Leaving the compressor, the discharge line should be led downwards where possible. This will avoid accumulation of oil or liquid refrigerant in the discharge head of the compressor during idle periods. Where the condenser is above the compressor, it is necessary to have adequate velocity in the line from the compressor, or oil separator, to ensure that oil is carried forward. A non-return valve may be fitted as a safeguard to prevent oil flowing back. Horizontal lines should sloped downwards in the flow direction. Traces of oil, which enter the condenser, will settle on the cooling surfaces and fall to the bottom as a liquid or become dissolved in the condensed refrigerant. Either way, the two liquids will then pass to the expansion valve and into the evaporator. Here, the refrigerant will change to a vapour but most of the oil will remain as a liquid, containing dissolved refrigerant. Slight traces of oil pass out as a low-pressure vapour with the suction gas. It is necessary to limit the build-up of liquid oil in the evaporator, since it would quickly accumulate, reducing heat transfer and causing malfunction.
With ammonia, oil sinks to the bottom and does not go into solution with the refrigerant. Ammonia condensers, receivers and flooded evaporators can be distinguished by the provision of oil drainage pots and connections at the lowest point. Automatic drainage and return of the oil from these would have to depend on small different densities and is very rarely fitted. The removal of oil from collection pots and low-point drains is a periodic manual function and is carried out as part of the routine maintenance. Some ammonia systems are designed to return the oil by maintaining gas velocities, for example direct expansion and liquid overfeed evaporators. The halocarbons are all sufficiently miscible with oil to preclude the possibility of separate drainage and they are in any case rarely used with flooded evaporators.
The most common method of returning oil from the evaporator to the compressor is to keep it moving, by ensuring a minimum continuous fluid velocity in all parts of the circuit by using direct expansion evaporators. This dynamic circulation method is the decisive factor in the design of nearly all halocarbon evaporators.
The critical section of the circuit is where there is no liquid refrigerant left to help move the oil, i. e. the evaporator outlet and the suction line back to the compressor. Suction lines should be sloped downwards towards the compressor but not as to allow liquid to flood the compressor. Minimum gas velocities of about 3.5 m/s are required. Where the evaporator is below the compressor vertical sections which provide sufficient entrainment velocities, typically at least
7 m/s are required to ensure that oil droplets will be carried back by the dry refrigerant gas to the compressor. Figure 11.1 illustrates piping arrangements, which can be used to convey oil upwards.
In some situations a suction accumulator (see Chapter 9) is used to prevent large quantities of oil and/or refrigerant from suddenly entering the compressor on start up, or immediately after defrost for example. This system behaviour is termed liquid slugging and compressors are invariably designed to handle this to some extent.
With some small cooling circuits which can work at reduced gas flow when capacity controlled, it may not be possible to maintain the minimum velocity to carry oil back to the compressor, and it will settle in the circuit. This is particularly true with speed-controlled compressors where much reduced speed is allowed. Reversing refrigerant flow type circuits (i. e. cooling/heat pump) are another example of this. Arrangements must be made to increase or reverse the gas flow periodically to move this oil.
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