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AG 31-011 • REFRIGERANT PIPING DESIGN 26 www.DaikinApplied.com
Discharge Line Sizing
Discharge lines contain gaseous refrigerant that moves the
oil along the piping back towards the compressor. Oversized
discharge lines increase the initial cost and can reduce the
refrigerant gas velocity to a point where oil is not returned to
the compressor. Undersized discharge lines reduce system
capacity. Oil movement in discharge lines is further complicated
by risers, where gravity is working against oil return.
How to Size a Discharge Line
Size minimum capacity discharge line for a single riser and
the pressure drop for the following indoor process chiller with
remote air-cooled condenser.
The system:
• Uses R-22
• Has Type L copper pipe
• Evaporator operates at 20°F (-6.7°C) Saturated Suction
Temperature
• Superheat is 15°F (5.6°C)
• Condenser operates at 110°F (48.9°C)
• Discharges at 140°F (60°C)
•
Capacity is 250 tons (176 kW) circuits with up to 33%
turn down
• Discharge line equivalent length for the horizontal runs is:
— Bottom 15 ft (4.6m)
— Top 10 ft (3.0m)
• Single pipe riser discharge line equivalent is 110 ft
(33.5m)
Step 1 – Estimate the Discharge Line Size
To determine the discharge line pipe size for a 250 ton (211 kW)
unit use Table 7 on page 42 in Appendix 2. According to the
table, a 4-1/8 inch (105 mm) pipe will work for a 276.1 ton (970
kW) unit with 20°F (-6.7°C) Saturated Suction Temperature.
Note, the table conditions (equivalent length and condensing
temperature) are different than the design conditions.
Step 2 – Correct For Actual Operating Conditions
Sizing the pipe for full load requires a correction for the 110°F
(43.3°C) actual condenser temperature. Referring to the
correction factors at the bottom of Table 7 .
Actual Capacity = Table Capacity × 1.04
Actual Capacity = 276.1 Tons × 1.04 = 287 Tons
(Actual Capacity = 970 kW × 1.04 = 1009 kW)
Step 3 – Calculate the Actual ∆T
Using Note #5 in the table, we can calculate the saturation
temperature difference based upon the actual design conditions.
Air Cooled
Condenser
Discharge line
inverted trap
(can be replaced
with a check valve)
Discharge
Line
Liquid
Line
Riser
Discharge
riser trap
only at base
Hot gas bypass
top connection
to avoid refrigerant
collection
Sight
Glass
TX
Valve
Solenoid
Valve
Filter-
Drier
Chiller
Discharge
Line
∆ T
Actual
= ∆ T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆ T
Actual
= 1°F
∆ T
Actual
= 0.56°C
= 0.86°F
110.0 ft
100.0 ft
250.0 Tons
287.0 Tons
1.8
= 0.48°C
35.5 m
30.5 m
879 kW
1009 kW
1.8
Discharge Line — Step 3
Pressure Drop
Actual
= Pressure Drop
Table
∆ T
Actual
∆ T
Table
Pressure Drop
Actual
= 3.05 PSI
Pressure Drop
Actual
= 21.03 kPa
= 2.62 PSI
0.86°F
1°F
= 18.03 kPa
0.48°C
0.56°C
Liquid Line — Step 4