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Installing
Refrigeration Equipment
| Technicians are often tasked to installation
refrigeration systems. Therefore, it is important for you
to understand the basic requirements applicable to the installation
of the various types of the equipment.
When installing a refrigeration or air-conditioning plant,
you must not allow dirt, scale, sand, or moisture to enter
any part of the refrigerant system. Since air contains moisture,
its entrance into the circuit should be controlled as much
as possible during installation.
Most maintenance problems come from careless erection and
installation. All openings to the refrigerant circuit—piping,
controls, compressor, condensers, and so on—must be
adequately sealed when work on them is not in progress.
The R-12 refrigerant is a powerful solvent that readily
dissolves foreign matter and moisture that may have entered
the system during installation. This material is soon carried
to the operating valves and the compressor. It becomes a
distinct menace to bearings, pistons, cylinder walls, valves,
and the lubricating oil. Scoring of moving parts frequently
occurs when the equipment is first operated, starting with
minor scratches that increase until the operation of the
compressor is seriously affected.
Under existing specifications, copper tubing and copper
piping needed for installation should be cleaned, deoxidized,
and sealed. When there is a question about cleanliness of
tubing or piping to be used, each length of pipe should
be thoroughly blown out. Use a strong blast of dry air when
blowing out, and clean the tubing with a cloth swab attached
to copper wire pulled back and forth in the tube until it
is clean and shiny. Then the ends of the tubes should be
sealed until connected to the rest of the system. |
Effects
of Moisture
As little as 15 to 20 parts of moisture per million parts
of R-12 can cause severe corrosion in a system.
The corrosion results from hydrochloric acid formed by
R-12 in contact with water. A chemical reaction takes place
between the acid and the iron and copper in 6-28.
the system to form corrosion products. A strong acid combined
with high discharge and compressor temperature can cause
decomposition of lubricating oil and produce a sludge of
breakdown products.
Either the corrosion or the oil breakdown products can
plug valves, strainers, and dryers and cause a serious casualty.
NOTE: The formation of ice from a minute quantity
of moisture in expansion valves and capillary tubes can
occur when operating below 32°F.
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Location
of Equipment
Adequate space should always be left around major portions
of equipment for servicing purposes; otherwise, the equipment
must be moved after installation so serviceable parts are
accessible. Compressors require overhead clearance for removal
of the head, discharge valve plate, and pistons with side
clearance to permit removal of the flywheel and crankshaft
where necessary. Water-cooled condensers require a free area
equal to the length of the condenser at one end to provide
room for cleaning tubes, installing new tubes, or removal
of the condenser tube assembly. Space is needed for servicing
valves and accessory equipment.
A low-temperature screw or helix compressor system. (1)
Compressor; (2) Oil separator and reservoir; (3) Oil collar;
(4) Oil filters (5) Hot -gas discharge line.
Service openings and inspection panels on unitary equipment
require generally at least 18 inches of clearance for removal
of the panel. Air-cooled condensing units should be placed
in a location that permits unrestricted flow of air for
condensing, whether the condenser is in a unitary piece
of equipment or separate. Inadequate ventilation around
air-cooled condensers can cause overloading of the motor
and loss of capacity.
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Refrigeration
Piping
Certain general precautions for the installation of refrigerant
lines should be followed. When the receiver is above the cooling
coil, the liquid line should be turned up before going down
to the evaporator. This inverted loop prevents siphoning of
the liquid from the receiver over into the cooling coil through
an open or leaking expansion valve during compressor shutdown
periods. If siphoning starts, the liquid refrigerant flashes
into a gas at the top of the loop, breaking the continuity
of the liquid volume and stopping the siphoning action. Where
the cooling coils and compressors are on the same level, both
the suction and liquid lines should be run to the overhead
and then down to the condensing unit, pitching the suction
line toward the compressor to ease oil return. On close-coupled
installations, running both lines up to the overhead helps
to eliminate vibration strains as well as provide the necessary
trap at the cooling coil.
Prepare pipe and fittings with care, particularly when
cutting copper tubing or pipe to prevent filings or cuttings
from entering the pipe. The small particles of copper should
be completely removed since the finely divided copper may
pass through the suction strainer. The tube should be cut
square, and all burrs and dents should be removed to prevent
internal restrictions and to permit proper fit with the
companion fittings. If a hacksaw is used to cut, a fine-toothed
blade should be used, preferably 32 teeth per inch. The
use of a hacksaw should be avoided whenever possible. When
making silver-solder joints, brighten up the ends of the
tubing or pipe with a wire brush or crocus cloth to make
a good bond. Do not use sandpaper, emery cloth, or steel
wool for this cleansing, as this material may enter the
system and cause trouble.
Acid should never be used for soldering, nor should flux
be used if its residue forms an acid. Use flux sparingly
so no residue will enter inside the system and eventually
be washed back to the compressor crankcase. If tubing and
fittings are improperly fitted because of distortion, too
much flux, solder, and brazing material may enter the system.
The temperature required to solder or braze pipe joints
causes oxidation within the tubing. The oxidation eventually
will be removed by the refrigerant flow after the system
is in operation. The oxide breaks up into a fine powder
to contaminate the lubricant in the compressor and to plug
strainers and driers. To eliminate this possibility, provide
a neutral atmosphere within the tube being soldered or brazed.
