AAON M2-036 - Manual (Page 41)

41
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.
Liquid refrigerant traveling upwards in a
riser loses head pressure. If the evaporator is
below the condenser, and the liquid line
does not include risers, the gravitational
force will increase the pressure of the liquid
refrigerant. This will allow the refrigerant to
withstand greater frictional losses without
the occurrence of flashing prior to the TXV.
A moisture-indicating sight glass may be
field installed in the liquid line to indicate
the occurrence of premature flashing or
moisture in the line. The sight glass should
not be used to determine if the system is
properly charged. Use temperature and
pressure measurements to determine
liquid sub-cooling, not the sight glass.
Liquid Line Routing
Care should be taken with vertical risers.
When the system is shut down, gravity will
pull liquid down the vertical column, and
back to the condenser when it is below the
evaporator. This could potentially result in
compressor flooding. A check valve can be
installed in the liquid line where the liquid
column rises above the condenser to prevent
this. The liquid line is typically pitched
along with the suction line, or hot gas line,
to minimize the complexity of the
configuration.
Liquid Line Insulation
When the liquid line is routed through
regions where temperature losses are
expected, no insulation is required, as this