MET 214 Module 7

MODULE 7

BOILERS

• A boiler is a device in which steam is generated.

• The boilers are divided into two types:1. Fire Tube Boiler and2. Water Tube Boiler

• Superheated steam is required, the wet steam is removed from the steam space and is piped to a super heater. This consists of a long tube or series of tubes which are suspended across the path of the hot gases from the furnace.

• If a control of degree of superheat is required, as in some of the larger boilers, then an attemperator is fitted. The control of the degree of superheat is obtained by the injection of water or steam into the superheated steam. If an attemperator is fitted, the super heater is generally divided into two parts .

• The first part is called the primary super heater. Then comes the attemperator followed by the second part of the super heater called secondary super heater.

• The economizer is really a heat exchanger in which the feed water being pumped into the boiler at a higher temperature. Hence , less energy is required to raise the steam, or, if the same energy is supplied, then the more steam is raised. The result is higher thermal efficiency.

• Auxiliary equipment fitted to boilers:a. A pressure gauge – this will record the gauge

pressure of the saturated steam formed in the steam space.

b. A water gauge glass – this will record the water level in the boiler. Often, two are fitted in case one breaks.

c. A pressure relief valve- this is fitted as a safety precaution and is set to blow- off at a particular pressure

FIRE TUBE BOILERS• Example of fire tube type boiler is Lancashire

Boiler. Heat from the hot flue gases passes through the tubes and water surrounds the flue

• The basic elements of the boiler are a large steel shell through which pass two large-bore cylinders called flues.

• This boiler is provided with three gas passes. The first pass is through the boiler. The second pass is back under the boiler. The third pass is round the sides of the boiler.

Fire tube boiler

• These passes are an attempt to extract the maximum amount of energy from the hot flue gas before they are released to atmosphere.

• The quality of steam produced by a boiler in one hour is referred to as its evaporative capacity.

• The evaporative capacity of a boiler will depend upon its design, the type of fuel and furnace and on the quality of fuel.

• The Lancashire , like other shell type boilers, can be worked up to a pressure of about 1.7 MN/m2 (17 bar).

• The general range of sizes of the economic boiler are from the small size of about 3m long and 1.6m diameter to the large size about 6.6m long and 4m diameter. Equivalent evaporation ranges from about 900 kg steam/h to about 14000 kg steam/h.

WATER TUBE BOILERS• In water tube boiler Water is going to pass

inside the tubes and steam circulates around them.

• In most water-tube boilers the water circulation is by natural convection but there are a few designs in which forced convection is employed.

Water tube boiler

• The headers are connected by a large number of water tubes. Between the water tubes and the drum a super heater is fitted. Baffles are introduced across the water tubes to act as deflectors to the flue gas stream. The high end of the tubes is at the hottest end of the boiler and the low end of the tube is the cool end of the boiler.

• A very large water –tube boiler of modern design called the radiant heat boiler

• The boiler is fired with pulverished coal.• One of the difficulties of a pulverized fuel burning

boiler is that the ash in the coal is also pulverized and hence blown into the furnace and passes up with the flue gas out of the boiler. This dust ash must be removed so that it does not pollute the atmosphere. The dust ash is removed in the precipitator.

• One type of precipitator gives a vortex motion to the flue gas. The dust is thus flung out of the gas and is collected for disposal. This type of precipitator is called a cyclone precipitator.

• Two of the main pollutants appear to be sulphur oxides(Sox) and the other nitrogen oxides(Nox ). These can form acids and compounds which have a corrosive effect on surroundings and buildings and also have a contributory effect on the formation of what is generally referred to as acid rain.

• Problems:• Boiler calculation.

COAL FIRING• Coal was the fuel most generally used for boiler

firing in the industrial nations of the world.• The bed consists of a thick layer of fine inert

particles of such natural substances as sand or limestone's. If such a bed has air blown through it, the air being evenly distributed through a grate at this base by means of a device such as a plenum chamber, then, at a particular velocity and mass flow of air, the bed will begin to behave like a fluid , or, in other words, it becomes a fluidised bed.

