heat to the stack

f) Compounding of steam turbines:

The method in which energy from steam is extracted in more than single stage is called Compounding. A multi-stage turbine i.e having more than one set of rotors and nozzles is called compounded turbine.

1. Velocity Compounding of Impulse Turbine: The velocity compounded impulse turbine has moving and fixed blades. The moving blades are keyed to turbine shaft and fixed blades are fitted to casing. The high pressure steam from boiler is expander in nozzle where pressure energy is converted into kinetic energy. The high velocity steam is directed on first set of moving blades and as steam flows over the blade it imparts some of its momentum to blades and loses some velocity. Some part of high K.E is absorbed by blades and there is no change in velocity of steam as it passes through fixed blades. The steam then goes to next set of moving blades and this process is repeated until all the energy of steam is absorbed. The figure below shows the velocity compounding of impulse turbine.

2. Pressure Compounding of Impulse Turbine (Rateau turbine): This is used to solve the problem of high blade velocity in the single-stage impulse turbine. It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. In this type of compounding, steam is expanded more than once (as in velocity compounding). Here the high pressure steam is fed to nozzle where it is partially expanded i.e., pressure decreased velocity increased and when this steam is passed over the set of blades, where almost all its velocity is absorbed and pressure remains constant during this period and this process is repeated until condenser pressure is achieved.

 

g) Components of Nuclear Reactor:

There are several components common to most types of reactors:

Fuel: Uranium is the basic fuel. Usually pellets of uranium oxide (UO₂) are arranged in

tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. In a 1000 MWe class PWR there might be 51,000 fuel rods with over 18 million pellets. Moderator: Material in the core which slows down the neutrons released from fission so

that they cause more fission. It is usually water, but may be heavy water or graphite. Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. In some PWR reactors, special control rods are used to enable the core to sustain a low level of power efficiently. (Secondary control systems involve other neutron absorbers, usually boron in the coolant its concentration can be adjusted over time as the fuel burns up.) PWR control rods are inserted from the top, BWR cruciform blades from the bottom of the core.

Coolant: A fluid circulating through the core so as to transfer the heat from it. In light water reactors the water moderator functions also as primary coolant. Except in BWRs. there is secondary coolant circuit where the water becomes steam. (See also later section on primary coolant characteristics.) A PWR has two to four primary coolant loops with pumps, driven either by steam or electricity - China's Hualong One design has three, each driven by a 6.6 MW electric motor, with each pump set weighing 110 tonnes.

Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding holding the fuel and conveying the coolant through the surrounding moderator.

Steam generator: Part of the cooling system of pressurised water reactors (PWR & PHWR) where the high-pressure primary coolant bringing heat from the reactor is used to make steam for the turbine, in a secondary circuit. Essentially a heat exchanger like a motor car radiator. Reactors have up to six 'loops', each with a steam generator. Since 1980 over 110 PWR reactors have had their steam generators replaced after 20-30 years service, 57 of these in USA.

Containment: The structure around the reactor and associated steam generators which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any serious malfunction inside. It is typically a metre-thick concrete and steel structure.