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It might be guessed that to guarantee the proper speed ensuring the correct oil flow, if the compressor is working at partial loads, the
diameter shall be reduced, therefore a too much high speed is obtained when the compressor is working at full load.
This is cause of excessive pressure drops.
Thus, the refrigerant line sizing, results often in a problem of balance between two opposite needs. In some cases (especially when
the suction line is upward oriented), this balance cannot be easily reached, and therefore it is necessary to use a precaution on the
plant, known as a "dual inclination".
The dual inclination consists in installing two parallel lines, one of each (marked as "A" in fig. 1) is sized in order to allow the oil return
at the minimum capacity, the other one (marked as "B") is sized to ensure that the sum of the two sections is at least equal to that of a
single line dimensioned in accordance to the maximum cooling capacity.
Between the two vertical sections a sump is realized. The sump is sized to guarantee that at minimum loads, it will be filled with oil
closing the entrance to the line "B".
The sump, however, must not contain excessive amounts of oil, but it must have a minimum volumetric capacity. When the cooling
capacity grows, the greater flow creates a higher pressure allowing to overcome the oil cap and therefore also the line "B" works.
There are two other important precautions to be taken into account in making suction lines an oil trap 6 meters shall be created or a
small downward line inclination shall be installed in case of horizontal sections, to ensure a better oil flow.
If the circuit is equipped with two evaporators in parallel, one independent from each other, and one of the two evaporators is not
working, oil accumulation must be avoided anyway.
If the two evaporators are installed at the same level a downward line on both evaporators outlet shall be created, before joining the
common section.
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