Use gas-bled nitrogen through the tubing during soldering
or brazing and for a sufficient time after the bond is made
until the heat of the copper has been reduced below the
temperature of oxidation.
All joints should be silver-soldered and kept to a minimum
to reduce leaks. Special copper tube fittings designed for
refrigeration service should be used since these are manufactured
with close tolerances to assure tight capillary joints in
the brazing process.
SAE flare joints are generally not desired, but when necessary,
care should be taken in making the joint. The flare must
be of uniform thickness and should present a smooth, accurate
surface, free from tool marks, splits, or scratches. The
tubing must be cut square, provided with a full flare, and
any burrs and saw filings removed. The flare seat of the
fitting connector must be free from dents or scratches.
The flare can best be made with a special swivel head flaring
tool, available as a general stores item, which remains
stationary and does not tear or scar the face of the flare
in the tubing. Oil should not be used on the face of the
flare, either in making up the flare or in securing it to
the fitting, since the oil will eventually be dissolved
by the refrigerant in the system and cause a leak through
the displacement of the oil. The flare joint should always
be tightened with two wrenches—one to turn the nut
and the other to hold the connecting piece to avoid strain
on the connection and cause a leak.
Where pipe or tubing has to be bent, bends should be made
with special tools designed for this type of work. Do not
use rosin, sand, or any other filler inside the tubing to
make a bend. Threaded joints should be coated with a special
refrigerant pipe dope. In an emergency, use a thread compound
for making up a joint; remember R-12 and R-22 are hydrocarbons,
which dissolve any compound containing oil. A compound containing
an acid or one whose residual substance forms an acid should
not be used. The use of a thick paste made of fresh lethargy
and glycerin makes a satisfactory joint compound; however,
the joint should be thoroughly cleaned with a solvent to
eliminate oil or grease. Thread compounds should be applied
to the male part of the thread after it has entered the
female coupling one and one-half to two threads to prevent
any excess compound from entering the system.
When securing, anchoring, or hanging the suction and liquid
lines, be sure and allow enough flexibility between the
compressor and the first set of hangers or points where
the lines are secured to permit some degree of freedom.
This flexibility relieves strain in the joints of these
lines at the compressor due to compressor vibration.
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Multiple
Compressors
Parallel operation of two or more reciprocating compressors
should be avoided unless there are strong and valid reasons
for not using a single compressor. In a situation where two
compressors must be used, extreme care in sizing and arranging
the piping system is essential.
An acceptable arrangement of two compressors and two condensers
is shown in figure 6-47. An equalizer line connects the
crankcase at the oil level of each machine. Therefore, the
oil in both machines will be at a common level. If machines
of different sizes are used, the height of the bases beneath
the machines must be adjusted so the normal oil level of
both machines is at the same elevation; otherwise, the oil
accumulates in the lower machine.
This arrangement is called a single-pipe crankcase equalizer.
It can be used only on those machines with a single equalizer
tapping entering the crankcase in such a position that the
bottom of the tapping just touches the normal oil level.
Another method of piping to maintain proper oil level in
two or more compressors uses two equalizer lines between
the crankcase—one above the normal oil level and one
below. The double equalizer system must be used on compressors
having two equalizer tappings. A single equalizer line on
machines having two equalizer tappings should never be used.
The lower oil equalizer line must not rise above the oil
level in the crankcase and should be as level as possible.
This is important since the oil builds up in one crankcase
if the line rises. The upper equalizer line is a gas line
intended to prevent any difference in crankcase. pressure
that would influence the gravity flow of oil in the lower
equalizer line or the level of oil in the crankcase. This
upper line must not dip, and care should be taken to eliminate
pockets in which oil could accumulate to block the flow
of gas. Valves in the crankcase equalizer lines are installed
with the stems horizontal, so no false oil levels are created
by oil rising over the valve seat and minimize flow resistance.
It is poor practice to skimp on piping when making up these
equalizer lines. Oversize piping is preferred to undersize
piping. General practice indicates the use of oil equalizer
lines equal to the full size of the tapping in the compressor.
The discharge lines from the compressors are also equalized
before they enter the condensers. This, in effect, causes
the individual condensers to function as a single unit.
This is the most critical point in the piping system. It
is here that pressure drop is extremely important—a
pressure drop of 0.5 psi being equal to a 1.0 foot head
of liquid. Excessive pressure drop in the equalizer line
may rob one condenser of all liquid by forcing it into the
other condenser. One of the results may be the pumping of
large quantities of hot refrigerant vapor into the liquid
lines from the condenser of the operating compressor. This
could reduce the capacity of the system materially. For
this reason, the equalizer line should be just as short
and level as possible. A long equalizer line introduces
an unequal pressure in condensers if one of the compressors
is not operating. The refrigerant then accumulates in the
condenser of the non-operating compressor. The equalizer
line should also be generously sized and should be equal
to or larger than the discharge 1 line of the largest compressor
being used.
If the condensers are more than 10 feet above the compressor,
U-traps or oil separators should be installed in the horizontal
discharge line where it comes from each compressor.
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 implies—separate
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 pressures—the 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
1. 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.
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