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Oil Separator
An oil separator minimizes the amount of oil that circulates through the refrigeration system. Oil coats the inside of every refrigeration cycle component through which it passes. It reduces the heat transfer capacity and efficiency of the evaporator and condenser. This is one reason for separating as much oil as possible from the refrigerant before sending it into the system. Another reason is to slow down the accumulation of oil in places from which it is difficult to return the oil. This consideration is especially important where the refrigerant is not miscible. That is, the refrigerant does not dissolve in the oil and the oil does not dissolve in the refrigerant.
Oil separators are recommended for all systems using non-miscible refrigerants. Ammonia is non-miscible, while R-11, R-12, R-113, R-114, and R-500 are miscible. R-22 and R-500 have limited miscibility, but mix well with oil at normal comfort air conditioning temperatures.
Oil separators are also recommended for low temperature systems and those that use flooded evaporators. Examples are centrifugal liquid chillers and flooded refrigeration coils. Finally, oil separators are used on any split system or built-up system with suction or discharge risers and/or capacity reduction steps. These features often cause oil return problems. In practical terms, this means that oil separators are not normally found on residential and commercial comfort air conditioning systems. They are more often used on refrigeration and industrial applications. Centrifugal chillers are more likely to have an oil reclamation system than an oil separator.
The only purpose of oil in the refrigeration cycle is the lubrication of the compressor. Therefore, the oil separator is installed in such a way that the oil is confined to the compressor. As shown here, it is usually installed in the discharge (hot gas) line, as close to the compressor as practical. It is installed downstream of the muffler, if a muffler is provided. The oil return line is connected directly to the compressor crankcase in some designs. In others, it is piped to the compressor suction inlet.
Even though oil separators, when properly applied, are very effective in removing oil from the refrigerant, they do not remove all the oil circulating with the discharge gas. Some oil still finds its way into the system. Therefore, when oil separators are used, some means must be provided to periodically return the accumulated oil to the compressor. Oil drains are provided for this purpose at the bottom of receivers, condensers, evaporators, accumulators, and other vessels used on ammonia systems. Automatic methods are also used.
The compressor oil comes into contact with the refrigerant in the compressor. As a result, a certain amount of oil will be carried along with the compressed refrigerant that leaves the compressor through the discharge line. Two basic types of oil separators are the impingement type, shown here, and the chiller type.
In the impingement type, the oil-laden discharge gas is sent through a series of screens or baffles. Since the area within the separator is larger than the discharge line, the hot gas flow slows down. This allows the oil to stick to, or impinge on, the screens. It drains off the screens into the sump at the bottom of the separator. A float valve in the sump maintains a liquid seal between the high and low-pressure sides of the cycle. This valve automatically returns oil to the compressor through an orifice.
The chiller type of oil separator can be water-cooled like the one shown here, or refrigerant-cooled. The "oil cooler," as it is sometimes called, is similar in construction to the water-cooled condenser. Water is circulated through the tubes while the oil-laden discharge gas passes through the shell. The oil precipitates on the cold water tubes and drops down into the sump. From there, it is automatically returned to the compressor through a float valve. The water flow through the separator must be regulated so that the refrigerant vapor is not condensed into liquid on the surface of the tubes. If it is, liquid refrigerant will be sent back to the compressor inlet, causing slugging, oil foaming, and oil dilution. All these conditions damage the compressor and rob the system of efficiency.
An advantage of the oil separator is that it helps insure adequate oil return to the compressor. It also improves system efficiency by minimizing the amount of oil in circulation and by keeping the oil at the compressor, where it belongs. However, it does not prevent or correct problems for systems that trap oil because of improper sizing or design. In these systems, the oil separator only delays the appearance of the problem.
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Refrigeration