Principle of a Refrigerator


  • Three basic laws of physics are involved in working of refrigerators
    1. A cold object is placed next to a hot object, the cold object will become warmer and the hot object will become cooler.
    2. The boiling point of a liquid is directly proportional to the pressure it is subjected to. (Water boils at 100°C at 1 atmospheric pressure, but it boils at 46°C itself at 0.1 atmospheric pressure and conversely water vapour at 50°C and 0.1 atmospheric pressure can be condensed back into liquid by increasing the pressure to 1 atmospheric pressure)
    3. Evaporation produces cooling, as the vapour absorbs heat and heat is given off or lost by a liquid as it condenses.
  • All refrigerators make use of refrigerants to bring about their action.
  • Refrigerants are liquids or liquefied gases with very low boiling points. For example, ammonia, a common refrigerant used boils at -33.34 °C. These basic components are the evaporator, compressor, condenser, and metering device.
  • The evaporator is where liquid refrigerant evaporates. As stated already, the evaporation of this refrigerant requires heat. Because the temperature of the food is higher than the evaporator, heat leaves the food and enters the evaporator, increasing the temperature of the liquid refrigerant.
  • Since when a liquid reaches its boiling point, the temperature stops rising and begins to boil when more heat is added, refrigerant with very low boiling points begins to boil and evaporate in the evaporator.
  • The evaporator stays at the same temperature (cold) until all the refrigerant boils away (evaporates).
  • This means that the evaporator would continue to absorb heat from food inside the refrigerator. The effect is refrigeration; warm air from inside the food of the refrigerator is absorbed as the refrigerant evaporates.
  • Eventually all the refrigerant will have evaporated and we won’t be able to cool food any more, if it is allowed to progress without the heat in the refrigerant being absorbed. This is where the compressor comes into play.
  • The compressor is like a pump for refrigerant. It creates a suction that draws the refrigerant out of the evaporator, keeping the low pressure in the evaporator needed to boil more refrigerant.
  • As the refrigerant is compressed, two things happen. Its pressure goes up, because it’s being squeezed into a smaller space,and as a consequence its temperature also increases. So while the compressor sucks in cold, low pressure refrigerant vapour, it pumps out hot, high pressure refrigerant vapour.This high pressure refrigerant vapour is pumped into a condenser, where it (logically) condenses.
  • The boiling point of the refrigerant vapour has increased because it is at a high pressure.The condenser looks very similar to the evaporator, and with good reason, they perform the same function.
  • Both are designed to move heat; the evaporator moves heat from the food to the refrigerant and the condenser moves heat from the refrigerant to the air blowing past its coils.
  • After the refrigerant turns back into a liquid, we’re ready to send it back to the evaporator and use it for more cooling. It is still at a high pressure. This is the big difference between the evaporator and condenser, the difference in pressure. Just connecting the condenser and evaporator together won’t work because the high pressure liquid won’t boil in the evaporator. This is where the metering device comes into play.
  • There are various kinds of metering devices.   The simplest is just an extremely thin piece of tubing. The idea is to just let a tiny trickle of refrigerant into the evaporator; just enough to replace what the compressor is sucking out without raising the pressure.
Last modified: Tuesday, 27 December 2011, 10:34 AM