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The above value represents the cooling capacity for a line with Δext=28mm, at Tas=5°C e Tc=50°C and with a
corresponding pressure drop equal to 0,016 K/m.Since this capacity is lower than the cooling capacity supplied by
the unit (30 kW), the pipe diameter will be undersized. The Δext=35 mm will then be chosen. In order to verify if
the choice is correct, the total pressure drop must be calculated again by using the formula [2]. Consequently,
having chosen Δext=35 mm only the P value will change, as follows:
P=54,37x1,11=60,35 kW
And then:
As shown, the new total pressure drop is much more smaller than the maximum value suggested by the ASHRAE.
It should be noted that it's important to verify that the compressor oil return on the suction line is ensured.
A small oil amount tend to escape towards the refrigerant circuit, and therefore it's fundamental that even this small oil
amount could flow through the pipes reaching the compressor through the suction line. In this section, the oil can
flow by gravity if the pipe is oriented downward otherwise it is carried by the refrigerant if the pipe is perfectly
horizontal or even upward oriented.
To ensure a correct oil flowing the refrigerant speed shall not be lower than the minimum allowed value. This, however,
shall also be possible if the is working at partial load due to a reduced power demand from the plant.
In such cases, very common for unit of big capacity, it is necessary to size the plant according to the minimum
compressor capacity, and not in accordance to the full load capacity.
In Tables 3 and 4, the allowed capacities at partial load condition are shown, both for the suction and discharge lines.
Table 3: Minimum capacity (kW) ensuring the oil flowing in vertical sections of the suction line (R410a refrigerant).
The cooling capacity shown in table 3 refer to 40°C of condensing temperature: in case of different temperature values of
the liquid line, the capacities must be adjusted according to the following factors:
Table 3.a: Correction factors for different liquid temperature.
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