A scroll compressor is a device for compressing air or refrigerant. It is
used in air conditioning equipment, as an automobile supercharger and as a
vacuum pump. A scroll compressor operating in reverse
is known as a scroll expander, and can be used to generate mechanical work from
the expansion of a fluid, compressed air or gas. Many residential central heat
pump and air conditioning systems and a few automotive air conditioning systems
employ a scroll compressor instead of the more traditional rotary,
reciprocating, and wobble-plate compressors.
History Léon Creux first patented a scroll
compressor in 1905 in France and the US. Creux invented the compressor as a
rotary steam engine concept, but the metal casting technology of the period
was not sufficiently advanced to construct a working prototype, since a
scroll compressor demands very tight tolerances to function effectively. The
first practical scroll compressors did not appear on the market until after
World War II, when higher-precision machine tools enabled their
construction. They were not commercially produced for air conditioning until the
early 1980s. Design
A scroll compressor uses two interleaving scrolls to pump, compress
or pressurize fluids such as liquids and gases. The vane geometry may be
involute, Archimedean spiral, or hybrid curves.
Often, one of the scrolls is fixed, while the other orbits eccentrically
without rotating, thereby trapping and pumping or compressing pockets of fluid
between the scrolls. Another method for producing the compression motion is
co-rotating the scrolls, in synchronous motion, but with offset centers of
rotation. The relative motion is the same as if one were orbiting.
Another variation is with flexible tubing where the archimedean spiral acts
as a peristaltic pump, which operates on much the same principle as a toothpaste
tube. They have casings filled with lubricant to prevent abrasion of the
exterior of the pump tube and to aid in the dissipation of heat, and use
reinforced tubes, often called 'hoses'. This class of pump is often called a
'hose pumper'. Since there are no moving parts in contact with the fluid,
peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals
and glands makes them comparatively inexpensive to maintain, and the hose or
tube is a low-cost maintenance item compared to other pump types.
Applications Air conditioner compressor
Vacuum pump Superchargers for automobile
applications, e.g. Volkswagen's G-Lader Engineering comparison to other pumps
These devices are known for operating more smoothly, quietly, and reliably
than conventional compressors in some applications. Unlike pistons, the
orbiting scroll’s mass can be perfectly counterbalanced, with simple masses, to
minimize vibration. The scroll’s gas processes are more continuous.
Additionally, a lack of dead space gives an increased volumetric efficiency.
= Rotations and pulse flow = The compression process occurs over
approximately 2 to 2½ rotations of the crankshaft, compared to one rotation for
rotary compressors, and one-half rotation for reciprocating compressors.
The scroll discharge and suction processes occur for a full rotation,
compared to less than a half-rotation for the reciprocating suction process,
and less than a quarter-rotation for the reciprocating discharge process.
Reciprocating compressor have multiple cylinders, while scroll compressors only
have one compression element. The presence of multiple cylinders in
reciprocating compressors reduces suction and discharge pulsations.
Therefore, it is difficult to state whether scroll compressors have lower
pulsation levels than reciprocating compressors as has often been claimed by
some suppliers of scroll compressors. The more steady flow yields lower gas
pulsations, lower sound and lower vibration of attached piping, while
having no influence on the compressor operating efficiency.
= Valves = Scroll compressors never have a suction
valve, but depending on the application may or may not have a discharge valve.
The use of a dynamic discharge valve is more prominent in high pressure ratio
applications, typical of refrigeration. Typically, an air-conditioning scroll
does not have a dynamic discharge valve. The use of a dynamic discharge valve
improves scroll compressor efficiency over a wide range of operating
conditions, when the operating pressure ratio is well above the built-in
pressure ratio of the compressors. If the compressor is designed to operate
near a single operating point, then the scroll compressor can actually gain
efficiency around this point if there is no dynamic discharge valve present.
= Efficiency = The isentropic efficiency of scroll
compressors is slightly higher than that of a typical reciprocating compressor
when the compressor is designed to operate near one selected rating point.
The scroll compressors are more efficient in this case because they do
not have a dynamic discharge valve that introduces additional throttling losses.
However, the efficiency of a scroll compressor that does not have a
discharge valve begins to decrease as compared to the reciprocating compressor
at higher pressure ratio operation. This is a result of under-compression losses
that occur at high pressure ratio operation of the positive displacement
compressors that do not have a dynamic discharge valve.
The scroll compression process is nearly 100% volumetrically efficient in pumping
the trapped fluid. The suction process creates its own volume, separate from
the compression and discharge processes further inside. By comparison,
reciprocating compressors leave a small amount of compressed gas in the
cylinder, because it is not practical for the piston to touch the head or
valve plate. That remnant gas from the last cycle then occupies space intended
for suction gas. The reduction in capacity depends on the suction and
discharge pressures with greater reductions occurring at higher ratios of
discharge to suction pressures. Reliability
Scroll compressors have fewer moving parts than reciprocating compressors
which, theoretically, should improve reliability. According to Emerson
Climate Technologies, manufacturer of Copeland scroll compressors, scroll
compressors have 70 percent fewer moving parts than conventional reciprocating
compressors. In 2006 a major manufacturer of food
service equipment, Stoelting, chose to change the design of one of their soft
serve ice cream machines from reciprocating to scroll compressor. They
found through testing that the scroll compressor design delivered better
reliability and energy efficiency in operation. However, many refrigeration
applications rely on reciprocating compressors, which appear to be more
reliable in these applications than scroll compressors. These applications
include supermarket refrigeration and truck trailer applications.
