AG 31-011 • REFRIGERANT PIPING DESIGN 24 www.DaikinApplied.com
How to Size Suction Lines (continued)
Step 5 – Confirm Oil Return at Minimum Load in the Riser
Calculate the minimum capacity.
Min Capacity = Capacity
Full
× Turn Down
Min Capacity = 50.0 Tons × 0.2 = 10 Tons
Actual Refrigerant Temperature = SST Temperature + Superheat
Temperature
Actual Refrigerant Temperature = 40°F + 10°F = 50°F
Using
50°F (10°C) refrigerant temperature the minimum allowable
capacity is 15.7 tons (55.2 kW). The table is based on 90°F
(32.2°C) condensing temperature. The bottom of
has
correction factors for other condensing temperatures.
Min Allowable Capacity
Actual
= Min Allowable Capacity
Table
× Correction
Factor
Min Allowable Capacity
Actual
= 15.7 Tons × (0.8) = 12.6 Tons
(Min Allowable Capacity
Actual
= 52.2 kW × (0.8) = 44.16 kW
Since the Min allowable capacity (12.6 tons) is greater than the
minimum capacity (10 tons), a 3-1/8 inch (79 mm) suction pipe
is too big for minimum flow in a riser. A minimum capacity of
25 tons (88 kW) (for example, two tandem scroll compressors)
would have worked with this riser.
The solution is to reduce the riser pipe one size and repeat
Step 5 to confirm minimum condition is met.
We decrease the riser pipe to 2-5/8 inches (67mm) while
leaving the horizontal pipes at 3-1/8 inches. Using
we check the minimum capacity of a 2-5/8 inch (67 mm) riser.
According to the table, the minimum allowable capacity is 10.1
tons (35.5 kW) at 90°F (32.2°C) condenser temperature.
Min Allowable Capacity
Actual
= Min Allowable Capacity
Table
×
Correction Factor
Min Allowable Capacity
Actual
= 10.1 Tons × (0.8) = 8.1 Tons
The minimum allowable capacity is now less than the
minimum capacity so a 2-5/8 inch (97 mm) riser is sufficient
for this system.
Step 6 – Calculate the Suction Line Pressure Drop with the
New Riser Size
Suction line pressure drop is the sum of the 3-1/8 inch (79 mm)
horizontal piping and the 2-5/8 inch (97 mm) vertical piping.
The equivalent length of the vertical pipe is given at 42 ft (12.8 m).
, the capacity for a 2-5/8 inch
(97 mm) line is 35.8 tons (125.87 kW). To calculate the vertical
pipe suction line temperature drop use Note #3 in
.
Corrected Vertical Suction Line Capacity = Table Capacity × 0.902
Corrected Vertical Suction Line Capacity = 35.8 Tons × 0.902 = 32.3 Tons
shows the pressure drop for 40°F saturation
temperature change with a 100 ft equivalent length is 1.93 PSI
(13.3 kPa).
The same approach is used again to calculate the horizontal
3-1/8" piping. In this case the equivalent length of horizontal
piping was 22 ft (6.7m).
