AG 31-011 • REFRIGERANT PIPING DESIGN 30 www.DaikinApplied.com
Hot Gas Bypass Line Sizing
Hot gas piping should be sized using the discharge gas line
sizing tables found in Appendix 2 (
). It is best to
undersize hot gas bypass lines, keeping them as short as
possible, to limit the line volume. During OFF cycles, the vapor
refrigerant will condense and may create a slug of refrigerant
when the hot gas bypass valve opens. A rule of thumb is use
one line size smaller than the recommended discharge table
line size because hot gas bypass lines are short. Once the
line size is selected, the actual temperature and pressure drop
should be checked. The line pressure drop should be small
relative to the pressure drop across the valve. The line should
be pitched 1/8 inch per foot (10.4 mm/m) in the direction of
refrigerant flow.
The hot gas bypass valve and solenoid should be located as
close to the discharge line as possible. This will minimize the
amount of hot gas that may condense upstream of the valve
and solenoid.
The hot gas bypass line should be routed above the
evaporator and introduced to the ASC from the side to reduce
oil scavenging. The line should be insulated and a check valve
added if the ambient temperature is lower than the saturated
suction temperature.
Hot Gas Bypass Valves
Hot gas bypass (HGBP) valves used with distributor-type
DX coils should be externally equalized. Their purpose is to
maintain minimum suction pressure to the compressor. This is
best done when the valve is responding to suction pressure.
Over sizing the HGBP valve may cause:
• System inversion
• Loss of oil management
• Prevent the compressor from cycling OFF (overheating)
•
Poor efficiency
Hot gas valve selection is based on;
• Refrigerant type
•
Minimum allowable evaporating temperature at reduced
load – typically 32 to 34°F (0.0 to 1.1°C) for chillers and
26 to 28°F (-3.3 to -2.2°C) for air conditioners
• Minimum compressor capacity
• Minimum system capacity. For air conditioning
applications, minimum load with hot gas bypass use
should be limited to approximately 10% of a system’s
capacity. Some process applications will require
unloading down to zero
• Condensing temperature at minimum load – typically
80°F (26.7°C).
Hot gas bypass valves must be sized for the difference
between the minimum compressor capacity and the minimum
system capacity. If the minimum system capacity is zero, then
the hot gas bypass valve should be sized for the minimum
compressor capacity.
The example provided here is based on Sporlan products.
For other manufacturers, refer to their installation and
application guides.
Sporlan valves begin to open at approximately 6°F (3.3°C)
above the minimum evaporator temperature and remain open
at the rated capacity of the minimum evaporator temperature.
The actual pressure which the valve will open at depends on
the refrigerant.
When remote condensers are used, always layout and size the
condenser piping before selecting the HGBP valve. During light
loads, when the HGBP valve is open, the remaining velocity in
the discharge line may be so low that oil becomes trapped.
Figure 19: Hot Gas Bypass Accessories
Photos courtesy of Sporlan Division – Parker Hannifin Corporation
Aux SideConnector
Discharge Bypass Valve
Solenoid Valve
Discharge Bypass Valve