Why do we need a compressor in a refrigeration system?

At first ,we have to correct this question.All refrigerator system dont need compressor.It is needed only in vapor compression cycle,not in vapor absorption cycle.

Now again think of a scenario. Suppose you have a refrigerant tank attached to an evaporator.The refrigerant is at a higher pressure in the tank.You open the tank and boom! Refrigerant expands,get cooled and take heat from the evaporator.So why need a compressor?

Got confused?

The answer is simple.Let cooling was done using the refrigerant.Now how it will be compressed again to run a new operation? We will need new refrigerant tank to do so.But that would be highly costly.

So we have to compress the vapor refrigerant to use it again.

Now now, evaporator releases the vapor at a low pressure and temperature.We need to remove the heat energy stored in in vapor .But will it be easily removed?

Of course not.The temperature of refrigerant vapor is always less than the environment of the condenser.So ,we somewhat need to increase its temperature.For this reason,we compress it ,so pressure and temperature both are increased.So it can easily reject heat at the condenser and become liquid.Here ,one thing might seem interesting.In evaporator and condenser,there is not significant change of pressure.So , refrigerant is still at high pressure at the condenser.

So the purpose of compressor is two:

It increases temperature of the vapor

It smooths the refrigerant flow(compressing it ,it can be easily expanded later)