• If particles of coal are added to the fluidised bed, they become well mixed throughout the bed. If the temperature of the bed is high enough, then the coal will burn.

• A device is required for the removal of the coal ash at the base of the bed through the bed gate. A further device, sometimes referred to as an arrester, is located at the top of the combustor to intercept any small ash particles.

OIL FIRING: oil burner is fitted into the side of the boiler furnace. A large boiler, as in a power station, can employ as many as 32 oil burners.

GAS FIRING:Many of smaller boilers of the shell type are gas

fired. In these boilers burning gas jets are fed into tubes which pass through the water space in the boiler much in the same way as illustrated in the economic boiler. Gas fired boilers can be either horizontal or vertical.

EVAPORATION• The objective of evaporation is to concentrate

a solution consisting of a non- volatile solute and a volatile solvent.

• Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution of thick liquor.

• Evaporation differs from drying in that the residue is a liquid – sometimes a highly viscous one – rather than a solid.

• Some of the most important properties of evaporating liquids are as follows:

1. Concentration – although the thin liquor fed to an evaporator may be sufficiently dilute to have many of the physical properties of water, as the concentration increases, the solution becomes more and more individualistic. The density and viscosity increase with solid content until either the solution becomes saturated of the liquor becomes too sluggish for adequate heat transfer.

2. Foaming – some materials, especially organic substances, foam during vaporization. A stable foam accompanies the vapor out of the evaporator, causing heavy entrainment.

3. Temperature Sensitivity- Many fine chemicals, pharmaceutical products, and foods are damaged when heated to moderate temperatures for a relatively short times.

4. Scale- Some solutions deposit scale on the heating surfaces. The overall coefficient then steadily diminishes, until the evaporator must be shut down and the tubes cleaned. When the scale is hard and insoluble, the cleaning is difficult and expensive.

5. Materials of Construction- Whenever possible, evaporators are made of some kind of steel. Many solutions, however, attach ferrous metals, or are contaminated by them. Special materials such as copper, nickel, stainless steel, aluminum, impervious graphite, and lead are then used.

6. Single and Multiple Effect Operation- when a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This method utilizes the steam ineffectively. If the vapor from one evaporator is fed into the steam chest of the second evaporator and the vapor from the second is then sent to a condenser, the evaporation becomes double effect.

TYPES OF EVAPORATION• The main types of steam heated tubular

evaporators in use today are:1. Long-tube vertical evaporatorsa. Upward flow (climbing-film)b. Downward flow(falling-film)c. Forced Circulation2. Agitated-film evaporators

• Types of long tube vertical evaporator:• 1.falling film evaporator.• 2. forced circulation evaporator.• 3.agitaged film evaporator.

PERFORMANCE OF TUBULAR EVAPORATORS

• The principal measures of the performance of a steam-heated tubular evaporator are the capacity and the economy. Capacity is defined as the number of pounds of water vaporized per hour. Economy is the number of pounds vaporized per pound to steam fed to the unit.

• Evaporator capacity- q= UA∆T

HEAT PIPE

• A term heat pipe as the name implies, is a device for transferring heat from a source to sink by means of evaporation and condensation of a fluid in a sealed system.

• The heat pipe is a very simple and very efficient heat transfer device. It can be considered a super- thermal conductor that transmits heat by the evaporation and condensation of a working fluid. It can easily transmit 5000 times heat transmitted by best known metal conductor and with a temperature drop of less than 5⁰C per meter length of heat pipe.

• The heat pipe was first invented in 1942 by Richard S.Gauler, a general Motors Engineer.

• No immediate use were found for it and the patent expired in 1959. in the early 60s the idea was revived by the Atomic nergy Commission and Natural Aeronautics and Space Administration.

• Different Types of Heat Pipes-• A simple vertical pipe contains a simple vertical

pipe. It consists of a container,(pressure and vacuum tight) a wick structure and small amount of working fluid as mentioned earlier. In this system the wick capillary action brings the condensate back to an evaporator region and cycle is completed.

• Another type of heat pipe is a wickless pipe.