AAON CC-063 - Manual (Page 20)

20
The liquid lines have been chosen to
maintain velocities between 100 and 350
fpm. The suction line diameters are selected
to limit velocities to a 4,000 fpm maximum,
while a minimum velocity restriction is
imposed by the ability to entrain oil up
vertical suction risers (ASHRAE Handbook
- Refrigeration).
Acceptable pressure loss criteria are applied
to each of the lines: The total equivalent
length of the liquid line available is
determined such that 3°F of liquid sub-
cooling remain at the TXV. This includes
the pressure losses in horizontal and vertical
sections, accessories, elbows, etc.
Recall that the available sub-cooling for the
cycle is assumed as 10°F. To maintain at
least 3°F sub-cooling as a factor of safety to
avoid flashing at the TXV, we consider a
maximum pressure loss equivalent to a 7°F
change in saturation temperature. Pressure
losses in the suction line are not to exceed
2°F.
When to Use Predetermined Line Sizing
The line sizes presented are not the only
acceptable pipe diameters, they are however
appropriate for general comfort cooling
applications, and satisfy common job
requirements. Examine the conditions,
assumptions, and constraints used in the
generation of the predetermined pipe
diameters to ensure that this method is
applicable to a particular case. Do not
assume that these line sizes are appropriate
for every case. Consult ASHRAE
Handbook – Refrigeration for generally
accepted system piping practices.
How to Use Predetermined Line Sizing
First, read the previous section, When to Use
Predetermined Line Sizing, to decide if this
method is applicable. Next, consult Table 3
for pipe diameters. Examine Figure 3 to determine the
acceptable line dimensions associated with
the pipe diameters determined in Table 3.
The figure is shown as total available riser
height versus total equivalent line length for
the liquid line. The curve identifies a region
of acceptable piping configuration when the
predetermined line sizes are selected for any
model in the table. A piping configuration
above
the curve falls outside the
assumptions used to determine the line size
and will result in a loss of sub-cooling, and
additional pressure losses in the suction and
hot gas lines.
The total equivalent line length definition
includes the height of vertical rise, pressure
drop through elbows and accessories, and
horizontal
line length, so elbows,
accessories, and vertical rise must be
considered when determining horizontal
length available from the total equivalent
line length.
Figure 3 is presented in terms of the liquid
line, but it assumes that the suction line
length is similar, as these lines are
commonly routed together to minimize the
space and cost required for split system
installation.
Before using this table read the
When to Use Predetermined Line
Sizes section. Do not assume that
these line sizes are appropriate for
every case. Consult ASHRAE
Handbook
–
Refrigeration for
generally accepted system piping
practices. The AAON Refrigerant
Piping Calculator in Ecat32 can be
used for job specific line sizing.
CAUTION