How a jet engine works

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Jet Engine animation

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Various defrosting techniques used in Refrigeration and Air conditioning

In every AC and refrigeration unit, we want a fluent transfer of heat. Four methods which can be implied to get rid of Frost which is a hindrance to the heat transfer and has a negative impact on refrigeration systems.

These include

1.      Defrosting using Hot Water

2.      Electric heating

3.      Defrosting through reverse refrigeration cycle

4.      Defrosting through hot gases

Defrosting using Hot Water:

As the name clearly indicates, in this method water with high temperature is passed to melt the water vapors which are present deposited as frost. The heat flows from hot water and melts the frost.

Electric Heating:

In this apparatus, some coils are present inside the evaporator. These coils do not contain any fluid. These coils are used for Electric heating purpose. When the current is passed through these coils, heat is generated due to the resistance of the coils which can be calculated as

                                               

P = I² R

This heat then melts the frost from the evaporator.

Defrosting through reverse refrigeration cycle:

This apparatus has been provided with a four way valve system. By this valve, we can bring appreciable changes in the apparatus. Evaporator can be converted to Condenser and the condenser then behaves as Evaporator.

By doing so, if evaporator is converted into condenser, it will release heat to the surrounding. This heat will melt the pre existing layer of the frost.

Defrosting through Hot gases:

The compressed gases (refrigerant) have a high temperature inside the compressor. In this apparatus, we possess a solenoid valve. By opening this valve, these high temperature, high pressure refrigerant can be introduced inside the evaporator.

By doing so, the heat from these hot gases will flow to the low temperature side and will eventually result in melting of the frost.

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Nomenclature of Gear

Different terms used in gears are explained below one by one.

Pitch Circle: It is a theoretical circle, which by pure rolling action would give the same motion as the actual gear. The pitch circles of a pair of mating gears are tangent to each other. The diameter of the same circle is called as  Pitch Circle Diameter.
Pinion & Gear: Of the two mating gears the smaller gear is known as pinion while the larger is oftenly known as gear.

 

Pitch: The circular pitch p is the distance, from a point on one tooth to a corresponding point on an adjacent tooth, measured on the pitch circle, . Circular pitch is equal to the sum of the tooth thickness and the width of space.

Pitch Point: The common point between two pitch circle is denoted as pitch point.

Module: The module m is the ratio of the pitch diameter to the number of teeth of the gear.

Diametral pitch P: The diametral pitch P is the ratio of the number of teeth on the gear to the pitch diameter. Diametral pitch is reciprocal of module.

Addendum & Dedendum: The radial distance between the top land and the pitch circle is known as adendum denoted by "a". The radial distance from the bottom land to the pitch circle is known as  dedendum  "b". The whole depth  is the sum of the addendum and the dedendum. 

The circle that is tangent to the addendum circle of the mating gear is clearance circle. The clearance c is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear. The backlash is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth measured on the pitch circles of the mating gears.

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Steam Power Plant

Steam turbine is the main component in power plants in both large and small units, and industrial steam systems. The steam turbine has been tailored for large fossil, nuclear, combined-cycle, geothermal, and small power facilitates and mechanical-drive service. The turbine island - including condensers, cooling systems, and cooling towers, instrumentation and control system, condensate/feed water pumps-make up water treatment systems, valves, pumps. Piping will continue to anchor the next generation of power plants.  A single shaft steam turbine can produce more thant 1000 MW of power. Thus steam turbine has no match with other machines.

 

 

Fossil fuel is burnt in the furnace of the boiler and hot gases pass over the water tubes, heating the water inside the tubes. The water tubes are connected to the main boiler drum where high pressure, saturated steam is produced. The steam is then raised to a temperature higher than saturation temperature in the super heater. High pressure and high temperature super heated steam then enters the steam turbine. The steam expands in the turbine from a high pressure to low pressure area of the condensor. The heat energy is converted to mechanical energy to drive a generator to produce electricity. The exhaust steam from the
turbine is condensed inside a condenser by circulating water of the condenser tubes. The condensate is sent back to the boiler by condensate pumps. The steam power plant needs a large quantity of water for producing power and much more water to cool the exhaust steam from the turbine. So it is imperative that steam plants are located near river, lake or sea.

Schematic Diagram of Steam Power Plant  B Boiler,S Super Heater, T Turbine, C Condenser P Pump.

 

 The steam explands in the turbine from a very high pressure to a low pressure of the condenser (less than atmospheric) with a large enthalpy drop. A very large quantity of water has to be circulated in the condenser and that heated up by condensation of steam so circulating water is cooled in cooling tower-a visible sight for the power plant from outside. Power produced by the turbine is mass flow rate of steam multiplied by the enthalpy drop.

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Classification of fire

For identification, fire are classified according to their properties, which relate to the nature of fuel. The properties of fuel directly corresponds to best means of combating a fire. Without a source of fuel there is no fire hazard, but almost everything in an environment could be a fuel. Fuels occur as solids, liquids, vapors and gases.

Fire can be classified as following

Class A fires: Fuel is solid materials such as wood, plastic, textiles and their products: paper, housing, clothing etc. Solid fuels includes wood, building and synthetics used in furniture.

Class B fires: Fuel is in the form of flammable liquids and gases. Flammable liquids have a flash point below 100 ^oF.

Class C fires: Fires that are caused due to electricity as burning, natural and generated electricity play a large role in causing fire. Electrical lines and equipment can cause fires either by short circuiting or by resistances generating heat.

Class D fires: Fire that is caused due to combustible, easily oxidized metals such as aluminum, magnesium, titanium, and zirconium is categorized as “class D fire”.

Special Categories: This type of fire is caused due to extremely active oxidizers or mixtures, flammables containing oxygen, nitric acid, hydrogen peroxide, and solid missile propellants

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CAUSES OF FIRE HAZARDS

SPONTANEOUS COMBUSTION:

              These are rare but can happen. It happens by self-heating (increase in temperature due      to exothermic internal combustion).

HYPERGOLIC REACTIONS:

It includes pyrophor hypergolic fuels. It occurs when mixing fuels. Oxidizers produce such a rapid heat buildup, causing immediate combustion.

MISHANDLING FLAMMABLE LIQUIDS ANG GASES:

They should be handled with care because any shock can increase its internal molecular kinetic energy causing increase in temperature.

NATURAL AND GENERATED ELECTRICITY:

Electrical lines & equipment cause fires by a short circuit that provides a spark, by arcs, or resistance heat. Lightning strikes start many fires every year.

HEAT FROM HOT SURFACES:

Irons in textile manufacturing & dry-cleaning, coffee pots space heaters, hotplates, etc., all create hot surfaces. Boilers, steam equipment, radiators, pipes, flues/chimneys. Surfaces exposed to direct sunlight become hot surfaces.

ENGINES AND COMPRESSORS:

  Engines produce heat, especially in exhaust pipes. Compressors produce heat           through friction, transferred to their housings.

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