The traps or separators prevent the oil from
draining back to the compressor head on shutdown.
Should a single compressor or multiple compressors
with capacity modulation be used in an instance of this
kind, another solution may be dictated. When a
compressor unloads, less refrigerant gas is pumped
through the system. The velocity of flow in the
refrigerant lines drops off as the flow decreases. It is
necessary to maintain gas velocities above some
minimum value to keep the entrained oil moving with
the refrigerant. The problem becomes particularly
acute in refrigerant gas lines when the flow is upward.
It does not matter whether the line is on the suction or
discharge side of the compressor; the velocity must not
be allowed to drop too low under low refrigerant flow
conditions. Knowing the minimum velocity, 1,000
feet per minute (fpm), for oil entrainment up a vertical
riser and the minimum compressor capacity, the
designer of the piping can overcome this problem
using a double riser.
The smaller line in the double riser is designed for
minimum velocity, at the minimum step, of
compressor capacity. The larger line is sized to assure
that the velocity in the two lines at full load is
approximately the same as in the horizontal flow lines.
A trap of minimum dimensions is formed at the bottom
of the double-riser assembly, which collects oil at
minimum load. Trapped oil then seals off the larger
line so the entire flow is through the smaller line.
If an oil separator is used at the bottom of a
discharge gas riser, the need for a double riser is
eliminated. The oil separator will do as its name
impliesseparate the major part of the oil from the gas
flowing to it and return the oil to the compressor
crankcase. Since no oil separator is 100 percent
effective, the use of an oil separator in the discharge
line does not eliminate the need for double risers in the
suction lines of the same system if there are vertical
risers in the suction lines. When multiple compressors
with individual condensers are used, the liquid lines
from the condenser should join the common liquid line
at a level well below the bottoms of the condensers.
The low liquid line prevents gas from an "empty"
condenser from entering the line because of the seal
formed by the liquid from other condensers.
NOTE: A common water-regulating valve should
control the condenser water supply for a multiple
system using individual condensers, so each condenser
receives a proportional amount of the condenser water.
Frequently, when multiple compressors are
installed, only one condenser is provided. Such
installations are satisfactory only as long as all of the
compressors are operating at the same suction
pressure. However, several compressors may
occasionally be installed which operate at different
suction pressuresthe pressures corresponding, of
course, to the various temperatures needed for the
different cooling loads. When this is the case, a
separate condenser must be installed for each
compressor or group of compressors operating at the
same suction pressure. Each compressor, or group of
compressors, operating at one suction pressure must
have a complete piping system with an evaporator and
condenser, separate from the remaining compressors
operating at other suction pressures. Separate systems
are required because the crankcase of compressors
operating at different suction pressures cannot be
interconnected. There is no way of equalizing the oil
return to such compressors.
The suction connection to a multiple compressor
system should be made through a suction manifold, as
shown in figure 6-47. The suction manifold should be
as short as possible and should be taken off in such a
manner that any oil accumulating in the header returns
equally to each machine.
Evaporative condensers can be constructed with
two or more condensers built into one spray housing.
This is accomplished quite simply by providing a
separate condensing coil for each compressor, or a
group of compressors, operating at the same suction
pressure. All of the condensing coils are built into one
spray housing; this provides two or more separate
condensers in one condenser housing.
What type of acid is formed when R-12 is mixed
Air-cooled condensers should be located in
areas that provide plenty of clear space around
them for what reason?
On close-coupled systems, running refrigerant
lines up to the overhead helps eliminate what
To eliminate possible oxidation from occurring
while conducting soldering or brazing
operations, you should ensure what condition
exists within the tube or pipe?
U-traps or oil separators should be installed on
multiple compressor systems when the
condensers are how many feet above the