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6.6.3 - Use of pipe sizing diagrams
On page 23 of this document two pipe sizing diagrams are
shown. They allow an estimate of the cooling capacity, corre-
sponding to 1.5 K pressure drop for different pipe diameters,
based on the pipe length.
The following procedure can be used for pipe sizing:
1. Measure the length (in metres) of the piping under
consideration.
2. Add 40 to 50% to take account of special characteristics.
3. Multiply this length by the appropriate correction
factor from Table 1 (this correction factor depends on
the saturated suction and discharge temperatures).
4. Read the pipe size from diagrams “Discharge piping”
and “Liquid line piping”.
5. Calculate the equivalent lengths for parts included in
the piping under consideration (such as valves, filters,
connections).
The equivalent lengths are normally available from the
component supplier. Add these lengths to the length
caculated in step 3.
6. Repeat steps 4 and 5 is necessary.
The diagrams in the appendix can obviously be used to
calculate the actual pressure drops for the piping under
consideration:
7. Based on the pipe diameter and the cooling capacity
find the equivalent length, producing 1.5 K pressure
drop in the figures “Discharge piping” and “Liquid
line piping”.
8. Calculate the equivalent pipe length as described in
steps 1, 2, 3 and 5.
9. Calculate the length ratio from steps 8 and 7 (equivalent
length from step 8 DIVIDED by the equivalent length
from step 7).
10. Multiply this ratio by 1.5 to find the equivalent pressure
drops in °C.
6.6.4 - Discharge pipe sizing
The discharge piping must be sized to achieve reasonable
pressure drops: a variation of 1.5 K of the saturated tempe-
rature is normally accepted (approx. 60 kPa variation for a
condensing temperature of 50°C).
For most applications the refrigerant gas velocity is sufficient
to entrain the liquid refrigerant/oil mixture. Nevertheless,
Table 2 shows the minimum required cooling capacities for
different pipe diameters and different saturated discharge
temperatures.
This table is based on 8 K superheat, a saturated suction
temperature of 4°C and 8 K subcooling. Table 3 shows the
correction factors to be applied to the values from Table 2, if
the operating conditions are different from those previously
stated.
6.6.5 - liquid line sizing
The 30HZV compressors are supplied with an oil that is
fully miscible with refrigerant R-407C in the liquid phase.
Consequently low refrigerant velocities in the liquid lines
are not a problem.
The admissible pressure drops in the liquid lines depend
mainly on the subcooling level of the liquid refrigerant at
the condenser outlet. Pressure drops corresponding to 1.5°C
saturated temperature must not be exceeded.
Special attention must be paid to the liquid line sizing when
the expansion device ist positioned higher than the conden-
ser. It may now be necessary to increase the pipe diameter
to compensate for the additional pressure of the liquid refri-
gerant column. If the liquid refrigerant head is very high, it
may even be necessary to increase the subcooling to prevent
a phase change in the liquid line. This can be done e.g. by a
liquid-vapour heat exchanger or an additional coil.
At 45°C the volume mass of refrigerant R-407C in the liquid
phase is approximately 1050 kg/m
3
. A pressure of 1 bar
corresponds to a liquid head of: 100 000/(1050 x 9.81) = 9.7 m.
