Feed water is the water that is fed to the Boiler.  The feed water is a mixture of the condensed water also known as return water and fresh water (makeup water) to compensate for its losses.  The make up water is treated to remove its minerals. 

Chemical treatments are carried out to bring the conductivity, pH and alkalinity within specific limits.  The water is deaerated to remove dissolved oxygen and carbon dioxide.

The makeup water that is fed into the boiler depends on the total water losses.  Water leaks from the boiler and the pipelines as steam.  Some of the water is deliberately released via blowdown to remove the impurities which have settled at the bottom of the tank.

The Fusible Plug is a safety device in a boiler.  When the temperature of the boiler reaches abnormal levels, the plug fuses (melts) and provides an exit for the steam.  The jet of steam which exits the boiler will alert the operator to the high temperature.

A typical scenario is when the water level in the boiler falls below the safe limit.  This can cause the boiler to overheat and ultimately fail due to the metal walls softening. 

The plug is placed below the water level.  If the water level falls below the plug, the boiler starts to heat and the plug operates, releasing steam.

Boiler Mountings are the accessories which are mounted on the boiler for the effective control and safety of the boiler.  There are many accessories available today.

The mandatory boiler mountings are
1. Safety Valves - 2 Nos
2.  Water level Indicators - 2 Nos
3. Steam Stop Valve
4. Fusible plug
5. Blow Off Cock
6. Manholes and Mudholes

Mobile Boilers are boilers which are not fixed to any particular location.  They can be moved from place to place.  A common example is the locomotive boiler in railway engines.  These boilers move along with the Engine. 

Mobile boilers  are available in a variety of capacities.  They are also available in a variety of pressure ranges.  The Boilers can be used to provide both steam and hot water.

Mobile Boilers are mounted on a trawler which can be towed from one place to another.  These boilers are handy and can be installed at the locations where steam is required on a temporary basis.  They can be used for heating, sterilizing in plants and in hotels.  Many mobile boilers are powered by electricity.

Mobile Boilers come in both fire tube and water tube versions.

A boiler, as the name suggests, boils the water before turning it into steam at subcritical pressure - the pressure at which bubbles can form.  Steam generators, on the other hand, convert water into steam into steam without boiling at a super-critical pressure.

There are also constructional differences between a boiler and a steam generator.  A boiler contains many tubes which carry the water.  A steam generator has, generally,  only one tube in which the heating occurs. 

A waste heat recovery boiler is a device which recovers the heat produced by another industrial
process or equipment such as a genset, incinerator, furnaces, etc.  Many industrial processes produce heat which is rejected into the environment as waste.  This heat can be captured by the boiler and used to generate steam. 

The steam which flows through the exhaust is diverted by means of a diverter to pass through the boiler and then to the exhaust. 

This steam generated by the boiler can be used for a variety of functions such as to generate power, for other application such as in the case of a textile mill, for heating, etc. 

Other benefits of Waste Heat recovery Boilers are that they reduce pollution and the temperature of the exhaust gas.  This reduces the maintenance requirements of the exhaust systems. 

According to the standards of the ASME (American Society of Mechanical Engineers), Boilers can be classified on the basis of pressure into the following types. 

Low Pressure Boilers

Boilers with operating steam pressure not exceeding 1.021 atmosphere and a temperature of 394 K. 

Power Boilers

Power Boilers are those boilers whose pressure rating and temperature are above those of Low pressure boilers.

Miniature Boilers

These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.

Steam Traps are devices which are used to release condensate which may form in steam lines.  Steam traps allow only the condensate and prevent the useful steam from escaping. 

Condensate should be removed from steam lines.  Condensates can cause hammering in the pipelines and corrosion.

In addition to Steam, Steam traps also release air and other gases. 

There are many different types of steam traps.  The most simple can be a nipple in a pipeline.  Since condensate (water) is heavier than steam, it will collect at the lowest point.  The trap opens once sufficient amount of condensate has collected. 

Water present in a steam system moves at a very high velocity driven by the steam pressure.  In some cases, the acceleration is even greater than that of steam.  When this water at high speed hits a fitting such as valve or a bend, violent impact resulting in noise or a pressure shock which travels through the system is produced.  This is called water hammering

Mild cases of water hammering can cause noise or vibration.  Extremely severe cases can result in fracture of the pipe. 

Condensate Hammering is more damaging than water hammering.  In condensate hammering, a pocket of steam surrounded by cooler water condenses into water.  This causes a rapid reduction in volume.  The water surrounding the pocket are thus drawn inwards and a collision occurs.  This results in a severe a rapid over pressurization which can easily damage gaskets, valves and other components.  The consequences can be very disastrous even fatal.

Hammering can be removed by preventing water from entering the steam lines.  Reducing Carryover, ensuring that the steam traps are functioning properly are some of the precautions.  The steam velocity in a boiler should not be allowed to exceed limits.  Steam velocity has a direct bearing on the intensity of the hammering. 

