Dinc Ibrahim. Refrigeration systems and applications 2th edition
Подождите немного. Документ загружается.
Refrigeration System Components 111
Motor
start winding
Motor
main winding
Insulation
Motor-stacking (Stator)
Rotor
Crankshaft
Top
main bearing
Internal
motor overload
Oil groove
Connecting rods
Crankcase
Suction muffler
Oil spinner
Outboard bearing
Thrust
plate
Oil
grooves
Piston
Locking
pin
Valve
plate
Cylinder head
Rubber mtg grommet
Cylinder head
gasket
Suction muffler
cover gasket
Discharge tube
Suction chamber
cover
Valve plate
Suction valve leaf
Internal
spring mounting
Weldseam
Discharge valve
leaf assembly
Piston pin
Discharge
shock loop
Discharge
muffler assembly
Motor
fan blades
Compressor shell
Internal
suction pickup
Figure 3.3 A typical hermetic reciprocating compressor (Courtesy of Tecumseh Products Co).
example, the compressor capacity ranges from 1/12 HP to 30 BG in household refrigerators. Their
revolutions per minute are either 1450 or 2800 rpm. Hermetic compressors can work for a long time
in small-capacity refrigeration systems without any maintenance requirement and without any gas
leakage, but they are sensitive to electric voltage fluctuations, which may make the copper coils of
the motor burn. The cost of these compressors is very low. Also, Figure 3.4 shows two air-cooled
condensing units using a hermetic-type refrigeration compressor.
3.5.2 Semihermetic Compressors
In larger sizes, refrigeration compressors are often semihermetic, that is, although motor and com-
pressor are within one casing, this casing may be unbolted, and the refrigerant does not flow over
the motor windings. Access for maintenance is straightforward, but the need for external motor
cooling which aids efficiency in cooling applications is no advantage in refrigeration operations,
and the cost is substantially higher than for hermetic units. As large motors are more efficient than
small ones, overall efficiencies of up to 70% or more are theoretically possible, and in multicylin-
der compressors, capacity may be controlled by making one or more cylinders ineffective (e.g., by
holding the inlet valve open). Cylinder unloading at start-up is also a convenient way of reducing
starting torque.
These compressors (single or double acting) were developed to avoid the disadvantages of the
hermetic compressors. Semihermetic compressors are identical to the hermetic types, but the motor
112 Refrigeration Systems and Applications
Figure 3.4 New, high-efficient compact coil air-cooled condensing units using hermetic compressors (Courtesy
of Tecumseh Products Co).
(
b
)(
a
)
Figure 3.5 Semihermetic reciprocating compressors. (a) Single stage. (b) Two stage (Courtesy of Bitzer
K¨uhlmaschinenbau GmbH ).
and compressor are constructed in a fabricated enclosure with bolted sections or access panels
to facilitate servicing. These compressors are manufactured in small and medium capacities and
their motor capacities can reach 300 kW. For this reason they are cheap and another advantage
is that they are compact. Also, they do not have a leakage problem. Figure 3.5 shows new-type
semihermetic reciprocating compressors for medium- and low-temperature commercial refrigeration
applications. These compressors are available for alternative refrigerants (e.g., R-134a, R-404A,
and R-507). Figure 3.5a shows the cutaway view of a single-stage, octagon series semihermetic
reciprocating compressor with nominal motor powers of 60 and 70 hp. With integrated pulsation
mufflers and capacity control (100-50%), smooth running, efficient, and compact reciprocating
semihermetics are also available now for this category of capacity. They can be operated with
the refrigerants R-134a, R-407C, R-404A, R-507A, and R-22. Figure 3.5b shows a two-stage
semihermetic reciprocating compressor for extremely low-temperature applications and its main
Refrigeration System Components 113
feature is the two-stage compression in one housing. In two-stage compression, the compression
ratio is divided, thus avoiding extreme operating temperatures and achieving very reliable operation.
Particularly for commercial refrigeration applications with high load variations, an energy-efficient
operation at full and part load (up to four capacity stages) with all common refrigerants is possible
at reasonable cost. In addition to that, there are the recognized features of the octagon compressors
which even pay off double in tandem configuration.
3.5.3 Open Compressors
Open reciprocating compressors with a shaft seal and an external drive motor suitable for a range
of prime movers are also available up to about 2 MW duty (e.g., compressor in the condensing
unit given in Figure 3.6b). In these compressors, the crankshafts, which are externally coupled with
electric motors, extend through the compressor housings. Appropriate seals must be used where
the shafts come through the compressor housings to prevent refrigerant gas from leaking out or
air from leaking in (when the crankcase pressure is lower than atmospheric pressure). In order to
prevent leakage at the seal, the motor and compressor are rarely enclosed in the same housing.
