AAON CB-024, Installation Operation & Maintenance, Page: 21 / 56

21
A double riser used for heat pump operation
works differently than that described above.
The specific volume (ft
3
/lb) of refrigerant at
the discharge temperature (heating mode
line conditions) is significantly lower than
the
specific volume at the suction
temperature (cooling mode line conditions).
To compound the issue, the capacity in
heating mode is lower than the capacity in
cooling mode. The discharge velocity in the
riser during heating mode is much lower
than the suction velocity during cooling
mode. Often, a double riser is necessary to
get acceptable velocities for the discharge
mode and acceptable velocities for the
suction mode. A solenoid valve must be
added to the second riser. The solenoid
valve must be wired to the reversing valve
so that the solenoid valve opens in cooling
mode, and closes in heating mode. See the
diagrams below:
Figure 4 – Heat Pump Piping Schematic of
Suction Vapor Flow Down in Double Riser
Figure 5 – Heat Pump Piping Schematic of
Discharge Vapor Flow Up in Double Riser
Suction Line Insulation
The entire suction line should be insulated
with a minimum 1 inch thick Armaflex
insulation. This prevents condensation from
forming on the line, and reduces any
potential loss in capacity associated with
heat gain placing additional load on the
system.
Suction Line Guidelines
For proper performance, suction line
velocities less than a 4,000 fpm maximum
are recommended. The minimum velocity
required to return oil is dependent on the
pipe diameter, however, a general guideline
of 1,000 fpm minimum may be applied.
When suction flow is up, variable capacity
compressors require a minimum velocity of
2,000 fpm.
Heat pump vapor lines must be checked for
suction flow (cooling mode operation) and
discharge flow (heating mode operation).
The same line must be used for both modes
of operation.