= Vulnerabilities = Scroll compressors are more vulnerable
to introduced debris, as any debris need to pass through at least two closed
compression pockets. The scrolls that operate without radial and/or axial
compliance are even more prone to the damage caused by foreign objects.
However, scrolls do not have suction valves, which is one of the most
vulnerable parts of the reciprocating compressor to liquid flooding.
Scroll compressors utilize different methods of protection inside the
compressor to handle difficult situations. Some scroll designs utilize
valves at different points in the compression process to relieve pressure
inside the compression elements. A reciprocating compressor can run in
either direction and still function properly, whereas a scroll compressor
must rotate in one direction only in order to function. This can be important
during extremely short periods of power loss when a scroll compressor may be
forced to run backward from the pressure in the discharge line. Only single phase
scroll compressors would continue to run in reverse after the power comes back
on. If this happens, the scroll compressor will stop pumping. Running
scroll compressor in reverse for several minutes would normally not damage the
compressor. The three-phase compressor, as compared to single phase compressors,
would revert to operation in a forward direction at the end of a short power
interruption. However, it is important to properly wire the three-phase
compressor during the initial installation. If during the installation
the polarity is inadvertently reversed then the three-phase compressor would
run backward and the damage to the compressor may result if it goes
unnoticed for long period of time. Interestingly, one of the ways to
mitigate the flooded operation of the compressor on start up, is to actually
run the compressor for several minutes in the reverse direction before turning
the compressor in the forward direction. The short reverse run on the start up
would expel any liquid accumulated inside the compressor pumping element
back into the crankcase, as well as preheat the liquid stored in the
crankcase by dissipated motor heat. Expelling the liquid from the pumping
element and preheating any liquid refrigerant in the crankcase prior to
initiating the normal run in the forward direction significantly alleviates
problems with the flooded start. = Size =
Scroll compressors tend to be very compact and smooth running and so do not
require spring suspension. This allows them to have very small shell enclosures
which reduces overall cost but also results in smaller free volume. This is
a weakness in terms of liquid handling. Their corresponding strength is in the
lack of suction valves which moves the most probable point of failure to the
drive system which may be made somewhat stronger. Thus the scroll mechanism is
itself more tolerant of liquid ingestion but at the same time is more prone to
experience it in operation. Small size of a scroll compressor and quiet
operation allows for the unit to be built into high power density computers,
like IBM mainframes. Scroll compressors also simplify the piping design, since
they require no external connection for the primary coolant.
Partial loading Until recently, a powered scroll
compressor could only operate at full capacity. Modulation of the capacity was
accomplished outside the scroll set. In order to achieve part-loads, engineers
would bypass refrigerant from intermediate compression pocket back to
suction, vary motor speed, or provide multiple compressors and stage them on
and off in sequence. Each of these methods has drawbacks:
Bypass short-circuits the normal refrigeration cycle and allows some of
the partially compressed gas to return to the compressor suction without doing
any useful work. This practice reduces overall system efficiency.
A two-speed motor requires more electrical connections and switching,
adding cost, and may have to stop to switch.
A variable speed motor requires an additional device to supply electrical
power throughout the desired frequency range. Also variable frequency drive
associated with variable speed compressor has its own electrical
losses, and is a source of additional significant cost and often is an
additional reliability concern. Compressor cycling requires more
compressors and can be costly. In addition, some compressors in the system
may have to be very small in order to control process temperature accurately.
Recently, scroll compressors have been manufactured that provide part-load
capacity within a single compressor. These compressors change capacity while
running. Reciprocating compressors often have
better unloading capabilities than scroll compressors. Reciprocating
compressors operate efficiently in unloaded mode when flow to some of the
cylinders is completely cut off by internal solenoid valves. Two-stage
reciprocating compressors are also well suited for vapor injection when
partially expanded flow is injected between the first and second compression
stages for increased capacity and improved efficiency. While scroll
compressors can also rely on vapor injection to vary the capacity, their
vapor injection operation is not as efficient as for the case of
reciprocating compressors. This inefficiency is caused by continuously
changing volume of the scroll compressor compression pocket during the vapor
injection process. As the volume is continuously being changed the pressure
within the compression pocket is also continuously changing which adds
inefficiency to the vapor injection process. In case of a two-stage
reciprocating compressor the vapor injection takes place between the two
stages, where there is no changing volume. Both scroll and reciprocating
compressors can be unloaded from mid-stage compression, however
reciprocating compressors are also more efficient for this mode of unloading
than scroll compressors, because the unloaded port dimensions in case of
scroll is limited by the internal port size, which would not be the case for a
reciprocating compressor where unloading again occurs from between the two
stages. Emerson manufactures a scroll compressor
that is capable of varying the refrigerant flow as per requirement.
Instead of fixing the scrolls together permanently, the scrolls are allowed to
move apart periodically. As the scrolls move apart, the motor continues to turn
but the scrolls lose the ability to compress refrigerant, thus motor power
is reduced when the scroll compressor is not pumping. By alternating the two
different working states: the loaded state and the unloaded state. A solenoid
contracts and expands the rotating scroll and/or the fixed scroll, using
axial compliance. The controller modifies the load time, and the unload
time, matching the capacity of the compressor to the load requested. This
type of scroll compressors while offering variable capacity control,
normally down to 20% of the full flow, can suffer from a significant loss of
efficiency especially toward the lower range of the capacity control.
See also Vacuum pump
Gas compressor Pump
Compressed air battery References
External links Copeland Compressors 111, video showing
how the scroll compressor works