Figure 3.6a shows an open-type reciprocating compressor which is suitable for all kinds of
refrigerants, including NH
3
, and Figure 3.6b shows a compact air-cooled condensing unit with an
open reciprocating compressor.
3.5.4 Displacement Compressors
These compressors use the shaft work to increase the refrigerant pressure by reducing the compres-
sion volume in the chamber. The compressors of this group are reciprocating, vane (rotary), and
screw (helical rotary) compressors.
3.5.4.1 Reciprocating Compressors
A great majority of reciprocating compressors which compress the refrigerant gas only on the for-
ward stroke of a piston are built to be single acting in a large-capacity range, up to hundreds of
(a) (b)
Figure 3.6 (a) Open type reciprocating compressor and (b) air-cooled condensing unit with an open type
reciprocating compressor (Courtesy of Bitzer K¨uhlmaschinenbau GmbH ).
114 Refrigeration Systems and Applications
Figure 3.7 An internal view of V-type six-cylinder reciprocating compressor (Courtesy of Grasso
Products b.v.).
kilowatts. Models of these compressors may be single-cylinder or multicylinder in V (Figure 3.7),
W, radial, or line form. The power required for the compressor can be provided either directly by
a motor or indirectly by a belt or a gear drive. In these compressors, cylinder clearance volume,
compression ratio, amount of suction superheat, valve pressure drops, and the refrigerant-oil char-
acteristics are the main parameters which affect their efficiencies. The selection of cooling method
is dependent on the discharge temperature. For example, when the discharge temperature is low, as
in R-134a compressors, air cooling is usually chosen. Water cooling is used where high discharge
temperatures occur.
Note that Danfoss Maneurop hermetic reciprocating compressors are specially designed for appli-
cations with a wide range of operating conditions. The concept has proven its reliability and
durability in low-, medium-, and high-temperature applications. Suction gas enters the compressor
and cools the electrical motor. The circular valve design and profiled piston provide for an efficient
compression process. Discharge gas passes through an internal muffler to eliminate gas pulsation
which reduces sound level and vibration. The internal discharge line runs through the oil sump
taking care of an oil temperature high enough to evaporate eventual liquid refrigerant entering the
compressor.
3.5.4.2 Rotary Compressors
In reality, rotary compressors are of four general design configurations: (i) rolling piston, (ii) rotating
vane, (iii) screw, and (iv) scroll. Therefore, rotary compressors have a rotary or circular motion
Refrigeration System Components 115
instead of a reciprocating motion. They operate on rotors which rotate on an eccentric shaft. Gas
enters through a space between the rotor and the cylinder through a suction port. The gas is
compressed as the rotor revolves because of the eccentrical assembly of the rotor and the cylinder.
A discharge port on the opposite releases the compressed air. The two more commonly used rotary
compressors include the rolling piston-type and the rotating-vane-type. Both are very similar in
size, performance, and applications. Rotary compressors are popular in domestic refrigeration and
suited for applications where large volumes of vapor are circulated and where a low-compression
ratio is desired. In fact, these work as positive displacement pumps.
3.5.4.3 Vane Compressors
There are two major types of vane compressors, single-vane (rotary) and multivane. A rotary
compressor simply consists of a bladed, eccentric rotor in a cavity. As the rotor turns, the blades
extend and retract, sealing off the cavity into segments of varying size. The gas enters the intake port
where the segments are large, is compressed as the cavities are reduced, and is discharged where
the segments are small. These compressors are commonly used in domestic refrigerators, freezers,
and air conditioners. The possible maximum compression ratios achieved are on the order of 7:1.
Small systems and some ammonia systems also employ compressors of this type. In multistage
systems in which each stage has a low-compression ratio, vane compressors can be used as boosters.
Figure 3.8 shows the cutaway view of a rotary vane compressor. These compressors have some
basic advantages which are as follows:
• Simple, compact design. Sturdy construction with few moving parts, easy to access and main-
tain, easy to replace parts, very reliable, and durable.
• Single-stage compression. The nature of the design produces sufficient compression in a single
stage, resulting in a very high-compression ratio during cycle, as well as better energy efficiency,
reduced risk of fault, and reduced maintenance requirements.
• Direct axial coupling to the motor. Direct coupling is possible because the high-compression
ratio permits low-rotation speeds, eliminating the need for transmission or gears. Fewer parts
mean lower energy dissipation and simplified maintenance.
• Low-rotation speeds. Lower speeds reduce vibration, thus diminishing noise and wear, lowering
temperature, and eliminating the need for foundations.
