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Table 4: Minimum capacity (kW) ensuring the oil flowing in vertical sections of the discharge line (R410a).
In this case also, the cooling capacity shown in table 4 refers to a saturated evaporating temperature equal to -5°C: for
different temperature values the capacity the capacities must be adjusted according to the following factors:
Table 4.a: Correction factors for different suction temperature
It might be guessed that in order 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 due to 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 consist 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 in order 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 in account in making sunction 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 evaporator in parallel, one independent from each other, and one of the two evaporator
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. If the two
evaporators are installed at different levels there are two possible solutions shown in figure 2.
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