Notes

Refrigerator is the reverse of a heat engine …

Refrigerator is the reverse of a heat engine. Here the working substance extracts heat Q₂ from the cold reservoir at temperature T₂, some external work W is done on it and heat Q₁ is released to the hot reservoir at temperature T₁. In a Refrigerator the working substance (usually, in gaseous form) goes through the following steps:
(1) sudden expansion of the gas from high to low pressure which cools it and converts it into a vapour-liquid mixture,
(2) absorption by the cold fluid of heat from the region to be cooled converting it into vapour,
(3) heating up of the vapour due to external work done on the system, and
(4) release of heat by the vapour to the surroundings, bringing it to the initial state and completing the cycle.