• Low cycle temperature. Lower temperatures reduce wear, oil consumption, and leakage caused
by distension of parts. Less energy is needed for cooling and the purity of delivered air is
enhanced.
• Low need for maintenance. With fewer parts suffering little wear, single-stage rotary vane units
offer cleaner and more reliable operation, significantly reducing maintenance needs.
3.5.4.4 Screw Compressors
Surprisingly enough, the screw compressor was invented in 1878. However, commercial application
developed slowly because of its inability to match tight tolerances with existing manufacturing
equipment of the time. Over the past 10 years, several manufacturers have introduced chillers
with screw compressors and have moved away from older reciprocating technology. Screw
compressor technology offers many benefits over reciprocating types, including higher reliability
and improved performance. In addition to these benefits, some noteworthy characteristics make
the screw compressor the compressor of choice for future chiller developments and designs
(Duncan, 1999).
116 Refrigeration Systems and Applications
Figure 3.8 Cutaway view of a rotary vane compressor (Courtesy of Pneumofore SpA).
(
a
)(
b
)
Discharge port
Discharge port
Suction side
Figure 3.9 Screw compressor. (a) Dual rotor. (b) Mono rotor (Duncan, 1999) (Courtesy of ASHRAE ).
Screw compressors are also positive displacement refrigeration system components. Both single-
screw and twin-screw compressors are widely used in refrigeration applications. A single-screw
compressor consists of a single helical rotor (shaft) and a pair of gate rotors that then mesh together,
and with the casing form a sealed volume wherein compression takes place. There are two different
rotary screw compressor designs. One is a twin rotary screw design, in which there is a male and
a female rotor that mesh together (see Figure 3.9a). The other is a single rotary screw design, in
which two gate rotors are placed on both sides of the main compressor rotor (Figure 3.9b).
Reciprocating compressors have, until now, carried the workload in applications requiring
temperatures below −35
◦
C. This was the technology of choice, mainly because cascading
Refrigeration System Components 117
refrigeration systems was the only choice. The screw compressors were developed specifically
for use in applications of −40
◦
C and below (down to −50
◦
C). Originally designed for larger
applications, this technology is now available in chambers requiring only a single 15 hp or larger
compressor. The development of this advanced screw-style refrigeration system offers the following
benefits:
• better performance per hp,
• improved reliability,
• reduced costs,
• fewer moving parts,
• less vibration, and
• less refrigerant loss.
By design and function, the screw compressor has far fewer moving parts than the reciprocating
style. Engineered with no valves and rolling element bearings, the total number of parts is dras-
tically reduced as well. This reduction of parts is important because it dramatically improves the
compressor’s reliability rate and increases its expected life span.
Note that screw compressor technology greatly reduces the risk of refrigerant loss because of
the decrease in vibration within the entire system. Any structural breakdown within a refrigeration
unit may cause loss of its valuable refrigerant. With the accelerating costs of R-22, R-134a, and
R-507/404A, product loss becomes a crucial operating factor.
A twin-screw compressor consists of two helically grooved rotors (containing a pair of inter-
meshing screws) and operates like a gear pump (Figure 3.10). The male screw is directly coupled
to the electric motor and this drives the compressor. With the absence of the suction or discharge
valves, the gas is drawn into the compression chambers between the gear teeth and the cylinder
wall and the helical movement of the gears forces the gas to travel parallel to the rotor shaft. Single-
screw compressors are also available and these consist of a helical gear on the main rotor shaft
and a pair of planet wheels, one on either side to separate the high and low pressures. Oil flooding
(a) (b)
Figure 3.10 A large-capacity double-screw compressor. (a) Complete view. (b) Internal view (Courtesy of
Grasso Products b.v .).
118 Refrigeration Systems and Applications
provides lubrication and restricts leakage of refrigerant gas. These compressors are used mainly in
heat reclaim and heat pumping applications.
A screw compressor (Figure 3.10) has a male rotor (with four lobes) which drives a female
rotor (with gullies) in a stationary casing with the inlet port at one end and the outlet port at the
other. The rotating elements open a void to the suction inlet of the vane compressor, take in a
volume of gas, and then seal the port. More rotation decreases the volume between the rotors and
compresses the refrigerant gas. The gas is discharged at the low-volume, high-pressure end of the
compressor through the outlet port. In general, the male rotor is directly driven. On the other hand,
the female rotor rotates along with the male rotor, either through a gear drive or through direct
rotor contact.
