HEAT AND THERMODYNAMICS - iI

Dr. V. MathivananAssociate Professor of

Physics

E.mail: mr.mathivanan@rediffmail.com

SCHEMATIC OF A FOUR STROKE OTTO ENGINE

Suction (charge exchange stroke)When the piston moves down, fuel/air mixture is drawn through the intake valve.

Compression (power stroke)The cylinder volume is compressed.

Expansion (power stroke)In the petrol engine, the fuel/air mixture is ignited by a spark plug.

Exhaust (charge exchange stroke)The exhaust gas is expelled when the piston moves up.

Schematic of a four stroke diesel engine

·Suction (charge exchange stroke) When the piston moves down, air is drawn through the intake valve.·Compression (power stroke) The cylinder volume is compressed.·Expansion (power stroke) In the diesel engine fuel is injected under high pressure and the mixture ignites spontaneously.·Exhaust (charge exchange stroke) The exhaust gas is expelled when the piston moves up.

Refregirator

Coefficient of performance

Coefficient of performance (COP) is defined as the ratio of quantity of heat Q2 removed per cycle from the contents of the refrigerator to the energy spent per cycle W to remove this heat.

Freon liquid absorbs the heat energy from the food materials

Due to absorption of heat, the liquid freon becomes low pressure freon gas molecules

The freon gas moleucules enters through the compressor and gets compressed – becomes high pressure freon gas molecules

The high pressure freon gas molecules loses their heat energy to the coil to beocme freon liquid

This a cyclic process which continues and to keep the things in the refrigerator cool

How an air conditioner works

1 Warm air from the room is sucked in through a grille at the base of the machine2 The air flows over some chiller pipes through which a coolant fluid is circulating. This part of the machine works just like the chiller cabinet in a refrigerator. It cools down the incoming air and a dehumidifier removes any excess moisture.3 The air then flows over a heating element (similar to the one in a fan heater). On a hot day, this part of the unit may be turned right up so the HVAC works as a heater.4 A fan at the top blasts the air back through another grille into the room. If the heating element is turned down, the air re-entering the room is much cooler, so the room gradually cools down.

5 Meanwhile, coolant (a volatile liquid that evaporates easily) flows through the chiller pipes. As it does so, it picks up

heat from the air blowing past the pipes and evaporates, turning from a cool liquid into a hotter gas. It carries this heat

from inside the room to the outside of the building, where it gives up its heat to the outside air. How? Just like in a refrigerator, the coolant flows through a compressor unit and some condensing pipes, which turn it back into a cool liquid

ready to cycle round the loop again.What happens to the heat?

6 In the unit outside the building, there are lots of metal plates that dissipate the heat to the atmosphere. An electric fan blows air past them to accelerate the process.

7 Over time, the heat inside the building gradually pumps away into the outside air.

Uses of air conditioners:

Air Conditioning of Residential and Official Buildings

Industrial Air Conditioning

Spot Cooling

Spot Heating

Printing

Textiles

Precision Parts and Clean Rooms

Photographic Products

Computer Rooms

Air Conditioning of Vehicles

Food Storage and Distribution

NUMERICAL PROBLEM 1

NUMERICAL PROBLEM 2

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