AAON F-060 - Manual (Page 29)

29
Determining Refrigerant Line Size
The piping between the condenser and low
side must assure:
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 expansion valve
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.
The pipe sizes must be selected to meet
the actual installation conditions, and not
simply based on the connection sizes at
the evaporator and/or condensing unit.
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 will lose head pressure. If the
evaporator section 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
installed in the liquid line by special order to
indicate the occurrence of premature
flashing or moisture in the line. The sight
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