Jetson FWCD Series, Installation Operation & Maintenance, Page: 45 / 96

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expansion valve inlet is extremely low. On
start, the suction pressure may plunge into the
freezing range causing a nuisance fault. The
maximum ambient temperature limit for the
FWCD is 110°F.
Line Sizing Guidelines
Liquid Lines
Pressure drop should not be so large as to cause
gas formation in the liquid line, insufficient
liquid pressure and the liquid feed device, or
both. Systems are normally designed so that
pressure drop in the liquid line from friction is
not greater than that corresponding to 1 to 2°F
change in saturation temperature.
Sufficient sub-cooling must be maintained at
the expansion valve. To provide proper
operation throughout the range of operating
conditions, the liquid-line pressure drop should
not exceed the unit’s minimum sub-cooling
value less 5°F. To achieve this objective, keep
these liquid line considerations in mind:
1)
Select the smallest, practical line size for
the application. Limiting the refrigerant
charge improves compressor reliability.
2) When designing the liquid line for a typical
air conditioning application (i.e., one with
an operating range of 40°F to 110°F),
remember that every 10 feet of vertical rise
will reduce sub-cooling by 2.8°F, while
every 10 feet of vertical drop will add 1.1°F
of subcooling.
3) Provide a 1-inch pitch toward the
evaporator for every 10 feet of run.
4) If the liquid line must be routed through an
area warmer than outdoor air temperature,
insulate the line to prevent the refrigerant
from flashing. A liquid line filter drier must
be installed as close as possible to the
compressor chiller. The filter drier should
be changed whenever the system is opened
for service. FWCD Series compressor
chillers do not include a filter-drier as
standard, but one may be ordered if the
installing contractor desires a factory type.
5) A moisture-indicating sight glass permits a
visual check of the liquid column for
bubbles. Sight glasses are included on the
FWCD Series compressor-chiller. Never
use the sight glass to determine whether the
system is properly charged! Instead, either
charge the system based on the required
sub-cooling or calculate the amount of
refrigerant needed and add it based on
weight.
Discharge (Hot Gas) Line
Limit the pressure drop in the discharge line to
6 psig whenever possible to minimize the
adverse effect on unit capacity and efficiency.
While a pressure drop of as much as 10 psig is
usually permissible, note that a 6-psig pressure
drop reduces unit capacity by 0.9 percent and
efficiency by 3.0 percent.
Pitch discharge lines in the direction of hot gas
flow at the rate of 1/2-inch per each 10 feet of
horizontal run. Discharge line sizing is based
on required velocity to provide good oil
movement. Basic discharge line parameters
are:
•
Maximum allowable pressure drop 6
psig (°F)
•
Maximum Velocity 3500 fpm
•
Minimum Velocity (at minimum load)
o
Horizontal lines 500 fpm
o
Vertical lines (up flow) 1000
fpm
To design the discharge line properly, follow
the recommended guidelines:
1)
Choose the shortest route from the
compressor to the condenser.
2) Use different pipe sizes for horizontal and
vertical lines to make it easier to match line
pressure drop and refrigerant velocity to
discharge-line requirements.
3) To assure proper oil entrainment and avoid
annoying sound levels, size the discharge
line so refrigerant velocity equals or