For industrial refrigeration applications, such as process chillers, the high-temperature compact
screw compressors provide an ideal solution. The integrated oil separator and oil reservoir signif-
icantly reduce the installation time, complexity, cost, and space required. Such compressors are
available in sizes ranging from 50−140 hp and are equipped with the dual capacity control system
and auto-economizer and can be used with the common refrigerants R-134a, R-407C, and R-22
(R-404A, R-507A in special applications). The operation with or without economizing is possible.
Figure 3.11 shows the cutaway view of a hermetic rotary type screw compressor which is
commonly used in small-scale refrigeration applications, particularly in household and commer-
cial units.
Figure 3.11 Internal view of a hermetic rotary screw compressor (Courtesy of Hartford Compressors).
Refrigeration System Components 119
(
a
)(
b
)(
c
)
Figure 3.12 A hermetic scroll compressor. (a) Complete view. (b) Cutaway view. (c) Internal view (Courtesy
of Carlyle Compressor Company).
3.5.4.5 Scroll Compressors
The scroll compressor (Figure 3.12) uses one stationary (fixed) and one orbiting scroll to compress
refrigerant gas vapors from the evaporator to the condenser of the refrigerant path. The upper scroll
is stationary and contains the refrigerant gas discharge port. The lower scroll is driven by an electric
motor shaft assembly imparting an eccentric or orbiting motion to the driven scroll. That is, the
rotation of the motor shaft causes the scroll to orbit (not rotating) about the shaft center.
This orbiting motion gathers refrigerant vapors at the perimeter, pockets the refrigerant gas, and
compresses it as the orbiting proceeds. The trapped pocket works progressively toward the center of
the stationary scroll and leaves through the discharge port. Maximum compression is achieved when
a pocket reaches the center where the discharge port is located. This happens after three complete
orbits. The compression is a continuous process. When gas is being compressed in the second orbit,
another quantity of gas enters the scrolls and a quantity of gas is being discharged at the same time.
This ensures a smooth compression process with low noise and low vibration compared to other
compression technologies. Studying this time lapse series carefully gives a true picture on how the
trapped gases are progressively compressed as they proceed toward the discharge port.
Scroll compressors are a relatively recent compressor development and are expected to eventually
replace reciprocating compressors in many cooling system applications, where they often achieve
higher efficiency and better part-load performance and operating characteristics.
3.5.5 Dynamic Compressors
These compressors increase the refrigerant pressure through a continuous exchange of angular
momentum between a rotating mechanical element and the fluid subject to compression. The main
types are centrifugal and turbo compressors.
120 Refrigeration Systems and Applications
3.5.5.1 Centrifugal Compressors
Centrifugal compressors are often used in place of positive displacement compressors for very large
capacities, or for high-flow, low-pressure difference applications, and are available, designed for
refrigeration use, in the 300 kW–20 MW range (e.g., 400−10,000 tons). Centrifugal compressors
are also appropriate to multistage refrigeration applications, where two or more compression stages
may be incorporated within the same turbine housing with interstage gas injection between the
rotors. These compressors produce compression by means of a high-speed impeller connected to
an electric motor or gas engine. Figure 3.13a shows the cutaway view of a centrifugal compressor
which uses hybrid bearings. The incorporation of hybrid bearings in compressor designs allows the
refrigerant itself to be used as the lubricant. Figure 3.13b shows a chiller unit with a centrifugal
compressor using hybrid bearings.
The centrifugal compressors available in the market use R-123, R-22, and R-134a. This usu-
ally calls for semihermetic designs, with single or multistage impellers. In refrigeration industry,
multistage centrifugal compressors are now manufactured with cast iron, nodular iron, and cast
steel casings for discharge pressures up to 40 bar. With up to eight wheels in a single casing, the
compressor has a capacity of 42,000 m
3
/h and 9000 kW.
Note that refrigeration systems using ammonia as the refrigerant are not generally available with
centrifugal compressors. Only open-drive screw or reciprocating compressors are compatible with
ammonia, largely because of its corrosive characteristics and reactions with copper.
The selection of single stage, multistage, open, or hermetic designs is largely a function of
individual manufacturer preference and the application. For example, centrifugal compressors are
limited in their compression ratio per impeller. Therefore, applications calling for high-temperature
lifts (such as with ice thermal storage) may require multistage designs.
The operating principle of a centrifugal compressor is the same as that of a centrifugal pump,
but the refrigerant gas is pumped instead of a liquid. A rotating impeller imparts velocity to the gas,
flinging it outward. The housing slows the gas flow, converting a portion of the kinetic energy (the
velocity pressure) into a static pressure. These compressors are commonly used for large-capacity
(a) (b)
Figure 3.13 (a) Cutaway view of a centrifugal compressor. (b) A chiller unit with centrifugal compressor
(Courtesy of Trane Company).