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with symbols having the same meaning as in formula [1]
In order to verify if the selected diameter could be correct the following maximum pressure drops shall be considered according to
ASHRAE:
Liquid line: ΔPmax = 0,5÷1 K
Suction line: ΔPmax = 1 K
Discharge line: ΔPmax = 1 K
Example:
How to choose the proper tube diameter.
Input data:
Equivalent length calculated: Leq=60 m
Saturated suction temperature: Tas=5°C
Condensing temperature: Tc=50°C
Cooling capacity at the evaporator: Pf=30 kW
Since the values of cooling capacity indicated in Table 1 are relative to pressure drop values equal to 0.02 K / m , first check the
pressure drops that would occur on the line using the diameter corresponding to the cooling capacity of the unit. The equivalent
length is about 60 meters, then with a pressure drops of 0.02 K/m the total pressure drops are equal to: T = 60x0,02 = 1.2 K
These pressure drops are higher than the maximum specified by ASHRAE. To comply with the recommended value (K1) it is
necessary to reduce the pressure drops length down to 0.016 K/m, using the formula [1].
Refer to Table 1 and to search in the discharge line column (with suction temperature equal to 5 ° C) a cooling capacity close to the
unit one.
P = 29.7 kW at line with Δext = 28mm.
This capacity is relative to a condensing temperature of 40 °C. To obtain the corresponding 50 °C capacity, this value shall be
multiplied to a factor shown in the table 2a:
(discharge line, Tc = 50 ° C) factor = 1.11.
The cooling capacity delivered by the line with Δext = 28 mm (corresponding to pressure drops of 0.02 k/m) is therefore P = 29,7
x1,11 = 32.97 kW.
In conclusion, the data we need to use the formula [1] are the following:
ΔT=0,016x60=0,96 K
ΔTtab=0,02 K
Le=1m
Leff=60m
P=32,97 kW
As a consequence:
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 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 tends to escape towards the refrigerant circuit, and therefore it is 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.
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