The piping should be as per design.  Sagging of the pipeline can cause water hammering.  Damaged insulation can cause condensate formation in the pipelines.  Insulation should be checked and replaced if damaged. 

Vacuum Pumps are used in Boiler Systems to evacuate the air from the piping.  Air reduces the heat transfer and acts as a thermal insulator.  It allows impedes the flow of steam.  The Vacuum pump is used to evacuate air from the pipelines. 

The gases present in the air such as oxygen and carbon dioxide can dissolve with air and cause corrosion.  Using a Vacuum pump can remove these gases and lower the dissolved gas level in the system preventing corrosion. 

Once air is removed from a system and a vacuum is created, steam requires a very low pressure to flow across the system. 

Since the pressure of the system decreases, the boiling point of water also decreases.  This results in a reduction of the fuel consumption.  Vacuum pumps can also be used for lifting the condensate  to the receiver.  This is necessary to prevent water hammering.

Vacuum pumps are specified in the amount of air they can move at a given vacuum. They can produce vacuums of the range of 5, 10 and 15 inches of Hg.

The Stroboscopic Effect in Fluorescent lamp is a phenomenon which causes running or moving equipment to appear stationary or appear to be operating slower than they actually are.

In an AC supply, the voltage drops 100 times a second to zero volts as the supply frequency is 50 Hz.  When a Fluorescent lamp is operating with an AC supply, the light intensity drops 100 times a second.  This flicker is not noticeable to the human eye due to the persistence of vision. 

When a worker in a factory observes a running machine, say a flywheel under the illumination of a fluorescent light, the flywheel may appear to be stationary or to be operating at reduced speed.  This can result in accidents and is highly dangerous.

A sewing machine whose needle moves up and down may appear to be stationary and the operator can prick the fingers.  These are some examples where the stroboscopic effect in the Fluorescent lamps can prove to be dangerous.  When using fluorescent lamps around rotating or moving machinery, two lamps powered by two different phases should be used.  This ensures that both the lamps do not flicker due to the zero crossing at the same time. 

If another phase is not available, a capacitor can be added in series to one lamp.  This ensures that there is a phase lag between the two lamps. The Stroboscopic effect can be eliminated by  using electronic ballasts where the supply to the lamps is of a very high frequency of the order of kiloHertz.

Optical Cables are used extensively in the field of telecommunication.  They have numerous advantages over conventional communication on wires.  They are efficient, quick and secure.  Optical Communication Cables are designed to provide high efficiency of transmission.  They are also designed to withstand the challenges of the external environment such as corrosion, heat and physical stress.
The Optical Cable has the following main components. 
The Core provides the pathway for the light to travel.  It is made of glass or transparent plastic material. 
The cladding is the layer that covers the core.  The function of the Cladding is to reflect the light which may come out of the core back into the core.  This results in Total Internal Reflection which ensures that there is no loss of the light signal. 
The Buffer is a coating which is outside the Cladding.  The Buffer serves to protect the optical fibre. 
Aramid Yarn Protection
The Aramid yarn which surrounds the buffer provides crush protection to the cable.
Protective Jacket
The protective jacket offers mechanical protection to the cable. 

A Ballast in a fluorescent light is necessary to get the light glowing.  The Ballasts generates a high voltage by means of the would coil which acts as an inductance.  When the starter interrupts the supply to the inductance, a powerful voltage is generated.  These are called magnetic ballasts. 

However, magnetic ballasts take time to get the lamps to start.  They also have an undesirable hum.  They also produce a flicker before the tube lights up continually.

Electronic eliminate the problem of initial flicker and hum.  They also reduce power consumption.
Electronic Ballasts work by converting the AC supply into DC first.  This rectified DC is then chopped by a chopper circuit to generate high voltages to start the discharge in the lamp.  The chopped AC waveform is t a very high frequency of the order of kHz.  This ensures that the flicker is reduced to a minimum.

The efficiency of the lamp is also increased.  Electronic Ballasts are 10% more efficient than magnetic ballasts. Electronic Ballasts can generate harmonics.  This, however, is insignificant as the amount is very small.

Conductivity in metals is due to the presence of free electrons in the atomic lattice.  When the metal is heated, the atoms in the lattice vibrate.  This results in reduced movement of the electrons as they hit against the vibrating atoms.

This results in an increase in resistance of the metals. 

Most Metals have positive temperature coefficient of resistance. There are , however, exceptions such as carbon and semiconductor metals such as Silicon and Germanium.  Some Alloys have zero temperature coefficient of temperature which means that the resistance does not change with increase of temperature.  Manganin is an example.

When a semiconductor is heated, the conductance increases and the resistance decreases.  Semiconductors, thus, have a negative temperature coefficient of resistance.
When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy.  These electrons are able to overcome the attraction of the nucleus and leave the atom.  Thus they become free electrons which can conduct. 
The number of electrons increase exponentially and this results in a large drop in the resistance. 
Electronic devices will behave erratically above a certain temperatures.  Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function. 
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band.