AAON V3-E - Manual (Page 28)

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Modulating Hot Gas Reheat Piping:
1. Run a hot gas reheat line from the
condensing unit and connect it to the inlet of
the stub-out on the reheat coil. The inlet
connection is the top (or highest) stub-out of
the reheat coil. Connect the hot gas line
from the outdoor unit to the upper stub-out
connection of the reheat coil.
2. Run a liquid line from the discharge of the
reheat coil through a tee connection. Run a
liquid line from the condenser, through a
check valve to the other side of the tee. Run
a liquid line from the tee to the liquid line
stub-out of the evaporator coil.
3. Run a suction line from the evaporator
coil outlet stub-out to the condensing unit.
Determining Refrigerant Line Size
The piping between the condenser and low
side must ensure:
1. Minimum pressure drop, and
2. Continuous oil return, and
3. Prevention of liquid refrigerant slugging,
or carryover
Minimizing the refrigerant line size is
favorable from an economic perspective,
reducing installation costs, and reducing the
potential for leakage. However, as pipe
diameters
narrow, pressure-reducing
frictional forces increase.
Excessive suction line pressure drop causes
loss of compressor capacity and increased
power usage resulting in reduced system
efficiency. Excessive pressure drops in the
liquid line can cause the liquid refrigerant to
flash, resulting in faulty TXV operation and
improper system performance. In order to
operate efficiently and cost effectively,
while avoiding malfunction, refrigeration
systems must be designed to minimize both
cost and pressure loss.
Equivalent Line Length
All line lengths discussed in this manual,
unless specifically stated otherwise, are
Equivalent Line Lengths. The frictional
pressure drop through valves, fittings, and
accessories is determined by establishing the
equivalent length of straight pipe of the
same diameter. Always use equivalent line
lengths when calculating pressure drop.
Special piping provisions must be taken
when lines are run underground, up vertical
risers, or in excessively long line runs.
Liquid Line Sizing
When sizing the liquid line, it is important to
minimize the refrigerant charge to reduce
installation costs and improve system
reliability. This can be achieved by
minimizing the liquid line diameter.
However, reducing the pipe diameter will
increase the velocity of the liquid refrigerant
which increases the frictional pressure drop
in the liquid line, and causes other
undesirable effects such as noise.
Maintaining the pressure in the liquid line is
critical to ensuring sufficient saturation
temperature, avoiding flashing upstream of
the
TXV, and maintaining system
efficiency. Pressure losses through the
liquid line due to frictional contact, installed
accessories,
and vertical risers are
inevitable. Maintaining adequate sub-
cooling at the condenser to overcome these
losses is the only method to ensure that
liquid refrigerant reaches the TXV.
Line sizes must be selected to meet
actual installation conditions, not
simply based on the connection sizes
at the condensing unit or air handling
unit.
CAUTION