Pressure Drop
Total
= Pressure Drop
Vertical
+ Pressure Drop
Horizontal
Pressure Drop
Total
= 1.78 PSI + 0.41 PSI = 2.19 PSI
(Pressure Drop
Total
= 12.21 kPa + 2.78 kPa = 14.99 kPa)
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.2°F
64.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.67°C
19.5 m
30.5 m
176 kW
181 kW
1.8
Suction Line — Step 3
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 0.42°F
22.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.23°C
6.7 m
30.5 m
176 kW
181 kW
1.8
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.84°F
42.0 ft
100.0 ft
50.0 Tons
32.3 Tons
1.8
= 1.01°C
12.8 m
30.5 m
175.9 kW
113.6 kW
1.8
Suction Line — Step 6
Pressure Drop
Actual
= Pressure Drop
Table
∆
T
Actual
∆
T
Table
Pressure Drop
Actual
= 1.93 PSI
Pressure Drop
Actual
= 13.3 kPa
= 1.16 PSI
1.2°F
2°F
= 8.1 kPa
0.67°C
1.1°C
Suction Line — Step 4
Pressure Drop
Actual Vertical
= Pressure Drop
Table
∆
T
Actual Vertical
∆
T
Table
Pressure Drop
Actual Vertical
= 1.93 PSI
Pressure Drop
Actual Vertical
= 13.3 kPa
= 1.78 PSI
1.84°F
2°F
= 12.21 kPa
1.01°C
1.1°C
Pressure Drop
Actual Horizontal
= 1.93 PSI
Pressure Drop
Actual Horizontal
= 13.3 kPa
= 0.41 PSI
0.42°F
2°F
= 2.78 kPa
0.23°C
1.1°C
Pressure Drop from the Riser = Pressure Drop ×
Total Pressure Drop = Actual Pressure Drop + Riser Pressure Drop
Total Pressure Drop = 3.23 PSI + 8.6 PSI = 11.83 PSI
(Total Pressure Drop = 59.35 kPa + 22.81 kPa = 82.16 kPa)
Saturated Pressure
TX Valve
= Saturated Pressure
120°F
– Total Pressure Drop
Saturated Pressure
TX Valve
= 433.0 PSIA + 11.83 PSIA = 421.17 PSIA
(Saturated Pressure
TX Valve
= 2985.0 kPa + 82.15 kPa = 2902.85 kPa)
Refrigerant Pressure Drop
ft
Pressure Drop from the Riser = 20.0 ft ×
Pressure Drop from the Riser = 6.1 m ×
= 8.6 PSI
0.43 PSI
ft
= 259.35 kPa
9.73 kPa
m
Liquid Line — Step 4
Liquid Line — Step 5
Subcooling = Actual Saturation Temperature – Saturation Temperature
TX Valve
Subcooling = 120.0°F – 117.8°F = 2.2°F
Liquid Line — Step 7
Subcooling Requirement = TX Valve Temperature + Minimum System Temperature
Subcooling Requirement = 2.2°F + 4.0°F = 6.2°F
Liquid Line — Step 8
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.2°F
64.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.67°C
19.5 m
30.5 m
176 kW
181 kW
1.8
Suction Line — Step 3
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 0.42°F
22.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.23°C
6.7 m
30.5 m
176 kW
181 kW
1.8
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.84°F
42.0 ft
100.0 ft
50.0 Tons
32.3 Tons
1.8
= 1.01°C
12.8 m
30.5 m
175.9 kW
113.6 kW
1.8
Suction Line — Step 6
Pressure Drop
Actual
= Pressure Drop
Table
∆
T
Actual
∆
T
Table
Pressure Drop
Pressure Drop
Actual
= 13.3 kPa
= 1.16 PSI
1.2°F
2°F
= 8.1 kPa
0.67°C
1.1°C
Suction Line — Step 4
Pressure Drop
Actual Vertical
= Pressure Drop
Table
∆
T
Actual Vertical
∆
T
Table
Pressure Drop
Actual Vertical
= 1.93 PSI
Pressure Drop
Actual Vertical
= 13.3 kPa
= 1.78 PSI
1.84°F
2°F
= 12.21 kPa
1.01°C
1.1°C
Pressure Drop
