AAON F-060 - Manual (Page 32)

32
suction line. Connect the drip line a
minimum of 1-inch above the bottom of the
drain leg.
Insulate the entire length of the hot gas line
with a minimum 1-inch thick Armaflex
insulation.
Hot Gas Reheat Guidelines
Maintain velocities below a maximum of
3500 fpm. A general minimum velocity
guideline is 2000 fpm.
Predetermined Line Sizes
To aid in line sizing and selection, AAON
has predetermined line sizes for the liquid
and suction lines in comfort cooling
applications.
In order to generate this information, the
following cycle assumptions are made:
Saturated suction temperature = 50°F,
Saturated condensing temperature = 125°F,
Sub-cooling = 10°F, Superheat = 15°F.
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 4000 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 practices.
How to use predetermined line sizing
First, read the previous section entitled
(When to use predetermined line sizing) to
decide if this method is applicable.
Next, consult Table 8 for pipe diameters.
Examine Figure 3 to determine the
acceptable line dimensions associated with
the pipe diameters determined in Table 8.
The figure is shown as total available riser
height versus total equivalent line length for
the liquid line. This 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 bypass 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,