How 12V Fridges Work

12v fridges, 24v fridges, compressor fridges and household fridges all use a similar process for cooling. It is known as the 'vapour-compression' cycle. Put your nerd hats on, recall a bit of high school physics, and we're away....

The Aerosol Can Principle

Have you ever noticed when you spray an aerosol can (e.g. deoderant, hair spray) that the canister gets cold while in use? Why is this so?      The can contains a liquid stored under high pressure. The liquid (mainly a propellant such as propane) has a low boiling point (e.g. 5 degrees celsius), but because it is under pressure, it remains as a liquid at room temperature.      When you press the button, in addition to some of the contents being ejected from the can, the pressure inside the can dramatically decreases, and some of the liquid contents evaporates into gas to maintain an even pressure. This change of state from a liquid to a gas requires energy - which comes in the form of heat absorbed from its surrounds -  then can, the air, and your hand touching it.      (For the uber geeks out there who want more info - this is known as the Joule-Thomson effect. There are actually two different effects taking place with aerosols - one is the expansion of the gas outside the can (think about a CO2 fire extinguisher where the spray is very cold), and the other is the expansion of gas inside the can as described here - where the can gets cold).

 

Fridges vs Aerosol Cans

The cooling process for a fridge works on a similar principle to that of an aerosol, but it must keep cycling around, and therefore it has a way of getting the compressed gas back into the 'can' as a liquid at room temperature. There are a whole pile of pipes on the back of the fridge. In these pipes are a special refrigerant gas - normally R134A. There's also a couple of special sections of pipework:


1. Compressor    2. Condensor      3. Expansion Valve      4. Evaporator

Outside the fridge

  1. The gas enters the compressor as low-pressure vapor, at or slightly above the temperature of the refrigerator interior. The compressor (which comprises a cylinder attached to a motor) compresses the gas, in the process superheating the vapour.
    1. Think of it like a bike pump - you know when you start pumping really fast, the end of the bike pump (near the outlet) gets really hot. All that energy of you pumping, compresses the gas (air), but also makes it really hot.
  2. The superheated vapour then travels under pressure through through a set of coils called the 'condenser'. The condenser has fins which passively cool the superheated vapour by exposure to air in the room. At this stage, the lower temperature (which is now around room temperature) causes the vapour to liquify, while still under pressure.
  3. This liquid refrigerant is then forced through an expansion valve (basically a pin-hole sized constriction in the tubing). Because only a small amount can get through, the other side of the expansion valve is at a lower pressure. This causes the some of the liquid (about half) that has passed through to immediately evaporate (turns into a gas). The heat absorbed by this evaporation is drawn mostly from the remaining liquid refrigerant - much like the aerosol can.

Inside the fridge

  1. This cold and partially vaporized refrigerant continues through the tubes of the evaporator unit (inside the fridge). So we now have some cold tubes inside the fridge itself.
  2. A fan blows air from the refrigerator or freezer compartment ("box air") across these coils or tubes and the refrigerant completely vaporizes, drawing further latent heat from the box air. The box air is hotter than the evaporator. The air is cooled as it passes over the cold tubes, but this air also energizes the liquid, turning the remainder to gas and expanding, absorbing even further heat from the fridge.
  3. This cooled air is returned to the refrigerator or freezer compartment, and so keeps the box air cold.
  4. Refrigerant leaves the evaporator, now fully vaporized and slightly heated, and returns to the compressor inlet to continue the cycle.

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