We have already studied about various topics related to oil and water seperator on various types of ships. One application we saw was in the bilge handling system of the ship. But that is not the only area where oil needs to be separated from a system on a typical ship.
Oil in the compressor of a refrigeration system on ship is also a very common problem. Oil mainly enters the pipelines through the lubrication system of the compressor. In order to prevent this ingress of oil into the compressor system, oil separators and oil rectifiers are used.
Oil separators are generally fitted in the discharge line of the hot gases. These oil separators are generally of impingement type as shown in the figure. The separator’s arrangement comprises of an enclosed vessel that has a series of baffle plates, also known as knitted wire mesh. It is though these perforated plates that the vapors pass.
Oil Seperation Process
The enclosed vessel is at the discharge side of the gas exhaust. The oil laden gases enter the separator at a very high speed. Sudden confrontation with a large area reduces the speed of the vapors. However, the velocity of the oil particles remains the same and they impinge on the baffle plates. The oil moves down the baffles due to gravity and gets collected at the bottom of the vessel. A float valve fixed at the bottom controls the flow of the oil from the bottom of the vessel to the compressor crankcase.
The main disadvantage of using a separator in the exhaust gas line is the passage of refrigerant from the separator to the compressor crank case, when the compressor stops. To avoid this, the separators are placed in high temperature regions. One more way to prevent this problem is to drain the oil from the separator into a receiver containing heating elements, which boils off the refrigerant and then allow the oil to pass to the compressor crankcase.
A little bit of oil carry over is always there in the compressor system. Some of the refrigerant mixes with this oil and form a solution. However, the generation of this oil is helpful because the oil moves with the refrigerant throughout the circuit and thus increases lubrication. If the oil doesn’t mix with Co2 then it needs to be removed at the evaporator manually. If the oil and Co2 is not removed, the loss of lubrication from the oil sump and accumulation of oil in the system occurs. Oil rectifiers are used to remove this oil automatically from the evaporator.
The oil rectifier removes the oil automatically from the evaporator into a heat exchanger in which liquid refrigerant and oil mixture is vaporized and CO2 is released. Heat is provided to the heat exchanger by passing warm refrigerant from the condenser to the evaporator. Oil and vapor is separated by this process and the oil goes to the sump and the Freon goes to the compressor.
Marine Auxiliary Machinery , 7th Edition - McGeorge