Actual Horizontal
= 1.93 PSI
Pressure Drop
Actual Horizontal
= 13.3 kPa
= 0.41 PSI
0.42°F
2°F
= 2.78 kPa
0.23°C
1.1°C
Pressure Drop from the Riser = Pressure Drop ×
Total Pressure Drop = Actual Pressure Drop + Riser Pressure Drop
Total Pressure Drop = 3.23 PSI + 8.6 PSI = 11.83 PSI
(Total Pressure Drop = 59.35 kPa + 22.81 kPa = 82.16 kPa)
Saturated Pressure
TX Valve
= Saturated Pressure
120°F
– Total Pressure Drop
Saturated Pressure
TX Valve
= 433.0 PSIA + 11.83 PSIA = 421.17 PSIA
(Saturated Pressure
TX Valve
= 2985.0 kPa + 82.15 kPa = 2902.85 kPa)
Refrigerant Pressure Drop
ft
Pressure Drop from the Riser = 20.0 ft ×
Pressure Drop from the Riser = 6.1 m ×
= 8.6 PSI
0.43 PSI
ft
= 259.35 kPa
9.73 kPa
m
Liquid Line — Step 4
Liquid Line — Step 5
Subcooling = Actual Saturation Temperature – Saturation Temperature
TX Valve
Subcooling = 120.0°F – 117.8°F = 2.2°F
Liquid Line — Step 7
Subcooling Requirement = TX Valve Temperature + Minimum System Temperature
Subcooling Requirement = 2.2°F + 4.0°F = 6.2°F
Liquid Line — Step 8
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.2°F
64.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.67°C
19.5 m
30.5 m
176 kW
181 kW
1.8
Suction Line — Step 3
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 0.42°F
22.0 ft
100.0 ft
50.0 Tons
51.5 Tons
1.8
= 0.23°C
6.7 m
30.5 m
176 kW
181 kW
1.8
∆
T
Actual
=
∆
T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆
T
Actual
= 2°F
∆
T
Actual
= 1.1°C
= 1.84°F
42.0 ft
100.0 ft
50.0 Tons
32.3 Tons
1.8
= 1.01°C
12.8 m
30.5 m
175.9 kW
113.6 kW
1.8
Suction Line — Step 6
Pressure Drop
Actual
= Pressure Drop
Table
∆
T
Actual
∆
T
Table
Pressure Drop
Actual
= 1.93 PSI
Pressure Drop
Actual
= 13.3 kPa
= 1.16 PSI
1.2°F
2°F
= 8.1 kPa
0.67°C
1.1°C
Suction Line — Step 4
Pressure Drop
Actual Vertical
= Pressure Drop
Table
∆
T
Actual Vertical
∆
T
Table
Pressure Drop
Actual Vertical
= 1.93 PSI
Pressure Drop
Actual Vertical
= 13.3 kPa
= 1.78 PSI
1.84°F
2°F
= 12.21 kPa
1.01°C
1.1°C
Pressure Drop
Actual Horizontal
= 1.93 PSI
Pressure Drop
Actual Horizontal
= 13.3 kPa
= 0.41 PSI
0.42°F
2°F
= 2.78 kPa
0.23°C
1.1°C
Pressure Drop from the Riser = Pressure Drop ×
Total Pressure Drop = Actual Pressure Drop + Riser Pressure Drop
Total Pressure Drop = 3.23 PSI + 8.6 PSI = 11.83 PSI
(Total Pressure Drop = 59.35 kPa + 22.81 kPa = 82.16 kPa)
Saturated Pressure
TX Valve
= Saturated Pressure
120°F
– Total Pressure Drop
Saturated Pressure
TX Valve
= 433.0 PSIA + 11.83 PSIA = 421.17 PSIA
(Saturated Pressure
TX Valve
= 2985.0 kPa + 82.15 kPa = 2902.85 kPa)
Refrigerant Pressure Drop
ft
Pressure Drop from the Riser = 20.0 ft ×
Pressure Drop from the Riser = 6.1 m ×
= 8.6 PSI
0.43 PSI
ft
= 259.35 kPa
9.73 kPa
m
Liquid Line — Step 4
Liquid Line — Step 5
Subcooling = Actual Saturation Temperature – Saturation Temperature
TX Valve
Subcooling = 120.0°F – 117.8°F = 2.2°F
Liquid Line — Step 7
Subcooling Requirement = TX Valve Temperature + Minimum System Temperature
Subcooling Requirement = 2.2°F + 4.0°F = 6.2°F
Liquid Line — Step 8