How a Fridge Works!

Hello,

Here is a summary of all the information and concepts that you need to understand and explain how a fridge works.

Key Thermal Physics Points/Concepts for the refrigeration cycle

Thermal energy is the sum of Ek and Ep of a substance KMT = particles move (Ek) high energy = more motion. Temperature is a measure of the motion of particles (Ek) Heat Transfer is the transfer of thermal energy thermal energy is transferred from high temp to low temp 0 K is absolute zero. The temperature at which kinetic energy of particles would be zero

Heat transfer (Q) in joules Specific heat capacity is a property of a material. Amount of energy it takes to raise the temperature (kinetic energy) of the substance. Liquid water 4187 J/Kg C Q=mcpt

Phase changes melting/freezing, evaporating/condensing Phase changes release or absorb large amounts of energy stored or released by potential energy of the molecules. Latent heat is the thermal energy associated with a phase change. Latent heat of vapourization of water is 2260 KJ/Kg

Liquids are non compressible, gases are Compressed gases increase in temperature (gain thermal energy) by the work done by the compression process Expanding gases decrease in temperature (lose thermal energy) by the work done by the gas during expansion Higher pressure =  higher evaporation temperature By compressing and expanding a working fluid thermal energy can be transferred from a location of low kinetic energy to a location with high kinetic energy = refrigeration cycle Compressor, evaporator, condenser and expansion valve make up the major components of the simple refrigeration cycle.

Figure 1: Diagram demonstrating the refrigeration cycle that functions in a fridge



Choosing a working fluid (refrigerant)

Many different types of refrigerants are available and are selected for their specific properties to function properly with a specific design.


Figure 2: graph of various refrigerants boiling pressure versus temperature

From Figure 2 if one were to want and evapourator temperature of -15C then the pressure would need to be dropped to around 150 KPa and for a corresponding condensor temperature of 40C the pressure would need to be increased to about 1000 KPa. Increasing the pressure of the gas is the job of the compressor. It is what drives the cycle and consumes the electrical input energy.


A note about efficiency. All refrigeration cycles have a maximum theoretical efficiency (amount of useful energy in vs. useful energy out). This theoretical efficiency increased as the temperature of the refrigerant becomes closer to the ambient temperature. So most efficiency improvements with these systems have been done in this way.

But what is the consequence of having a smaller temperature gap?  What other factors must compensate?


Compare 361A model
vs.


compare 361E

How much bigger is the more efficient unit compared to the inefficient one?...

Heat pump?

Let's have a chat

Hope this is helpful and interesting! Go wow your parents/guardians with your knowledge of heat transfer and explain the refrigeration cycle, why it might be time for a new air conditioner or even better a heat pump!

RZ