What is the oil trap and why we use it in split air conditioning system?

- Oil trap exist  to avoid oil  escape from compressor to the other  parts of refrigerant cycle 

- that causes compressor damage and low Efficiency in refrigeration cycle  

Oil traps are used when the pipe length is long; upto300 ft where as without one will take upto150 ft. P trap is given after every10 ft of vertical height.

Oil separator minimizes the amount of oil circulating in the cooling system. Oil to coat the inside of each cycle of the cooling components, through which it passes. This reduces the heat transfer efficiency and capacity of the evaporator and condensor. This is one of the reasons for separation as much oil as possible with the refrigerant before sending it to the system. Another reason is to slow down the accumulation of oil in those places where it is difficult to return the oil. This consideration is particularly important where the refrigerant is not mixed. The refrigerant does not dissolve in oil and oil is not soluble in refrigerant.

Oil trap is used in vertical suction risers.
Double suction risers are used if the capacity of the plant is large for the oil to get entrained along with the gas back to the compressor for lubrication.

Oil traps are usually used in split air conditioning systems with refrigerant piping going through high vertical risers.
Compressor oil travels to the condenser and back into the system if there is a vertical riser.
To avoid this, oil traps (nominally1 trap after every10 feet) are used to block the compressor oil to travel into the refrigerant piping system and block the path.

Oil Traps are used as a medium to enhance oil return mechanism.When refrigerant is released from the compressor at high pressure , the compressor oil is also pumped along with the refrigerant.This may cause poor heat tranfer rate as oil foms a coating on evaporater coil.This also reduces the compressor efficiency & life due to improper wear & tear occuring due to the lack of oil in compressor.

 

I agree with  Muhammad Sharif answer .