SLD/DLD/PLD AIR COOLED CONDENSING UNITS
4
2) Practical pressure drop (usually
equivalent to 2 F maximum)
3) For proper oil return, all horizontal lines
should be sloped downwards toward
compressor at 1 inch per 10 feet.
Suction line risers must be trapped at the
bottom of the rise and at every 20-foot
increment for proper oil return. The proper
balance is to design suction lines for
approximately 1,200 ft. per min. velocity in
risers and approximately 600 ft per min.
velocity in horizontal lines. “P” traps should
be installed at the bottom of all risers in
suction lines for proper oil return.
Table 1 lists common suction line sizing
which can be used with-but not instead of -
ASHRAE- guidelines.
Suction lines should not be exposed to heat
unless insulated. Insulation also prevents
sweating and subsequent damage to goods.
B) LIQUID LINE
Avoid excessive liquid line pressure drop by
using Table 1 and ASHRAE guidelines to
size the liquid line. Install a liquid line
solenoid valve at the evaporator.
On parallel compressor systems, if one
compressor cycles off due to part load
conditions, suction lines must be sized to
allow for oil return under reduced load
conditions.
2) LINE FABRICATION &
SOLDERING
Copper pipe should be cut with a wheel type
cutter and not with a hacksaw. Using a
hacksaw produces copper filings which can
cause problems if it gets into the system.
Also, if the pipes to be used are not capped
and perfectly clean, they should be cleaned
with a clean lint free rag before fabrication
into the system.
Soft solders should be avoided wherever
possible, as in most cases they require the
use of a flux. Most of the low temperature
solder flux consists of heavy wax type
materials which if allowed to enter the
system cause excessive service problems in
the form of wax at expansion valves which
looks like moisture but cannot be removed
by the average dryer core. Where silver
brazing must be used between copper and
brass or copper and steel joints, care should
be taken to avoid excessive use of flux lest it
be introduced into the system to create
problems at a later date. Easy-Flo or silver
solders which contain sufficient silver
content to still retain joint strength and yet
require minimum use of flux are
recommended here.
For copper to copper joints phos-copper
solders with 4 to 15% silver content are
recommended. No flux is required, and the
resultant joints are of maximum strength
without brittleness.
Nitrogen should be used to exclude the
oxygen within the pipes during soldering in
order to prevent the creation of large
quantities of copper oxide. Copper oxide is
a good abrasive and if it gets into the
compressor it can cause excessive wear
and/or shorting out of electric motor
windings.
3) LEAK TESTING
When all refrigeration connections have
been completed, the entire system must be
tested for leaks. With all valves in the
system open, pressurize the system to no
more than 175 PSIG with refrigerant and dry
nitrogen. The use of an electronic type leak
detector is highly recommended.
III. SYSTEM EVACUATION
CAUTION:
DO NOT
use the
compressor as a vacuum pump.
DO NOT
start the
compressor while it is in vacuum.
