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AG 31-011 • REFRIGERANT PIPING DESIGN 32 www.DaikinApplied.com
How to Size a Hot Gas Bypass Line
Size the hot gas bypass line and valve for the following air
conditioner with a fin tube condenser.
The system:
• Uses R-407C
• Capacity is a 30 ton air conditioner with tandem scroll
compressors
• Minimum capacity is 5 tons (17.6 kW)
• Minimum compressor capacity of 15 tons (52.8 kW) or
one compressor
• Evaporator operates at 26°F (-3.3°C)
• Condenser operates at 120°F (48.9°C) that drops to 80°F
(26.7°C) during minimum load
• Equivalent length is 10 ft (3.0 m)
Step 1 – Estimate HGBP Valve Capacity
HGBP Valve = Minimum Compressor Capacity – Minimum System
Capacity
HGBP Valve = 15 Tons – 5 Tons = 10 Tons
(HGBP Valve = 52.8 kW – 17.6 kW = 35.2 kW)
Step 2 – Select a HGBP Valve
Table 3 on page 31 shows the Sporlan rating table for
ADRHE series of HGBP valves. Given a 10 ton capacity, 26°F
evaporator temperature, 80°F condensing temperature we can
see a ADRHE-6 can deliver 9.43 tons (33.1 kW) and can use a
5/8, 7/8, or 1-1/8 inch solder connection.
Step 3 – Estimate HGBP Piping Size
Table 10 on page 45 can be used to determine the hot
gas bypass line size for R-407C. For 10 tons 1-1/8 inch line
delivers 8.5 tons (29.8 kW) at 20°F (-6.67°C) SST and table
rating conditions. The equivalent length of this application is
only 10% of the table rating condition. A 1-1/8-inch (29 mm)
pipe will deliver much more capacity at such a short length.
Let’s consider a 7/8-inch (22 mm) line which delivers 4.2 tons
(14.7 kW).
Sizing the pipe for full load requires a correction for the 80°F
(26.7°C) actual condenser temperature. Referring to the
correction factors at the bottom of Table 10 ;
Actual Capacity = Table Capacity × 0.787
Actual Capacity = 4.2 Tons × 0.787 = 3.31 Tons
(Actual Capacity = 14.8 kW × 0.787 = 11.65 kW)
Step 4 – Calculate the Actual ∆T
Using Note #5 in the table, we can calculate the saturation
temperature difference based upon the actual design conditions.
Step 5 – Calculate the Actual Pressure Drop
The top of Table 10 shows the pressure drop for 1°F (0.56°C)
saturation temperature change with a 100 ft equivalent length
is 3.3 PSI.
A 7/8-inch (22 mm) line provides a satisfactory pressure
drop and keeps the line volume to a minimum. For point of
comparison, a 1-1/8 inch (29 mm) line would have provided a
pressure drop of 0.65 PSI (4.48 kPa). This would have been
an acceptable pressure drop, but the volume would have been
greater. A 5/8-inch (16 mm) line would have had a 13.5 PSI
(93.1 kPa) drop and the refrigerant velocity would have caused
excessive noise.
In addition to the HGBP valve we require:
• A 7/8 inch (22 mm) solenoid
•
An ASC for the distributor
• A new nozzle for the distributor
Recall that the HGBP valve begins to open at 6°F (3.3°C)
above SST, or in this case 32°F (0°C). By the time SST is 26°F
(-3.3°C) the HGBP valve will be passing the equivalent of 10
tons (35.2 kW) of R-407C refrigerant from the discharge line to
the inlet of the evaporator.
∆ T
Actual
= ∆ T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆ T
Actual
= 1°F
∆ T
Actual
= 0.56°C
= 0.732°F
10.0 ft
100.0 ft
10.0 Tons
31.1 Tons
1.8
= 0.40°C
3.0 m
30.5 m
35.2 kW
11.7 kW
1.8
HGBP Line — Step 4
Pressure Drop
Actual
= Pressure Drop
Table
∆ T
Actual
∆ T
Table
Pressure Drop
Actual
= 3.3 PSI
Pressure Drop
Actual
= 28.8 kPa
= 2.42 PSI
0.732°F
1°F
= 16.3 kPa
0.40°C
0.56°C
HGBP Line — Step 5
∆ T
Actual
= ∆ T
Table
Actual Length
Table Length
Actual Capacity
Table Capacity
1.8
∆ T
Actual
= 1°F
∆ T
Actual
= 0.56°C
= 0.732°F
10.0 ft
100.0 ft
10.0 Tons
31.1 Tons
1.8
= 0.40°C
3.0 m
30.5 m
35.2 kW
11.7 kW
1.8
HGBP Line — Step 4
Pressure Drop
Actual
= Pressure Drop
Table
∆ T
Actual
∆ T
Table
Pressure Drop
Actual
= 3.3 PSI
Pressure Drop
Actual
= 28.8 kPa
= 2.42 PSI
0.732°F
1°F
= 16.3 kPa
0.40°C
0.56°C
HGBP Line — Step 5