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How Does a Fridge Work

First law of thermodynamics states that work done is proportional to heat and heat is proportional to work done.

It means any work done can be converted into heat energy and heat energy can be converted into work done completely which is not possible.

Second law of thermodynamics corrects the first law of thermodynamics. No energy goes one form into another form without any loss. That is the second law of thermodynamics. Many scientists explained same points in different ways.

The main theme all of them is same that 100% heat can be not converted into 100% work done. For example when we cook our food with the help of gas burner we apply heat outside the cooking pot. Suppose we apply 100J heat to cook the food. Some of heat let 20J going into environment and 30J heat is wastes by the pot we only getting 50J heat in our cooking system inside. Here we are losing 50J means 50% heat although we applied 100J heat to work with.

According to second law of thermodynamic there will loss must converting heat energy into another form of energy. No one can claim he made a 100% efficient engine. If claim first of all we should consider he is false man because there is a loss between converting energy one form to another form although it may be small loss.

In the second law of thermodynamics also states that heat cannot be converted into work done without using an engine. Generally we know temperature goes higher to lower point. To convert heat into work done there is needed an engine which takes heat as input and converts it into work done partially and remaining energy which it cannot convert into work done rejects into sink which loss. Each engine has its own material of substance; the substance takes the heat as input and does work done and reject loss heat into sink. In petrol engine petrol is the material substance; diesel is the material of substance in diesel engine.

The engine which can transform more heat energy into work done is high efficient engine.

The efficiency of an engine = Output Energy/Input Energy

Suppose input heat of an engine 100J, work done 80J and sink heat 20J.

Then the efficiency of the engine

Efficiency Refrigerator

Before learning about refrigerator operation it is important to know about heat engine operation.  Refrigerator works reverse heat engine.

Heat Engine: The heat engine is an engine where heat energy is inserted as input and mechanical energy is taken as output.

The working principle of heat engine is “the working substance of engine takes heat energy from higher temperature source and does work with the partial input heat finally reject remain amount of heat into lower temperature sink”

The diagram shows the operation.

Input heat from the high temperature source = Q1

Rejected heat from the sink at lower temperature = Q2

Work done by the engine W = Q1 – Q2

How Does a Fridge Freezer Work:

Figure: Refrigeration Cycle

Refrigerator is a common machine in our daily life. We preserve fish, meat, drinking water, ice cream, other beverage with the help of refrigerator.

The machine which always maintains a stable low temperature than environment inside the system is called refrigerator.

Refrigerator works inverse the heat engine. Heat engine receives heat from higher temperature source and releases heat to lower temperature sink doing partial work done. Refrigerator receives heat from lower temperature and does work and releases heat to higher temperature. In the block diagram shows a refrigerator removed heat Q2 to the compressor and the compressor does the mechanical work. After doing the mechanical work the compressor rejects Q1 heat to the higher temperature. In our household refrigerator Q2 is the heat removed from the refrigerator by vaporization coil. Q1 heat is rejected in the air and W is the work done by the motor of the compressor.

If we draw the P-V characteristics curve for the refrigerator it appears at the opposite curve of Carnot cycle. From the P-V curve ADCBA is work done path by the refrigerator.

Co-Efficient of Performance of Refrigerator (COP):

Co-efficient of performance of a refrigerator is the ratio of heat removed and supplied work.

Heat removed by a refrigerator = Q2

Supplied work = Q1 – Q2

COP refrigertor

The co-efficient of performance is denoted by K.

Q2 = the heat removed by the vaporization coil

W = the work done by the compressor

Q1 = the rejected heat in the condenser coil

Md Ebrahim Shah
 

Electrical and Electronic Engineer, Researcher of Renewable Energy Technology, Lecturer in Physics and Founder of engineersblogsite.com. His passion is to describe electrical and electronic technology in easy way as people can get clear conception and upgrade the technology for the better human life.

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